SUSY06 Wednesday Night SmackDown!

By cjohnson | June 17, 2006 7:28 pm

frank wilczek Well, Wednesday night at the SUSY06 conference featured presentations by Frank Wilczek, Lenny Susskind, Andre Linde, and Burt Richter, and ended with a panel discussion. Slides and notes can be found here. It was about “Naturalness”, and -among other aspects of naturalness- featured a lot of discussion about the Landscape scenario, and the use of Anthropic arguments in fundamental physics. The first three speakers gave excellent presentations of their points of view on the topics, each using Anthropicity (is there such a word?) to varying degrees. You’ve possibly read about the nature of the discussion (especially as it pertains to what some researchers in string theory are pursuing) in an earlier post of mine (see the comments, especially).

lenny susskindandre lindePeople get very passionate (understandably) about the whole topic, although the strangest thing about it to me is the way so many people seem to want everyone in the field to “choose sides”, as though this is some sort of war for the future of physics. It is not a war. It is an interesting and important discussion that will simply fade away in due course, when other interesting and more urgent discussions take its place. New experimental results and new understanding of theory will inevitably help make this happen. This is the way it should be with all discussions in science, of course, and this is no different.

richterI was wishing I could get across to you the nature of the discussion that took place on Wednesday, because it was interesting -with good points made on both sides- and because all four characters are so engaging and entertaining in their own ways. Here’s one aspect I particularly loved: At one point everyone in the audience seemed sort of in the thrall of the whole Anthropic view after three excellent talks about it, only to have the bubble burst and splatter everywhere when Burt Richter got up and, speaking from notes with no slides, just declared the whole thing to be a theological discussion. Laughter erupted along with a round of applause! This was -I think- not because everyone agreed with him (he later refined his criticisms to make more productive points), but because it was such a great slap-in-the-face reawakening after about two hours of the other view.

It was basically a loud fart in a quiet cathedral, during evensong. Excellent.

The great news is that some of the panel discussion was recorded, and can be found here [see update 2, below] (thanks chimpanzee; shot of panel borrowed from that site too).
panel on naturalness
There, you can see some of the friction and heat of the discussion on the video. So have a look at it (it is interesting and informative to see the bigshots arguing -and how they argue- even if you’re not working in this field), and then, sure, come back here and let’s have a polite shouting match about the landscape if you like. It’s been awhile since we’ve gone over this and maybe there’s some new ideas to be added… and new research to discuss.

The usual rules about stepping away from the temptation to make personal attacks apply.


[Update 1: Peter Woit has written about Burt Richter's remarks on his blog, using the transcripts of Richter's notes as a guide. I note that during the panel discussion itself, Richter was able to shape and focus his remarks considerably, making criticisms of specific "Naturalness" ideas while being more accepting of others. There was some useful give-and-take during the discussion, in other words, which the transcripts of his talk don't betray. You can see a little bit of that in the video clip, which is unfortuantely incomplete. See also the first part of my comment addressed to Peter.]

[Update 2: The complete panel discussion has now been uploaded and can be found here. Warning: It is a 300Mb file.]

  • Moshe

    Excellent Clifford, I think theorists (and people in general) should have the ability to hold contradictory viewpoints simultaneously, especially in the face of uncertainty. I thought therefore it may be nice exercise for me to defend the viewpoint I find less plausible, the anthropic one. Also every good shouting match needs someone to shout at…

    The stongest argument I find for this viewpoint is that there may well be no choice involved. The CC looks fine tuned, the electroweak scale shows some preliminary signs of being fine tuned as well. If true the only explanation better than “that’s the way things are” will involve those being environmental parameters.

    On the theoretical side, inflation looks pretty strong, and having precisely 62 e-foldings seems artificial, so it is likely that our Hubble patch is a small part of the universe. The viewpoint that some parameters change from place to place may well end up being inevitable. Of course we’ll have to understand both theory and experiment better before we decide.

    There is also the landscape of string vacua, which may eventually be important in all that, but the above arguments are independent of this or any other details of any microscopic theory. Maybe for the sake of clarity these two issues can be decoupled (as if I have a chance…).

    Incidentally, I am having problem viewing the file, so I am running the distinct risk of inarticulately repeating what is (no doubt brilliantly) presented there. In any event those words were said many times before, hopefully this is just a starting point for a fascinating and penetrating discussion…

    (Also, one day I’d like to see a discussion regarding whether or not a complete theory of science and its practice was really finalized in 1935, or perhaps we are still allowed to think about such issues).

  • Sean

    “Naturalness” is exactly the right label for such a discussion. All that multiverse ideas do is to re-calibrate one’s idea of what is natural and what isn’t, nothing more nor less. If you believe that the low-energy laws of physics we just happen to observe locally can be confidently extrapolated far beyond the horizon into unobservable areas, you will have one notion of “natural,” which will act as a guide as you formulate new hypotheses about unknown physics. If you think there are a wide variety of regions with different low-energy laws, your notion of “natural” will be something else, and your favorite hypotheses for what comes next will be correspondingly different. That’s all. It’s really nothing worth having kittens about. Data will still ultimately decide between models, once the data are available.

    Nine times out of ten, when a scientist says something stupid about anything, you can trace the problem back to [Karl] Popper. — Daniel Davies

  • Rob Knop

    It’s really nothing worth having kittens about.

    But I really *like* kittens!!

  • amanda

    I would really like to see some commentary on a point made by Schellekens in his recent arxiv paper, namely, that numbers like 10^120 have to come from somewhere, and if they don’t come from a multiverse, then they have to come from mathematics. The point is: why would the second possibility be better than the first? I must say that I have always been very anti-anthropic, but Schellekens slowed me down a bit. That is, I am in that somewhat uncomfortable state of uncertainty described by Moshe.

    Another point: anti-anthropics like me often say that non-anthropic arguments should always take precedence over anthropic ones. But what does this mean in practice? That a not very plausible non-anthropic account of [say] the arrow of time should always be preferred to an anthropic argument with better technical arguments behind it?

  • Babe in the Universe

    Einstein said that insanity is doing something repeatedly and expecting different results. You go, Bertie!

  • Plato
  • Peter Woit

    “Data will still ultimately decide between models, once the data are available.”

    The problem with the string theory anthropic landscape is exactly that it’s not science because this statement is not true in this case. I’ve looked very carefully at the writings of the landscapeologists and nowhere is there a calculation or plausible proposal for a calculation that would allow one to use experimental data to test the kinds of models they are looking at. The problem here isn’t that people are being fuddy-duddies, insisting on a 1935 model of what is science, and not getting with it and being willing to adopt a new post-modern 21st century idea of what is science. The problem is exactly what Richter says: at the highest levels of particle theory, people are giving up on doing science. And they’re not doing it quietly, but doing everything they can through public lectures, popular books, etc. in order to convince others to join them.

    This isn’t some minor, easily self-correcting problem, that will get fixed as new data come in. There’s a very real danger that, as a serious intellectual endeavor, particle theory will just die, replaced by something very different, not a science, but an elaborate exercise in making excuses for failure. There’s very good reason for people to be upset about this.

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  • Clifford


    I’d like to point out two things in response to what you wrote above, and on your blog:

    (1) Note my statement in the main text anticipating that “New experimental results and new understanding of theory” will make the difference in this game. By the latter, I include the possibility of better understanding of string theory (which, as I keep reminding you, is still very under-developed) which may or may not help us determine whether it has anything to do with Nature, and I also include the possibility that some other theoretical framework comes along which is demonstrably better at doing the job, and we just move on, using that new theory. You seem to be taking the pessimistic view that all we’ll ever have is the current state of theoretical development, and will have to make do with that forever. This seems extremely unlikely to me.

    (2) It is important to note that all of the non-Richter speakers were -to different degrees- talking about using naturalness as an important tool and guide alongside (not instead of) what might be referred to as “traditional’ theory. I say again, this is not a battle of absolutes, of choosing one or the other….. it’s just a matter of using all the tools, ideas, and approaches available to us at the time to understand Nature. When something better comes along, everybody adopts the new tools and moves on. This is the way science has always proceeded through history, more or less, and I see nothing different here.

    In my opinion, characterizing this as a crisis for particle physics is an overblown storm in a teacup.



  • Science

    ‘… the possibility of better understanding of string theory (which, as I keep reminding you, is still very under-developed) …’ – Clifford

    This is a revelation. I was under the impression that more money and more resources and more brainpower had been poured into string theory than into any other theory in the history of physics.

    I thought string theorists had been developing the ideas, having revolutions, claiming that it is the only theory of quantum gravity, and that LQG and other alternatives are nonsense because they are under-developed.

    If string theory is under-developed now, that is news.

    If it is so under-developed, perhaps the string theorists should stop hyping it so much.

  • Peter Woit


    You’ve pretty much completely ignored everything I wrote, which was not about prospects for theoretical developments getting us out of the current situation, or about naturalness. It was agreeing precisely with Richter that prominent physicists have given up on science and this is a very bad thing.

    Naturalness is an independent technical notion from the anthropic landscape, and I’d agree with what Richter has to say about it:

    “Naturalness may be a reasonable starting point to solve a problem, but it doesn’t work all the time and one should not force excessive complications in its name.”

    There are many reasons a calculated number can come out to be much smaller than what one thinks the natural scale for that number is other than because of technical naturalness or anthropics.

    You make up something I’ve never said or thought:

    “You seem to be taking the pessimistic view that all we’ll ever have is the current state of theoretical development, and will have to make do with that forever.”

    I certainly look forward to the day when we have some new more successful theoretical ideas. What I object to is the current campaign by Susskind and others to abandon standard ideas about how to do science that in the past have led us to more successful theoretical ideas (like insisting that one’s ideas ultimately be testable, so that one can abandon them and move on to something else if they’re wrong). Susskind sees himself in a war with much of the rest of the theoretical community over this. If he wins, we’re in trouble.

    As to whether this is a crisis, we may just have to disagree. But I think it is undeniably historically unparalleled. If you know of an example of a major high-energy physics conference during the past century where a prominent highly-respected physicist stood up and accused his colleagues of having abandoned the scientific method in favor of theology, I’d like to hear about it (other than Glashow 10-20 years ago, who was just perhaps ahead of his time…).

  • chimpanzee

    CVJ, I plan on video-Blogging the entire “Naturalness” session, so the entire video will be available to the Physics community..over your video-iPod (or Sony PSPv, or Archos AV500..the latter is a French-based portable video-player solution)


    You’re right, it’s good to see/hear the audience response, presentation/exposition by the speakers (facial expressions, intonations & emphasis). Text transcripts misses the “emotion” of the presentation. This is the power of video-blogging.

    [The rest of long comment -about the technology stuff- was moved to the thread where it belongs. -cvj]

  • Clifford

    Science (commenter #10)…. I’m happy to have provided you with a Revelation. However, evidently you’ve not been reading anything I’ve been saying in these discussions since July/August last year on this blog, since I’ve said nothing new today or yesterday on this matter: That string theory needs a lot more understanding and development is extremely old news. I’ll remind again:- research in string theory is not confined to what you hear about in the press and in the fancy public talks and media. It is not confined to worrying about whatever controversy is artifically blown out of proportion by a select few practitioners or detractors. There is a lot to learn in many areas of the field, and there are people getting on with doing that. Not having heard about such effort does not mean that it does not exist.



  • Science

    Thanks Clifford,

    It shocks me if string theory can be claimed to be highly-developed (for stamping on rivals), and claimed to be under-developed (for the purpose of defending problems in it). (This sounds like ‘doublespeak’.)

    I’ve had papers rejected on that basis, alleged under-development of an alternative idea compared to string theory. Perhaps I will resubmit the papers citing your statement that string theory is “under developed”, and those journals will apologise and be less patronising…

  • Vince


    How’s Toronto?

  • anon

    “although the strangest thing about it to me is the way so many people seem to want everyone in the field to “choose sides”, as though this is some sort of war for the future of physics. It is not a war.”

    Clifford, you are being disingenous.

    It is a war. A war for physics funding. A war for the soul of physics. We either insist on physical theory that can be corroborated by experiments, or we accept String Theory

  • Moshe

    Sean, thanks, that comment thread is golden. Again, discussion of what science actually is, how is it practiced, why is it useful and how is it different from other activities, all those questions are very interesting and are studied professionally by a large community of scientists (do they blog?). Would be nice to go beyond the “half-digested chunks of Popper” mentioned in that thread (yes, one would have to avoid a certain basin of attraction…).

  • Clifford

    Science (comment #14)

    It is only “doublespeak” if spoken by the same person.

    Please tell me: Why you are assuming that anyone who speaks about string theory speaks for the entire community of string theorists? I don’t understand this. It is hard to make progress in nuanced argument or discussion if one party is assuming this.

    anon (comment #16)

    LOL! With your sense of drama, you should write screenplays for the Hollywood studios to be made into Bruckheimer productions. :-) Seriously, if you wish to go ahead and voluntarily paint yourself into such a tiny and narrow corner, that’s up to you. Just remember that no one is forcing you to make those choices.


  • Eugene Stefanovich


    the miopic attempts to suppress alternative points of view and force everybody to march in step are way too common. The fact that somebody has an alternative way to look at thing is not a minus, but a plus, even if this alternative view does not bring immediate benefits. The history of science is full of examples when alternative routes lead to significant progress. Take Feynman’s path integrals, for example.

    I see a big danger in the modern trend to “streamline” theory and to “cut out extra branches” from the tree of science. Like any complex system interacting with environment, physics can only benefit from diversity. Imagine that 100 million years ago some higher power decided that dinosaurs are the most promising species and cleaned up the Earth from those pesky mammals. Where would we be now?

  • Science

    “It is only “doublespeak” if spoken by the same person.”

    From memory I fear you are spot on! George Orwell did seem write about this particular caveat: the mainstream can use different spokespersons when delivering totally conflicting messages. It makes it far harder to spot.

    “Why you are assuming that anyone who speaks about string theory speaks for the entire community of string theorists? I don’t understand this. It is hard to make progress in nuanced argument or discussion if one party is assuming this.”

    My fault again. I wrongly assumed string theory was developed to the point of having a consensus in the string theory community. I expect a community for the very cohension which makes for a community, to have some consensus at least on fundamentals of the subject, with a range of views on unresolved issues. But they should surely all have the same views on the degree of development so far?

    (It now sounds as if there’s no consensus on how to measure progress in string theory research? If so, there is very little meaning to statements saying it has made great or little progress. Who decides? Could all the string theorists decide the issue by voting?)

  • Clifford

    Hi Science, (comment #20)

    In your last two paragraphs, you seem to be asking the community of string theorists to do something that no other field does. And how does anyone in any field of science objectively agree on how much progress has been made? Would you not have to know what the result was in advance of doing the research? And voting?! Are you kidding?!

    When I write some slides for a presentation about my opinion of research outcomes and research directions in my field, I do not call up Witten or Susskind or any other senior person in the field and ask them to check my slides. My opinion is my opinion. Theirs are theirs.

    On specific objective facts, such as “Is X=Y?”, I may of course check with the relevant people, or the papers that they have written if I am not sure of a result, but that’s not what we are talking about. We’re talking about opinion on progress. Questions like “Given that X=Y, what does it all mean for the future of science?”…. etc, are not things that should be pre-agreed on by some elected representatives before sending them out to woo the crowds. That’s just silly. This has never been the way science works.

    So when you hear any of us out there giving a talk, please assume only what you see: that the opinion given is *the speaker’s* opinion. We are not elected representatives of the community.

    On this point, I’ve also been saying this several times on the blog during these discussions. It is very important to make this distinction. Despite the fact that I have been saying this clearly several times, it seems to be convenient for people to forget this, as it makes a much better news story to lump the whole community together in some “vast conspiracy”. Let’s try to stop doing this. It is a bit silly, if you think about it.



  • Science

    Thank you, Clifford. A diversity of views is healthy!

  • Cynthia

    After all, physics does fall under the broad heading of the natural sciences. Consequently, physicists should have every reason to deeply abhor any hint of the universe having unnatural tendencies. Likewise, physicists should strive unrelentingly to reveal a universe which is fundamentally natural. However – if the cosmological constant is truly fine-tuned, then physics might have to face the harsh realization that the universe is fundamentally unnatural.

    Because the landscape is a way of removing this cosmic unnaturalness, the landscape appears to becoming an attractive option for many thinkers of cosmology. By recasting the cosmological constant within the framework of the landscape, this seemingly unnatural, fine-tuned constant becomes transformed into a less unnatural,less finely-tuned constant.

    Nevertheless, the landscape still fails to address the oscillating boundary between natural and unnatural. Therefore, I will argue that the landscape is still a mere Band-Aid without a real cure for this cosmic ailment called “unnatural-fine-tuning.”

  • Jack

    Oh dear, this comment thread got off to such a good start, and yet so quickly degenerated into the usual “it isn’t science” arguments that we have all heard so many times before. I suggest that if people really want to say something about the panel discussion, they should either [a] point us towards things we have not heard before or [b] talk about the technical details of what was actually said at the panel session. A cogent demonstration that [eg] Linde’s arguments do not make sense would be far more of a blow against anthropics than the same old stuff about what should count as science and what should not……

  • JoAnne

    I was at the last half of this panel discussion (After driving down from the Bay Area, I arrived too late on Wed to see the whole thing!) and quite frankly found it rather dull. As did most of the people I talked to afterwards. A common comment was that the panel was overloaded with Anthropic-minded theorists, with no theorists giving a balanced opinion on the other side. (Most people I spoke with were keenly disappointed about that.) Burt did fine, of course, but the discussion could have used another theorist talking about the definition of science and the interplay between theory and data. And the discussion afterwards quickly delved off-topic and into the irrelevant. Although my favorite moment of the discussion was when Lenny Susskind claimed one could not calculate the temperature of planets from data. Hmmmmm…..I also enjoyed when Lenny said that oxygen existed on earth because human life needed it – hmmmm again – I thought evolution had a different casuality!

  • chimpanzee

    The complete “Naturalness” panel-discussion is up here. File is 300mb, it’s largest video-podcast file I’ve ever done (took 3.5 hrs of work using a quad G5, 2 hrs to upload @400kbps). It’s also available over “SUSY ’06″ video-podcast over iTunes Music Store, just do a search on “susy06″ or “supersymmetry” in iTunes Music Store.

    I listened casually to the Q&A, & the exchanges should complement the above posts. A key exchange is between B. Richter & A. Linde, where AL responds to BR’s point that “Anthropic Principle is an Observation, not an Explanation”. Both are saying, each other is “missing the point”. Hmm. I still can’t figure out AL’s point-of-view. It almost sounds like a “belief mechanism”.

    I’m reminded of what a Caltech CS (computer-science) prof told me about Physics research back in ’99: “It gets very religious”. I.e., a battle between Idealogies. My ex-classmate (Stanford PhD, Geophysics) warned me about my own arguments for my Interdisciplinary Initiative: “Don’t make it too Idealogical”. I.e., Idealogy can sound religious & turn off scientists. A Caltech DNA Computing post-doctoral scholar emailed me “Cosmology is Religion”.

    I talked to T. Plehn (on LHC evening-session/Thu) at the end of the conference, & he is leaning towards BR. Myself, I take the view that Data takes priority: “it starts with the Data”, it’s Data driven. If I were doing Theory, I would *definitely* find a small group of Experimentalists to collaborate guide my theoretical “probes”.

  • Haelfix

    Unfortunately in physics there are plenty of examples of parameters that just so happen to be unnatural. For instance in nuclear physics as Nima emphasized a few times, you have situations where the binding energy w.r.t to pion exchange in deuterium comes out to be about 2 Mev when you would expect it to be about 100.

    In other instances, say EM you have things like positrons that just so happen to appear at exactly the right place to make things 100% natural.

    I’d venture to guess the majority of physicists are comfortable with a little finetuning (say on the order of one or two magnitudes, eg something like the little hierarchy problem in msusy), but 30 orders of magnitude, much less 60 is an enormous leap of faith. We’ve spent 20 years now working on these problems, and just abandoning the direction now when we are so close to getting results is highly presumptous

  • Plato

    It’s sort of like Cosmologists being circumvented to the beginning of this universe and we have these fine methods by which we wil deduce high energy photons as measures of the early indication of the beginning of this universe.

    Heaven forbid if we tend to the quark Gluon plasma and find such efforts to meausre this state of existance? How did you arrive here?

    Well, that’s fine.

    So one askes then of this universe, had it a beginning then, how did it become so?

    Are we not supposed to ask this question and from it deduce all the possible scenarios that could have allowed such a expression?

    So the “bulk perspective” is devised, and with it, all the geometrical propensites to the universe now in expression?

    So cosmologists are taught different now? :)

  • Plato

    More on name.

    So, no great annoucements, just trying to piece it together?

    So then of course, how is such measures today, taking our views “so very close” to this beginning? What is “the trigger?” Does no one like the perspective in regards to the “arrow of time” as listing “strings” in the “microsecond realm,” as part of that inflationary universe?

    Is what I am saying all about some Voodoo science? :)

  • Jack

    Looking at Richter’s stuff, two points stand out, one bad, and one good.
    Bad: the accusation of metaphysics. Umm, Burt, what do you think your remarks amount to if not more metaphysics?
    Good: the point that more effort should go into trying to show that the parameters of our Universe are somewhere close to being the most probable ones. He’s clearly right, the landscape will never go anywhere unless people can start computing such things. Some work is being done in this direction, there should be a lot more.

  • wolfgang


    Hawking seems to propose some sort of ‘inverse anthropic principle’ as announced e.g. in this press release

    It seems very reasonable to me. What do you think about it?

  • Clifford

    Hi wolfgang,

    At first glance at the news article to which you link, I don’t see what is different about what they are doing *in practice* from what the standard classical landscape statistics searches aim to do: figure out the features of the landscape to see if one can then apply conditional probabilities to assess the likelihood of some vacuum or class thereof. They’re instead wanting to do a path integral over all vacua. Nice words, but using what action principle? Also, without knowing both the action principle and the classical features of the landscape, I don’t understand how one can safely ignore any contributions to the path integral as insignificant. In other words, what is the measure in the sum over paths? How do you know that a type of configuration that makes only a tiny contribution does not at the same time have a huge number of representatives, thereby making a significant contribution in any case? So it seems that the existing program of classical explorations is essential making this “quantum” approach viable.

    But I’m most bothered by the lack of a believable action principle. Doesn’t this just boil down to the usual search for a better understanding of string theory’s mechanism for choosing vacua? If you don’t have it, the proposal seems to be missing something crucial, no?

    As described in the article, what they seem to want would be like saying that you can measure the anomalous magnetic moment of the electron, and from that number, deduce the Lagrangian of QED, and then use it to predict other physics like Bhabba scattering, etc. Nice…but it seems like an inverse problem that is…problematic.

    Of course, I’ve not read the actual paper, so there could be a lot more going on than all that.



  • wolfgang


    the way I understood the text, one performs the path integral over all histories compatible with what we observe now (basically fixing the end-state rather than an initial state).
    In the usual ‘landscape’ approach one would estimate probabilities over all possible histories or vacua. At least this is how I understood this so until now 8-) After what you wrote I am not so sure I understood it correctly.
    I assume Hawking talked about this in Beijing and perhaps some participant can tell us more about it.

  • chimpanzee

    The audio-only version of the panel-discussion is now available here. You can click on the upper-right icon at this URL, to get to the “SUSY ’06″ video-podcast (or do a search in iTunes Music Store on “susy06″ or “supersymmetry”).

    Just subscribe to above podcast, & hook up your iPod

    [Irrelevant long babble about tech stuff deleted, once again.

    Chimpanzee, for the umpteenth time, please try and distinguish threads which are about physics from threads which are about tech stuff. I created a thread elsewhere ("SUSY06 Goes HiTech) where you can put discussions about tech stuff. This is your last warning, ok? Last warning. -cvj]

  • Thomas Dent

    One point that seems to have been missed is that if you believe Vafa’s ‘swampland’ hypotheses then string theory could be rather easily falsifiable, if we find a low-energy field theory that does not satisfy his properties for moduli space. Susskind pushed this a bit further by pointing out that quintessence is very difficult to achieve. So all we have to do is measure the equation of state of the universe and show it is not a cosmological constant…

    Even if you don’t believe Vafa’s hypotheses, it is still true that one could discover a low-energy field theory that causes huge difficulties for string theorists in trying to get anything resembling it.

    You might even say, field theory contains more models (in fact a continuous infinity) than string theory – but people don’t complain that field theory is untestable.

    Richter’s remarks were a disappointingly blunt instrument, it sounded from the beginning as if either he doesn’t understand the anthropic principle as Weinberg used it, or he doesn’t want to. And it also sounded as if he hadn’t noticed that the great majority of conference talks were experimental or phenomenological and had nothing to do with landscaping.

    Now there are lots of objections to be made to statistical-type arguments – that we don’t understand the prior distribution, and that we can’t calculate reliably the probability that intelligent observers will emerge – but it seems to me a logically valid method.

    If you knew the probability distribution of values of some observable X from your underlying theory, the probability of *observing* a certain value of X depends on the probability that observers will exist given that particular value.

    P(Observe X) = P(Theory gives you X)*P(Observers exist given X)

    There is a problem with the use of ‘atomic principle’ or ‘stellar principle’ or ‘weak symmetry breaking principle’ as recently fashionable, since they don’t actually give you the probability P(Observers exist), and they disagree among themselves. Also to ask for ‘Life as we know it’ makes no sense. Observations could be made by anyone or anything, not specifically by carbon-based life forms with ten fingers… so I think some of the current work being done using such arguments is of poor quality, but that doesn’t mean that the approach is necessarily unscientific.

    So what has this to do with ‘naturalness’? The usual naturalness or fine-tuning argument implicitly assumes some probability distribution over the continuous space of parameters of field theory and then asks what kind of field theory could give what we observe with non-tiny probability. It is not so clear how this is better than anthropic-type statistics since we still assume that Nature has some sort of roulette wheel to choose a distribution of parameters.

    The alternative is just to get on and measure stuff and interpret it within an effective field theory and see what field theory models gets ruled out. That’s what a lot of people at SUSY were talking about and will be for many years to come. And I don’t know any string theorist who is *against* measuring things, or anyone who wants the budget for measuring things (which is much greater than the budget for theorists!) to be cut. The question is whether you go on to think about what a fundamental theory could be. On that you can be pessimistic or optimistic depending on the state of play in experiment and theory, but I think you ought to go on thinking about it until it is absolutely certain that no progress can be made.


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  • Kenny Easwaran

    I suppose it makes sense that physicists have to deal with a lot of questions like these about “what is science”, because they are after all trying to do science. But their main job is to do science. So it seems plausible that people having these arguments should pay at least some attention to people whose job it is to think about “what is science”. Obviously, the models by people like Popper, Hempel, Kuhn, etc are far too simplistic. But isn’t there at least some relevant stuff in the recent literature? I don’t know much about philosophy of physics – I think the ones I know of like David Albert and Tim Maudlin work more on space and time in relativity than these issues. But surely there are some others who work on this stuff. And some people in more general philosophy of science who assess the strength of general probabilistic arguments. For instance, I know of someone at Stanford who right now is writing a dissertation on fine-tuning arguments – surely some of the arguments he considers should be part of this debate? (The philosophy of biology literature also sounds like it has a lot of discussion about elimination of theories and seemingly related issues.)

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