Our First Guest Blogger – Lawrence Krauss

By Mark Trodden | November 14, 2005 1:32 pm

After the incredible response to two of our recent posts (Krauss on Intelligent Design, Religion (and String Theory); and From the Sublime to the Ridiculous), Sean, JoAnne, Clifford, Risa and I asked Lawrence Krauss if he would be interested in submitting a post summarizing his views on the issues raised regarding string theory, religion, and the popularization of science.

Lawrence is an extremely well regarded member of the physics community, whose research, popular writings and remarkable efforts to defend science against pseudoscience and political distortion have earned the respect of all of us. We were therefore delighted when Lawrence agreed to make time to do this and we welcome him as Cosmic Variance‘s first guest blogger.

We look forward to a high level of discussion regarding this post and, since it might need saying explicitly – keep it polite please folks! Here is Lawrence’s post.

__________________________

The contributors asked me to write and clarify some of my thoughts, especially since in one way or another I, and my writing, have been the subject of a few blog threads. I have already tried to respond; eventually hopefully clearly, within the threads to the concerns, but perhaps a single reasonably cogent monologue bringing some of these ideas together may be worthwhile. We will see:

On popularizing: the most important thing to attempt to get at is the difference between what we know, and what we don’t know; and how we can tell the difference.

On String Theory: A mammoth and very deep and original enterprise which, to date, has not been particularly successful, in my opinion. While string theory has been a fruitful stimulation of new mathematical ideas, as a search for physical theory it hasn’t been productive. This is not to say that it one day will not be so. It simply hasn’t achieved the goals it originally had, and thus far has not been able to make contact in any useful way with either experiment or observation, nor has it yet explained any of the fundamental theoretical puzzles that drive particle physics. Is it interesting? Yes! Should theorists continue to investigate it? Yes! Might it be of vital importance if it leads anywhere? Yes.

Is it worth talking about to the general public? I am not sure. As a study in the kinds of things that physicists sometimes like to think about, and why they think about them, yes; and that is what I have tried to talk about in my most recent book, and no doubt others have as well. As a demonstration of what is likely to be the underlying reality beneath what we observe in the world today, no; namely I personally do not think there is any compelling evidence that these ideas are going to be correct. Nor do I think it fair to place the large set of ideas that are currently being explored under the single banner of a “theory”. What seems clear is that as these ideas are explored in greater depth, the mathematical complexities increase and the possible connections to the world we measure seem to have decreased. I don’t know where things are headed, but frankly I see no reason for great optimism. And, as I try to emphasize to popular audiences, it is important to realize that most theoretical ideas, even great ones, are wrong. So in some sense it is important to keep that fact in the back of one’s mind whenever any new ideas are being discussed.

The good news is that ultimately science has been able to determine which ideas are wrong, and one hopes in the case of string theory this might be possible too, although, as Ed Witten himself has pointed out, it may turn out to be impossible for string theory to make any contact with the measurable world. It is also vitally important that those who are going to devote most of their productive years trying to work on an idea have faith that it is going to pan out. There is nothing wrong with that – it is required to keep up one’s motivations – but it might not pan out. That is the way it goes in science. It was in this context that I think the example of the dual-string model and QCD, which so irked Clifford, is relevant. It is not to make fun of infinities; rather it (a) demonstrates some of the subtleties of mathematics, which lord knows is a difficult subject to try and popularize, and (b) it illustrates what I said in one of the blog comments, namely “My point was not to use infinities to argue against anything.. but to point out that canceling infinities, as the dual-string did, was not by itself a guarantee that it was right, but that a completely different theory ended up coming along and replaced it.. as could easily happen again… I didn’t use it to argue that anything was flawed.. but merely that it is a mathematical problem that needs to be solved, but not every solution of it needs to correspond to reality.”

On String sensitivity: I understand that young people who currently work on string theory probably feel that they work under an undue burden, placed upon them by the original hype associated with the remarkable results in the mid 1980’s, which has continued and sometimes escalated since. I also understand that they may be entranced with various aspects of the ideas that have been developed. That is fine. But it simply is not yet on a par–in almost any sense–with any of the other significant, successful, and well-defined theoretical and experimental developments in physics in the past century. That is neither a bad nor a good thing; it is a fact. And relating one’s own excitement is fine, and good, but it should be tempered with a dose of realism, especially when discussing things to a popular audience, which cannot discern science from pseudoscience in general, much less the finer details of particle physics.

It is bad for science to give the impression that we know more than we do. Moreover, I hope this explains some of the sensitivity of others who do not work on string theory – namely, there are truly great and wonderful developments in theoretical and experimental physics that have simply been far more important and successful at describing nature and which have, in addition, led to technological advances. There have also been concrete discoveries, like dark energy, that are astounding and about which we currently have no clear understanding, so that other areas where we may have little understanding, such as the inconsistencies between GR and quantum mechanics, while important, are perhaps not the most overpowering immediate concerns.

On Extra Dimensions: I continue to remain neutral, if skeptical, here. While the notion of large undetectable extra dimensions is fascinating, and the fact that they can exist and have remained undetected is really fascinating, my own impressions, based on my understanding of particle physics data, is that they don’t smell right as a solution of the hierarchy problem. The apparent unification of couplings, large top quark mass, etc, provide at least suggestive evidence to me that there really is a large scale involved in unification, and also that supersymmetry seems to be suggested at some level. The research I did for my new book also made me frankly more skeptical from a theoretical perspective as well; namely, if I think about what the ground state of M-theory might be, the likelihood of some single, relatively isolated, relatively flat brane on which we live existing embedded in a higher dimensional and large space, seems unlikely to me. But we shall see.

On ID and Science: As many of you know who have followed any of my writing in this regard, the reason I took up this cause a bunch of years ago, and have spent many unfortunate hours defending science against attacks rather than doing what I prefer to do, which is getting people excited about science, is that I viewed the attack on evolution as an attack on science as a whole. The more I learned, the more I saw this as a campaign that was based on fear of the fact that God is not an explicit part of the scientific method. For some, this implies that science itself is immoral, and if you read much of the literature, in particular from the Discovery Institute, you will see this expressed explicitly. I also saw this campaign as not merely one by well-meaning but misinformed individuals, but rather by people who were very well schooled in public relations, who had a mission, and wanted to achieve it however possible. And since scientists, by nature, tend to be miserable at public relations, it seemed important to try and counter this in whatever ways possible.

My own awareness for the necessity of being respectful of religious beliefs has increased tremendously during this process. It has also become more clear to me that scientists tend to, whether they want to or not, appear patronizing about this, and also tend to make the philosophical leap from the fact that science deals with natural causes and effects to the statement that there can be no purpose in the universe. Whatever one’s personal perspective on this, and I see no evidence for purpose myself, this is a personal philosophical or religious notion, not a scientific one. In my piece in the NYT in May – the one that provoked the wrath of the Cardinal, Archbishop of Vienna – I used the example of Lemaitre and the Big Bang to point out that science functions independently of questions of purpose.

Now, how does all of this relate to string theory and the source of all the concern in one of the blog posts and the resulting comments? Well; it is the point I mentioned at the end of one of them, when responding to Clifford. I paraphrase: “the context in which I referred to ID was actually to make a point that I am beginning to think is actually relevant… namely that when physicists refer to ‘string theory’ it is in the context of ‘field theory’… namely as a technical replacement of one physical and mathematical framework for dealing with relativistic quantum mechanics with another.. but unfortunately in the context in which we complain about IDers saying Evolution is ‘just a theory’, the popular use of the term string theory is unfortunate.. because ‘string theory’ is not a theory in the context in which we claim evolution or general relativity is… i.e. something that has been tested time and again against experiment and observation.. calling it the string hypothesis would not be inappropriate in this sense..”

Unfortunately, string theory and extra dimensions are often held up as examples of science being indistinguishable from religion. I have tried, even in my last NYT piece, to explain some of the differences, but in a statement I made elsewhere that got someone very upset, I do believe that saying, other than tongue-in-cheek, that the current ideas are so beautiful that they must be correct–without any recourse to empirical data, is almost indistinguishable from religion.

  • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean

    Lawrence, thanks for chiming in. It’s only a matter of time before you have a blog of your own, I’m sure.

    Too many interesting things going on here, so let me just register my own quick reactions to your main points:

    — Whether or not string theory has been successful to date depends both on what you think its goals are, and how much progress one should expect. Of course it has not made direct contact with the real world, and it’s still hard to tell how it ever will, and that’s the ultimate test. Maybe it won’t, and we’ll move on to something else. But we have learned an incredible amount, working under the difficult situation of having very few experimental clues. The theory could have led nowhere, but it keeps leading us in surprising new directions — certainly more than enough progress to keep people from giving up.

    — As far as achieving goals is concerned, I think the primary goal is obviously to quantize gravity, no matter what people say in their more excitable moments. And in that direction, string theory is by far our best current idea, well worth pursuing on that basis alone.

    — I agree that separating what we know from what we’re speculating about is one of the most important goals of talking to the public. At the same time, I think it’s perfectly clear that we should be talking about string theory, and being honest about what we know and don’t know about it. Successful or not, the ideas behind string theory are both deep and incredibly exciting, and more importantly are what a large number of theoretical physicists today are actually thinking about. Why should we hide our work from the public until it’s all finished? We should trust them to be able to understand when we explain the speculative ideas we are thinking about.

    — I’m never sure what is meant by “respecting religious beliefs.” Personally, I don’t agree with such beliefs, and I firmly believe that scientific reasoning ultimately comes squarely into conflict with them. And I’m not reluctant to say so, since I think it’s the truth. At the same time, I think we should be very respectful of the individual people who have such beliefs; no reason not to have a rational discussion. I think there is overwhelming reason to believe that there is no purpose in nature, and it would be patronizing to not admit this just so we didn’t hurt people’s feelings.

    — I completely agree that saying that an idea is so beautiful that it must be correct is just silly, even if the silliness is coming from people who are much smarter than me. I tend to think that such utterances are in the spirit of Einstein expressing that he would have felt sorry for the Lord if general relativity hadn’t been correct; somewhat tongue-in-cheek. Hopefully.

  • Elliot

    To me the key aspect of string theory, whether its present lines of development prove to be correct or incorrect is to transcend the notion of point particles (zero dimensions) and theorize that there is at least one if not more than one dimension to the fundamental constituents of the universe. Doesn’t this conceptual shift actually put the theory (or one of its descendants) in a better position to remove infinities from equations describing these fundamental phenomena?

    Elliot

  • http://www.math.columbia.edu/~woit/blog Peter Woit

    As an example of why Lawrence might have a problem with some of the current attempts to promote string theory to the public, see the following news article about an event held in Princeton yesterday:

    http://www.dailyprincetonian.com/archives/2005/11/14/news/13792.shtml

    as well as some commentary about this at my weblog. I don’t think this kind of thing does anything to help the public understanding of physics, quite the opposite, and I know of very few people besides Lawrence who have been willing to publicly criticize it.

  • http://blogs.discovermagazine.com/cosmicvariance/clifford/ Clifford

    Hi Lawrence,

    Thanks for preparing the post. I very much agree with much of the caution (tested vs not tested) that you express, and have said many similar things several times on this blog.

    As to your second and fourth paragraphs, I am sure that it is important to talk about this to the public. This is why I was disappointed that you did not say those careful things in your public presentation, (and in fact it is very dangerous not to be careful when discussing such thing) and hence my earlier post, and the summary I gave in comments here , here and here.

    Here’s hoping that some of the people who were at that presentation will read your careful words here, and thus get closer to the correct story. Also, I’m glad that this discussion means that several new people (readers of this blog) will see this, rather than the more misleading (intended or not) statements discussed previously.

    Best Wishes, and thanks again.

    -cvj

  • http://blogs.discovermagazine.com/cosmicvariance/clifford/ Clifford

    Hi Peter,

    I note that the article that you pointed us to ends with the following:

    “Superstrings may be purely philosophical and may have no measurable contributions to our universe,” Foster said. However, their existence could be proved if they are large enough to create new, detectible particles when they collide.

    “If supersymmetry exists, the subatomic particles physicists found over the past 50 years are like various notes that can be played on superstrings,” the final slide read, as Foster and Liebeck performed a duet believed to have been played by Einstein.

    About his other analogies, I can’t comment. I don’t fully understand them!

    But he does seem to allow for (and clearly point out) the possibililty that the enterprise may come to naught. Is that not significant? The extracts on your blog’s post don’t mention these lines. I’m puzzled about that. Please help me.

    Cheers,

    -cvj

  • Hektor Bim

    The problem is the presentation of the problems of Einstein’s theory, coupled with strings as the only possible solution.

    I think it is frankly inappropriate to present string theory as “solving the problems of Einstein’s theories”, when to date it has done exactly nothing for our understanding of the physical universe. Mathematical contributions, sure. But no physical contributions at all, really.

    This is what annoys people who don’t work in string theory, and it happens again and again. String theory is presented as somehow correct, a physical theory, a solution, when to date at least, it is none of those things. At this stage, it is a speculation with no supporting evidence, based on extrapolations of known physics. It is inappropriate to include it next to Einstein’s theories, which had rapid and complete experimental verification.

  • http://www.math.columbia.edu/~woit/blog Peter Woit

    Hi Clifford,

    I don’t think it’s difficult to understand what the analogies were trying to explain (perturbative superstrings better behaved than perturbative quantization of GR as a QFT in the ultraviolet), or that they were unlikely to actually transmit any information about this and would instead just confuse people.

    In my posting I did quote Foster’s line that “Superstrings may be purely philosophical and may have no measurable contributions to our universe”, and commented that this is highly misleading and gives people the wrong idea about what science is. If superstring theory ends up involving “no measurable contributions to our universe”, it’s not “purely philosophical”, it’s wrong.

    The last line that you quote is something the reporter says that she copied off the final slide, it would be interesting to hear in her words what she thought it meant. I actually think it is again quite misleading: as your well aware, the existence of supersymmetry does not imply the existence of superstrings (although I’ll readily agree it would be some evidence that superstring theory might be on the right track).

    So I take it you have no problem with this kind of performance and think it should be encouraged?

    Peter

  • http://blogs.discovermagazine.com/cosmicvariance/clifford/ Clifford

    Whoops, I see a flaw in the last sentence…. susy does not imply strings, of course. that is certainly wrong.

    Cheers,

    -cvj

  • http://blogs.discovermagazine.com/cosmicvariance/clifford/ Clifford

    Ah,… just came back from your blog….. where I posted the above comment, and did not see yours here before I repeated my post here. Sorry. Indeed, susy does not imply strings, and he is wrong about that. Very wrong.

    But I do think that he was clear to state that strings may not be correct….

    And I think that “purely philosophical” in this context is universally understood to be the same as “wrong”.

    It would be nice if he was more accurate in his comments, I agree. I don’t see him being misleading about what other people’s research is about though. That would be bad. If he makes an analogy between this research and something that is not at all science (and in fact is damagaing science) and then does not explain the analogy, that would also be bad. If he claims (I can’t see it in a quick read) that string theory is the only solution to these problems, then he is also wrong to do that, since we don’t know that…… but if he is just trying to give some analogies about what is going on in string theory research and what sort of research people are doing with that field, then it is fine.

    I don’t think wrong statements should be encouraged in general, no.

    Cheers,

    -cvj

  • http://www.anthropic-principle.ORG island

    If origins science sticks with what we know, then we live in a finite expanding, yet flat universe and I’ll buy that from day-one without assuming inflationary theory nor a cosmic singularity either.

    If, as it appears, the goal of the big bang was an aboslutely flat and perfectly symmetrical structure, then it barely missed, but every action since then has increased the ongoing the effort “toward” *apparent* purposeful goal of absolute equilibrium.

    It requires an unfounded leap of faith to assume beyond what we know that humans can be here for any other purpose and our actions clearly demonstrate that we are very good at satisfying it.

    Sticking to what we know, my observation has been that we’ve carried a problem that never got resolved into higher physics under the assumption that this would resolve itself later, and this still seems to be the mentality.

    Nobody understands why antiparticles have postive mass.

    Nobody resolved the negative mass solutions for GR, but particles that comprise a negative pressure vacuum give the appearance of negative mass, due to the “anti-gravity” effect that comes from having less pressure and less density than matter.

    Won’t the wave functions in this background affect the expansion of the field and the corresponding creation and annihilation operators?

    Doesn’t this process require the condensation of vacuum energy before you can have a high-energy photon interaction to make it real?

    Won’t the rarefying effect of condensation increase negative pressure and cause vacuum expansion?

    Doesn’t the offset increase between positive mass-energy and negative pressure hold the universe flat as it expands?

    How can we just skip over a resolution between Relativity and Quantum theory if we carry this flaw into QFT and beyond?

    Sticking to what we know… the anthropic principle has VERY strong thermodynamic implications in only the observed universe.

  • Moshe

    Quickly before I will have to run: I really enjoyed reading this piece and feel grateful for Lawrence for taking time to write it. On first reading I cannot see anything I particularly disagree with… and to one of the points, maybe string theory being called the string framework or string activity or something is not a bad idea, this will also emphasize the fact that it is not really one idea (that is being carefully guarded by the believers) but a diverse and interesting process that is still developing.

    Hope things run smoothly here, ‘fraid I will have to decouple now…

  • Arun

    LK wrote:

    It has also become more clear to me that scientists tend to, whether they want to or not, appear patronizing about this, and also tend to make the philosophical leap from the fact that science deals with natural causes and effects to the statement that there can be no purpose in the universe. Whatever one’s personal perspective on this, and I see no evidence for purpose myself, this is a personal philosophical or religious notion, not a scientific one.

    Why is science worth doing? One has only so much lifespan, so many waking hours, so many working hours – why do science in that small period? I do not think there is a scientific justification for doing science; it boils down to personal perspective. Yes, there is the survival of the species, future generations, etc., but to the extent one is free agent, why should these matter? Then, if we are into respecting extra-scientific personal perspectives, then any religious-type perspective that does not attempt to dictate the content of the universe ( e.g., “any number assertions in the Scripture to the contrary will not make fire cord”) is also worthy of respect?

  • http://eskesthai.blogspot.com/ plato

    Is it worth talking to the general public

    Why would it not matter what the building blocks of nature are to us?

  • http://eskesthai.blogspot.com/ plato

    on Sean’s point

    — As far as achieving goals is concerned, I think the primary goal is obviously to quantize gravity, no matter what people say in their more excitable moments. And in that direction, string theory is by far our best current idea, well worth pursuing on that basis alone.

    I think Lee spoke to this as well….yet he was not satisfied either by it’s success, although success has been implied by use of string theory? Did it lead to other scientifc avenues in regards to technologies developement?

    By it’s theoretical nature does “extra dimension” not imply the way evotvos handles it procedures? This also is a comment to the ideas of “theory” as a establish process in scientifc dealings. If you cannot adhere to this process, then those looking over your shoulder and reading, will have been mislead by the the words chosen?

    Does this go to the susy comments brought forward here? Statements about a superfluid as misleading?

  • http://www.math.columbia.edu/~woit/blog Peter Woit

    A couple comments more directly about what Lawrence had to say, my earlier comment was somewhat tangential.

    1. I’ve been thinking a bit about his question about string theory: “Is it worth talking about to the general public?” As far as my own blogging goes, this doesn’t really come up since my concept there is to write assuming I’m talking to professional colleagues, although with the hope that interested members of the public will sometimes tune in and get something out of it, even if they find much of it mystifying. The people here at Cosmic Variance are trying to directly engage the public, which is great, but it seems to me they do need to seriously think about Lawrence’s question.

    I know some string theorists disagree, but I think an accurate characterization of particle theory these days is that it is a victim of its own success, with no promising ideas about how to get beyond the all too successful standard model. Absent the usual pattern of important new clues coming from experiment keeping theorists honest and on the right track, a certain degree of dysfunctional behavior has emerged in the way particle theory is being pursued. Do you really want to invite the public in to see this? As an analogy: if your kitchen is a complete mess and family members are having a loud, ugly argument about what to do about it, do you really want to invite the neighbors in to show them around? One possible tactic is to keep the kitchen door closed, show off the incredibly beautiful and elegant front rooms (e.g. the standard model), and tell people that you’re working on the kitchen, things aren’t going that well, so it’s in no shape for public viewing. Another is to throw the doors open, invite them in to see the mess and hear all the arguing, hoping that they’ll appreciate your family’s openness and straight-forwardness.

    I can see arguments for either of these tactics, but I think the tactic that many have actually adopted is a third and indefensible one: to keep people out of the kitchen while claiming everything is going just great in there, showing off drawings of what you wish your kitchen looked like, and giving the misleading impression that you’re making progress toward getting the kitchen to look like the drawings. The problem with this is that sooner or later your neighbors are going to find out what has been going on, and you will have lost all credibility with them.

    2. While I agree with just about everything Lawrence has to say, I have a different perspective on the point he makes here and elsewhere that saying a theory must be true because it is beautiful is not distinguishable from religion. Of course this is true: he’s right that what finally makes something a tested scientific theory is to confront it with experiment. But at an earlier, more exploratory stage when you can’t yet do that, picking one idea over another because it is more beautiful is actually a very rational, scientific way to proceed. Occam’s razor is important, and one should be looking for simple, elegant, beautiful ideas in preference to complicated, ugly ones, unless experimental evidence makes the ugly ones unavoidable. The problem with string theory is that it is no longer a beautiful idea about how to unify the standard model and gravity. All constructions that even partially reproduce the standard model are hideously ugly, with no experimental evidence to back them up. What is happening these days is that an increasingly large number of string theorists, in desperate attempts to get the standard model, have ended up working with increasingly hideous constructions.

  • zevatron

    Sean wrote…

    — I’m never sure what is meant by “respecting religious beliefs.” Personally, I don’t agree with such beliefs, and I firmly believe that scientific reasoning ultimately comes squarely into conflict with them. And I’m not reluctant to say so, since I think it’s the truth. At the same time, I think we should be very respectful of the individual people who have such beliefs; no reason not to have a rational discussion. I think there is overwhelming reason to believe that there is no purpose in nature, and it would be patronizing to not admit this just so we didn’t hurt people’s feelings.

    …and I think he’s right. “respecting religious beliefs” in the context here (this overall discussion) really means respecting and not patronizing a well-meaning person who professes religious belief.

    However, if it was the case that someone’s religious belief was that the world is flat (literally, not in the Thomas Friedmann sense) then I don’t think that belief is worthy of respect. Perhaps he or she is just a well meaning person who is mis-informed (and in this case woefully so). If also they attempted to present flat-earth geology as an viable alternative to geology then no respect is accorded.

    Whenever I get to riled up thinking about the ridiculousness and idiocy of some religious people attempting to have ID taught in science class or saying that Dover PA now voted itself onto God’s “pay-no-mind” list, I think that there are also people like my parents who are religious, well-meaning, but just not informed and are un-interested in the way science works. They’re not active pro or anti ID people, but just don’t think much about it or don’t have the time to. This is why public outreach about science is so important, because they are open to learning about it.

    Now, whether I still attend Mass on Sunday is something they DO worry about…

  • Lee Smolin

    Hi,

    I agree with most of what Lawrence said. But since I was mentioned, I should clarify why I and most people who work on quantum gravity, apart from string theorists, find it impossible to agree with Sean’s statement that string theory is “by far our current best idea” about quantum gravity.

    -Any acceptable quantum theory of gravity must incorporate the basic lesson of GR that the geometry of spacetime is dynamical and defined without reference to any fixed background-hence background independent. String theory has as yet no such formulation. Furthermore, there are several theories that provide precisely such a theory, such as loop quantum gravity, spin foam models and dynamical triangulation models, about which many non-trivial results have been proven.

    – Even at the background dependent level GR is not recovered because all backgrounds on which consistent propagation of worldsheets have been shown are static in that they have timelike or null killing fields. So, there is no evidence that string theory exists even on backgrounds that are time dependent, which is the generic case in GR. We know that the Einstein equations on the target space are (up to higher order terms) a neccessary condition for consistent worldsheet propagation. But sufficient conditions include also canceling the tachyon instability. In all known cases this imposes another condition which is worldsheet supersymmetry (or something equivalent) which in turn requires that the background be static. So it is not true that string theory predicts or incorporates GR as a low energy limit, because all solutionis to GR seem to be ruled out as giving inconsistent propagation except static ones.

    Thus, while there is talk about string theory on time dependent backgrounds there are no actual examples. Given the importance of supersymmetry in consistent string dynamics, it is plausible to me that this is because there is only consistent worldsheet propagation on static backgrounds.

    -The precise matching of black hole thermodynamics to string states is impressive but only seems to work for special black holes that have positive specific heat (and are hence near extremal.) This is because no black holes are involved at all, instead what is counted are states of non-gravitational systems of branes in flat space with gravity turned off that have, by virtue of BPS symmetry the same quantum numbers of certain positive specific heat black holes. This can’t work for negative specific heat black holes because they are not BPS and because they can’t have the same thermodynamics of orindary systems with positive specific heat. There is no evidence string theory can describe precisely generic negative specific heat black holes. I suspect this may be because there is no consistent worldsheet propagation on backgrounds with horizons-because they do not have global timelike killing fields.

    -On top of this, while there is very non-trivial evidence for all orders finiteness, it is still not proved (please don’t jump on me again about this unless you have a new paper with a full proof.)

    -Since Sean’s statement is comparative, it is important to say that background independent approaches exist, and many key results have been shown about them. LQG, in its spin foam formulation, continues to advance and is now, in my view, the “best model.” There were recently big advances on the hardest problem, that of getting known physics form the low energy limit announced in talks by Rovelli, Freidel, Markopoulou and others at the loops05 meeting (now with talks on line at http://loops05.aei.mpg.de/), and there were predictions for corrections to the CMB described in the talk there by Hofmann. Elimination of both black hole and cosmological singularities has recently been shown in related models. This is on top of proofs of uv finiteness and the fact that the theory is background independent. Black hole entropy is understood for real negative specific heat black with real horizons.

    In my view the second best model is the Loll-Ambjorn causal dynamical triangulations model, with stunning recent progress described by Loll at loops05.

    If string theory overcomes the problems mentioned I would be the first to cheer But for the present I believe that the background independent appraoches are moving faster and are more worthy of our time.

    Thanks,

    Lee

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  • http://countiblis.blogspot.com/ Count Iblis

    Kaku told in a recent interview that he believes that if Einstein had not invented GR, it wouldn’t have been invented by others before 1970. This is because, as Wald explains in his textbook, the obvious way to write down a relativistic theory of gravity is using retarded potentials in a flat spacetime. The idea that spacetime could be anything else than flat would not have entered the heads of physicists anytime soon.

    To me theories like string theory, LQG etc. look more like naive contrived attempts to come up with a TOE, similar to what would have happened had GR not been discovered by Einstein.

  • Elliot

    I think there continues to have two different threads embedded here. 1) Is string theory the best existing approach to quantum gravity? and 2) how do practicing scientists most effectively communicate to the lay public what is “going on” in fundamental research in physics? I’d like to humbly suggest that although they are deeply interrelated, they are really two different questions. I like Peter Woits kitchen analogy but it appears that there is no clear consensus on whether of not the kitchen is clean or dirty or if it is better for the family to clean it up or call in a cleaning service or demolish it.

    Elliot

  • Fyodor

    So, upon further reflection, would Prof Krauss agree that there is in fact nothing in common between the string theorist’s “fascination with things unseen” and religious sentiment? That they are in fact utterly different things which the magic of the English language [yes, folks, ONE set of words can have TWO meanings, incredible as that seems…] allows ignorant people to confound? That a great deal of florid “popular science writing” is nothing but the raising of such simple confusion to an art form?

    Look at the Princetonian event which Peter Woit described. Everyone laughs at idiots talking about quantum pasta. But nearly all of the popular writing about science I see is just as idiotic. Prof Krauss, remember that dancer waving her arms around and claiming that some deep insight into “space” was being vouchsafed? Pretty silly, wasn’t it? But is it really much more silly than seeing analogies where none exists?

  • http://eskesthai.blogspot.com/ plato

    Okay, maybe just quantum gravity then?

    Would this make everyone happy? As far as keeping out of the kitchen, I don’t think so. If you had taken your synoptic views in book form to the public, what would make you think they could not come for a closer look and see what you are doing?

    Oh! Maybe targeting specific readers are you? About 1% of the population, this should make you really rich?

    That should cuts sales down quite a bit, if you think you are selling to a select few who you think are in the kitchen?

    So what are the problems that exist then? For the lay person a historical perspective.


    The first is that the geometric interpretation of general relativity is not fundamental, but just an emergent quality of some background-dependent theory. This is explicitly stated, for example, in Steven Weinberg’s classic Gravitation and Cosmology textbook.

    The opposing view is that background-independence is fundamental, and quantum mechanics needs to be generalized to settings where there is no a priori specified time. The geometric point of view is expounded in the classic text Gravitation, by Misner, Wheeler and Thorne.

    The two books by giants of theoretical physics expressing completely opposite views of the meaning of gravitation were published almost simultaneously in the early 1970s. The reason was that an impasse had been reached, a situation which led Richard Feynman (who himself had made important attempts at understanding quantum gravity) to write, in desperation, “Remind me not to come to any more gravity conferences” in a letter to his wife in the early 1960’s.

    http://en.wikipedia.org/wiki/Quantum_gravity

  • Rof

    Thanks, Lawrence, for a thoughtful and well-written article.

    The significant question about which there is disagreement appears to be: Should physicists be talking to the public about string theory?

    I believe that, in the light of Lee’s posting, it is instructive to consider the question: Should physicists be talking to the public about causal dynamical triangulations, and about loop quantum gravity?

    More specifically, when the public are told about string theory, should they be told about possible alternatives? And if not, why not? And if so, why hasn’t that been the case so far?

  • http://wb.homedns.org/wb/page1.html Wolfgang

    Lee,

    > In all known cases this imposes another condition which is worldsheet supersymmetry (or something equivalent) which in turn requires that the background be static.

    Can you please elaborate why supersymmetry requires the background to be static?

  • http://thomas.loc.gov X

    Regarding Lee Smolin’s comment in #17, even if string perturbation theory is defined on arbitrary background spacetimes, what can we get other than S-matrix elements? Is that all the content of a quantum theory of gravity?

  • Moshe

    Hey guys, I have a few minutes before dinner, so I have to be quick. I disagree with lots of what Lee mentioned above as statements of fact about string theory (many other issues are simply a matter of individual judgement).

    Since the subject is close to me heart, let me only concentrate on the sentence “there is no evidence that string theory exists even on backgrounds that are time dependent”, which is incorrect on many levels. There is a lot of work on string theory on such backgrounds, which tend to be singular and therefore interesting…

    In particular we have AdS/CFT and it is complete and non-perturbative definition of string theory in spaces with a negative cosmological constant, this includes each and every physical question one may dream up, time dependent or not. So I am not sure what Lee means exactly.

    His evidence cited here is a technical point about string perturbation theory, (which is only one corner of the subject) which I believe is misguided, worldsheet (unlike spacetime) SUSY and the resulting absence of tachyons have nothing to do with spacetime Killing vectors, and therefore does not require spacetime to be static. (in fact worldline SUSY is a beautiful structure which arises when you try to describe in this language any particle with spin, nothing to do with string theory…).

    I am not sure you guys want this thread to concentrate on this issue, maybe there will be a better time and place for this…

  • Aaron

    Any acceptable quantum theory of gravity must incorporate the basic lesson of GR that the geometry of spacetime is dynamical and defined without reference to any fixed background-hence background independent.

    Insert sound of Aaron banging his head against the wall here.

  • Aaron

    “Any acceptable quantum theory of gravity must incorporate the basic lesson of GR that the geometry of spacetime is dynamical and defined without reference to any fixed background-hence background independent.”

    Insert sound of Aaron banging his head against the wall here.

  • Aaron

    Sorry about that — the comment system was acting funny.

    And I still haven’t seen any computation of black hole entropy in LQG that doesn’t pretty much assume that the answer is proportional to the area. I’d be happy if someone could point one out to me.

    But that does bring to mind a point I’ve been wondering about. Do LQGers expect that LQG is only consistent with that bizarre value of the Immirizi parameter? If not, what physics is described by the other consistent background independent theories of whatever?

  • http://eskesthai.blogspot.com/ plato

    moshe,

    I just thought that for any symmetrical breaking action it would need to break from something much purer? Is there no such thing as supergravity?

    I thought such a thing as a superfluid, would be quite beautiful, and a place for such bubbles to emerge.

    So Lee you talk about “new” theoretical developement. Here is something to contrast it. Your point of view?

    Sir Michael Atiyah


    If theory is the role of the architect, then such beautiful proofs are the role of the craftsman. Of course, as with the great renaissance artists, such roles are not mutually exclusive. A great cathedral has both structural impressiveness and delicate detail. A great mathematical theory should similarly be beautiful on both large and small scales.

    Anyway Lawrence, is there anything you would like to say so far about the discussion?

  • Lee Smolin

    Hi,

    Thanks very much for the comments. Let me begin by answering Wolfgang’s questions and then address Moshe’s and Aaron’s comments. As I said before, lets try first to agree on matters of fact. Perhaps I’m not aware of a result, in which case I look forward to being educated.

    “Can you please elaborate why supersymmetry requires the background to be static? ”

    The basic reason is to expect so is that the algebra of supersymmetry transformations closes on the hamiltonian, which is the generator of time translation symmetry.

    Now on to Moshe’s comments:

    “There is a lot of work on string theory on such backgrounds, which tend to be singular and therefore interesting…”

    I agree there is interesting work, but that is not the same as there being explicit examples of consistent quantum world sheet propagation on time dependent backgrounds. Am I wrong that there are none?

    “In particular we have AdS/CFT and it is complete and non-perturbative definition of string theory in spaces with a negative cosmological constant, this includes each and every physical question one may dream up, time dependent or not. So I am not sure what Lee means exactly….”

    AdS spacetime has 10 global symmetries, so that is not a counterexample. Nor is the issue that we can’t ask time dependent questions on a time independent background. We can also argue about which version of the AdS/CFT conjecture is supported by the present results, but it is certain that the very strong form in which “each and every physical question one can dream up, time dependent or not” can be answered by measuring something in the dual CFT is a conjecture, not yet shown. The non-perturbative existence of N=4 SYM is also an open conjecture as there is still, so far as I know, no known lattice regulator that preserves supersymmetry.

    “His evidence cited here is a technical point about string perturbation theory, (which is only one corner of the subject) which I believe is misguided…”

    String perturbation theory is one part of the subject, but it is the only part where one can compute explicit quantum gravity corrections to ordinary processes.

    “…worldsheet (unlike spacetime) SUSY and the resulting absence of tachyons have nothing to do with spacetime Killing vectors, and therefore does not require spacetime to be static….”

    I agree in principle that worldsheet supersymmetry does not imply spacetime supersymmetry. But tell me, are there explicit examples with worldsheet supersymmetry on a time dependent background? If not we should try to understand whether there is a reason. If there were an example, would it imply the existence of a killing fields in the induced metric on the worldsheet, but not on the spacetime from which the metric was induced? If so this could not be generic.

    If I am wrong about this I would be happy to be educated. Or maybe there is an extension of string theory to time dependent backgrounds that does not have a perturbative description in terms of worldsheets. But if this exists it has yet to be constructed.

    “in fact worldline SUSY is a beautiful structure which arises when you try to describe in this language any particle with spin, nothing to do with string theory…”

    Certainly, I’ve nothing against SUSY as a mathematical structure, and I’ve devoted lots of time to it myself, but no one is claiming that worldline supersymmetry leads to a consistent theory of particle physics or quantum gravity.

    Re Aaron’s point: “And I still haven’t seen any computation of black hole entropy in LQG that doesn’t pretty much assume that the answer is proportional to the area” None of the calculations usually cited (Ashtekar, Baez, Corichi, Krasnov and others….) make that assumption. Please read the papers, these are careful people and they state their assumptions explicitly.

    But I agree with your worry, Is “LQG … only consistent with that bizarre value of the Immirizi parameter? ” I’ve been wondering about this too.

    Thanks,

    Lee

  • http://quantumfields.blogspot.com the one intelligently designed

    The problem of quantum gravity is so fascinating that young people can not help try attacking it and thats what I also wanna do. And unless people come up with more ideas about it quite a few people will keep working on strings.
    That said, one must take a course on astrophysics from LK to get the feel what it means in science when we say something is well tested (in contrast to Motl’s belief system), and how is it to work in a field where you are confronted with numbers to comapre your theory with on a daily basis.
    There probabaly is something in the physics popular writings that the common people ussualy dont have a clear idea of the real accomplishments of physics. For example, they would put the general big bang theory and the brane world scenario into the same category of speculation, on part of phycists, whosse job is to come up with new and marvelous ideas anyway. (Sure its just my personal expereince of talking to freinds, and does not have real statistical significance)
    Ofcourse there is nothing wrong with coming up new beautiful ideas. As I understand , Eienstien wrote GR based only on his theoretical insights and demanding some general properties for a theory of gravity ( relativistic invariance, equivalance principle ect) and then looking for internal consistencies. Probabaly string theorists find themselves in same situation. But the thing is that people would have not belieived in it as a hard fact of nature if there were no mercury’s prehilion shift and solar eclipse tests.

  • Scott

    I am not so sure as Sean, and most everyone else here seems to assume, that the goal of the string framework is to arrive at a theory of quantum gravity. As far as I can tell the “goal” of the programme (if programmes and not just individual theorists can be said to have goals) has been to explain masses, coupling constants ect in the standard model and that the apparent existance of gravitation in the framework is merely a highly motivating factor for study within the programe. Finding quantum gravitational correction to known processes seems very impractable as the corrections are so small that they likely will never be observed. So, assuming the overall hope is for a connection with experiment, that the goal would then be to calculate the parameters of the SM. It would be interesting to hear from the string theorists reading this blog, if they considered the goal to be one or the other( or if they even make a distinction). As most of them seem to think of themselves as particle physics I expect that they are more concerned with explaining the parameters of the SM.

  • Moshe

    Lee, worldsheet SUSY (which is usually needed for classical stability) does not imply that the spacetime has to be static. This is the technical point I was making in response to your post, and it is actually correct. So this leaves the possibility of string backgrounds that have worldsheet SUSY but are not static, and indeed there are many, too many to mention here, various orbifolds of flat space and coset constructions are a few such examples. They are stable classical backgrounds (=worldsheet theories) of the superstring. Incidentally there are also consistent (indeed solvable) classical string backgrounds that include horizons (Witten’s black hole is an example, but there are many more).

    But, again this is just one method of investigating string theory. Another one I mentioned is AdS/CFT that requires you to fix the asymptotic geometry to be AdS, but in the interior of the geometry absolutely anything is allowed to happen, time dependent or not. So for example one can calculate dynamical quanitities like various transport coefficients in strongly coupled gauge theory using the gravity dual.

    In the context of AdS/CFT, I am not sure what is the distinction between “time dependent questions” and “time independent background”, because you see, AdS/CFT is *background independent*. The AdS space itself with its isometries is just the vacuum of the theory, one can ask questions about excited states as well, including time dependent questions. There is a large body of literature on the subject (Alex Buchel and Andrei Starinets are experts around you)

    I am not motivated to enter into a discussion what is proven and what is not, since basically nothing interesting is proven to mathematical rigour, starting with QED with massless electrons. If there are some reasons to doubt AdS/CFT correspondence for specifically time-dependent questions that would be interesting to hear.

    best,

    Moshe

  • Moshe

    Again, dear bloggers, not sure you want us to divert attention from the main subject here, this is getting quite technical…I’ll be glad to postpone further discussion to a more appropriate time.

  • http://eskesthai.blogspot.com/ plato
  • http://golem.ph.utexas.edu/~distler/blog/ Jacques Distler

    Lee says:

    The non-perturbative existence of N=4 SYM is also an open conjecture as there is still, so far as I know, no known lattice regulator that preserves supersymmetry.

    To which Moshe responds:

    I am not motivated to enter into a discussion what is proven and what is not, since basically nothing interesting is proven to mathematical rigour, starting with QED with massless electrons.

    Even by some much lower standard of rigour, there is far more reason to doubt the nonperturbative existence of the Standard Model (a chiral gauge theory) than N=4 SYM.

    A quick search of hep-lat (if Lee were interested) would turn up a fair-sized literature on N=4 SYM on the lattice (see, e.g. Catterall or Kaplan and Unsall). Crudely, fermion-doubling (the reason there’s no satisfactory lattice regulator for chiral gauge theories, like the Standard Model) is, in the case of extended supersymmetric theories, your friend.

    And, of course, the very striking nonperturbative checks on S-duality by Vafa and Witten (among others) would be rather hard to understand if the theory didn’t actually exist nonperturbatively, wouldn’t they?

  • Aaron

    Re Aaron’s point: “And I still haven’t seen any computation of black hole entropy in LQG that doesn’t pretty much assume that the answer is proportional to the area” None of the calculations usually cited (Ashtekar, Baez, Corichi, Krasnov and others….) make that assumption. Please read the papers, these are careful people and they state their assumptions explicitly.

    All the ones I read explicitly assumed that the degrees of freedom are located on the horizon. I have not seen any construction of a black hole state in LQG.

    This thread in spr is relevant.

  • Moshe

    Lee, also apologies in advance for not being able to engage in a conversation in the next few days…mainly meant to quickly point out that one technical point, which I did…

  • Ugo

    “Any acceptable quantum theory of gravity must incorporate the basic lesson of GR that the geometry of spacetime is dynamical and defined without reference to any fixed background-hence background independent.”

    Hello,

    I’m not an expert in physics but nevertheless was wondering about this particular issue.
    I’m very intrested in understanding why any quantum theory of gravity should necessarily be derived from a General Relativity framework.
    Isn’t it possible to formulate a “relativistic” theory of gravitation on minkowski space-time and then try to quantize it as is done for the other field theories using canonical or path integral quantization ?.
    Does this approach lead to inconsistencies and that’s why it has never be carefully examined or is there another reason why nobody cares to take a non-geometric approach to the quantization of gravity.

  • http://atdotde.blogspot.com Robert

    Lee,

    insisting on background independende is a nice philosophical principle but not more. For the longer answer check here:

    http://golem.ph.utexas.edu/string/archives/000621.html

    Everybody agrees that gravity without symmetries is harder than with symmetries. This is not any different in string theory than elsewhere. For example I am not aware (but that might be my fault) of any analytical solutions to Einstein’s equations without symmetries. And all the textbook examples have Killing vectors. So, is that a problem of GR?

    Re tachyons: The effective description of a ball on a mountain top contains a tachyon: The ball can roll down. Of course again, there is no inconsistency with balls on mountain tops it’s just that small deviations do not stay small. And situations with instable modes (aka tachyons) are slightly harder to analyse.

    There are many things you can argue about string theory, but these ones are not too strong.

  • http://www.math.uni-hamburg.de/home/schreiber Urs

    Can you please elaborate why supersymmetry requires the background to be static?

    The basic reason is to expect so is that the algebra of supersymmetry transformations closes on the hamiltonian, which is the generator of time translation symmetry.

    Unbroken target space supersymmetry means existence of a constant spinor. Existence of a Killing spinor implies existence of a Killing vector. But not necessarily a timelike one, as far as I am aware.

  • http://iso42.blogspot.com Wolfgang

    Robert,

    > Everybody agrees that gravity without symmetries is harder than with
    symmetries.

    Sure. But Lee’s argument (as I understand it) is that the current superstring theory works only in highly symmetric (static) backgrounds and that there is a reason why this is so (insert his argument about supersymmetry here).

  • http://iso42.blogspot.com Wolfgang

    Ugo,

    > Isn’t it possible to formulate a “relativistic” theory of gravitation on minkowski space-time

    gravity is attractive, so you would have to consider a spin-0 or spin2 bosonic field
    or a combination of both.
    spin-0 alone does not reproduce some effects we know and spin-2 leads more or less straight back to GR due to consistency arguments.

  • Aaron

    Unbroken target space supersymmetry means existence of a constant spinor. Existence of a Killing spinor implies existence of a Killing vector. But not necessarily a timelike one, as far as I am aware.

    That’s not the argument that Lee’s referring to. Essentially, in target space, the SUSY generators have to close on a time translation.

  • Anonymous

    Jacques has already pointed out that there are recent papers on lattice constructions of gauge theories with high SUSY. See also hep-lat/0302017 and hep-lat/0307012.

    There have also been attempts to formulate lattice constructions of chiral gauge theories, see hep-lat/0503011 and hep-lat/0510073. A proof of whether these truly work appears to be elusive, but perhaps they are a step in the right direction.

    Lattices aside, does anyone really have strong doubts about whether asymptotically free gauge theories make sense?

  • Lee Smolin

    Hi everyone, thanks for all the very useful comments. I’ll coment where I can, I’m sure someone will let me know if I get something wrong.

    To Jacques, Thanks very much for the references for N=4 SYM on the lattice. Of course I’m interested, this is important progress. These were in the last year and I missed them, thanks for pointing them out.

    To Urs, here is what I understand in 3+1 dimensions: A killing Weyl spinor gives a null killing vector. Two of them, as in a Dirac spinor make two null killing vectors, whose sum is a timelike killing vector.

    To Aaron, Yes, the older LQG results about black holes assume the presence of an horizon and study the states that live on the horizon. That is recognized as a limitation. But this is not the same as assuming that the entropy is proportional to the area. There are a few recent works that explore going beyond that, for example, Husain and Winkler, gr-qc/0410125, gr-qc/0412039 gr-qc/0503031 , gr-qc/0505153.

    To Robert, I agree its hard to write solutions to GR with no killing fields, but you can establish lots of properties of solutions we can’t write down. If string theory is a theory of gravity shouldn’t we be able to prove there is a consistent string worldsheet propagation on any generic solution? If not, why not? Shouldn’t there at least be consistent worldsheet propagation on simple spatially homogeneous models like Bianch IX or arbitary FRW?

    Re tachyons, if you know the full non-perturbative definition of a theory and have proved stability and consistency, and then expand around a semiclassical state and find tachyons it just means that you are expanding around the wrong vacuum. But if you only have a perturbative description and it has tachyons you cannot be sure there is any non-perturbative formulation with a stable vacuum. For example, lambda phi^4 with the wrong sign of lambda does not exist in any dimension because there is no stable ground state. We don’t know if there is any full non-perturbative formulation of string theory, so we cannot be sure whether the tachyon rolls to disaster or to some unknown non-perturbative ground state.

    On background independence, it is much more than philosophy, it has motivated a large body of results concerning approaches to quantum gravity that implement some form of background independence. I don’t need to convince you about the philosophy but I would like to convince you to take an interest in those results.

    To Ugo, that approach of doing Feynman perturbation theory for gravitons on flat spacetime was thoroughly investigated and it was known definitively by the mid 70’s that it fails.

    To Moshe, as you say, there have been studies of time dependent string backgrounds, orbifods of flat spacetime, coset constructions, linear dilatons etc. My understanding is that till recently, these either had problems with stability or either the string or Einstein frame metric was still static. Can you point us to the best case currently, so we can discuss that? So far as the AdS/CFT examples are concerned, I agree you can calculate many things, having to do with states propagating on an AdS X something background, we are not disputing that. By Witten’s black hole do you mean the large (positive specific heat) black holes in AdS? I wasn’t aware you could compute consistent string amplitudes on that, or for that matter on AdS X S^5 (generically, there are some results in extremal cases). If I’m wrong please give references.

    Thanks, Lee

  • http://www.math.coumbia.edu/~woit/blog Peter Woit

    Anonymous,

    I don’t think there’s any reason to doubt that pure YM theory on the lattice makes sense: the lattice formulation is very straightforward, and there’s a lot of numerical evidence that the continuum limits exists and behaves as predicted by continuum perturbation theory.

    Chiral and supersymmetric gauge theories are another story, since finding a lattice formulation with any hope of working is not at all easy. As far as I know, the various proposals out there are not particularly amenable to numerical simulation, so little is known about whether these proposals actually avoid the various problems one might worry about them having in the continuum limit.

    This is getting far away from the original topic of this thread. In an attempt to bring things back to something related to the thread topic, I’ll mention that the unsolved problem of carefully understanding the non-perturbative behavior of chiral gauge theories has always seemed to me a good example of the kind of thing that hasn’t gotten the attention it deserves due to the way particle theory has been dominated by string theory for the last twenty years.

  • Doug

    Elliot wrote:

    To me the key aspect of string theory, whether its present lines of development prove to be correct or incorrect is to transcend the notion of point particles (zero dimensions) and theorize that there is at least one if not more than one dimension to the fundamental constituents of the universe. Doesn’t this conceptual shift actually put the theory (or one of its descendants) in a better position to remove infinities from equations describing these fundamental phenomena?

    I sure wish one of you leading experts (Clifford, Lawrence, Lee, Peter, others) would address Elliot’s incisive question. If you think that it’s possible that the public can understand and care about what physicists are doing in the kitchen, then maybe it’s important to listen to them as well as blab on in your own venacular, oblivious to their curiosity and keen desire to understand.

    Sean writes:

    But we have learned an incredible amount…The theory could have led nowhere, but it keeps leading us in surprising new directions…the primary goal is obviously to quantize gravity…it’s perfectly clear that we should be talking about string theory

    Ok, then why not talk about it in terms of transcending the zero dimensional point particles and removing infinities. What is it that you have learned about transcending the zero dimensional constituents of the universe? That if you take those constituents and cook them into different kinds of pasta, they are no longer zero dimensional? Really, is your only choice to be so doggone condescending, or else to retreat into the netherland of professional jargon? Can you talk to us as if we had some brains for crying out loud, but without the use of your esoteric terms!

    It’s not that hard. For instance, the natural numbers are like zero dimensional point particles, are they not? But when we interpret them as describing a relation between quantities, as a ratio, a vibration, lo and behold we transform them from 0D numbers to 1D numbers. The relation of two natural numbers, as a rational number, has dimension, the dimension of the signed integers; that is, n/m…1/2,1/1,2/1…m/n is the same thing as -n…-1,0,1…n, without the need for imaginary numbers.

    Thus, a vibration, 1/t, transcends the zero dimensions of a point – we move from a point to a line, and the infinities are gone, because a line formed by a 1D vibration is bound, just as the number 1/2, interpreted as -1, is bound. Whoa, ok, that’s easy enough to understand, and it’s fascinating, but don’t stop there! Tell us, guys, what about 2D and 3D vibrations? What do they do for us? Is this one of the surprising new directions Sean is talking about? Tell us about these n-dimensional vibrations, please?

    But don’t be condescending. Remember, just because we don’t have the specialized knowledge that you do, it doesn’t follow that we are stupid. We know that increasing the dimension of the integers leads to complex numbers, and then to quaternions and then to octonions. We also know that the octonions are not only three dimensional (four if you count the zero dimensional scalars), but that they are very curious too (we read also read Baez, you know).

    What we really want to know is how this idea of yours, the vibration idea, plays with these numbers. Put down your colander and pasta, and pick up this vibrating octonion. Now this is really cool. The 3D vibration has the scalar, 1/1, three linear vibrations, three planar vibrations, and one cubic vibration, as a set, you say? All as one particle? What’s the frequency of this vibration? What’s its radius? Does it have an multiplicative inverse? Is the Clifford algebra of these n-dimensional vibrations, or rational numbers, commutative and even associative, due to the bi-directional nature of the vibrations?

    Come on guys, talk to us. There are at least two of us (right Elliot?) who are keen to hear what you have to say.

  • Moshe

    Lee, have to go and catch a flight, so let us not raise new issues which may be interesting to discuss on some other occasion. You mentioned some specific issue to do with string worldsheet making sense only on static backgrounds (and w/o horizons) because of worldsheet SUSY. I think that technical statement is incorrect, and I would be eternally grateful if you will correct it wherever it appears.

    Reference: Witten’s black hole is in two dimensions and is the classic paper
    Phys.Rev.D44:314-324,1991; there are 788 follow-up papers also… You can also look at Tseytlin 1994 review hep-th/9410008 for an assortment of string vacua which are time dependent and exactly solvable. I don’t think most of them are static in either frame.

    Finally, my point was that AdS (times something) is not chosen as a background to expand around, one describes in the CFT also all excited states as long as they are asymptotically AdS. So this describes also time dependent processes, and this is not just statement of principle, there is a large body of literatutre doing time dependent calculations in this framework. You should really talk to Andrei or Alex, they know everything there is to know about the subject.

    best,

    Moshe

  • http://blogs.discovermagazine.com/cosmicvariance/mark/ Mark

    Doug, just one small comment. I can’t see that any of the experts have been condecending here. In fact, your comment is one of the first in this thread that is a little insulting (“blab on in your own venacular, oblivious to their curiosity and keen desire to understand”).

    Part of the reason we have this blog is public science education. We do a lot of it, and so I for one don’t appreciate your tone. If you’d like something answered, please feel free to ask, but don’t be rude; it brings down the level of the entire discussion.

  • Aaron

    To Aaron, Yes, the older LQG results about black holes assume the presence of an horizon and study the states that live on the horizon. That is recognized as a limitation. But this is not the same as assuming that the entropy is proportional to the area.

    I guess everyone will have their own opinion on that point.

    There are a few recent works that explore going beyond that, for example, Husain and Winkler, gr-qc/0410125, gr-qc/0412039 gr-qc/0503031 , gr-qc/0505153.

    I don’t see any lqg in those papers. I don’t see any calculations of black hole entropy, either.

  • http://eskesthai.blogspot.com/ plato

    I will try to keep a low profile now in this conversation, but hopefully the foundational perspective you are sharing, has been caught here?

    Thanks,

  • http://www.math.uni-hamburg.de/home/schreiber Urs

    On the physical viability of static or stationary backgrounds:

    SUSY backgrounds are interesting for particle physics phenomenology. For that purpose it should be fine if they are Minkowski in 3+1D since accelerators tend to be insensitive to cosmological scales.

    I’d expect that a static CY x Minkowski SUSY background might describe the universe nicely locally, on the scale of accelerators, while it may be just a local approximation to a non-static non-SUSY cosmologically valid background.

    (Just like we use Minkowski field theory to compute accelerator data even though we live in a world which is non-Minkowski on large scales.)

    Even though everybody is talking about the “landscape”, I don’t have the feeling that cosmological issues are anywhere near tractable with current string technology. Please correct me if I am wrong.

    That’s not the argument that Lee’s referring to. Essentially, in target space, the SUSY generators have to close on a time translation.

    Surely the statement is not supposed to be that SUGRA only has stationary solutions because some of the SUSY generators close onto the Hamiltonian constraint?

    I think the question is rather if solutions that do not break all SUSY have to be stationary. So the question is if the _preserved_ susy squares to time translation. This is the case precisely if the solution has a Killing spinor that squares to a timelike vector.

  • Doug

    Mark,

    Point taken. Sorry, for letting that sentiment show. It’s just that only having the unintelligibility of the professional jargon on the one hand, or the oversimplified analogies on the other, is so frustrating.

  • The one intelligently designed

    Exact Supersymmetry can be mintained using Fuzzy regularization

  • http://blogs.discovermagazine.com/cosmicvariance/mark/ Mark

    Thanks Doug. I expect one of the string theory or LQG people will get around to addressing the point (I’m certainly not expert enough in these topics to do a good job). It’s not just jargon though. The issues involved in some of the topics here are highly technical and the language used is that suited to them. Explaining them in other terms can be very difficult. However, I think some of the questions, such as the one you mention, can be addressed in more simple terms.

    I’ll let others take a real stab at it, but I would say that the extended nature of strings does mean that infinities are easier to deal with (are sometimes just absent) in string theory. However, we are already pretty happy with how we deal with some infinities in field theory (regularization and renormalization work well), and those infinities that arise in gravity (e.g. curvature singularities in black holes or the big bang) haven’t really been shown to be any easier to deal with in string theory yet. So while it’s a promising direction to head in, we don’t know if it helps yet. The fact that it seems to be a consistent theory of quantum gravity is, to my eye, the most important thing.

    Cheers.

  • http://golem.ph.utexas.edu/~distler/blog/ Jacques Distler

    Doug,

    “Removing infinities” is the wrong way to think about renormalization theory. A proper answer to your questions is way longer than would be possible in the comment section to this blog. I wrote a long post over on my blog, explaining why String Theory is the only viable approach to a theory of quantum gravity (in response to some of the comments, I wrote a followup post explaining why one frequently-cited “alternative” doesn’t work).

    Perhaps these posts are still too technical; they were written with an intended audience of, say, a physics graduate student. But they’re probably a better start at an explanation than anything I could post as a comment here.

  • Anonymous

    Peter wrote:

    Chiral and supersymmetric gauge theories are another story, since finding a lattice formulation with any hope of working is not at all easy.

    I didn’t claim otherwise. What I did claim is that I don’t think anyone really believes that an asymptotically free, or a conformal, continuum field theory in these cases is not actually well-defined. We don’t have a satisfactory nonperturbative definition of them, but it doesn’t cast doubt on whether they exist. It just means we don’t yet have an adequate toolkit for understanding these things. It seemed that Lee was insinuating that AdS/CFT isn’t really a nonperturbative definition of string theory on backgrounds with negative cosmological constant because the N=4 SYM (or related field theories for deformations of AdS) might not be well-defined. I think that 99% of people with experience in working with field theories would disagree with this statement. A theory like QED is nonperturbatively suspect because of the Landau pole, but QCD-like theories and conformal theories are not.

    I’ll mention that the unsolved problem of carefully understanding the non-perturbative behavior of chiral gauge theories has always seemed to me a good example of the kind of thing that hasn’t gotten the attention it deserves due to the way particle theory has been dominated by string theory for the last twenty years.

    Wow. I don’t think people have been ignoring this. I just don’t think anyone has a clue how to make good progress. Even for non-chiral gauge theories like QCD we don’t have much understanding of non-perturbative behavior. The lattice can only do so much, although it has made a lot of progress in recent years. I would say that the only 4D theories where we “carefully understand” non-perturbative behavior are SUSY ones, and string theory has been very useful in some of this.

  • Arun

    Moshe,

    Do we have to distinguish between solving the effective action (the GR-like equations) that arises from string quantization in a flat background versus string quantization in a general background?

    -Arun

  • http://eskesthai.blogspot.com/ plato
  • http://eskesthai.blogspot.com/ plato
  • Lee Smolin

    To Anonymous,

    Your post raises for me an interesting point about the attitude we take towards problems which lots of smart people have tried and failed, over several decades, to solve. You say, “What I did claim is that I don’t think anyone really believes that an asymptotically free, or a conformal, continuum field theory in these cases is not actually well-defined. We don’t have a satisfactory nonperturbative definition of them, but it doesn’t cast doubt on whether they exist. It just means we don’t yet have an adequate toolkit for understanding these things.”

    I have a background in quantum field theory and I certainly share the expectation that asymptotically free or conformal gauge theories should exist. But unlike you I believe we should focus attention on things like this where hard work has not payed of because, fairly often, it turns out we are all wrong. I was very influenced by Feynman who told me, when I was a grad student, that these kinds of cases are opportunities that good scientists exploit by exploring the possibility that what everyone has expected, but failed to show, might be wrong.

    At the very minimum, I think we must always maintain a clear distinction between what has been shown and what we hope will be true. And I certainly think that doubt should always be cast in cases like these. Doubt if free, whereas certainly about things that turn out to be wrong can be very costly.

    Thus, you misunderstand me when you say,

    “It seemed that Lee was insinuating that AdS/CFT isn’t really a nonperturbative definition of string theory on backgrounds with negative cosmological constant because the N=4 SYM (or related field theories for deformations of AdS) might not be well-defined. I think that 99% of people with experience in working with field theories would disagree with this statement….”

    No, I was simply saying that to my knowledge N=4 had not been non-perturbatively defined. I was not expressing any view about the likelihood of this being done, nor was I insinuating anything. I was glad to hear from Jacques that progress was recently made about this and I will study those papers.

    But science is not a democracy and whether 99% of people believe something is not an argument. If anything, if so many good people believe something they have failed to prove, it suggests that something is being overlooked.

    Thanks, Lee

  • http://atdotde.blogspot.com Robert

    Urs,

    {Q,Q}=2H and the existence of Killing vectors coming from Killing spinors is the same. If psi is a Killiing spinor, you can form

    j_m = psi-bar gamma_m psi

    (for some nice version of ‘bar’) and that is a Killing vector. If psi were a physical field, j would be its current. So It should better have the same mass (in terms of representation theory ot the Poincare group) as psi has. If psi is Weyl it means it is chiral, so it has to be massless. So j has to me light-like. If psi is Dirac and has positive mass then j points along the flow of that particle and thus has to be time-like.

  • http://www.math.coumbia.edu/~woit/blog Peter Woit

    Anonymous,

    I see Lee has responded to the first part of your last comment. As for the second part, about the non-perturbative formulation of chiral gauge theories

    “I don’t think people have been ignoring this. I just don’t think anyone has a clue how to make good progress.”

    Sure, no one has a clue how to make good progress on this. But neither does anyone have a clue about how to make good progress on other major problems. The fact that no one has a clue how to make progress on the fundamental problems of string theory doesn’t mean people have stopped working on string theory. Instead they do what they can: they try to learn more about the problem by doing calculations that won’t make good progress but that they hope might teach them something that will allow them to make good progress. I see a huge amount of this kind of work on string theory, very little of it on chiral gauge theory.

  • Anonymous

    Lee wrote,

    No, I was simply saying that to my knowledge N=4 had not been non-perturbatively defined. I was not expressing any view about the likelihood of this being done, nor was I insinuating anything.

    Sorry for misunderstanding your point.

    Lee also wrote,

    But unlike you I believe we should focus attention on things like this where hard work has not payed of because, fairly often, it turns out we are all wrong.

    Here you misunderstand my point. I don’t say that we shouldn’t focus attention on such things. I was just emphasizing that, while a proof is lacking, there is a large amount of substantive information that suggests that we do have a nonperturbative definition of string theory on asymptotically AdS backgrounds. Of course putting this on a better footing is worthwhile, and understanding gauge theories nonperturbatively is (in my opinion) even more worthwhile. But I think almost everyone in the field would agree that understanding gauge theories nonperturbatively would be a huge advance. Many people chip away at this problem all the time in small ways. The lack of a major breakthrough isn’t, I think, because people are not trying. It’s just that it’s really hard.

    Similarly, Peter wrote:

    The fact that no one has a clue how to make progress on the fundamental problems of string theory doesn’t mean people have stopped working on string theory. Instead they do what they can: they try to learn more about the problem by doing calculations that won’t make good progress but that they hope might teach them something that will allow them to make good progress. I see a huge amount of this kind of work on string theory, very little of it on chiral gauge theory.

    I just don’t agree. Maybe people don’t specifically focus on chiral gauge theory, but as I said above, the non-chiral case is not well-understood either, and there is a huge amount of effort directed at understanding QCD. Many people working on stringy things are doing it with the goal (or at least a goal) of better understanding nonperturbative aspects of gauge theories, and there are all sorts of non-stringy approaches as well. I can agree with you that in the chiral case there is probably less effort, but that’s because there’s an even easier case we don’t understand.

    I’m not going to say that everyone in the field makes good use of their effort. In fact, I would say that a large fraction of the work done in high-energy physics (both theoretical and phenomenological) is misdirected. Still, I think it’s nonsense to say that people don’t care about nonperturbative gauge theory or don’t try to make progress on it.

    I think at some level I don’t disagree too much with either of you. But I think that if there’s a problem it’s that people don’t always follow the most promising routes, not that they don’t have the right goal.

  • http://golem.ph.utexas.edu/~distler/blog/ Jacques Distler

    Still, I think it’s nonsense to say that people don’t care about nonperturbative gauge theory or don’t try to make progress on it.

    These are very hard problems, but the past 12 years have seen a huge advance, starting with the work of Seiberg in the early 1990s, in our understanding of nonperturbative gauge theory dynamics.

    Most of those results have to do with nonchiral supersymmetric gauge theories, but a significant fraction (and, certainly, the most interesting results) have to do with chiral theories.

    It’s true that supersymmetry (and, in many cases, stringy methods) have been indispensible tools in this indeavour. Direct results on the nonsupersymmetric case have been few and far between. Still, most people believe quite strongly that many of the qualitative features found in the supersymmetric case will carry over to the nonsupersymmetric case.

    Interest has ebbed and flowed, but averaged over the past 12 years, nonperturbative gauge theories have been hotbed of activity. It’s a very odd, and distinctly perverse view of recent history to say that they have been neglected.

  • Doug

    Thanks Mark, thanks Jacques. I think the gulf is just too wide to bridge, though. There was a time when physicists could explain to the public how their goal was to find the fewest interactions among a few particles in order to classify the kinds of forces that exist in nature that would enable them to develop a classification of particles according to the kinds of interactions in which they participate.

    This was easy enough to understand. But now that the classification is nearly complete, with everything except the one classification that troubled Newton, gravity, it seems that there is more disagreement than ever. The unflattering image of the kitchen scene that Peter painted is what the public is seeing.

    We can’t understand the details of the problems, let alone the merits, or lack thereof, of different approaches to the solutions – your discussions come across like incomprehensible babble, even though to you it must make sense.

    Nevertheless, I can hold a metal ball in my hand, and feel its weight. I can toss it into the air and watch its trajectory, as it falls to the earth. I can feel a magnetic tug on it. I can charge it and discharge it, heat it and watch it glow. I can crush it, melt it, vaporize it, and analyze its composition. I can even simulate it and its properties in a computer, even to the point of simulating the light and images reflecting off its surface. I can do all these things and more, but what I can’t do is understand its fundamental properties.

    To a degree, I can understand its ball properties, its magnetic properties, its electrical properties, its molecular properties, its atomic properties, and even its quantum properties, and that’s pretty good, but I still cannot understand its fundamental properties. What’s it made of, ultimately? What is a fermion or boson made of? What is charge? What is mass? Why is there no magnetic charge? Why do photons propagate? Why is there quantum spin?

    Does your field theory answer “fields?” Does your string theory answer “strings?” Does Jacques point of the neverending story mean that we can’t get there from here? Does Sean’s view that there is no purpose to it anyway mean that the answers don’t exist in any form? Does Peter’s conviction that we are wasting time and money even thinking about strings mean we should start over?

    Weinberg rejects Kuhn’s view that real scientific progress toward an ultimate reality is only an illusion, and so do I, but honestly, it’s hard to believe it sometimes, when we stop by the kitchen.

  • http://eskesthai.blogspot.com/ Plato
  • http://thomas.loc.gov X

    Doug,

    If you have some background in all this, it would be helpful. But over time,
    you can build enough so that this jargon is not impenetrable. You may
    never be able to make a meaningful statement using these terms, but
    you will be able to understand statements made by the physicists (kind
    of like having a weak grasp of a language). The only question is how
    much do you want to invest in this effort? For it will take effort.

  • Elliot

    Doug, thanks for revisiting this on my behalf. I think the responses are satisfactory. I am however still interesting in a response to item 20 above. The discussion is on two levels. 1) which approach to QG is the correct (mainly a scientific discussion) couple with 2) how does this get presented to the public at large. I would like to suggest an abstract concept — “The intelligent lay person” not disimilar to the “reasonable person” in law. who I think the scientist should be targeting with their explanations of science at this level. I am not going to describe this person in detail but lets assume that this person can understand abstract concepts if well explained without having to fully understand the mathematics.

    My impression is that when there is scientific disagreement it is not only confusing but difficult for lay people to “get” the underlying concepts.

    I am not suggesting any type of scientific democracy should ensue. The truth should win however unpopular but I’d like to see a (I’m going hate myself for using these words in the morning) fair and balanced description and presentation of the various approaches to QG without the border wars that seem to inevitably break out whenever these topics are introduced.

    My views are from the outside and I am sensitive to the passions of those who toil on these highly challenging problems for many years. But as I said elsewhere science should be the search for truth. Law is the realm of advocacy.

    Regards

    Elliot

  • agm

    So, everyone likes to talk shop (not that I’d accuse anyone here of being as bad as teachers, but the analogy is possible). Nonetheless, X at 70 completely missed Doug’s point at 68. This thread was in large part motivated by Krauss’s defense to Clifford’s post about a talk Krauss gave (linked in number 4). Krauss’s response contained several points, but many of the people here have totally ignored the question of how to deal with the kitchen. Telling someone to go learn some string theory before they can talk here is the exact opposite of the popularization/public outreach championed by our hosts here. Arguing in technical jargon, while nice when done well and politely, as here, also misses that point.

    More to the point, the question of whether to make string theory a public topic has already been decided. The question is what to discuss in public, and what this thread has shown so far does not encourage me for the prospects of building public support, wider participation and discussion, or rebuilding funding sources (which goes not to whether this is valuable research but to whether it will be carried on long term, since you have to pay for utilities and infrastructure and grad students somehow).

  • http://home.comcast.net/~jcgonsowski/psychophysics/ John G

    Progress on the nonperturbative front would be nice but from my Doug-like view it seems more like it would be a computational advance than a fundamental understanding advance. Anonymous mentioned people need to follow through more in the promising areas. What are the promising areas? Doug, being a reader of Baez, mentioned octonions. Not sure if you need to vibrate the octonion, it can make a nice fermion representation space just sitting there. Where does it fit in the grand scheme of things. You can think of it in the D4 Triality that string theory uses to get E6 orbifolded fermions. E6 is nice. Baez would like E6/F4 for a spin network but he and Tony Smith agreed it would be tough to do foamy things with it. Smith though, as Doug suggests likes to drop down to Clifford Algebra. Clifford Algebra gives Smith some nice 8-fold periodicity foamy structure. The 26-dim E6/F4 is also nicely used by Smith for bosonic strings and d-branes. Jump up a level to E7/E6 and you find 27-dim bosonic M-theory that Horowitz-Susskind and later Smolin wrote about. So Clifford Algebra spin foam and bosonic M-theory would be my Smithish suggestions for where promising areas need more exploration. That recent SO(10) Baez paper is nice too since SO(10) has a nice spacetime and nice bosons to place under those E6 fermions. That’s my Doug-like/Oz-like hand-wavy analysis.

  • http://iso42.blogspot.com Wolfgang

    agm et al.,

    > The question is what to discuss in public

    I guess it depends on what the target audience is and what the goals are.

    If it is about funding then it might help to mix some religious overtones into the presentation ( “the God particle” etc.), especially in the US.
    If the goal is to surprise the audience then Michio Kaku has some good stories to tell…
    If the goal is to make some extra bucks at the end of a career one should follow the well-known advice to avoid formulas and stick mostly to anecdotes.

    If the audience is interested to learn about physics, I would recommend to pick up some text books (starting perhaps with the Feynman lectures) and avoid popular science books and op-ed articles in the NYT. But of course this requires work and dedication …

  • Elliot

    Wolfgang,

    I find your comment very condescending and really misses the point. A lot of people here understand basic physics including relativity, basic QM, field theory, standard model, basic cosmology etc. The topic of interest is not physics or how hard people are willing to work, but where things are on the cutting edge of research into theories of quantum gravity in general and string theory in particular. Funding certainly plays into the equation as well as exciting children about possibly considering the field as a future endeavor. Also having a well informed public enriches our culture.

    Regards,

    Elliot

  • http://www.math.coumbia.edu/~woit/blog Peter Woit

    Jacques,

    Note that I did not anywhere say that non-perturbative gauge theories in general have been neglected. Obviously there has been a huge effort in this area since Seiberg-Witten, with quite a lot of success (in the supersymmetric case). I was explicitly talking about chiral gauge theories, non-supersymmetric ones as context should have made clear. There we’re in complete agreement that “Direct results on the nonsupersymmetric case have been few and far between”.

  • http://iso42.blogspot.com Wolfgang

    Elliot,

    > A lot of people here understand basic physics including relativity, basic QM, field theory, standard model, basic cosmology etc.

    as I wrote, it depends on the target audience. If you talk about people who understand physics I suggest arXiv.org as a good starting point to learn about the current status of theoretical physics. (hep-th for string theory)
    There are many good overview and introductory articles not only technical papers.

  • Elliot

    Wolfgang,

    Thank you for the suggestion.

    Elliot

  • http://scientiaestpotentia.blogspot.com/ A

    As in the American Physical Society’s back page article, I am concerned about the strain that “intelligent design” might put upon public support of good science. It is not only worth speaking to the public, but absolutely necessary! The public pays my salary (as probably yours), and hold very dear the honor and duty of doing good science on their behalf.

    On the other hand, “an attack on evolution is an attack on science” is likely a generalization. It’s not immediately clear from Prof. Krauss’ essay that he is certain on this point, but I want to point it out. Generalizations are dangerous.

  • Richard

    Just this morning I found myself once again cringing while hearing physics mangled in the media and placed in inappropriate context, this time on the local public radio station. A New Age guest mingled these expressions in one sentence: “quantum mechanics,” “out of body experience,” and “mediums.”

  • http://thomas.loc.gov X

    agm, this is near the cutting edge. The physicists can turn around and explain it; and explain it without the jargon; but if a non-physicist wants to follow it in real time, I’m sorry, can’t be done without effort. This is true of any other technical field as well.

    One thing a discerning John Q. Public would catch on is that the fact of controversy means that there are plenty of unknowns and undecided things. If he also catches on that it means that there are plenty of interesting (and hard) questions to answer, then, in my opinion, this experiment in exposing the kitchen has worked.

  • Anonymous

    I’m with X: I’m not quite sure what this kitchen analogy is all about. If the field were all nice and tidy everywhere, even the kitchen, the field would be dead. There can only be a confusing mess and a lot of debate when dealing with hard problems. And no one is going to say we shouldn’t deal with hard problems. So why shouldn’t things be a confused mess with people disagreeing with each other about what is the best thing to do? Seems to be (to some extent) a sign of a healthy field.

  • http://www.math.coumbia.edu/~woit/blog Peter Woit

    Anonymous:

    The problem is that the mess in the kitchen is pretty horrendous, caused by an expensive, failed 20 year-long renovation project. Until recently, any family member who pointed out this out was dismissed as not knowing what they were talking about. This situation does seem to be changing.

  • http://blogs.discovermagazine.com/cosmicvariance/mark/ Mark

    There’s an interesting and somewhat frustrating thread going on here. It concerns what we mean by the public popularization of science.

    I am somewhat sympathetic to what Doug and Elliot have had to say about the discussion here – maybe it has strayed far away in places from the level the public can grasp – but this is a blog, not a tightly controlled classroom, and anyone can post and discuss here and it is up to them what level they choose to converse at.

    The thing is, we desire (at least I do) the public to be able to get the big picture of what we care about; the main issues we’d like to address, and the main techniques we are using. However, the idea that one should be able to converse fluently with experts about the intricacies of what they are doing, without years of dedicated study, is just hopeless. This is a natural thing – since what does it mean to be an expert if it doesn’t require intense, long term study.

    Some experts here are trying very hard to get across the basic picture. Others aren’t trying. Our blog tries to bring the excitement and the big questions to the public when it can, but one just cannot expect to become fluent in, for example, the technical issues facing Loop Quantum Gravity, or whether string theory formulated on an asymptotically AdS manifold constitutes a legitimate nonperturbative description of some time-dependent backgrounds.

    I say this as someone who spends inordinate amounts of time on public science education. But I say it because some of the comments here seem to be annoyed at experts for not explaining the extreme technical subtleties of what they do in layman’s terms. This occurs because there are no layman’s terms for some concepts. They require a whole career to grasp. we are working very hard on getting some of these ideas across. Some people can be condescending when answering such questions, but I don’t intend to be – I merely want to explain the obstacles.

    Bear with us, ask questions, and try to take part in the conversation at whatever level suits you. We’ll try to be helpful when you do – it’s what we’re here for.

  • Elliot

    Mark,

    Thank you for your reply. I would like to say that it is not annoyance at all. On one hand it is great to be able to witness key contributors to the field unabashedly fire away at whatever (or whoever) they want. On the other its a little like getting invited to a family dinner where there are clearly some conflicts and sitting there while these are displayed in public. Its a little uncomfortable for outsiders.

    That being said I totally with the comment above by X that if everything were tidy the field would be dead and there wouldn’t be much excitement or interest.

    It is precisely because these are hard and unresolved problems that I choose to spend some of my time here. It is a wonderful drama.

    Elliot

  • http://eskesthai.blogspot.com/ Plato
  • http://blogs.discovermagazine.com/cosmicvariance/mark/ Mark

    Hi Elliot – well put. I understand and sympathize. Cheers,

  • Aaron

    I’m not sure if I should feel guilty for participating in the thread hijacking or not, but it does drive me nuts when Smolin (especially) continually posts just plain incorrect information about string theory. If he’s going to make technical points, it doesn’t exactly demand a technical response, but it’s a lot of work to detechnicalize it, so just responding in kind is easier. Not only that, we only just recently had a rather long exchange on many of the same topics and I don’t think anyone’s particularly interested in having that rehashed at length.

    So, back to the main point of the thread: I do think that string theory is oversold. I don’t think there’s any reason to hide the problems of string theory from the public. For all its many problems, it’s still the best game in town. And, even if it’s wrong, it has inspired phenomenological models, a new nonperturbative ideas in SUSY QFTs, a better understanding and implementation of QCD strings, lots of fun mathematics, new solutions of GR and probably other stuff I’m forgetting. When it’s all wrung out, I doubt it will be a completely useless endeavor. But, I think every popular article on the subject should really be prefaced by the statement that this all could be utterly wrong.

    I do think that there’s a lot more turmoil going on in the field than gets communicated in the popular press. I don’t know what I think about that, though. Somewhere between Woit’s knee-jerk negativism and the happy-fun1 Brian Greene Nova shows, there’s a truth to be found, but it’ll be tough to communicate it well.

    1. Still the best thing about string theory ever written, even if nobody else agrees with me.

  • http://eskesthai.blogspot.com/ Plato
  • http://www.math.uni-hamburg.de/home/schreiber Urs

    Robert,

    yes, I understand that. It’s the Weyl condition that I did not take into account. A Weyl spinor sure enough gives a null vector. All right.

  • Doug

    Mark,

    I appreciate this blog (and its precursor) tremendously, not just for the unprecedented opportunity it affords me to peek into the kitchen from time to time, but also for the chance to “speak up” and “voice” my own opinion and perspective. So, many thanks to Sean and you and Clifford and your colleagues for making this possible.

    When Elliot posted his initial comment, which I quoted, I thought it was right on target to the issue. His question was a layman’s question expressed in layman’s terms, however the issue it addresses is the central issue – the motivation for string theory has to do with the MAJOR assumption of the Newtonian system of physical theory: that every physical object can be represented as a composite of particles.

    If this general assumption is false, then there are major philosophical implications that we all need to face and the public has a right to know. Decades spent on an unsuccessful kitchen renovation project could turn into several more decades of wasted effort. My point is that it’s not all a matter of professional dialog over the technical details involved in the renovation. Granted, the owners can’t expect the architects and engineers to stop their deliberations and find a way to teach the owners to understand the technical issues – that’s not possible, but it’s also not necessary.

    What they can do is address the problems in general terms. If physical objects cannot be represented as a composite of particles, and the behavior of a particle cannot be shown to be governed entirely by its interactions with other particles, what does this mean? What I meant to say was that demonstrations of flour and pasta in colanders on the one hand, and undecipherable professional dialog on the other, leaves a huge gap in the middle unaddressed, and that goes directly to the point of this thread made by Lawrence – “Is it worth talking about [string theory] to the general public?”

    Is it worth talking about? Krauss’s answer to this question is just baffling. On the one hand, he thinks it is – to help the public understand what physicists “sometimes like to think about,” but on the other hand, “no,” because he doesn’t think that it’s likely that these ideas are related to the underlying reality of the structure of the physical universe! For crying out loud, why would we be more interested in the manifolds, orbifolds, and the myriad of other folds that you guys banter about among yourselves, than we would be in your conclusions that “there is [no] compelling evidence that these ideas are going to be correct?”

    Why can’t that conclusion be discussed in more general terms? Point particles lead to trouble. Vibrations seemed to be a way out, but decades of intense effort cannot find it. Some think we need to rethink the nature of space and time (background-free, Smolin; emergent spacetime, Gross; natural origin of integers, Atiyah).

    Sean wrote:

    I think it’s perfectly clear that we should be talking about string theory, and being honest about what we know and don’t know about it. Successful or not, the ideas behind string theory are both deep and incredibly exciting, and more importantly are what a large number of theoretical physicists today are actually thinking about. Why should we hide our work from the public until it’s all finished? We should trust them to be able to understand when we explain the speculative ideas we are thinking about.

    But I would add, at this point, with all due respect, Mark, many of us are tired of the endless speculative ideas. Speculation, ad hoc solutions, is not good science, just ask Newton. It seems as if the time has come to face the sobering truth that we are floundering here. “Something is missing,” as Gross puts it. We are “stuck” according to Weinberg, and, most revealing of all, Atiyah, observes:

    If…a final theory emerges soon from strings, we will discover that the universe is built of fantastically intricate mathematics.

    Understanding the tower of “fantastically intricate” mathematics may be the only way a human being can come to understand nature in the final analysis, but I hope not, and I think it’s fair to say that Atiyah, and all wise men everywhere share the same faith. Ironically, though, if it turns out that this is so, what does that imply about intelligent design? Does it take more intelligence to design a system of physical structure from which great things proceed from that which is small, simple and easy to understand, or does a “fantastically intricate” basis call for fantastically advanced intelligence?

    On the other hand, if there is no intelligent basis for the structure of the physical universe, could the “fantastically intricate mathematics” have emerged from first principles?

    I think the answer to Krauss’ question “Is it worth talking about [string theory] to the general public?” should not only be answered affirmatively, but officially.

  • Anonymous

    Doug wrote:

    If this general assumption [that the world is made up of point particles] is false, then there are major philosophical implications that we all need to face and the public has a right to know.

    I hope you’ll forgive me for saying so, but this sort of statement is rather exasperating. I think it highlights a common problem in the dialogue between “the public” and scientists. On the one hand, we want to combat the notion that science is a collection of “facts” that have been established and that one can memorize. We want to highlight that science undergoes continual revision, that we must constantly test our ideas against the real world, that no claim is sacred and anything is subject to change in light of new evidence. On the other hand, we can get so caught up in trying to make people understand this that we emphasize too much the uncertainties and the arguments, and people get a misleading view that science doesn’t really know anything. Of course the truth lies somewhere in between. The Newtonian view of physics, Einstein’s special relativity, Maxwell’s classical electrodynamics, the quantum mechanics of the atom: these things are all right. They have all been revised, they have all been found to apply only in certain regimes, but this does not change the fact that they are extremely good descriptions of the world that continue to work very well in their regimes of validity. So there can’t be any “major philosophical implication” of overturning them, unless your philosophy is broken. It is simply always true that anything we learn has some domain of validity and might be shown false once one probes beyond that domain. But this doesn’t render what we have already learned untrue! For this reason, I think it’s essential that we educate people about the things that we do know scientifically about the world. In doing so we should stress the process that showed that we are correct, and we should stress that what is known applies only in some limited domain of accuracy. This is the way to make people understand what science is all about and what it really achieves. To talk too much about the “kitchen,” to use the metaphor that’s dominating this thread, is to confuse people about the extent to which we really know things about the world. Of course the public should have access to information about current research, and we should try to interest them in what we are doing now, but this is secondary to making them understand that great progress has already been made, progress that will not be overturned, only refined. I think this confusion over what science really is and to what extent old ideas get overturned is largely responsible for things like the Intelligent Design mess.

    When people demand that they “have a right” to know all the details of current science, I am baffled. All the information is there, but if we were to express all of it in terms that non-experts could understand, we would spend far more time popularizing than doing actual work, and progress would grind to a halt. It is in this sense that the “kitchen” is healthy. If you genuinely feel a need to understand what is going on in science, the information is there for the asking, but you cannot expect it to be all immediately transparent. There is a balance for scientists to strike between doing science and explaining science, and while the balance might not be struck in the best way, it will never be possible for every new development to be immediately communicated in a clear way to the public. Some of us do our best, others of us do not.

  • http://golem.ph.utexas.edu/~distler/blog/ Jacques Distler

    Woit wrote:

    I was explicitly talking about chiral gauge theories, non-supersymmetric ones as context should have made clear.

    Sorry, it wasn’t clear to me, at all, that when you use the phrase “chiral gauge theories,” you mean “nonsupersymmetric chiral gauge theories”. Thanks for the clarification (added to my Peter Woit Lexicon).

    Doug wrote:

    Understanding the tower of “fantastically intricate” mathematics may be the only way a human being can come to understand nature in the final analysis, but I hope not…

    You make it sound as if the fact that our physical theories are couched in mathematical terms is defect, rather than a virtue. I really think the opposite is true: that our mathematical descriptions (however elaborate) are simpler than the myriad phenomena they describe.

    When we describe (in introductory physics classes), blocks and springs and baseballs, we’re tallking about phenomena with which we have a great deal of personal experience, and hence, before hitting the mathematical description, we have built up a certain intuition as to how they work. In explaining those phenomena, it’s helpful to appeal to that intuition. The further we delve into the quantum regime, the less intuition we bring from our ordinary experience, and the more we need to rely on mathematics.

    That’s not to say that it’s impossible to suggest useful metaphors for what the equations are saying (pasta wouldn’t be my choice; see Feynman’s “QED” for a masterful example of making a complicated subject apprendable in plain English).

    But it’s pretty darned difficult to explain why approach A doesn’t work, or approach B runs into technical difficulties, based on the metaphors we use to describe them. To attempt such an explanation would be to confuse the metaphor for the thing itself.

  • http://eskesthai.blogspot.com/ Plato

    I have to remind myself sometimes of the many books that have appeared on the scene over the past years.

    You came into my life, I read what you had to say. If you now want to talk about it, that’s okay.:)

    Your own blog? THis is a good idea. Because you wrote a good book I won’t hate you or call you a money grabber, or one who likes the light.

    The fact that you would summarize your views is important, and it is just as important that I recognize whether your words are supported. What did you think? You could hide behind all this information and not be seen?

    Currently I am reading Robert B Lauglin’s, A Different Universe.

    Preface:


    My views are considerably more radical than those of either of my predeccessors, however, becuase they have been sharpened by recent events. I am increasingly persauded…..

    I will read your book too, Lawrence. The factual details of those extra diemnsions are really alluring I know. Fact or fiction, and of course we are drawn to the historical perspective.

    Was your view all inclusive in this range and detailled the perspective properly? The science that you had encounterd, revealled a truly represntative view of a geometrical basis that we lost track of at planck length?

    Please correct me here as I wonder about the emergence of the right geometry, from quantum gravity. Fromwhat is inside that blackhole?

    What are these buiding blocks of nature then?

    Robert Lauglin gives perspective on this yet what discerning force keeps it all together for us as we get ever closer to the begnning of the universe. How is it turned inside/out? Doug asks a good question.

    Doug has this thing about Octonians and now that he is opening up, it is nice to see his interest has blossommed. He was a little stagnated for a while there. ;)Yes, thank you people of Cosmic Variance for hosting Lawrence.

  • http://eskesthai.blogspot.com/ Plato

    When we describe (in introductory physics classes), blocks and springs and baseballs, we’re tallking about phenomena with which we have a great deal of personal experience, and hence, before hitting the mathematical description, we have built up a certain intuition as to how they work. In explaining those phenomena, it’s helpful to appeal to that intuition. The further we delve into the quantum regime, the less intuition we bring from our ordinary experience, and the more we need to rely on mathematics.

    This is very useful Jacque as to clarity of methods adopted. I thnk sometime analogies or metaphors are good things to use as long as your logic blends within th eanalogy just right.

    As you point out Feynman’s toy model as a pathway of possibilties? It just help to see where these interactive features would help define a bunch of ideas about the physics. Feynman idea, was gotten from Dirac?

    I think this is charismatic in the sense, that the creative abilites would help direct physics to the deeper levels, that math now in turn speaks for. What types of math?

    Yet, we are still talking about a depth of perception that is not readiy available for us all as you suggest in math spoken or used.

    Interesting.

  • http://blogs.discovermagazine.com/cosmicvariance/mark/ Mark

    Thanks for the comment Doug. I was trying to compose a detailed reply, but I think Anonymous has actually done a very nice job of it (thanks Anonymous!).

    Best wishes,

  • David

    Anonymous wrote (in #66):

    “Still, I think it’s nonsense to say that people don’t care about nonperturbative gauge theory…”

    If this is true it is not reflected in job opportunities. It is very tough these days for anyone not working on phenomenology/string theory/astrophysics or something with extra dimensions.

  • Anonymous

    David, I don’t entirely disagree. I said “I’m not going to say that everyone in the field makes good use of their effort. In fact, I would say that a large fraction of the work done in high-energy physics (both theoretical and phenomenological) is misdirected.” And if I were to start expressing opinions about a lot of research in the field, non-anonymously, I would offend many of the people I’ll be applying to for jobs in a year or two.

    On the other hand, a large number (not the majority, but a substantial fraction) of the people doing phenomenology, string theory, or “something with extra dimensions” are working at some level on nonperturbative gauge theory. There’s a relatively small but healthy lattice gauge theory community (some more phenomenologically oriented than others), there’s the large contingent of string theorists studying AdS/CFT and its generalizations, there are phenomenological approaches to holographic models of gauge theories, there’s a-maximization, continued study of monopoles and vortices in SUSY gauge theories, nonperturbative orientifold correspondences between non-SUSY and SUSY gauge theories, continued phenomenological study of sum rules as they relate to QCD data, and other directions beyond these. Just because a lot of these ideas aren’t immediately going to, say, prove confinement in non-SUSY QCD, doesn’t mean they aren’t teaching us useful things that allow incremental progress.

  • David

    “Just because a lot of these ideas aren’t immediately going to, say, prove confinement in non-SUSY QCD, doesn’t mean they aren’t teaching us useful things that allow incremental progress.”

    We can agree on that. But one can get an impression that string theorists/higher dimensional types are only interested in nonperturbative gauge theory to the extent that they can say something about it from strings/brane models. Ok, I know that is an unreasonable statement and I don’t really believe it myself. But still, there is something to it. Let me illustrate with an example. For a long time there were fundamental problems with incorporating chirality into lattice gauge theory; it seemed an insurmountable problem. But during the last decade a theoretical breakthrough occured which solved this problem. So we now for the first time have a framework for nonperturbative formulation of chiral gauge theories. More work is still needed to complete this construction, but the developments to date nevertheless represent a major breakthrough. However, no one outside the lattice community bats an eyelid at this. Now imagine if the same progress had instead been achieved through an application of string/brane model techniques. You can bet that the stringers or higher dimensional folks would be trumpeting this as a damned important result and further vindication of their research program. Hopefully the point I’m trying to get at is clear.

  • Moshe

    David, if I understand things correctly Neuberger’s fermions are a great improvment over- but still the same basic idea as- Kaplan’s fermions, which are the chiral fermions living on a brane embedded in higher dimensional space. Note that also the two ideas Jacques mentioned above to define N=4 on the lattice (topological twist and deconstruction) are also deeply rooted in string theory.

    Just pointing this out, you draw your own conclusions.

  • David

    Moshe,
    Neuberger fermions (also called overlap fermions, although that is a dirty word in some quarters) live in 4 dimensions, but you are right that historically they had their origin in Kaplan’s domain wall model. However, let me point out that (i) the domain wall perspective is not needed to obtain them; e.g. the overlap Dirac operator could have been guessed as a solution to the
    Ginsparg-Wilson relation in 4D (and with hindsight this guess would have been easy to motivate based on topological considerations), and (ii) in any case, the fifth dimension in the domain wall model was just a mathematical trick; it was not ascribed any “physical reality”. (At least that’s my perspective, and i think it is shared generally in the lattice community.)

    “Note that also the two ideas Jacques mentioned above to define N=4 on the lattice (topological twist and deconstruction) are also deeply rooted in string theory.”

    Sure, I am happy to acknowledge the origin of these ideas in string theory. The point I was getting at was more a “sociological” one, basically this: On the one hand it would be career suicide for a young person these days to work directly on nonperturbative formulation of chiral gauge theories, but if that person instead did strings and used string/brane techniques to derive nonperturbative gauge theory results then that would be a completely different matter…

    At any rate, I’m glad to see that at least one person outside the lattice community noticed these developments ;-)

  • Moshe

    David, I am afraid I am just not smart enough to understand anything about sociology, I will have to stick to physics…

    But, as we somehow found ourselves talking about interesting physics (what are the odds?) I am wondering if you could very briefly summarize where the overlap fermions currently stand, and what is some of the work that still needs to be done. Just curious, if you have the time.

  • http://eskesthai.blogspot.com/ Plato
  • David

    It occurred to me that it might be worthwhile to say a bit more about the developments re. chiral gauge theory on the lattice and clarify what role the brane scenario mentioned by Moshe plays in this, in case anyone is interested (and well knowing that this is way off topic from the original post, but the discussion seems to have died down here anyway).

    There were two lines of development which eventually led to conceptually different but mathematically equivalent frameworks. The first of these has its origins in continuum work of Callan and Harvey from 1985. The idea there was that the fermion action for a single chiral (Weyl) fermion can be obtained from the Dirac fermion action for an infinite number of fermion flavours, parameterised by a continuous flavour parameter s running over the whole real line, and with a specially chosen mass term which is nontrivial in flavour space (involving the differential operator d/ds). This mass operator then has a Weyl fermion as a zero-mode localised at s equal to zero, and with the other modes being heavy; they become infinitely massive and decouple in the limit where a certain parameter is sent to infinity
    and one is left with the usual Weyl fermion action. So this is a way to regularise a Wely fermion action by packaging it into a Dirac fermion action with infinite number of flavours. The continuous flavour parameter s can also be thought of as the coordinate for a fifth dimension if one wants to.

    Kaplan’s accomplishment (referred to by Moshe above) (hep-lat/9206013) was to find a way to carry this idea over to the lattice setting. His setup is a bit different though: the flavour parameter s now takes discrete values in an interval from 0 to L, and the mass matrix in flavour space is such that there are light left- and right-handed Weyl fermions localised at s equal to 0 and L, respectively, and coupled via heavy modes. In the limit where L becomes infinite and the spacing between the discrete values of s goes to zero the heavy modes decouple, and the left- and right-handed Weyl fermions disconnect from each other as well. So a chiral fermion theory based on the left-handed Weyl fermion at s=0 can be constructed. If you want to, you can think of the multi-flavour Dirac fermion theory as a theory for fermions living in a 5D ”bulk” with 4D branes at each end of the 5th dimesional interval [0,L] ; then Weyl fermions on the two branes decouple in the mentioned limit. In any case, Kaplans work inspired the subsequent ”overlap” formulation lattice chiral gauge theories by Narayanan and Neuberger (hep-th/9411108 and ref.’s therein), which is closer in spirit to the continuum formulation of Callan and Harvey. I’ll spare you further details on this, including the reason for the name ”overlap”.

    The second line of development originated in the work of P. Hasenfratz and collaborators on a class of lattice Dirac actions called ”fixed point” (or ”classicaly perfect”) actions. I wonÂ’t go into the details of these except to say that they arise as fixed points under lattice RG blocking transformations. The discovery of Hasenfratz et.al. (hep-lat/9801021) was that the lattice Dirac operator in such an action satisfies what is known as the Ginsparg-Wilson (GW) relation. This relation had been discovered way back in 1982 by G&W as an acceptable way for a massless lattice Dirac operator to break chiral symmetry without losing the good properties that are associated with chiral symmetry (roughly speaking). It offered a way out from the no-go theorem of Nielsen and Ninomiya, which (roughly) states that any otherwise sensible chirally symmetric lattice Dirac operator will be afflicted with ”fermion doubling” (”spurious fermion species in the spectrum”) and unusable for constructing a Weyl fermion action. But no solutions of the GW relation were found at the time, and it was basically forgotten until the discovery of Hasenfratz &co. Shortly after that Luscher discovered (hep-lat/9802011) that lattice Dirac actions satisfying the GW relation have an exact symmetry – a lattice-deformed version of continuum chiral symmetry – and this allows a natural decomposition of the action into left- and right-handed pieces, i.e. lattice chiral gauge theories can be constructed.

    So how did these two new ways to formulate chiral gauge theories on the lattice come together? Well, Neuberger discovered (hep-lat/9707022) that the vector version of the overlap formulation could be described by a lattice action for just one flavour of Dirac fermion (i.e. no need for the flavour parameter s); the corresponding Dirac operator became known as the ”overlap” or ”Neuberger” operator. Moreover, he found that it also satisfied the GW relation (hep-lat/9801031). So then one can construct the chiral gauge theory from this a la Luscher, by decomposing the action in to left/right-handed pieces, taking say the left piece and constructing the chiral gauge theory via path integral just as in the continuum setting. It turns out that the resulting theory is mathematically equivalent to the original overlap formulation, involving infinite number of flavours as described above. The upshot is that any lattice Dirac operator satisfying the GW relation can be used to construct a lattice chiral gauge theory, and this can be done in two conceptually different but mathematically equivalent ways: either via decomposition of the Dirac action into left and right and use path integral in usual way, or go via the overlap formalism with infinite number of heavy flavours as regulators.
    Finally I should mention that the construction of lattice chiral gauge theories is not yet complete –there are issues regarding anomaly cancellation which remained to be sorted out. This is where the focus of current research efforts lie.

    So I hope that by now the role of the brane scenario mentioned by Moshe is clear: yes, you can make such an interpretation if you want to in one of the two lines of development of lattice chiral gauge theory, although it is arguably more natural to make an interpretation in terms of an infinte number of heavy regulatior fields in the spirit of Pauli-Villars. But in any case this goes back to the old idea of Callan and Harvey from 1985; it is not an outcome of recent string/branes research programs.

  • David

    Moshe, I just saw your #102 now after I had submitted my #103. By coincidence you request is (hopefully) fullfilled there, to some extent. I’ll be happy to tell you more about the current situation if you like; there are some (for me) very interesting things that still remain to be done.
    And yes, sticking to physics rather than sociology sounds like a good idea :)

  • Moshe

    Thanks David, this is very useful, I was aware only some parts of this narrative. I am not particularly concerned about attribution, that tongue-in cheek comment about braneworlds was in response to your “trumpeting” comment…However I should say that even is string theory the higher dimensional perspective is not always the most useful one. In some cases, where the “higher dimensions” are non-geometrical they ought to be thought of simply as internal degrees of freedom (which sometimes arrange themselves into KK towers).

    I would benefit from your thoughts about where this is going, but it is up to you how much time you want to devore to this.

    best,

    Moshe

  • Doug

    In 92, Anonymous writes about the scientist’s dilemma wherein the attempt to convey to the public the sense that the progress of science toward an ultimate understanding of the structure of the physical universe is real and meaningful seems to be defeated by the very process of scientific investigation that necessarily entails a healthy skepticism. This, he fears, leads to the “misleading view that science doesn’t know anything.” However, I think this is overstating the case significantly. The point I have continually stressed is that there is a need to respect the interested public’s ability to understand, in general terms, the state of mankind’s struggle to comprehend the nature of the physical universe.

    They learn of the advances from Newton to Feynman in school, from high school through university level and beyond. They read, they study and they understand enough of the subject to discuss many aspects of its struggle intelligently. Nevertheless, this is a far cry from being able to write a paper on it, or engage in a dialog with professionals on its technical aspects, especially since those have become almost exclusively clothed in obtuse mathematics.

    Yet, this is not relevant to the real issue, which is not one of deciding if educating the public on the ins and outs of string theory is worth the effort or not, as Krauss has framed it, but rather is one that pertains to the public’s evaluation of the scientific community’s efforts. Elliot’s question in 2 addressed the core issue that lies at the center of mankind’s struggle to understand nature: is the assumption that the physical universe is composed of fundamental particles valid or not? The concept of replacing point particles with vibrations is much more than a technical tactic to resolve the difficulties point particles present in Feynman diagrams. It goes right to the heart of our most basic assumptions since Newton.

    However, the prospect that we can no longer assume that matter is fundamental doesn’t imply that all progress to this point is invalid. As Anonymous puts it:

    It is simply always true that anything we learn has some domain of validity and might be shown false once one probes beyond that domain. But this doesn’t render what we have already learned untrue!

    In this sense, the ancient Greek’s knowledge of the elements, which they regarded as fire, air, water, and earth, is not rendered untrue by our probing beyond their domain and discovering that energy, gas, liquid and solid states of matter are due to the properties of atoms, which we can classify in a periodic table of 100 some odd elements. Likewise, the discovery that matter cannot be the fundamental constituent of the physical universe will not render untrue what we have learned about how particles interact, once they exist!

    Nevertheless, the fact that the efforts of string theory are leading physicists like Smolin to conclude that the problem lies in our understanding of the nature of space and time, and physicists like Gross to expect that the major revolution in our understanding of space and time will characterize the missing pieces that are currently frustrating our most diligent efforts, legitimizes Elliot’s question, regardless of his professional qualifications to discuss the details of those efforts. That’s my point.

    For the ancient Greeks, the universe was a harmonious whole, contained in the sacred secrets of four numbers. In contrast, today, although we know a lot more and the domain of our knowledge is much broader, “our theories of physics, the laws of physics, are a multitude of different parts and pieces that do not fit together very well. We do not have one basic structure from which all is deduced,” as the oft quoted Feynman put it, and it is this basic structure that we want, so Elliot’s question,

    Doesn’t this conceptual shift [from point particle to vibration] actually put the theory (or one of its descendants) in a better position to remove infinities from equations describing these fundamental phenomena?

    is not asking, as Jacques and others interpreted it, how do we remove infinities, but rather, it’s more like this: “Tells us string theorists, can we successfully transcend the zero-dimensional point particle and say now, with confidence, that the fundamental constituent of the physical universe is a multi-dimensional entity, or not?”

    Not only has the question not been addressed in this thread, but, apparently, it has not even been understood correctly. Ironically, this leads us to the sociological question, “how does the public effectively communicate to the scientific community?” It may not be as interesting a question to scientists as how the new ways to formulate chiral gauge theories on the lattice came together, and that’s understandable, but we all have a common interest in the deep and thought provoking answer, regardless of our professional status.

  • http://eskesthai.blogspot.com/ Plato

    That was very good Doug:)

    Since Wolfgang invoked the God particle, I have been quite restless, because even though the math equations lead to a deeper introspection of the world we cannot see, and becomes not so intuitive, critical thinking becomes quite tense and something was released in a quiet moment.

    Where did all those bits and pieces, make such a complete picture? Where did the picture come from?

    So sure I speculate too. I show Susskinds leanings as well as to the nature of reality and what is unfathomable, becomes imparted with Godly powers. Had we known, that it would be part and parcel of something “that already existed?” This is what PLato already thinks:)

    He knows about your Angels and Demons :) Please take it in good humour and then, wonder, about the potential of how such negativity arose in the beginning. The anti?

  • http://eskesthai.blogspot.com/ Plato

    So you see even under these “sociological foundations of good and evil,” there is physics involved. It just gets all dressed up.

    You got to remember where you are, as you think about the opening of this thread.

    Although, getting to the brane world happenings are very interesting to me, a simple explosion to branes from strings needed to be understood for the “junior partners?”

    All the while, we are exposing our narrative styles here:) Even Lawrence.:)Those, who would reject “string theory” as evil doers?

    Surely we don’t want to be any become evangelistic about how inherently good we are, while that natural part of our expressions, recognizes, that we can become better human beings as we struggle. Become Better string theorists? :)

  • http://nigelcook0.tripod.com/ Nigel Cook

    The problem is that people are used to looking to abstruse theory due to the success of QFT in some areas, and looking at the data is out of fashion. If you look at history of chemistry there were particle masses of atoms and it took school teachers like Dalton and a Russian to work out periodicity, because the bigwigs were obsessed with vortex atom maths, the ‘string theory’ of that age.

    Eventually, the obscure school teachers won out over the mathematicians, because the vortex atom (or string theory equivalent) did nothing, but empirical analysis did stuff.

    Mesons

    Pions = 1.99 (charged), 1.93 (neutral)
    Kaons = 7.05 (charged), 7.11 (neutral)
    Eta = 7.84

    Baryons

    Nucleons = 13.4
    Lambda = 15.9
    Sigmas = 17.0 (positive and neutral), 17.1 (negative)
    Xi = 18.8 (neutral), 18.9 (negative)
    Omega = 23.9

    The masses above for all the major long-lived hadrons are in units of (electron mass)x137. A statistical Chi-squared correlation test confirms they are close to integers. The mechanism is that the charge of the bare electron core is 137 times the Coulomb (polarisation-shielded) value, so vacuum interactions of bare cores of fundamental particles attract 137 times as much virtual mass from the vacuum, increasing the inertia that much too.

    Leptons and nucleons are the things most people focus on, and are not integers when the masses are in units of (electron mass)x137. The muon is about 1.5 units on this scale but this can be explained by a coupling of the core (mass 1) with a virtual particle of similar size for an average of half the time, just as the electron couples increasing its magnetic moment to 1 + 1/(2.Pi.137). The mass increase of the muon is 1 + 1/2 because the Pi is due to spin and the 137 shielding factor doesn’t apply to bare cores in proximity.

  • http://golem.ph.utexas.edu/~distler/blog/ Jacques Distler

    Doug wrote:

    …is not asking, as Jacques and others interpreted it, how do we remove infinities, but rather, it’s more like this: “Tells us string theorists, can we successfully transcend the zero-dimensional point particle and say now, with confidence, that the fundamental constituent of the physical universe is a multi-dimensional entity, or not?”
    Not only has the question not been addressed in this thread, but, apparently, it has not even been understood correctly.

    Umh. OK. I will try to answer that question.

    Perturbative quantum field theory is, in essence, a quantization of a theory of classical theory of point particles. Perturbative string theory is a quantization of a classical theory of strings (1D objects).

    However, there are strongly-coupled quantum field theories which are not mere quantizations of classical point particle theories. Indeed, there are quantum field theories which have no Lagrangian formulation and hence are not quantization of any classical system.

    I can’t tell you what strongly-coupled string theory is (well, in some cases I can), but it is not the quantization of a classical theory of strings. Moreover, many string theory backgrounds admit different weak-coupling limits in which the theory can be viewed as the quantization of a classical theory of 1D strings. But, in different limits, it’s a different kind of string that appears as the “fundamental degrees of freedom” that one quantizes.

    For instance, there’s a background with 16 unbroken supersymmetries in 6D Minkowski space (8 of each chirality). In some limits, it looks like weakly-coupled Type IIA string theory (compactified down to 6D). In some other limits, it looks like weakly-coupled E8×E8 heterotic string theory. In other limits, it looks like the SO(32) heterotic string. Or the Type I string. Or, in yet other limits, like the compactification of some 11-dimensional theory called M-theory.

    These different weak-coupling limits are all continuously related to each other. We could pick one and say that the theory is “really” the quantization of classical Type-IIA strings (say). But that would be a fake. It’s not “really” any of these. It’s a single, intrinsically quantum, theory (which we haven’t really understood yet) that has several different, and incompatible, classical limits.

    So, no, we haven’t definitively “replaced” point particles by 1D strings. We have replaced both of them by something infinitely more subtle and interesting.

  • http://www.math.columbia.edu/~woit/blog Peter

    Doug,

    “can we successfully transcend the zero-dimensional point particle and say now, with confidence, that the fundamental constituent of the physical universe is a multi-dimensional entity, or not?”

    The on-going debate among theorists is about how promising this kind of new hypothesis is, and the issue itself is not so simple: for instance it’s conceivable that at a fundamental level there are two equivalent dual descriptions of the universe, one involving fundamental multi-dimensional entities, one not.

    But I don’t think there’s any real debate about the fact that at the present time there is no experimental evidence for fundamental multi-dimensional entitities, and until that changes few serious physicists would claim that they could say with confidence that these exist.

  • http://thomas.loc.gov/ X

    I thought this is both interesting and an example of a good non-physics exposition of a technical subject. If I’m completely off-base, please delete this post.

    http://www.sciencenews.org/articles/20051105/bob8.asp

  • http://eskesthai.blogspot.com/2005/10/what-are-those-quantum-microstates.html Plato
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  • Elliot

    X,

    Actually I think this is a very good example. I am a little biased as I think Information Theory/Coding are fascinating areas and have this intuitive belief that they may actually be relevant to the main line discussion here. (but that is a topic for another day)

    Doug,

    I appreciate you reposing the question again. Let me rephrase it in a slightly different way. If one of the fundamental goals of current scientific inquiry is to come up with a consistent theory that integrates QM and Gravity, and the consensus is that this will involve quantizing gravity, all measureable phenomena will have some minimum value. (or they will register 0) Doesn’t this imply that there must be some structure at or around the planck scale vs. 0 dimensional point like objects? I understand that using classical techniques (assuming point particles and arbitrarily smaller distances) to calculate and probe these are one technique, but don’t people who “believe” that the resolution of incompatibilities between QM and Gravity will emerge with a quantum theory as opposed to QM being modified to fit the GR framework logically forced to abondoned the notion of 0 dimensional point particles at the phenomenolgical level

  • http://golem.ph.utexas.edu/~distler/blog/ Jacques Distler

    If one of the fundamental goals of current scientific inquiry is to come up with a consistent theory that integrates QM and Gravity, and the consensus is that this will involve quantizing gravity, all measureable phenomena will have some minimum value.

    Says who?

    I ses no reason whatsoever to assume that’s true. Maybe it is; most likely it isn’t, but it would be stupid to simply assume that it’s true, and try to build a theory around it.

    Doesn’t this imply that there must be some structure at or around the planck scale vs. 0 dimensional point like objects?

    There are no “structures” or “pointlike objects” (at least, not in the sense you mean those terms) at the Planck scale. The world is quantum-mechanical and it is almost certainly very quantum-mechanical at the Planck scale. Classical concepts (“structures,” “pointlike objects”) emerge at long distances, where the world behaves semiclassically.

    While I am (for the reasons explained in that blog post of mine that I linked to earlier) convinced that string theory is the only viable approach to a theory of quantum gravity, I am very uneasy arguing for it based on some hand-wavy claim that the exigencies of Planck-scale physics force us to abandon “point particles” in favour of “1D extended objects.” Particularly so because, ultimately, “string theory isn’t a theory of strings.”

  • Elliot

    Jacques,

    To be clear when I say not pointlike, I mean at least 1 dimensional or greater.

    Elliot

  • http://nigelcook0.tripod.com/ Nigel Cook

    The Higgs mechanism plus heuristic QFT leads to a lot of predictions.

    There was a crude empirical equation for their masses by A.O. Barut, PRL, v. 42 (1979), p. 1251. We can extend the basic idea to hadrons! The muon is 1.5 units on this scale but this is heuristically explained by a coupling of the core (mass 1) with a virtual particle, just as the electron couples increasing its magnetic moment to about 1 + 1/(2.Pi.137). The mass increase of a muon is 1 + 1/2 because Pi is due to spin and the 137 shielding factor doesn’t apply to bare particles cores in proximity, as it is due to the polarised vacuum veil at longer ranges. This is why unification of forces is approached with higher energy interactions, which penetrate the veil.

    This idea predicts that a particle core with n fundamental particles (n=1 for leptons, n = 2 for mesons, and obviously n=3 for baryons) coupling to N virtual vacuum particles (N is an integer) will have an associative inertial mass of Higgs bosons of:

    (0.511 Mev).(137/2)n(N + 1) = 35n(N + 1) Mev, where 0.511 Mev is the electron mass. Thus we get everything from this one mass plus integers 1,2,3 etc, with a mechanism.

    Accuracy tested against data for mass of muon and all ‘long-lived’ hadrons:

    LEPTON (n=1): Muon (N=2): 105 Mev (105.66 Mev measured)

    HADRONS

    Mesons (contain n=2 quarks):

    Pions (N=1): 140 Mev (139.57 and 134.96 actual)
    Kaons (N=6): 490 Mev (493.67 and 497.67 actual)
    Eta (N=7): 560 Mev (548.8 actual)

    Baryons (contain n=3 quarks):
    Nucleons (N=8): 945 Mev (938.28 and 939.57 actual)
    Lambda (N=10): 1155 Mev (1115.60 actual)
    Sigmas (N=10): 1155 Mev (1189.36, 1192.46, and 1197.34 actual)
    Xi (N=12): 1365 Mev (1314.9 and 1321.3)

    The mechanism is that the charge of the bare electron core is 137 times the Coulomb (polarisation-shielded) value, so vacuum interactions of bare cores of fundamental particles attract 137 times as much virtual mass from the vacuum, increasing the inertia similarly. It is absurd that these close fits, with only a few percent deviation, are random chance, and this can be shown by statistical testing using random numbers as the null hypothesis. So there is strong evidence that this heuristic interpretation is on the right lines.

  • http://golem.ph.utexas.edu/~distler/blog/ Jacques Distler

    To be clear when I say not pointlike, I mean at least 1 dimensional or greater.

    Same difference. You’re not going to get very far with me, pretending that physics is semiclassical at the Planck scale.

  • Elliot

    I think you misread my post. I am advocating the exact opposite that, physics is not classical or even semi-classical at all at the planck scale.

  • ksh95

    Jacques Distler said;
    Same difference. You’re not going to get very far with me, pretending that physics is semiclassical at the Planck scale.

    It’s comments like these that irk many of us outside of the string theory community. Personally, I’d bet the farm that physics isn’t semiclassical at the Planck scale, but the absolute undisputable fact of the matter, is that you have no idea what the nature of the universe is at those length scales. Any rebutals to that statement invariably have to start with …In my opinion…I find it hard to believe that…It seems to me that…

    The arrogance is astonishing

  • http://eskesthai.blogspot.com/2005/10/what-are-those-quantum-microstates.html Plato

    ksh95,

    Sorry Jacque…

    Jacque was right to make this comment not that he needs me to say it? He was being up front by suggesting that he would not respond to it. He recognized the limitation of views in that regard, as far as I could tell.

    It still has to be defined I think, so you’d be right there. Everyone understands the equation right?

    Elliot understood? :)

  • http://blogs.discovermagazine.com/cosmicvariance/clifford/ Clifford

    Hi ksh95,

    Thansk for your comment.

    I’m sorry you were offended by that remark of Jacques’ that you highlighted. I do have a lot of sympathy with the fact that all physicists (not just string theorists), arguably as a result of the nature of what they do and how they do it successfully, walk a thin line between confidence and arrogance in what they are doing, and unfurtunately often cross over it.

    But in this case I think Jacques deserves the benefit of the doubt. He did not mean to offend (I think), and I think that he was confidently stating that he considers it (in his opinion – yes you are right) extremely unlikely that physcis is classical at the Planck scale, and has decided to proceed under that assumption. There is no direct evidence for this, but it is one of the entire field’s (not just string theorist’s) strongest assumptions, and this is one area where nobody has yet come up with even close to a plausible idea of how to get classical physics to come out in that regime…..such an eventuality does fly in the face of everything we know to be true about Nature, since it would require us to revisit the formulation of quantum mechanics, which we believe to be firmly established as a property of Nature in its current form…. So far: We could well be surprised by real experiment one day…that would be wonderful. Until then, one has to proceed with some assumptions, and this is one of the most basic ones, so that’s what he was saying.

    That was the (physics) subtext of his remark…..I think!

    Cheers,

    -cvj

  • Moshe

    Arun (#60), never replied to your question. That is a good point: the way Einstein equations are derived in string theory is that you expand around the most general (weakly curved) background, and the conditions for consistency of the string propagation, without any further input, yield the Einstein equation for that background. The background is the most general one, one never has to specify that it is static,or anything else for that matter, except that it is weakly curved so corrections to Einstein action are small.

  • http://golem.ph.utexas.edu/~distler/blog/ Jacques Distler

    There was a physics subtext to my remark. It is explained in the blog post that I linked to up-thread. “We don’t have a clue,” is not really accurate. We do have a clue, if only a clue.

    If you disagree with the arguments in the aforementioned blog post, or if you disagree that they imply my remark above, then let’s have-at-it. But I don’t think you can just dismiss the arguments as mere “arrogance”.

  • Doug

    Peter writes, “There is no experimental evidence for fundamental multi-dimensional entities,” and Jacques writes, “We have replaced both [point particles and 1D strings] by something infinitely more subtle and interesting.” Of course, point particles have been the theoretical assumption for centuries and the motivation for replacing it, in physical theory, with multi-dimensional entities, comes from the experimental evidence indicating that it is not a fundamental entity. As Gross says, “Something is missing.”

    Now, from the layman’s point of view, this is clearly the issue the public needs to understand. Peter hits the nail right on the head: “The on-going debate among theorists is about how promising this kind of new hypothesis is.” But here’s the thing, this kind of new hypothesis, that maybe point particles are not fundamental, is not just another ad hoc invention to save a theory. Rather, it is a major shift in the paradigm that has been the basis for the Newtonian program of physics for centuries.

    As described by David Hestenes, that program seeks to explain the diverse properties of objects in our experience in terms of a few kinds of interactions among a few kinds of particles, but its great power lies in the fact that a precise mathematical formulation of its key concepts, particles and interactions, is found in the function x(t); that is, the expression of the continuous existence of a particle in some interval of time, is a continuous function of the variable t in that interval. When specified for all times in an interval, the function x(t) describes a motion of the particle.

    Now, granted, with the discovery of QM, the HUP and BCP twins modified the mathematical formulation of these key concepts to a certain extent, but still they remained the key concepts, until “this kind of new hypothesis” came along. So now the question is this: if the key concepts underlying the power of modern physics, that is the power of its mathematical formulation in the function x(t), are redefined (or even one of them is), does this not require a new formulation, maybe a new function based on the “new kind of hypothesis?”

    The motion of a point particle, whether translational, vibrational, rotational, or rotational vibration, even in light of QM, is subject to the power of the Newtonian program, as has been powerfully and convincingly demonstrated for centuries, but how can that program deal with “this new kind of hypothesis,” when particles are no longer fundamental? In other words, if there is nothing to move, how can its existence be a continuous function of the variable t in a given interval? I guess a layman would ask, “how can something that doesn’t exist move?”

    It seems to me that “this new kind of hypothesis” calls for a new formulation of its key concepts analogous to the function x(t), which has proven so suitable for point particles. Has this function been identified? If so, I hope it’s not too complicated. LOL.

  • ano nym

    If x(t) is “the function” for a point particle then “the function” for strings is x(s,t).
    I hope this helps you, but somehow I doubt it will …

  • rof

    If we say that the beta function has to vanish, we get what looks like Einstein’s equations. But if this describes reparametrization invariance on the string worldsheet, then it would seem that Einstein’s equations only have to be satisfied on the worldsheet. Why should we expect that the Einstein field equations are satisfied elsewhere in the target space?

    I hope there’s an answer more precise than “the worldsheet is everywhere”.

  • http://blogs.discovermagazine.com/cosmicvariance/clifford/ Clifford

    rof:- the spacetime coordinates are fields on the worldsheet. The spacetime metric appears on the worldsheet as a set of field dependent couplings of those worldsheet fields. The beta function condition is then a set of conditions on those couplings, which are spacetime metric components. So locality on the worldsheet (or being localized on the worldsheet) has nothing to do with location in spacetime. “The worldsheet is everywhere” would in fact be a wrong answer.

    Cheers,

    -cvj

  • Lee Smolin

    Deaer Moshe,

    Thanks for your last remark. Perhaps I can ask my question about time dependent backgrounds in the following, hopefully constructive way: You say, “the way Einstein equations are derived in string theory is that you expand around the most general (weakly curved) background, and the conditions for consistency of the string propagation, without any further input, yield the Einstein equation for that background… The background is the most general one, one never has to specify that it is static”

    Here is my understanding (and please correct me if I am wrong.) What you described is certainly the whole story for the bosonic string. But that has tachyons, and so could not describe the excitations of a stable ground state. So we have to impose additional conditions to cancel the tachyon to find a worldsheet theory that could descrive the excitations of a stable ground state. So here is my question: Are we sure that the extra conditions required to cancel the tachyon do not restrict the possible vacua to a small set of solutions of the Einstein’s equations ?

    My worry is that the answer is yes. My evidence for the worry is that in many cases the cancelation is accomplished by supersymmetry and, as I argued before, supersymmetry implies, by the closure of the algebra on the Hamiltonian, that the background has a timelike or null killing field.

    There may be exceptions to this, but am I wrong to say that there are many cases that work like this? If not, I conclude that in these cases it is not true that the acceptable backgrounds are generic solutions to the Einstein’s equations, because they must have killing fields. So my understanding is that while we can claim by the argument you made that generic weakly curved solutions to the Einstein equations are predictions of bosonic string theory, the same is not the case for supersymmetric string theory.

    Now, might there be other ways to cancel the tachyon leaving stilll generic weakly curved solutions to Einstein’s as good string backgrounds? My understanding is that a few examples have been explored, such as linear dilatons, coset constructions etc. but that either there was still a static metric (Einstein or string frame) or there were unresolved issues of instabilities. Is this right?

    Now, please tell me 1) Is there anything wrong with this argument? 2) Among the possible time dependent backgrounds that have been explored, which is the best case? 3) Even if there were shown to be a few consistent time dependent backgrounds, wouldn’t this still be a long way from what we want, which is showing that they are generic, which is what we have to have to claim that the generic solutions to Einstein equations are predictions of superstring theory.

    Thanks,

    Lee

  • rof

    Thanks, Clifford. That almost clears up my confusion. I wasn’t quite in the habit of thinking of spacetime as something that happens on the worldsheet. String theory would appear to be non-local in spacetime, even if it’s a local theory on the worldsheet.

    So, in string field theory, then, has anybody “derived gravity”? It seems that you couldn’t use the same approach there.

  • Aaron

    My worry is that the answer is yes. My evidence for the worry is that in many cases the cancelation is accomplished by supersymmetry and, as I argued before, supersymmetry implies, by the closure of the algebra on the Hamiltonian, that the background has a timelike or null killing field.

    The supersymmetry is on the worldsheet. Worldsheet supersymmetry does not imply spacetime supersymmetry. There exist tachyon free models in nonsupersymmetric spacetime. Of course, the ones I know of include a string scale cosmological constant for the usual reasons, but we can’t expect to solve all our problems at once.

    As for the rest, maybe Moshe can give you more detailed answers, but just type “time dependent” into an ArXiV search and you’ll get plenty of results.

  • http://blogs.discovermagazine.com/cosmicvariance/clifford/ Clifford

    rof: Most traditional formulations of string field theory do in fact use the same basic approach to how spacetime is encoded in the string. It is one of the reasons some of us believe that string field theory is not the best approach to understanding non-perturbative strings (although it is a powerful tool)…. It is too wedded to the string technology, and we know that string theory is not a theory of strings, since there are regions of the theory where there are no strings at all (or at least, where they cease to be the fundamental objects).

    Cheers,

    -cvj

  • Moshe

    Hi Lee, thanks for your last comment, it is certainly a pleasure talking physics!
    (and apologies for the technical level of this stream of comments, but this is the most efficient way to resolve technical issues).

    So, here is my understanding of the situation. The issue is that of classical stability, whether or not the spectrum of linearized fluctuations around specific background has tachyons. Now if space is exactly flat, the bosonic string has a tachyon with string scale mass, which means instability with string timescale. With worldsheet SUSY this disaster is avoided, one gets spectrum with no (classical linearized) instabilities.

    Now, let us talk about backgrounds that are almost flat, then just by continuity the only thing one has to worry about is the spectrum of modes that would have been zero modes in flat space. So my statement is that weakly curved solutions of string theory have tachyons if and only if the corresponding GR+matter solution is stable. So string theory at the classical level is as stable or unstable as the corresponding field theory it includes. On the classical level such instability is not such a disaster, one simply gets an unstable solution of Einstein equation, those are fine, they are not inconsistent (nor are they very interesting…)

    The point about worlsheet SUSY is a red herring I believe. Generally worldsheet structures are invisible from the spacetime viewpoint, they are just mathematical tricks (it is unnecessarily confusing to think about string worldsheet as embedded in spacetime, as one has to sum over worldsheet metrics, where the induced metric from spacetime is only one of the metrics to be summed over). Worldsheet SUSY is that mathematical trick allowing spacetime fields to have spin. It turns out that in string theory it also projects out the bosonic string tachyon, leaving the situation I described in the last paragraph. So all the set of models we mentioned, orbifolds, cosets etc. are consistent backgrounds of classical string theory with no tachyons.

    So to summarize,classically I think the statement that generic weakly curved solution of Einstein equation lifts to a classical solution of the full superstring theory, with no tachyons.

    I keep adding the adjectives “classical” and “weakly coupled” to everything, and at that level I think what I said so far is correct, hopefully someone will correct me if I am wrong. Now once we add quantum corrections there are several disasters that can happen, for example w/o spacetime SUSY various massless modes will start getting sources, so static solutions of the classical equations will no longer stay static (which sometimes is referred to as “instability”). If you are not interested specifically in static solutions maybe this point is not such a concern.

    More seriously for time dependent backgrounds, I think singularity theorems say that in many cases (generically?) time dependent backgrounds will tend to have spacelike singularity in the past or future, and then the background is no longer weakly curved everywhere. This manifest itself by certain singularities in string theory observables, also sometimes referred to as “instabilities” (one can detect certain lack of imagination…). As I said this is the reason why this is an interesting topic. It is certainly not clear that a generic classical solution of any theory, especially containing gravity, should lift to a full solution of the quantum theory. The question which do and which do not, and whether one can do it perturbatively or not, are very interesting open issues, not a lot is known about them currently.
    best,

    Moshe

  • David

    Moshe, re. #108: Thanks, and I get your points. I’ld be happy to tell you more about this, but I’m not sure it’s appropriate to do it here. So I’ll just make a quick description of what the current problem is and mention a couple of review articles for more details. Mathematically, the situation now is that we have a chiral fermion determinant line bundle over the orbit space of lattice gauge fields, and the problem is to show that it is trivialisable (and explicitly construct the trivialisation) in the case where the fermions live in representations which satisfy the usual (continuum) anomaly cancellation condition. This is what is needed to complete the construction of the lattice chiral gauge theories in an anomaly-free way. It has so far only been done for gauge group U(1); there is a partial result for U(1) X SU(2) but the general non-abelian case remains… A couple of places to look for more details would be hep-th/0102028 and hep-lat/0009033. (There’s been some more progress since then but overall the state of affairs hasn’t changed much.)
    Best,
    David

  • http://eskesthai.blogspot.com/2005/10/what-are-those-quantum-microstates.html Plato

    As to the physics involved?:)

    Question rasied there, that concerns here.

    http://blogs.discovermagazine.com/cosmicvariance/2005/11/18/a-particle-physicists-perspective/#comment-7238

    Thanks

  • Moshe

    Thanks David, this is a pleasure, the type of thing a blog is very good for.

  • http://eskesthai.blogspot.com/2005/10/what-are-those-quantum-microstates.html Plato
  • http://eskesthai.blogspot.com/2005/10/what-are-those-quantum-microstates.html Plato

    in regards to Lubos’s statement of, “there is no physics below planck length”

    Conformal invariance (also called “scale invariance”) is related to the fact that points on the surface of a string’s world sheet need not be evaluated in a particular order. As long as all points on the surface are taken into account in any consistent way, the physics should not change. Equations of how strings must behave when moving involve the Ramanujan function.

  • ksh95

    Distler said

    If you disagree with the arguments in the aforementioned blog post, or if you disagree that they imply my remark above, then let’s have-at-it. But I don’t think you can just dismiss the arguments as mere “arrogance”.

    If you reread my statement you’ll see that it is not the content of your statement I took issue with, it is the tone. And to tell the truth, I have no desire to have-at-it with anyone about issues that don’t effect my world, but never-the-less I’ll waste my time and respond.

    I may not know everything, but I do know some things. As a condensed matter theorist (formally trained in the art of wild speculation) I understand the process clearly. We publish some predictions, experimentalists verify said predictions. Some times the most far fetched ideas are correct, sometimes the most elegant and attractive ideas are wrong. The point is that physics is harder than most of us are smart, and if we aren’t guided by experiment it could be healthy to inject a dose of humility into our speculations.

    It is completly possible that the LHC discovers that mathematics no longer governs the universe below a certain length scale. Personally, I wouldn’t bet on it but it’s possible.

    Under these circumstances, it seems to me, that statements like “you won’t get very far with me saying…yadda, yadda, yadda”, should be replaced with statements like “Well, I believe blah, blah, blah and most professionals would agree that yadda, yadda, yadda is unlikely. But, we have to wait for experiment or, at a minimum, some accurate postdictions to be sure”.

  • http://golem.ph.utexas.edu/~distler/blog/ Jacques Distler

    It is completly possible that the LHC discovers that mathematics no longer governs the universe below a certain length scale. Personally, I wouldn’t bet on it but it’s possible.

    Yes.

    And it’s possible that, tomorrow, we will discover a room-temperture superconductor where the charge-carriers are angels. It’s “arrogant” to say that’s impossible.

    C’mon. There will always be experimental surprises. And there will be theoretical surprises, too: loopholes discovered in previously airtight-sounding theorems (string theory itself is such a loophole).

    Under such circumstances, I’m perfectly prepared to eat my words.

    But the mere possibility of such surprises should not reduce us to labelling every as-yet-not-experimentally-verified statement to the status of mere “opinion” or rank “speculation.”

    You wouldn’t do that in your field; please don’t expect me to do that in mine.

  • http://www.canonicalscience.com Juan R.

    On String Theory
    In my opinion, string ‘theory’ (if one can call ‘theory’ to an internally inconsistent mathematical goulash without claimed physical principles underlying because they “are being searched still”) has failed as a credible attempt to a theory of everything. Even if by ‘everything’ one only consider a derivation of the Standard Model from first principles and its unification with gravitation.

    Of course, string theory is especially simple and trivial (when compared to advanced stuff) and does not contain the adequate mathematical structure for being considered a true theory of everything. It is easily proven that from theories and models already available on literature (and others in press) one can obtain string theory framework on well-defined limits. Therefore, it is not so surprising for us —some string theorists are really astonished— that from early ideas of chemist Ilya Prigogine one can generalize non-critical string theory beyond usual limited trivial framework of wavefunctions and scattering.

    Still more interesting is when one revises all of that supposed ‘advanced stuff’ and one finds just trivial math, and even formulas incorrectly copied from previous work of others. Nanopoulos’ work “as time goes by…” [arXiv:hep-th/9406016 v1 3 Jun 94] is a fascinating example of how science may not ‘goes by…’!!

    For some basic ideas on why string theory has failed as a true theory of everything and why, even in his most advanced versions, it looks very trivial from rival theories, the reader can see “String theory is not a TOE” on sci.physics.strings.

    Today almost all of string theory research looks in the wrong way: symmetry groups, AdS, dualities, 10-11D spacetimes, non-commutative geometry, quantum states (even using TFD-Dpbrane), evolutors, and a large etcetera.

    Moreover, string theorists just are ignoring recent experimental data and advances in other fields. There are experiments proving that QED is not completely correct but other rival theories can explain them (see Discrepancies on QED )

    There are serious mathematical flaws in all the formalism of string theory (for example still nobody proven to me the absence of divergences). String theorists claim quantization of gravity, but far from rather trivial ideas (of course, 30 years ago the perspective was very promising) about a two-spin model in a flat background and similar ones, there is not real understanding of gravity on the string-brane side. In fact, it is highly probable —even if this sound ‘crank’— that GR cannot be considered the final approach to classical gravity (of course here other alternatives as LQG are not doing it better).

    Discrepancies in cosmological regimes, serious mathematical difficulties, and the unsolved character of the main fundamental problems of GR (e.g. problem of energy, problem of reference frames, the speed of gravity problem, the lacking of a consistent Newtonian limit!, etc.) would reconsider the status of the foundation of GR even before our attempt to quantize it.

    In fact, some other authors have obtained a first generalized gravitational theory that explains experimental data, and corrects GR errors. Interestingly, that new theory of gravity has been already quantized from basic principles in a non-perturbative manner without the typical problems of ‘absence of time’, wrong causality (in perturbative regimes as string) and similar ones.

    Note: Usual ‘Newtonian limits’ on literature are computed via incorrect limits (usual textbook ‘derivation’) incorrect math (wrong functional dependence for potentials in NC theory), via unphysical boundaries (Ehlërs approach), via appealing to NEW postulates does NOT derived from GR (Christian approach), etc.

    Note: it can be proven that the problem of aberration in experimental gravity is NOT solved via Carlip article [S. Carlip, Phys. Lett. A 267 (2000) 81-87.] or similar attempts.

    On String Math
    As a well-known part of string theory propaganda, it is claimed that string theory is providing new revolutionary ways on math. Well, this is not true. It is true that string theory use entire branches of math that particle physics does not use, but string theory has not invented or developed those branches. There exist some collaboration between mathematicians and string theorists, but the real impact of PURE string theory on math is insignificant. The current view of two Field Medals were awarded to research on string theory is so popular as incorrect.

    On String Marketing
    How many people still believe that main idea under string theory is that all ‘matter’ is composed of small one-dimensional objects and spacetime is 10D? When you explain to people that string theory is already NOT a theory of strings, they look you with a face… Still more interesting is that after of decades of explaining to us why the point-like objects of particle physics could not be correct, since, at small scales, pieces of nature “would be” extended objects, now some M-theoreticians claim that the fundamental item of nature are, exactly, point-like objects called D0-branes. Of course, D0-branes are not the ‘old’ particles of the Standard Model, but definitively are not the popular one-dimensional strings.

    I personally find very amazing the history of the field with almost all of previous -premature- claims proven to be completely wrong. It is sensible continue with that fascinating record of failure?

    Other example is that of ‘predictions’. In theory, string theory is a framework that predicts everything using a single parameter. Yes, that was the belief… but today we know that string theory can predict exactly nothing even if one use several dozens of hundred of parameters…

    Would not people who are paying string theory research know those important details?

    On Einstein achievements
    “the one intelligently designed” said

    Eienstien wrote GR based only on his theoretical insights and demanding some general properties for a theory of gravity ( relativistic invariance, equivalance principle ect) and then looking for internal consistencies.

    In fact, Einstein failed to obtain a consistent theory until he read a previous Hilbert paper where Hilbert obtained the correct field equations of GR. Then Einstein submitted the famous paper of day 25 without citing Hilbert. Still more interesting is that in that paper Einstein simply postulated the correct equations of GR. (see what is history of relativity theory?).

    Juan R.

    Center for CANONICAL |SCIENCE)

  • http://nigelcook0.tripod.com Nigel

    Just to remind ourselves of what Einstein and his verifier Sir Arthur Eddington wrote on ‘relativity':

    ‘The special theory of relativity … does not extend to non-uniform motion … The laws of physics must be of such a nature that they apply to systems of reference in any kind of motion. Along this road we arrive at an extension of the postulate of relativity….’ — Albert Einstein, ‘The Foundation of the General Theory of Relativity’, Annalen der Physik, v49, 1916.

    ‘The Michelson-Morley experiment has thus failed to detect our motion through the aether, because the effect looked for — the delay of one of the light waves — is exactly compensated by an automatic contraction of the matter forming the apparatus…. The great stumbing-block for a philosophy which denies absolute space is the experimental detection of absolute rotation.’ — A.S. Eddington, Space Time and Gravitation, Cambridge, 1921, pp. 20, 152.

    So the contraction of the Michelson-Morley instrument made it fail to detect absolute motion. This is why special relativity needs replacement with a causal general relativity:

    ‘According to the general theory of relativity space without ether is unthinkable.’ — Albert Einstein, Leyden university lecture ‘Ether and Relativity’, 1920. (A. Einstein, Sidelights on Relativity, Dover, 1952, p. 23.)

    ‘… with the new theory of electrodynamics [vacuum filled with virtual particles] we are rather forced to have an aether.’ — P.A.M. Dirac, ‘Is There an Aether?,’ Nature, v168, 1951, p906. (If you have a kid playing with magnets, how do you explain the pull and push forces felt through space? As ‘magic’?)

    ‘Children lose interest … because a natural interest in the world around them has been replaced by an unnatural acceptance of the soundness of certain views, the correctness of particular opinions and the validity of specific claims.’ — Dr David Lewis, You can teach your child intelligence, Book Club Associates, London, 1982, p. 258.

  • http://eskesthai.blogspot.com/2005/10/what-are-those-quantum-microstates.html Plato

    While my “Angel and Demons” was extreme, I guess, as lay people we want “clear presence.” This is what we can hope for?

  • http://eskesthai.blogspot.com/2005/10/what-are-those-quantum-microstates.html Plato

    oh nigel,

    My grandson who is nine, wants to a experiment with magnets. I thought of Einstein’s youth and the magic of the compass. Would it have a similar impact on him? :) I can only hope :)

    So, any ideas on what we can plant into society, that such a child will open new doors to a brighter future, for all of them. Make them wonder more. A new view that would hold their interest?

  • http://nigelcook0.tripod.com/ Nigel Cook

    Dear Plato,

    You may do OK in Holland, but in Britain A-level physics is declining:

    http://news.bbc.co.uk/1/hi/education/3580742.stm

    “There are more pupils taking A-level psychology than physics – and if current trends continue, the declining science subject will be overtaken by sociology…. Physics is in decline and other subjects, such as media studies and art, are increasingly popular despite the poor career prospects they offer. It’s a crazy situation,” says Dr Julia King, chief executive of the Institute of Physics.”

    I think the high level of abstruse maths which fails to tackle with mechanisms for forces, is not helped by marketing physics using wormhole and Hawking speculations. Hawking sells a lot of books in Britain, but fewer people study physics. I’m not saying I doubt hawking radiation, only that it’s not been found because it’s swamped by the natural background gamma radiation of space. The whole basis of the energy-time version of the uncertainty principle is going to be causal (random interactions between the gauge boson radiation, which consititues the spacetime fabric).

    Heuristic explanations of the QFT are required to further the basic understanding of modern physics. For example, Heisenberg’s uncertainty (based on impossible gamma ray microscope thought experiment): pd = h/(2.Pi), where p is uncertainty in momentum and d is uncertainty in distance. The product pd is physically equivalent to Et, where E is uncertainty in energy and t is uncertainty in time. Since, for light speed, d = ct, we obtain: d = hc/(2.Pi.E). This is the formula the experts generally use to relate the range of the force, d, to the energy of the gauge boson, E. Notice that both d and E are really uncertainties in distance and energy, rather than real distance and energy, but the formula works for real distance and energy, because we are dealing with a definite ratio between the two. Hence for 80 GeV mass-energy W and Z intermediate vector bosons, the force range is on the order of 10^-17 m. Since the formula d = hc/(2.Pi.E) therefore works for d and E as realities, we can introduce work energy as E = Fd, which gives us the strong nuclear force law: F = hc/(2.Pi.d^2). This inverse-square law is 137 times Coulomb’s law of electromagnetism.

    Surely the heuristic explanation of this 137 anomaly is just the shielding factor by the polarised vacuum?

    ‘All charges are surrounded by clouds of virtual photons, which spend part of their existence dissociated into fermion-antifermion pairs. The virtual fermions with charges opposite to the bare charge will be, on average, closer to the bare charge than those virtual particles of like sign. Thus, at large distances, we observe a reduced bare charge due to this screening effect.’ — I. Levine, D. Koltick, et al., Physical Review Letters, v.78, 1997, no.3, p.424. ;)

  • http://eskesthai.blogspot.com/2005/10/what-are-those-quantum-microstates.html Plato

    Hi Nigel,


    The ATLAS detector uses two large magnet systems to bend charged particles so that their momenta can be measured. This bending is due to the Lorentz force, which is proportional to velocity. Since all particles produced in the LHC’s proton collisions will be traveling at very close to the speed of light, the force on particles of different momenta is equal. (In the theory of relativity, momentum is not proportional to velocity at such speeds.) Thus high-momentum particles will curve very little, while low-momentum particles will curve significantly; the amount of curvature can be quantified and the particle momentum can be determined from this value.

    Maybe the psychology needed some “revisionistic ideas” in terms of the layering. As detection processes? UK awareness with physics people and government MP’s?

    Socialogical expansionistic processes, from the man on the street, to the man in his country? Reveals a deeper underlying math, that becomes hidden within government agendas and policies? Who knew poltical agendas could have become so compicated at it’s core. While realistic in all the “mazes” of our everyday thinking.

    Lessons learnt with John Nash and his bargaining processes exemplifying this underlying recognition? The further exposition in “new” nobel prize winners for game theory?

    You see? :)

  • http://nigelcook0.tripod.com/ Nigel Cook

    Dear Plato,

    Your link to 2005 Nobel laureates for economics (game theory) mentions their theory for winning a nuclear war:

    “US citizen Thomas Schelling and Israeli Robert Aumann have won the 2005 Nobel prize in economics for their work in an area known as game theory.

    “Professor Schelling has specialised in explaining strategies of international conflict, such as nuclear war. Professor Aumann has developed the theoretical underpinnings of bargaining, co-operation and conflict.

    “Professor Schelling, 84, a US citizen … emeritus professor of political economy at Harvard University, where he had taught for 20 years….

    “Thomas Schelling told reporters that “they (the Nobel committee) linked us together because he is a producer of game theory and I am a user of game theory.”

    “Indeed, both the USA and the former Soviet Union adopted such a strategy – known as mutually assured destruction – during the Cold War, when they developed long-range nuclear weapons but agreed not to develop defensive weapons such as ABMs.”

    http://news.bbc.co.uk/2/hi/business/4326732.stm

    This is interesting. They gave the Nobel Peace Prize to a pacifist who campaigned against nuclear weapons for 60 years a while ago, and now they’ve given the economics Nobel Prize for the deterrent use of nuclear weapons! How nice that nuclear physics is both good and bad.

  • http://eskesthai.blogspot.com/2005/10/what-are-those-quantum-microstates.html Plato

    In the one sense Nigel, those mathematically orientated are very much like these economists whose math is the basis and derivative of the reality called science. A society of scientists, much like, the socialogical foundation of the populations.

    While in some cases that math is very abtract and models created, how diverse would this reality of math become if it were exposed to the fashionable world call physics and supported?

    In game theory you might not have recognized this bargaining process, yet you had two superpowers engaged, and from it attitudes and adoptions. Now, the science has changed. What is this world like, that these two powers have been reduced in the arms race to now confront, the rogue states with weapons manufacture?

    The balance although it still exists and might have been in jeopardy with the Ukraine? Some kind of peace has been maintained. Yet, the dynamics and math has indeed changed. The cold war era had ended and with it a fear induced state. Now, what fear has this “new society” to worry about?

  • http://eskesthai.blogspot.com/2005/10/what-are-those-quantum-microstates.html Plato

    I was inspired by “Warren Siegel’s parodies,” to add a update on Angels and Demons.

    Will string theorist’s burn in hell? :(

    Some might use religion to distort the “empowerment” we all have? Place it outside of ourselves? While I say, it is quite feasible that such delusions, would have been sought to fool the public, by being anti-stringy?

    Is it a plot un-becoming? If one could have a delusion about it’s inception, then why not the reverse of it’s recognition? Claim it to be a positive thing about our comprehension about it’s negative impact?

    Have I followed the proper logic?

  • Lee Smolin

    Dear Moshe,

    Thanks very much for your #137. This is helpful, but can I query you on one point, where you say that on “…backgrounds that are almost flat, then just by continuity the only thing one has to worry about is the spectrum of modes that would have been zero modes in flat space. So my statement is that weakly curved solutions of string theory have tachyons if and only if the corresponding GR+matter solution is stable. So string theory at the classical level is as stable or unstable as the corresponding field theory it includes.”

    Can you fill in a few details of this argument? It seems to me that given that the cancellation of the tachyon involves a projection to a smaller state space (the GSO projection) and given that the projection implies spacetime supersymmetry which is broken as soon as there is any time dependence, you have to show that you can continue to impose a projection that eliminates a tachyon for any small non-supersymmetric deformation of the background geometry. Has this been done? If not, I don’t think you can use an effective field theory argument that assumes that the tachyon is absent from the deformed theory.

    On a related point, you say, “It is certainly not clear that a generic classical solution of any theory, especially containing gravity, should lift to a full solution of the quantum theory.” But for the classical theory to be recovered as the low energy limit, must it not be that every solution to the classical theory that is weakly curved on the Planck scale must lift to a coherent state in the quantum theory?

    Thanks, Lee

  • Moshe

    Hi Lee,

    Back to physics, very theraputic…

    The comment of continuity was just an intuitive comment, not really necessarily an EFT argument. Basically the spectrum of the string is continuous as a function of the background fields (w/o changing the GSO projection), so string scale tachyon cannot just pop up when you turn on arbitrarily small background fields. In any event, I assure you there are also more complete calculations, that was just an argument why this had to be the case. Incidentally, even for static backgrounds there are non-SUSY classically stable cases, but they are quantum mechanically unstable in the sense that they don’t stay static after including quantum corrections.

    As for the last paragraph, I mentioned that in the context of backgrounds which develope singularities, and then we don’t have a criteria for judging which background lift to the full quantum theory. My bet is that weakly curved backgrounds should be fine, but one cannot know for sure yet. (incidentally, is it always true that for time dependent background singularity thm. dictate there generaically exists future or past singulairty?)

    best,

    Moshe

  • Moshe

    Oh, one more point, the GSO projection by itself does not imply spacetime SUSY, it just allows for that possibility, if the background fields cooperate…

  • Lee Smolin

    Moshe, thanks very much. Some references when you have a minute would be appreciated. Thanks, Lee

  • Moshe

    Lee, not sure references to what, but soon enough we’ll be able to discuss in person.

    best,

    Moshe

  • http://eskesthai.blogspot.com/2005/10/what-are-those-quantum-microstates.html Plato
  • http://nigelcook0.tripod.com/ Darwin

    Plato,

    This isn’t the place to discuss “a genuine emergence of classical spacetime geometry from something more fundamental”. I wish you would persuade your colleague and mentor t’Hooft to put a review paper reviewing unorthodox work on the arXiv.org. Until there is a precedent by someone famous, they’ll delete everything submitted that looks crackpot.

  • Al Wloch

    My understanding of arriving at a physical stalemate is that, classicly speaking, individuals continually relate from their reference circle of origin. And how far can we technically “see around” or through our 3-D? Consider the various circling engines connected by a motor fanbelt…the belt leaps from one “world” to another. Ergo: the figure-eight concept arises. The 3 or 4 dimensions of half the hourglass figure-eight vis-a-vis the other half. One must consider the mirror-image to our perspectual reality!
    A

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Cosmic Variance

Random samplings from a universe of ideas.

About Mark Trodden

Mark Trodden holds the Fay R. and Eugene L. Langberg Endowed Chair in Physics and is co-director of the Center for Particle Cosmology at the University of Pennsylvania. He is a theoretical physicist working on particle physics and gravity— in particular on the roles they play in the evolution and structure of the universe. When asked for a short phrase to describe his research area, he says he is a particle cosmologist.

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