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If they are real, what exactly do gravitons do?

  1. Apr 19, 2009 #1
    If they are real, what exactly do gravitons do? I mean are they absorbed by other particles resulting in an attraction like photons and warp space-time in the process or do they just warp space-time which causes the attraction? If it's the latter how do the gravitons get "used up?"
  2. jcsd
  3. Apr 21, 2009 #2
    Re: gravitons

    You are essentially asking what is gravity....and how does it work...very serious questions.

    A graviton is the particle version of the continuous gravitational field...Gravitons are the quantum theoretical quantization of the classical gravitational field just as photons are to the electomagnetic field...emergence and absorption of gravitons is even more mysterious than the analogous process for photons....one particle carries EM and the other gravitational energy....at least with photons we can relate them to discrete electon energy levels.

    A difficulty arises in that quantum and classical descriptions diverge at singularities such as big bangs and black holes, so there appears to be a problem with each theory....

    Grasvitons can also be explained according to string theories...which was accidentally discovered after string theory was developed for other reasons....

    Some decent discussion but no absolutely firm answers at wikipedia:

    Ask "why" about five or six times with any theory of matter, energy, time, space,etc...and you usually end up at the level of "unknown, uncertain, or experimentally unverified" or close to it....

    Many of us can't even figure out which section to post such questions....I suspect this one may be moved!!!
  4. Apr 21, 2009 #3
    Re: gravitons

    This is the biggest problem with inventing a graviton. When one looks at relativity, one realizes that they know what is happening in nature. Space-time is being warped. With Quantum Field theory, they haven't got a clue as to what is really happening in nature. They just like the concept of fields, with particles being excitations of that field. How is a graviton responsible for attracting particles? There is some really fancy math, but no concrete explanation. There are no pictures, or diagrams. They have no idea. The only reason they invented the graviton was because everything else is regarded as a particle, so why not a particle for gravity?
  5. Apr 21, 2009 #4
    Re: gravitons

    So you gave it a fancy name and have sexy picture of what's going on, but that's not how science makes progress. Interestingly enough, Feynman wrote a lecture on (classical) gravitation entirely based on the (quantum) field theory picture.

    Also, there are plenty of sexy pictures and fancy names to "simplify" the mathematics of quantum field theory. Maybe as many of them as there are physicist and mathematicians actually. For the moment, either none of them is correct, or all of them are correct, depending on whether you are pessimistic or optimistic. But in any case, our understanding of Nature progress very little with those considerations.
  6. Apr 21, 2009 #5
    Re: gravitons

    all quantum field theories of gravity are mathematically equivalent and are supersymmetric and i've read that there is a gravitino of each theory but doesn't that fundamentally oppose supersymmetry?
    Last edited: Apr 21, 2009
  7. May 8, 2009 #6
    Re: gravitons

    Indeed, Feynman showed in his lectures on gravitation that one can for example,recover the number for the advance of perihelion of mercury by using field theory using gravitons rather than the usual general relativity. It is equivalent to use the general relativity tensor or treat the problem with gravitons and self interactions of gravitons.
  8. May 8, 2009 #7
    Re: gravitons

    It is equivalent mathematically. :smile:The concept of a particle compared to a tensor and/or space-time warping is very different.
  9. May 9, 2009 #8


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    Re: gravitons

    You are limiting the question to something like supergravity or theories derived from string theory. In supergravity the graviton always comes with a spin 3/2 partner, the gravitino.

    The SUGRAs are not all equivalent. Some of them are rich enough to embedd matter and gauge symmetries. Others are not. Some might be fully renormalizable or even finite, others not. Some can be derived from string theory, others not.

    And there are approaches to quantum gravity where the graviton is not the fundamental quantum object (LQG, CDT). In LQG the graviton is something like a derived concept that exists only in certain "semiclassical" or "IR" regimes.
  10. Jun 7, 2009 #9


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    Re: gravitons

    Whether or not gravitons actually get used up is an interesting question. Take for example the Sun and the Earth. Is the Earth using up some of the gravitons that might normally make their way to the planets beyond? If one were to make the Earth large enough to absorb a significant number of gravitons from the sun, there would be no way to know as the Earth would then be emitting it's own gravitons to make up for the "absorbed" ones from the Sun, and if the Earth were made small enough so it's own gravity did not contribute, it wouldn't be large enough to significantly affect the total quantity of gravitons from the Sun.
  11. Jun 7, 2009 #10
    Re: gravitons

    Consider Feynman's plate trick.

    To get a 1/2 Spin particle to it's original state (put your TV remote on your palm, with your hand in front of you, and your elbow pointing down) you have to rotate it twice.

    Keeping the buttons on the remote aimed upwards, turn it to the right and let your elbow rotate out and upwards.

    Note when you have the remote facing forwards again, your elbow/arm will be in the opposite state from when you started.

    Put the back of the hand holding the remote into the palm of your other hand, with that elbow oriented downwards.

    Fits nicely huh, they can share that state because the elbow state is different.

    Now continue rotating the remote to the right, under your elbow, and bring your elbow back down to the starting position.

    2 Rotations = back to normal.

    Can you put the back of the remote hand in the palm of the other hand with both elbows like that?

    They can't share that state, neat, but moving on.

    How could you rotate something once to reach the starting position?

    Grasp the remote, rotate it to the right and flip the buttons so they point down after half a rotation, and then bring it back around forwards again. (NOTE: this is not exact, just helping to consider these concepts)

    How could you rotate something halfway and have it arrive at the starting state?

    Is there anything you can think of that fits that description?

    Does a direction work? the x axis rotated 180 degrees... is the x axis.

    Just something to think about, Gravitons return to the same state when rotated 180 degrees. Spin 2.
  12. Jun 7, 2009 #11
    Re: gravitons

    This is exactly why gravitons don't make any sense, or haven't been found in nature. They don't predict the right things. Just because a satellite is positioned behind earth doesn't mean it is not affected by the sun's gravity.
  13. Jun 7, 2009 #12
    Re: gravitons

    I also just thought of this. Let's say, from a fantasy point of view, that we find a graviton. What does it do? If it does warp space, then the laws of Relativity are intact. There is still the correct prediction of time dilation, and everything else. If it doesn't bend space, then there are some big shoes to fill, because a graviton would have to somehow be responsible for time dilation and all kinds of other predicted phenomenon.
  14. Jun 7, 2009 #13
    Re: gravitons

    What about photons ?
  15. Jun 7, 2009 #14
    Re: gravitons

    What specifically is your question? What about photons?
  16. Jun 9, 2009 #15
    Re: gravitons

    The very fact that time slows as we approach any particle with mass should imply that time and gravity are very closely related. If we could explain the mechanics of how time slows it might also explain gravity.
  17. Jun 9, 2009 #16
    Re: gravitons

    The satellite will receive a diminished number of photons (there will be some light) since most are blocked by the earth...but some also are dragged by gravity and bend aound the earth as they pass on their way from the sun...and if the altitude above the earth increases and the satellite is further from the earth, more photons are received as they are closer to a straight line from the sun.....
  18. Jun 9, 2009 #17
    Re: gravitons

    but some also are dragged by gravity and bend aound the earth...

    This is key. This is why photons work. They match predictions and observations. But the only way light can hit the satellite is if we think of gravity.

    Now, try that same experiment with gravitons. How do they bend around earth? You have gotten rid of that clever mechanism by essentially burning Einstein's brilliant papers. Photons can still bend around earth, because they are now interacting with gravitons, but gravitons can't do that now (unless they somehow interact with themselves).

    Anyway, Even if light bends around the earth, most of the light is blocked. Is this the case for gravitons too? Does the gravity of the sun weaken as we pass behind other objects? I thought is was merely reinforced by the earth's gravity. Look at your diagrams with the sun and the earth, even the simplest matress-sheet ones. The earth is already in a slight decline thanks to the sun, but that same decline is not countered by the earth merely sitting there.
    Last edited: Jun 9, 2009
  19. Jun 9, 2009 #18
    Re: gravitons

    General Relativity predicts Gravitational Time Delay. This follows from the parameters of the Lorentz factor. We already know the mechanics of how time slows, and how gravity works (unless we look at black holes).
  20. Jun 9, 2009 #19
    Re: gravitons

    Here's another fun thing to think about: gravitons and black holes.

    If light cannot escape the event horizon of a black hole, then what properties would a graviton have to possess in order to escape and exert gravitational influence on outside objects?
  21. Jun 9, 2009 #20


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    Re: gravitons

    It may seem strange, but gravitons do not escape from black holes, either. The gravitational field = "static gravitons" outside of a black hole is the relict when matter was still present outside the black hole horizon. The spherical collaps of a star into a black hole does not affect its gravitational field. No gravitons are emitted; everything is just static. THe field before and after the collaps is identical.
  22. Jun 9, 2009 #21

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    Re: gravitons

    Which has nothing to do with the predictions of gravitons.

    By analogy with photons, just because a satellite is positioned behind earth doesn't mean it wouldn't affected by the sun's electric field if it were charged.
  23. Jun 9, 2009 #22
    Re: gravitons

    That's pretty much my point. It contradicts the notion that gravitons are "radiating" like photons, which is what most of the discussion so far describes.

    If gravitons exist at all, then each static gravitational field is maybe "a" graviton. They definitely don't shoot out of things and bounce around like photons. It just doesn't make any sense to think of them like that.
  24. Jun 9, 2009 #23


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    Re: gravitons

    Hi OB50,

    you should compare this to QCD, not to QED! In QCD with self-interacting gluons there is an exact mathematical construction of gluonic degrees of freedom. You can see that the gluon field is split into different quantum mechanical degrees of freedom, some of them describing something like "plane wave states", others decribing "static field configurations". In QCD you have nothing like a static electric field!!! Even the quark-antiquark potential is not a Coulomb field but it contains dynamical gluons.

    w/o too many formulas the situation is the following:
    In QED you have a differential operator D(x), something like a Laplacian; the inverse operator 1/D ~V(x-y) is something like the coulomb potential (as the kernel of an integral equation);
    In QCD (and this is much closer to QG than QED) the differential operator D[A] depends explicitly on the gauge field A; so does the inverse 1/D[A]. If you write A as an integral over plane wave states ("gluons") you have 1/D[A] as an integral operator with a kernel depending on plane wave gluon states. Therefore a "static" field in QCD is not "a gluon" but a very complex state consisting of different gluonic degrees of freedom. I expect that something similar will happen in QG.

    The question is therefore what you call a graviton? In my opinion it's simply wrong to talk about plane weaves only. They are not sufficient to describe QCD - and I am pretty sure they will not be able to describe non-perturbative aspects of QG. The approach even fails for perturbative QG due to the non-renormalizibility of the theory.

    So the whole discussion only makes sense if we agree on the following
    1) we must have a common understanding what gravitons are!
    2) if the majority wants to talk about gravitons as plane wave states only, then we must stop discussing non-perturbative aspects like gravitational fields of stars, black holes etc.
    3) unfortunately we have to accept that up to now nobody has a clear and mathematical sound understanding of the nature of QG
  25. Jun 9, 2009 #24
    Re: gravitons

    Which doesn't have anything to do with gravitons in gravity.

    The electric field depends upon the mediation of photons. They can 'bend' around the earth due to gravity. But you can't use gravity to explain gravity.

    Space-time is warped around the sun. This is analogous to an indentation in a bed sheet. Roll the earth around the sun. It causes a slight warp. The sun adds to the indentation in all directions around the earth, because the earth is already traveling in warped space-time.


    This is an example. From what I understand, the gravitons would strike the earth head on, but then what? How do they get around to the other side of our planet? If you set a basketball on the ground, and then shine a light on it, it casts a shadow. I think this same conclusion would be drawn from gravitons striking the earth. If the gravitons don't get around to the backside, then how does this match observations and predictions? It wouldn't. If they do somehow get around to the other side of our planet, I'm curious as to how.
  26. Jun 13, 2009 #25


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    Re: gravitons

    We know the precise relationship between gravitation and time but I don't believe we actually know the mechanics. By this I mean what is it about gravity that would cause the various physical effects we observe when measuring time change such as slowed orbits of planets or slowed decay of unstable subatomic particles. We know that time slows in the presence of gravity, but what is the mechanism?

    I would venture to say the answer must be broad and encompass a large number of phenomenon. What I mean is that it doesn't make much sense to say a graviton interacts with a photon which results in a "drag" slowing the photon down, as this could not also explain slowed decay of subatomic particles and so on. A larger paradigm would be needed to cover all events that describe the slowing of time.

    Or better still, a better explaination of "time dialation" might be needed. If gravity can cause time dialation and velocity can cause time dialation then what do these two things have in common that can cause time dialation.

    These questions may be more philosophical than anything, but interesting to ponder non-the-less
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