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Layman's question about gravitons

  1. Apr 25, 2008 #1
    Hello all. I have a quick question regarding gravity and I was wondering if someone could explain it to me in laymans terms, since I do not possess the mathematical background to understand higher physics.

    According to Einstein, if gravity is simply the "fabric" of spacetime -- akin to a 2-dimensional rubber sheet where matter resting on top warps the sheet, and creating the sensation of gravitational forces -- then how is it possible then that graviton particles should exist?

    It seems counter-intuitive to think that the background fabric of spacetime could have particles acting as carriers of the force, since the "warping" of space itself is the only explanation needed to explain gravity. After all, if we use the rubber sheet analogy, there is no need to explain gravity by introducing particles, since the presense of matter on the sheet is more than enough to account for the gravitational force.

    Thanks for taking the time to read this, can someone quickly explain it? I am very confused!
  2. jcsd
  3. Apr 25, 2008 #2


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    Theories of quantum gravity (which include gravitons) assume that the "warping" will turn out not to be the only explanation needed for gravity. People working on these theories hope that they will eventually be able to make predictions that differ from what classical general relativity would predict, and that can be tested by experiment. If this succeeds, then the new theories would supersede classical general relativity as the fundamental view of gravity. GR would probably continue to be used for practical calculations in areas where it already works, just as Newtonian gravity is still used for many practical calculations.
  4. Apr 28, 2008 #3


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    Layman's question too

    I greatly look forward to an explanation which would give some idea not only of what these gravitons are but from what theoreticians obtain such confidence and agreement that they even know something about them. For instance they appear agreed they have spin 2 (2 what? :confused:) though they never have been observed nor is there AFAIK any programme or hope of an observation. Nothing has ever been observed for which GR is not adequate.

    This kind of physics is clearly a somewhat different discipline from the experiment-relateable bulk of the science. However I think more valuable here than a philosophical discussion, we would get a better idea and feeling if someone were able to give us an idea why spin 2, why couldn't it be 1, 3, or 1/2?

    (I have read that the nearest thing to a physical prediction of string theory is that it predicts or allows this spin 2 graviton, which was already 'known' to exist, so the fact that gravity exists, as has been known for some time, was taken as a triumphant experimental confirmation of string theory! :rofl:)
  5. Jun 26, 2008 #4
    Its my understanding that matter particles all have spin 1/2, and the force particles have spin 0, 1 and 2. Its also on some wierd 720 degree measurement, where u need two complete revolutions to get the particle to look the same.

    Spin (with Paulis exclusion principle) explains why quarks form seperate well-defined protons and neutrons, and how with electrons these form atoms. Quarks even have colours to explain their interactions - red, green and blue.

    In the case of force carrying particles, that don't obey this exclusion principle, their antiparticles are the same as the particles themselves. We know they exist because they have a measurable effect.

    It appears these force particles (EM, weak & strong nuclear and gravity) all combine into one force at some grand unification energy. Its as if they are different states of the same invisible force particle.

    Hope that helps.
  6. Jul 5, 2008 #5
    Re: Layman's question too

    You don't need to "look forward" to that. It sounds like you're just saying you haven't learned about the theory. Go pick up some particle physics textbooks and you'll (eventually) get your explanations from them.

    You are correct that no one has observed gravitons, and I think it is not even certain that they exist. However, if they do exist they must have a certain spin due to the rules of particle physics. Again, read a book. Since you don't know what "spin 2" means, it sounds like maybe you don't even know what spin is, which is fine (everyone has to start somewhere), but if you want answer to these questions you are going to have to learn a little more.

    No, that isn't true. GR is not a complete theory of physics, it is just a description of "gravity", that is, how space and time interact with energy on large scales. GR won't tell you that opposite charges attract, nor that atoms are made of protons, neutrons, and electrons. In fact it tells us very little about the world of particles, which has been deeply explored by particle accelerators and is used every day by people in the modern semiconductor industry and many other places. For that kind of information, you need the standard model and quantum mechanics.

    As I understand it, the purpose of all that quantum gravity stuff is to try and find a way to reconcile particle physics (which basically means quantum) with "gravity stuff" (which basically means GR). The problem is that gravity has such a small effect on tiny particles that it's almost impossible to measure it. So, so far it is not completely understood how gravity affects the particle world. Gravitons, or at least some kind of quantum gravity concept, will need to be introduced. There is no "obvious" way that gravity makes sense in the quantum realm.

    No, it isn't. Quantum mechanics is one of the most widely tested, wildly successful theories of all of science. As mentioned earlier, it is used every single day by semiconductor manufacturers and other who work with tiny things. Many, many particle accelerator experiments have also confirmed it. Just because you personally aren't familiar with something doesn't mean it isn't well understood by scientists.

    Again, get a good textbook on particle physics and you'll have your answer. The purpose of introducing gravitons in some form (again, as I understand it) would be to provide some kind of possible idea of how particles might respond to gravity. So far, I don't know whether anyone has come up with any fully consistent hypothetical description of how gravity and particles might interact in any way that is consistent with other observations about particle behavior.
  7. Sep 9, 2008 #6


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    I come back after an absence to this forum and this thread. I think Xezlec has missed the point of my question.
    Eventually as you say, however if I had read some particle physics textbooks I would no longer be a layman presumably. It would be learning the theory not ‘about’ it, so I remind you title of the thread and my post. To put it another way is there a physically intuitive explanation at fairly popular level? Say Scientific American level. There are books or articles that make a fair stab at explaining the not exactly intuitive quantum mechanics to laymen. And things like quarks too... forces explained as exchange of virtual particles… Standard Model… General Relativity… Something about what these things were about and where they came from. Spin of electrons or photons does have some intuitively understandable obeservable physical manifestations I know of that entitle it to be called spin. But what gives similar confidence to think we know something of graviton spin?

    To reflect back Xelec’s
    Just because he can’t think of an explanation of that kind doesn’t mean that no one can. :tongue2:

    Blueprint's mention of 720 degree rotations suggests there may be one somewhere, though that hint is just tantalisng.
  8. Sep 10, 2008 #7
    Not only that, I didn't even realize you intended to ask a question. I interpreted your post to be saying something like: "These scientists are dumb. This obviously doesn't make sense because I don't understand it." I apologize if I misunderstood. Maybe I was a little testy that day.

    In any case, you appeared to claim that quantum mechanics was "clearly" different from other physics in the sense of not being based on evidence, and the main thing I wanted to say was that that is very, very, very false.

    Have you read them? Did they explain it? I think the problem is you are reaching the limits of what these kinds of coffee-table simplifications can do for you. Perhaps you want more understanding, but haven't yet resigned yourself to the fact that the stuff the simplifications don't explain is exactly the reason that they are simplifications. If you really want to understand why spin only occurs in certain values and what those numbers mean, I think you are going to continue to feel unsatisfied with the fuzzy answers you get until you finally make up your mind to just take the plunge and actually learn the painful complexity of details that together form the true answer to your question. I could be wrong but that's my guess.

    I am reminded of a story I read in a Philosophy of Religion class. There was an argument about whether everything had to have a single cause, and some philosopher proposed the following situation: you walk up to a bus stop and find 5 Eskimos standing there. You ask yourself what could be the cause of such a strange event. But what if it turned out there was no single satisfying answer? It's possible that one of them immigrated due to global warming and was going to school nearby and decided to catch the bus home, another is from Canada and works at the Canadian embassy in hopes of beefing up a resume to go into politics one day, and each Eskimo in turn has a different -- and completely unrelated -- reason for being there.

    I can relate to this story as follows. If you learned all of that by talking to all of them, let's say some stranger walks up and asks why all those Eskimos are standing there. You start explaining each one's story but he stops you, saying he doesn't want all that detail. He just wants the bottom line. What the hell can you possibly tell him? That's how I feel in this situation. All I can say is that each of these 10,000 Eskimos of physics has a story and probably a Nobel prize behind it for the leap of reasoning that it took to show how some experiment proved some amazing new fact. Or, I can explain each one and its reasoning in detail. But I don't know what else to do.

    Sure. I can try to help explain what little of this I understand, though I warn you that I'm still trying to learn it myself. However, the thing about simplified explanations is that they are simplified. That means they won't be completely correct and they probably won't completely make sense. If I said something that was both perfectly correct and didn't have any logical "holes", then it would be a literal physical law. And then you wouldn't be a layman.

    The other problem is that quantum physics is notoriously unintuitive. It is probably safe to say that every physicist who has lived since the dawn of quantum mechanics has tried and failed to find a more intuitive way of thinking about the subject. Experienced physicists today just tend to say "oh, well, our hunter-gatherer brains just aren't designed to understand things at the quantum level, so our intuition will never be up to the task. Tough luck."

    Have you at least read "The Elegant Universe"? It tries pretty hard to simplify these kinds of subjects. I personally didn't feel completely satisfied with its explanations, but I think that book is about the best anyone can ever possibly do.

    I believe, after studying a lot of math very very hard, that I may be coming close to seeing what it means to say that forces can be comprised of an exchange of virtual particles. Let me see what I can do. This is going to be horribly inaccurate, so I'll probably get a flood of corrections from people.

    To say things in a very oversimplified and foggy way: practically any kind of wave can be expressed as a moving distribution of "probabilities" of a particle being in different places. That probability is not really a probability in the typical sense of the word, since the particle actually sort of occupies all of those positions simultaneously up until its location becomes important for some reason, at which point the universe finally sort of uses the wave as a true probability and picks a random location for it to be. Oh, and this probability also has a constantly-rotating phase angle associated with it, and the probabilities add together in a funny way I can't explain without using details. The reverse is also true: any particle's position can be expressed as a wave.

    Then, I must explain that a virtual particle is like a particle that has some "probability" (in the above sense of the word) of existing somewhere at some time, but never is capable of actually coming into existence there and then because it would violate some conservation law, but still affects the world in some indirect way nonetheless, by virtue of how that probability wave produces other waves, smooshes together with other waves, and stuff. Kind of.

    Next, I could point out that forces must always be transmitted by waves. This one is a lot easier to explain, as it doesn't require any quantum. For example, if you push on an object, that force nudges the atoms touching you, and then they nudge the atoms touching them, and so on down to the end of the object. The same is true for electromagnetic and gravitation forces, but the wave is made up of something other than atoms: a field (instead of a force) "pushes on" empty space, and that sudden change then exerts a field in the empty space in front of it, and so on until finally the wave of fields reaches a charged object, at which point the field actually exerts a force on that object (in addition to exerting a field on the space it occupies, kind of).

    Finally, we can sort of tie it all together (loosely!) by saying that if a gravitational force exists between two stationary things, then there must be waves of fields propagating that force between them, and since all waves are in some (extremely complicated) way a representation of the probability of the existence of some particle at different points, there must be particles traveling between them, but all this particle motion would take energy coming from nowhere, but that energy is also going to nowhere since the objects aren't giving way to the forces at all, so the force between them that we observe must be the effect of particles that sort of exist in the sense that they have probability waves that make something true, but can't possibly really exist in the measurement sense and don't even need to. *passes out from lack of oxygen*

    I think that the above paragraph is particularly bad and wrong, due to my understanding falling off right about at that point. Maybe someone else can swoop in and clear that up a bit.

    Particles can be observed to have lots of different properties. The Standard Model is the final result of decades of individual observations and theories to simplify all the interactions that have been seen. I mean, I don't know what you expect here. There are millions and millions of events that have been recorded by particle accelerators and the standard model is the smallest set of rules anyone has come up with that completely describes them all. Don't ask what all those rules are, because if you knew those you certainly wouldn't be a layman anymore. And I don't know much about them anyway.

    Start with special relativity. Then add that an accelerated point of view feels exactly like a stationary point of view with a gravitational source nearby. Do a grotesque amount of math to flesh out that idea and you have a theory of propagating curvature of spacetime that is much too complex for my tiny brain to have ever accomplished a grasp of.

    Are you referring to the simple statement that it looks like "intrinsic angular momentum"? Let me show an example of how incomplete that description is.

    In answer to your "2 what?" question, the angular momentum represented by a particle with spin s is given by: [tex]\hbar \sqrt{s(s+1)}[/tex]. (I learned that from Wikipedia, along with most of the rest of the stuff I know). Now, I ask you, is spin a vector? Angular momentum is certainly a vector: it has some magnitude and some axis of rotation, right? So if spin really is angular momentum, it should be a vector too. But no, spin is just an integer multiple of 1/2. This is because no property of a particle is really definite; in quantum mechanics every particle is in a superposition of all possible states with different "probabilities". And because the quantum world is so complicated, it just turns out that the relative "probabilities" of different spin values for a given particle combine together to also tell you something about axis direction. However, what exactly they tell you, and why, is not easy to state without math.

    As far as I understand, the type of wave that propagates the 4-dimensional warping of spacetime that corresponds to gravity can only sensibly be connected to the probabilities of a particle with spin 2. This is apparently due to the details of how waves in quantum mechanics describe the properties of particles they correspond to. I think it has something vaguely to do with the number of degrees of freedom that are available to a spin-2 particle and the number of degrees of freedom involved in the gravitational force. I'm not going to be able to do any better than that, being fairly ignorant of that subject.

    Indeed not. I wish you the best of luck. If you ever do come across a simple, intuitive way of explaining advanced physics concepts, please post it here! We will all appreciate it very much.
    Last edited: Sep 10, 2008
  9. Sep 10, 2008 #8


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    At present, Einstein's theory of gravity as the warping of spacetime works for very large distances, but not very small distances.

    At small distances, we have the Standard Model of particle physics, which describes how particles like the electron and photon interact.

    However, we do not know how to use Einstein's theory of gravity together with the Standard Model of particle physics - the two theories seem mutually incompatible. So people are looking for a new theory that modifies one or both existing theories to make them compatible.

    The graviton is a particle proposed by some of these new theories. The graviton is not a particle like an electron or photon that moves on warped spacetime. It is a particle of spacetime itself. The movement of these particles of spacetime would be the warping of spacetime.
  10. Sep 13, 2008 #9


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    Thank you for your extensive answer.
    I was not challenging ornery quantum mechanics, which I understand very well holding up to exacting experiment. But does one call what is recounted in "The Elegant Universe" just 'quantum mechanics?'. And 'based on evidence' in the same way? Surely it would like to achieve that status but hasn't? I thought the reasons for taking it seriously was that it made some sense, which is apparently already difficult to achieve with such things, and had been shown not to, or at least had not been shown to, contradict known physics. And I think it was in that book that the spin-2 graviton was claimed as a Great Prediction. But as the graviton is not known experimentally, this boils down to: the prediction of this theory agrees with the prediction of another, lower-level, theory. (A more dramatic example of "This kind of physics is clearly a somewhat different discipline from the experiment-relateable bulk of the science." I remember was a Theory of Everything by Penrose, which predicted that all matter should move at the speed of light. But that discrepancy with observations was not considered at all fatal - things moving at other speeds would surely in the future sometime fall out as a by-the-way result of some ugly secondary interaction.) I am just saying there is this provisionally relaxed-standards difference with what is traditionally understood as those of physics, not that that makes it rubbish or that it will never get anywhere, though I know some physicists do think that.

    Correct me if I am wrong but I had the impression that gravitons came in in some sort of intermediate level theory. Not found in an ornery textbook called "Quantum Mechanics", nor quite a necessary deduction from the principles that are found there? Maybe from something implicit in them?

    If, for example, there was a theory that makes some successful verified experimental predictions, that all the elements of that theory are necessary to these predictions, and the same theory with no elements added predicts spin-2 gravitons then it is reasonable to believe in spin-2 gravitons even if they have not been observed yet until the matter can be settled experimentally. But is that the situation?

    In any case I know the confidence is high in this prediction, higher than in string theory etc. and I am still wondering what gives this confidence. (It was told me my a leading light that it would be very difficult to think about gravitation not meditated by spin-2 particles, 2 and no other!) Just because this is so specific you would think there must be some accessible explanation.

    I think one difficulty in my imagining this is the EM force is by contrast to gravitation very intimately involved in very varied studied interactions, collisions, spectroscopy, chemistry, conduction,... with spin essentiall in all, while gravitation is known only as a rather featureless force, just one equation in its classical part with no phenomena you can imagine spin entering into.

    Well you have a point there, I often was tempted to call some of these 'explanations without explanation'.

    But we musn't give up too easily surely. A lot of taxpayers' money is spent on LHC and they don't even grudge it and they believe in it, but they would be even happier if they understood what it was. You seem to be pessimist and undermining the whole popularisers' profession. Let it be a challenge to scientists with a communications bent. (If I can do it myself I will let you know - don't hold your breath.)

    Some parts of ornery QM can be reasonably well explained to the layman surely? I feel I have a reasonable grasp of the Casimir and Lamb effects as polarisation of the vacuum, evanescent virtual charge pairs allowed and required by the UP... Enough even to prepare for things not previously explained. I was reading 'Hydrogen' by J.S.Rigden and read about a thing I hadn't though of, hydrogen-like atoms (1 electron 1 nucleus) highly charged like a uranium atom with all but one electron knocked off. Thinks what would I expect the Lamb effect to be? Greater. So it is. So a popular understanding has even some modest qualitative predictive power!

    But I just thought of another conclusion from that. I could not calculate it - but then to the extent that it has been done that is highly specialised. I.e. hardly anyone knows it, so you could say that almost no physicists understand the Lamb effect! That seems to be a reductio ad adsurdam suggesting that you can perhaps obtain a satisfactory qualitative understanding without all the details necessary to professionals.
    Last edited: Sep 13, 2008
  11. Sep 14, 2008 #10


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    There are general theorems in particle physics that rule out (or at least really make unnatractive) spin > 2. So you are left with spin 2, spin 3/2, spin 1, spin 1/2 and spin 0 as the only possibilities for a fundamental candidate particle that mediates the gravitational force.

    Of those possibilities, only spin 2 correctly reproduces Einsteins field equations in the classical limit hbar --> 0. Thus the graviton. You can see it a bunch of ways too, for instance by counting polarization states or just by inspection of the tensor indices.

    You might try to make it a composite particle, but that too is heavily restricted by various powerful theorems and generally viewed to be unnattractive and contrived.

    So a spin 2 gauge boson remains the simplest and probably unique candidate for a quantum mechanical gravitational force carrier. Its a rather elegant situation, since starting from a manifestly quantum theory you can derive general relativity.
  12. Sep 14, 2008 #11


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    That is nice to know Haelfix. Would you agree with Xezlec that there is no way to give insight into or make sound plausible or rationalise or otherwise communicate a sense of why these other spins are ruled out to the layman who would not be capable of inspecting tensor indices or know without being told what these polarisation states are and their significance?

    The demand for it is there. The taxpayers. If you look in the pop science bookshelves in a bookshop and expect interest to be about how lasers and computers work you are reminded about G.H.Hardy's story about prime numbers outcompeting heavy wool - you will find many more books on string theory and TOE etc. than on these utilitarian subjects.
  13. Sep 14, 2008 #12
    OK, this is good; I think we are understanding each other now. The Elegant Universe was discussing String Theory, which is an (arguably?) untested, unproven, mostly hypothetical theory. I agree that there is a huge difference between a controversial (albeit cool and quite possibly correct) theory like String Theory with (arguably?) no real experimental evidence yet, and an established, well-tested theory like quantum mechanics. I think most real physicists see String Theory either as bad physics (because it turns out to be quite hard to test) or as a new idea that is just one of several untested possibilities competing to explain the final relationship between the four forces.

    The spin-2 graviton was sort of predicted by quantum mechanics, in the sense that it's the only natural way a particle-based theory of gravity really makes sense in a quantum framework, or something. Whatever Haelfix said; he obviously knows the subject better than I do. However, the graviton has never been observed, and there are a lot of mysteries surrounding it.

    When String Theory was developed (by reasoning about things other than gravity), it just accidentally turned out to "predict" the existence of a particle that would act exactly the way quantum mechanics expects a graviton to act, including the fact that it has spin 2. That's why the theory's proponents claim that it "predicts gravity", and why they argue that this is a Great Prediction of String Theory and should be seen as evidence that the theory is correct. Of course, this is a "prediction" of something that was already suspected to be true, so it makes some people uneasy to really call that a successful prediction. The way I understand it, it's a bit like going back through some sociological data from August 2001 and predicting the September 11th attacks... even though it's already 2008 now and we already know what happened.

    Funny, not only do I agree totally with what you are saying about the LHC, taxpayers, and popular science education, but I always thought someday I could help fill that role for at least a few people. I've been able to explain some things at the level of classical electromagnetics to some people I know, and I love doing that. I'm sorry if I'm being pessimistic about the quantum stuff. I guess maybe I'm just frustrated because I'm not able to come up with good answers there, nor am I able to think of any way you could make these things easy, nor have I ever heard any good explanations in the popular media, nor do I even understand this stuff fully myself.

    Maybe you're right; maybe I should be more hopeful that someday there will be a show on the Discovery Channel that a million people will watch and then they will be practically physicists, or at least close enough to understand the experiments being conducted in particle colliders and what the results mean. But after meeting high school graduates who can't add 3-digit numbers, witnessing how people in my country react to the metric system and simple high-school physics subjects, and seeing certain recent political events (I'll just leave it at that), it's just hard for me to be that optimistic about the potential intelligence and inquisitiveness of the American general public, and possibly the rest of the world too.
  14. Sep 14, 2008 #13


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    But there is always that hypothetical entity that has been postulated by many authors 'the intelligent reader'. Possibly examples of such entities could be found among the people who buy their books?

    There may be among these people who enjoy being bamboozled and buy them for that reason. People buy books on the lost Atlantis etc. for that reason. But I think there are two populations that overlap in parts but are distinct.
  15. Sep 15, 2008 #14
    Glad to read this. Now pray PF forums allow my submitted theory in the IR section to be posted, as I think (one can never be sure, that's why I need feedback :p) to have found an intuitive way of explaining gravity. If I'm right, it's not entirely the force we think it is though. :)
  16. Sep 15, 2008 #15
    I'm not sure but maybe you misunderstand me? When I said "explaining" I meant more simply communicating the laws of known physics to a reader. I wasn't suggesting coming up with new physical laws that are easier to understand. I assume you're saying you have some alternative to GR. What I meant was (for example) explaining GR in easier words.

    No offense intended though. I wish you the best of luck getting your theory posted.
  17. Sep 15, 2008 #16
    None taken. As far as I can tell, I'm not providing an alternative to GR, but simply an alternative explanation for what "warping spacetime" really means. As such I hope it's not only compatible with GR, but able to supercede it. Still, as it is a theory, it may have flaws that I overlooked, so I don't think it's suited to talk about it in the forums were speculation is not allowed.

    I realise that this kind of "advertising" isn't really helping either, but I'm rather excited about it and couldn't help myself upon reading your reply, as it provides a possible explanation for gravitons too :p

    Anyway, even if PF allows me to submit it, you shouldn't expect too much from it, as it could still be fundamentally flawed unless properly experimentally verified. In the meantime, let us stick to what we know and I'll try not to derail the topic any further.
    Last edited: Sep 15, 2008
  18. Sep 16, 2008 #17
    Thank you Atyy! You are the only person in this thread who answered the question.
  19. Sep 16, 2008 #18
    As I understand it, gravitons are associated with gravitational waves. But what about static gravitational fields? How do gravitons describe static gravitational fields.

    Also, if gravitons are particles of spacetime, then how can there be some other spacetime in which these particles of spacetime move? Gravitons move in a background spacetime. But they are supposed to be spacetime itself, right? How can they be both the particle that moves and the spacetime in which they move - all at the same time?
  20. Sep 16, 2008 #19


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    Friend, thats a hard question, b/c it involves solving the nonperturbative sector of the *quantum* field theory, which is precisely where the problem is (both calculationally and conceptually).

    Assume that you could solve it somehow, and you got some answer. You would presumably be able to resum the variables in such a way as to recast the full metric (rather than just the slight perturbation) into a dynamical form and lo and behold you would have transformed a background dependant theory into a background independant one. You would have essentially solved the quantum backreaction problem.

    The problem is you can't do that unambigously, b/c of the fundamental lack of constraints in the quantum theory.

    Note the problem is manifest classically as well (but there it isn't a problem). I can easily rewrite GR in terms of a background and perturb around classical solutions in a linearized fashion, I don't call the perturbation a graviton, but its essentially the same thing (at least by analogy). The only difference is we know the full solution already and linearizing only helps in solving certain hard problems by numerical methods, but the philosphy is the same and isn't a problem. You have an infinite series of ripples that more or less *define* the dynamics of the full metric tensor. The choice of original background will drop out or stay arbitrary and you can think of it as merely an initial condition in that case.
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