The Graviton In Our Time

By cjohnson | November 25, 2005 2:33 am

Today I heard* Monday’s edition of BBC Radio 4’s programme called “In Our Time”. This one was about the Graviton. You may recall me talking about this series before, back when I’d heard the one about Magnetism. My thoughts are roughly the same as before. A good and worthy (and very welcome) discussion of physics, but rather poorly executed overall. This is because the message got rather muddled. It started rather well, but it just kept going around the same tight circle, more or less, repeating the same tired stuff about the incompatibility of current gravitation theory (General Relativity) and everything else that the universe does, which is quantum mechanical at the core. The graviton arises as a key and natural idea about how to understand quantum gravity (at least from a particle physicist’s perspective). This is worth saying once or twice, maybe thrice, rather clearly, but then I think that’s enough in a forty minute programme when you want to cover a lot of ground. But they kept coming back to it, like a broken record.

One of the other problems I’ve found it that the host, Melvin Bragg, interrupts the speakers rather a lot and makes a mess of some of their explanations in doing so, sometimes trying out a poor analogy of his own that he believes might do the trick to explain something. I suspect that there might be a producer yelling “abort! abort!” in his headphones whenever a guest is beginning to sound a little too specialist and then Melvin Bragg tries to turn the conversations elsewhere, or “soften” the words a bit. This had the unfortunate effect of allowing misleading (and sometimes just plain wrong and/or silly) things to hang there, and even become real and re-enter the conversation. The guests, being obscure academics like all of us, appear to suffer from that problem of being so clearly pleased to be asked on to a primetime show like this that they don’t insist on being allowed to correct the inadvertent silliness, perhaps because they’re worried they’ll never be asked again. (We are all media tarts at heart, so I can see this being a factor, even if not consciously). I don’t really know why for sure (that was just a guess), but it seems to happen anyway. For example, have a listen to the program and try to spot when one of the guests valiantly attempts to explain what spin is. He’s about to trot out your favourite analogy which is innacurate, but an acceptable representation of some key attributes when you don’t have more time (you know…top spinning on axis, etc), when Bragg interrupts with something or other about fairgrounds and things, and the next thing you know, spin is candyfloss. Amazingly, nobody tries to clear this up. And it emerges again later: supersymmetry is some way of changing the candyfloss from one type to another. (…huh?)

I don’t know (as research colleagues) any of the people (Roger Cashmore, Jim Al-Khalili, Sheila Rowan) who were the guests and I am sure that they are fine people and good at what they do. They explained what they do rather well in fact (e.g. Sheila Rowan talked quite a bit about gravity waves, and quite interestingly and informatively too). But none of them, as far as I could tell from what they said, had much firsthand knowledge about what is actually going on in research involving gravitons, per se. This is the same weakness I pointed out about the Magnetism show last time. As a result, there were all sorts of simply bizarre alarmingly incautious statements (such as how the LHC might discover the gravitino and then this would prove that there were gravitons, as well as solve the dark matter problem. (….huh?! that assumes rather a lot, no…?) [update: or how we might produce gravitons directly at the LHC, and see them from missing energy signatures]) which were a bit embarrasing to hear [update: embarrassing largely because of the degree of overstatement about what the LHC can most likely do….unless we are really lucky….there is a wealth of dark matter candidates besides something like the gravitino, and we’ll probably need another accelerator to sort out the details….]. The usual over-blown claims about string theory were bandied about, with nothing beyond, say, 1995’s duality revolution mentioned as having any input (positive or negative) to the prospects for physics. The conflation with gravity wave research and graviton research (both interesting fields) was rather annoying, but I am pleased to say that all of the guests made important points to separate the two, once the host took a step back and let them talk.

Despite all that, I must say that it is nice (and welcome) to hear that the programme makers still have a commitment to discussing ideas from physics right along side other ideas from other fields. It was a bit like sitting in on a pub conversation, and being pleasantly surprised to hear what they were talking about. This is healthy. There was some good banter (such as Bragg giving Rowan a hard time about the fact that she keeps binging up rubber sheets every time she is going to try to explain something) and this was also good to hear mixed in with the other stuff.

I can only hope that the show’s producers will invite guests who not only have the historical persepective, but also guests who are actively doing research in the topic too. It lends a better balance to the proceedings and is a better resource to those learning about the topic afresh. This could make “In Our Time” into a truly great programme. Right now it keeps missing the mark, and being only “pretty good, considering”, at least in the area of science, for the reasons that I mentioned here and in the other post.


*Thanks for mentioning it, Ed Copeland!

CATEGORIZED UNDER: Science, Science and the Media
  • Scott

    “spin is candyfloss” well at first i didn’t realize how bad an analogy this is, and then i found out that candyfloss is cotten candy. Wow, just wow.

  • Wolfgang

    It could have been worse. I have heard people describe elementary particles as some sort of spaghetti (or was it strings?) spinning around in some 10-dimensional space.

  • Plato

    Is it wrong to move so far ahead theoretically to be called a charlatan, by those who recognized the limitiations of experimentally proving it?

  • damtp_dweller

    I’ve got to confess that I actually rather like In Our Time. Melvyn Bragg does seem far more at home when he’s discussing philosophy or history on the show than when science, particularly physics, is the central issue. Martin Rees and Roger Penrose have both been on several times and they seem able to steer the discussion in meaningful directions but when less notable scientists are on Bragg does seem to dominate the discussion with his constant attempts to simplify things.

    Still, it’s good that this sort of thing is discussed and In Our Time has quite a large audience over here.

  • Darwin

    Did they mention that by Einstein’s equivalence principle, the graviton would be involved not only in gravitational acceleration but also in all other accelerations?

    Rueda and Haisch, Physical Review A v49 p 678 (1994), suggested that the virtual radiation of electromagnetism can cause inertial mass, and in Annalen der Physik v14 p479 they do the same for gravitation in general relativity.

    Two electrons repel because gauge bosons exchanged between them are non-redshifted, unlike the gauge bosons pushing them together from the surrounding, receding universe. So repulsion is explained. An electron and a proton are pushed together because they shield one another, being opposite charges. It is all a matter of recoil due to the momentum of gauge bosons.

    The electron is emitting and receiving gauge bosons from charges in all directions, but the big bang means that those from the surrounding universe suffer redshift. Between two nearby similar charges, the exchange on facing sides of the charges where they couple is not redshifted, although it is redshifted where they are being pushed together on the far sides.

    The net exchange is like two machine gunners firing bullets at each other; they recoil apart. The gauge bosons pushing them together are redshifted, like nearly spent bullets coming from a great distance, and are not enough to prevent repulsion.

    In the case of attraction, the same principle applies. The two opposite charges shield one another and get pushed together. Although each charge is radiating and receiving energy on the outer sides, the inward push is from redshifted gauge bosons, and the emission is not redshifted. The result is just like two people, standing back to back, firing machine guns. The recoil pushes them together, hence the attraction force.

    The virtual radiation acts on fundamental particles of mass, quarks and electrons, which are always charged. It doesn’t ignore all the quarks in a neutron just because they have no net charge. A gauge boson going at light speed doesn’t discriminate between neutrons and protons, only the fundamental quarks inside them. Therefore, the background field of virtual radiation pressure besides causing inertia (and the contraction of moving objects in the direction of motion) also causes gravity (and the contraction in the direction of gravitational fields, the reduction in GR). The force strength for electromagnetism is then naturally related to gravity. Between similar charges, the electric field causing the radiation pressure adds up like a series of charged capacitors or batteries (remember, we’re talking of potential energy difference in volts, not current). In any line, there will be approximately equal numbers of both charges … full proof
    Feynman gravity home page

  • Plato


    not forgetting to mention the contributions to quantum gravity and the connection to this site has, eh? :)

    You help indirectly by site, shape perspective on the Planck epoch to now, and had me wonder? About it’s relevance, if we were to encompass such views with a history of “graviton bulk” perspectives.

    Would it be such a mistake to wrap all of it in one nice neat package, called the standard model?

  • http://1034:Incorrectkeyfilefortableusers;trytorepairit sisyphus

    Thanks, Clifford. Didn’t know about BBC site. Been listening for a couple of hours. Love it.

  • Lubos Motl

    Dear Clifford,

    I completely agree with many details you write. The incompatibility of QM and GR should not be repeated more than twice or thrice. This tension is important to understand why string theory is studied seriously, among other facts, but it is not too important to understand the very existence of gravitons.

    The graviton is, on the contrary, an example of a correct derivation from semiclassical gravity – a legitimate approximate unification of GR and QM. Its existence follows from the theories we have, even given some degree of ignorance of quantum gravity at higher energies, and at the semiclassical level, it is absolutely analogous to the photon.

    The only difference is the value of the spin, the geometric interpretation of the graviton, and ultraviolet divergences from loops.

    Physics fans are not being explained these basic things well enough, I am afraid.

    All the best

  • Kea

    Why, surely candyfloss is a much better analogy than, say, a merry-go-round! The latter is just as bad as the spinning top idea. You know how they do those funny hand twists when they make it and it seems to go ’round twice sometimes to finish a loop.

  • Plato

    A bit of correction here

    well Lubos, you and Clifford might have known, and a majority of the science people(?) even a layman like me(?)
    then why do you suppose, that any attempts here to define reality in such a way, causes such consternation amongst your colleques?

  • a.krug

    As a result, there were all sorts of simply bizarre statements (such as how the LHC might discover the gravitino and then this would prove that there were gravitons, as well as solve the dark matter problem. (….huh?!))

    Why the “huh?”? There are, after all, realistic dark matter gravitino scenarios.

  • Lubos Motl

    Clifford: if the LHC “sees” the gravitinos – which is usually not the easiest particle to see – it would quite likely confirm SUSY as well as the gravitons. The certainty about the existence of “gravitons” is much higher than the certainty about low-energy SUSY in the real world. There can be something totally surprising about quantum gravity and gravitons that we misunderstand but if some gravitinos agree with some of our models, I would claim that the uncertainty would evaporate.

    Dear Plato, interesting. Which consternation and which colleagues did you mean? Did you mean our colleague cook Nigel? 😉 Do I understand his comment on your blog well that he wants to revive the idea of the Maxwellian aether? Maybe the words “your colleagues” would be a bit of stretch after all.

  • Clifford

    Lubos: Here I defer to our resident phenomenologist, JoAnne. It seems to me, from things I’ve heard her explain very clearly, that things will most likely not be so clear at the LHC. Most likely it will be a discovery machine and we’ll need the ILC to properly work out what the hey we’ve discovered. I don’t think we should get into the business of over-promising what the LHC will do. String theory effort has (arguably) already been damaged by such over-promises.

    As to evaporation of the uncertainties you mention…. that would be nice, but again, you seem to be quite optimistic about how generous Nature might be to us, with regards clarity. Good. Optimism is good.


    [Update: I updated the post a bit, since I think that the word “bizarre” was a bit strong. I was trying to get across that there were a few off-the-cuff remarks about what the LHC might do, and that it was bizarre to hear this one clear statement about just one of the many things that the LHC might see, and very likely not clearly at all, unless we’re lucky… falling into the trap of promising too much, again. ]

  • Darwin

    I don’t think the mainstream string theory, the M-theory of Witten is deliberately wrong, but most charlatans think they are right.

    You can tell Witten is a charlatan because he said:

    ‘String theory has the remarkable property of predicting gravity’.

    (Quote: Edward Witten in the April 1996 issue of Physics Today. This was repudiated by Roger Penrose on page 896 of his book Road to Reality, 2004: ‘in addition to the dimensionality issue, the string theory approach is (so far, in almost all respects) restricted to being merely a perturbation theory’.)

    Witten is misleading people accidentally by being over-enthusiastic with no testable evidence. String theory does not predict for the strength constant of gravity, G!

    This means that my work on gravitational force mechanism which does predict gravity correctly is suppressed, as is Lunsford’s work which does away with the cosmological constant like my model.

    [snip! cvj asks that darwin posts the rest of his long -mostly off-topic- comment on his own blog. ]



  • Darwin


    Nigel’s page says: ‘The pressure due to the acceleration of the fabric of space, or gauge boson force-causing radiation, towards us produces gravity by pushing us from all directions equally, except where reduced by the shielding of the planet earth below us. Hence, the overriding push is that coming downwards from the stars above us, which is greater than the shielded effect coming up through the earth. This is the mechanism of the acceleration due to gravity.’

    He mathematically links quantum ideas with classical mechanism and gets a prediction which is testable. This is not Maxwell’s crackpotism! He is a crackpot but not as bad as string theorists. Who knows, he might even have guessed right? It is hard to disprove crackpots who only use experimental facts in mechanisms to prove other things an get the right answer!

  • Darwin

    [snip -see cvj’s comment below]

  • Clifford

    Darwin…. I’m not going to let this thread turn into another tedious discussion of why string theory is the scourge of all rational thought. Sorry. Nothing new has come out of these discussions here since we first had them in August. So that’s the last one for this thread, ok?

    Also, I’ll email your long long long comment above back to you and cut most of it out. You can post it on your own blog and put a comment here with the link to it.

    Lubos…. please humour me and reply (if you must) to Darwin on this matter elsewhere.

    Both:- I will use the delete option with no hesitation.



  • a.krug

    Clifford wrote:

    Here I defer to our resident phenomenologist, JoAnne. It seems to me, from things I’ve heard her explain very clearly, that things will most likely not be so clear at the LHC. Most likely it will be a discovery machine and we’ll need the ILC to properly work out what the hey we’ve discovered. I don’t think we should get into the business of over-promising what the LHC will do.

    Well, let’s not undersell it either. Discovery of apparently stable long-lived slepton-like particles would be a very strong hint of a gravitino LSP. It might take a long time to get conclusive evidence, but the LHC could point to a gravitino fairly quickly.

    (The ILC will be important to sort things out, but the LHC can do lots of precision stuff too, given enough eventual integrated luminosity. The luminosity upgrade will be important! This will be especially true if new particles are kinematically out of reach at the ILC….)

  • Clifford

    a.krug:- fair enough.


  • Darwin


    Lumos above falsely stated I’m interested in Maxwell’s ether, which is the opposite. My comment on Plato’s blog used Maxwell’s ether as an analogy to string theory, which has turned out useless for scientific purposes, just like the ether idea of top mathematicians like Maxwell.

    The mechanism for forces is gauge bosons. We are seeing the past with distance in the big bang! Gravity consists of gauge boson radiation at light speed, coming from the past just like light itself. Hence, there is apparent acceleration (variation in speed from 0 toward c with times past of 15,000,000,000 years toward 0), so F=ma gives outward big bang force. The 3rd law says there’s equal inward (Higgs field/gauge boson) force, which gets shielded by mass, proving gravity to within 1.65%.

    There’s no crackpotism here, unless you deem anything unorthodox as non-scientific.

  • Ayub Faridi

    Dear Clifford,

    From the point of view of a theoretical and experimental High Energy physics the mediators of four forces are gauge bosons. Still gravitons are a mystry. Is there a relation between Cuvature and graviton?Feild theory verses Geometry

  • Darwin

    The graviton is supposedly a spin 2 gauge boson, but it’s role is duplicated by the normal vacuum field: see Rueda’s papers:


    This doesn’t predict the actual mechanism for the force of gravity, the source of the coupling constant, but:

    “The principal conclusions of this paper are:

    “(1) Identity of inertial mass with gravitational mass, mi = mg naturally follows from the quantum
    vacuum inertia hypthesis. …

    “(2) The quantum vacuum inertia hypothesis is consistent with Einstein’s GR. We have already commented
    above, in particular in §8, on this interesting feature presented in §5 that puts the vacuum inertia
    approach of [1] within the mainstream thought of contemporary gravitational theories, specifically within
    that of theories of the metric type and in particular in agreement with GR.

    “(3) Newton’s gravitational law naturally follows from the quantum vacuum inertia hypothesis. …

    “(4) An origin of weight and a physical mechanism to enforce motion along geodesic trajectories for
    freely-falling objects is contained in the hypothesis. We have shown how this approach to inertia answers a
    fundamental question left open within GR, viz. is there a physical mechanism that generates the inertia
    reaction force when non-geodesic motion is imposed on an object and which can manifest specifically as
    weight. Or put another way, while geometrodynamics dictates the spacetime metric and thus specifies
    geodesics, is there an identifiable mechanism for enforcing motion along geodesic trajectories? The quantum
    vacuum inertia hypothesis represents a significant first step in providing such a mechanism.

    “(5) A physical basis is provided for the relativistic mass increase. …”

  • Plato
  • Lubos Motl

    Hi Clifford,

    it has become very popular among the phenomenologists to invent the most bizarre possible scenarios for what the LHC is gonna see. It’s because first of all they’re afraid that they won’t be able to say what they see; second of all, scenarios that make this happen offer many opportunities for their work; third of all, these are preliminary commercials for new colliders.

    My guess is that it will be rather clear pretty soon what the LHC is seeing. Let’s see.

    All the best

  • Lubos Motl

    Dear Charles (I hope that I can call you Charles, Prof. Darwin),

    too bad that your new reincarnation is a Cook instead of a great scientist. Edward Witten is absolutely correct that string theory predicts gravity. The existence of massless spin 2 particles and their interactions that agree with GR is a derived concept in string theory. Moreover, the theory also predicts its strength, namely G=1 (in Planck’s units). If you want different units, then it’s the same thing like criticizing your theory for its being unable to determine the age of the Earth and the life on it.

    This internet is a cool thing, is not it? It allows dead 19th century scientists to communicate with the 21st century physicists – as long as at least one Cook on the line suffers from schizofrenia.

    Al lthe best

  • Clifford

    Hi Lubos,

    I’m not sure where you’re going with this. There is nothing wrong with preparing oneself for the range of possible scenarios that can happen at a major experiment like that. It is not a bad thing to do in an of itself.

    In fact, there would be no way of knowing what the LHC is “seeing” if one did not do this to some extent…exploring the possibilities….the products of the collisions don’t come with helpful unambiguous signs around their necks saying what events produced them, you see. But you know that, of course.



  • Count Iblis

    I actually don’t understand at all how one could claim to be able to verify seeing the gravitino in the LHC, assuming they are indeed produced. Surely you need a lot of verifications of supersymmetry before you can claim that the particle that looks like the gravitino is indeed this particle and not some other particle that happens to have a lot of similar signatures?

  • Science

    Dear Lumos,

    You say: ‘Edward Witten is absolutely correct that string theory predicts gravity. The existence of massless spin 2 particles and their interactions that agree with GR is a derived concept in string theory. Moreover, the theory also predicts its strength, namely G=1 (in Planck’s units).’

    G=1 is not a prediction, but a normalisation. Which just shows how much you know about science. It’s less testable than Maxwells prediction of a solid elastic aether! At least Michelson-Morley could test it. ST just a perturbative theory which can’t be tested, so it predicts nothing.

    Look, Lumos, Witten can predict that a UFO containing little green men will land and save string theory, but it cannot be tested, so it isn’t science. Darwin gets gravity to within 1.65% from G = (3/4)H2/(PI.DENSITY(local)e3), which is proved from facts:

    Best wishes,

  • Count Iblis

    Science, dimensionful constants like G, h-bar and c are nothing more than conversion factors. You could just as well invent a formula for the ratio of the mile to the meter.

    It is unfortunate that in high school physics students are indoctrinated with the idea that meters, seconds and (inverse) kilograms are incompatible quantities.

  • Science

    Count Iblis,

    The fact based mechanism predicts 10 ms^-2 at Earth’s surface, compared to a measured 9.8 ms^-2. If you still don’t understand the agreement in the unification because of your problem with simple units of distance, time and mass, then the gravity mechanism tells you your mass within 1.65%, given that you know the earth’s mass accurately.

  • Science

    The general relativity consistent gauge boson radiation causing gravity and electromagnetism is DISPLACEMENT CURRENT. Catt shows that Maxwell got his interpretation of this ‘displacement current’ wrong, by ignoring the time it takes light speed electricity to flow along the capacitor plates. His co-authors Drs. Walton and Davidson mathematically worked out how the transmission line theory of Heaviside can be applied to explain the charging curve of a capacitor, which is compared to reality and is a correct prediction. Catt’s error follows from Heaviside’s false idea that


    [snip -cvj]

    …. see more here.


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