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Confused about gravitons

  1. Jul 13, 2008 #1
    According to particle physics, there is a certain particle called a graviton which is the messenger particle of gravity but it so happens that Einstein's theory of general relativity says that mass affects the space time fabric which is responsible for gravitational force. So it is like the apple on tissue paper analogy. Place an apple on a tissue paper and the tissue paper will bend downwards.

    The thing that I do not understand is that if general relativity says that if mass affects space time (gravity), then why do scientists keep searching for the graviton particle? Surely a graviton particle would somehow violate general relativity?

    Secondly, since gravity affects space and time, it seems intriguing that graviton particles can affect time? I know gravitons have something to do with string theory and trying to unify general relativity and quantum mechanics but still it seems strange.
  2. jcsd
  3. Jul 13, 2008 #2
    oh wait, its alrite, dont answer this i got the answer somewhere else haha.
  4. Jul 13, 2008 #3
    Gravitons? The Higg's Boson?
    We now have a quite workable (quantum) model to describe the way things are. It is a conceptual fairy-story, a mixture of mathematical rules around little characters, yet put together to provide predictive results of astonishing accuracy - except for gravity! We could build an alternative model with new characters 1/3 the size of the present ones, but it would only be saying the same thing in a different way.

    Despite its feebleness compared to the other forces, gravity does something the others do not. It reaches across galaxies! So if there must be little gravitons, there needs to be enough that each particle in the universe can interact with every other. This does strain credulity somewhat.

    Thus for me, noting the astronaut in orbit who "feels" weightless, it is not too hard to regard his situation as Newton's "in a state of uniform motion". I just left out the bit that says"in a straight line". Simply because we cannot tell any difference between the effects "an acceleration from being shoved along" from "acceleration due to gravity", we should maybe suspect that the space that astronaut was in really was "curved". After all, if the Earth were to suddenly disappear, the astronaut would shortly head off in a (nearly) straight line. Within a curved space, lines that curve exactly to match will be perceived "straight", just like your local dragstrip.

    Very qualitative I admit, but I am not so learned as others in this forum. :confused:
  5. Jul 14, 2008 #4
    You lost me there. Light reaches across galaxies too! And light is possible due to the EM force, another one of the four fundamentals. Likewise, gravity and EM follow the same inverse square law.

    Also, why does the idea of multiple gravitons for every massive particle bother you? Just think how many GLUONS there are out there - gigantic numbers for every quark in every nucleus in every atom.

    Plus, who's to say every particle gets one graviton? Maybe it takes more than one particle to make a graviton? Who knows?

    Anyway, he idea of gravitons is not conceptually difficult at all IMO. The problem is not how they work at _large_ distances, but how they work at _small_ distances. When you try to quantize gravity using QFT, and thereby come up with a workable model for quantum gravity, you get absurd results that don't jive with experiments, to say the least.
    Last edited: Jul 14, 2008
  6. Jul 15, 2008 #5
    Hi Peter

    OK - a piece at a time..
    Its in the unnecessary size of the numbers involved. For me, it blights the concept! For each particle that might have a gravitational effect on the rest, there has to be a whole universe worth of gravitons. Mathematically, they cannot be shared, if only because I can find one situation where I can keep splitting an affected mass down to its atoms. Going this way, for me, feels nearly equivalent to when the ancients held that the entire celestial sphere moved around once a year.

    That makes the situation worse! It leaves not enough to go around. If I can find just one situation that is physically realizable, that has gravitational effects between more than one separated bodies, then I can go on to point to all other bodies and, even to their constituent atoms.

    This won't do either! We have quite compelling evidence that gravity effects do work over large (galactic?) distances as well as small. If we are to have any theory at all that addresses it, it cannot be qualified to be truthful only for small distances.However much we are intrigued by speed of light observations, we should not too readily start postulating light-like properties to the gravity phenomenon. Gravity waves?

    I am not at all sure how one would "quantize gravity", or even if it deserves it.
    Last edited: Jul 15, 2008
  7. Jul 15, 2008 #6
    Ah, but you really didn't address my most important point, which was almost all that you just said is equally true of light. Do you have a problem believing in photons as well?
  8. Jul 15, 2008 #7
    Please define the graviton field for me
    Sure. Both QCD and a putative quantum theory of gravity are nonabelian, right ? The nonabelianity of QCD makes it asymptotically free and confined at the same time. Where are the nonabelian effects in gravity ? If they are at short distance, can you explain me how to reconcile with the fact that nonabelian effects act physically at large distance in QCD ?
  9. Jul 15, 2008 #8
    Re: Straying off-topic

    Would a photon with a frequency of 0Hz interact with matter like gravity?


  10. Jul 15, 2008 #9
    Re: Straying off-topic

    There is no such thing ! This is the limit where energy goes to zero, so this is also a limit where there is no coupling with the graviton.
  11. Jul 15, 2008 #10
    The only thing I can appreciate on light having properties in common with gravity is that both get to visit the farthest reaches of observable universe.

    I can make a pulse of light, as a spacetime event, (various methods), and I can measure its speed. Harder it is to suddenly create a mass presence pulse, but even if I could, I have no good reason to think that the effects would spread at the speed of light. I know Mr. Newton thought that gravitational effects, such as changes when masses split in collision, would propagate instantly through the universe. I am even less inclined to accommodate that notion.

    Gravity is a property of mass. We can destroy mass, and yield energy like light and its other wavelength friends. The light, it seems, can travel a bent path caused by a large mass. I am told the space was curved, and the light did continue in what was, for it, a straight path. For me, gravity is not light. It is not like light. We know so little about what it really is, we can only obliquely approach it by describing its effects with equations. Moreover, however successful is the Quantum Theory in providing us with a predictive model for most material behaviour (usually under high energy bombardment), it leaves out gravity.

    Earlier I described the quantum model as "a conceptual fairy-story, a mixture of mathematical rules around little characters, yet put together to provide predictive results of astonishing accuracy - except for gravity"!
    However much more I learn of it in the detail, and even, the more I appreciate the teachings of Mr. Feynman, it remains for me just that... a very useful predictive model just crying out for a better story to be put around it - one that includes gravity!
  12. Jul 15, 2008 #11


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    Well, in general relativity gravitational waves do move at the speed of light, and aside from a few weird spacetimes involving causal loops, I'm pretty sure all the realistic GR spacetimes do have a light cone structure where events can only be influenced by other events in their past light cone. We could also say that GR's picture of mass and energy curving spacetime is "a conceptual fairy-story, a mixture of mathematical rules around little characters, yet put together to provide predictive results of astonishing accuracy"--and the idea that gravitational effects propogate at the speed of light is one of the theory's predictions, so it seems likely to be true even if the conceptual model is revised by a theory of quantum gravity.

    Also, instead of thinking about a pulse of light, how about thinking the continuous electromagnetic force created by a charged object? In quantum electrodynamics, even in situations where there are no real photons (associated with electromagnetic waves created by accelerating charges), the force between charges is modeled in terms of "virtual photons" (which only appear in calculations and are never measured directly, so they are another 'fairy-story' which makes useful predictions). So in principle the situation could be similar with gravitation, where real gravitons would only be associated with gravitational waves caused by certain types of acceleration, while in other situations the gravitational force could be modeled in terms of virtual gravitons. Of course it's possible a theory of quantum gravity won't work like this, but I don't see why the idea is inherently more crazy than the electromagnetic analogue.
  13. Jul 15, 2008 #12
    Another thought I have had ...

    Consider small things falling about the Earth in orbit. There are a whole bunch of them all parked, giving Earth a unique equatorial ring. Big masses and little masses, it does not matter. The forces work out such that an astronaut leaving the space shuttle going for a spacewalk does not suddenly part company with the spacecraft and adopt a new orbit. It would seem he and his vehicle are in a state of uniform motion, in a space that is so curved the path is a circle. But the beam of photons from his flashlight does not agreeably follow the curved space. We know from viewing past solar eclipses that such beams are so affected.

    So now - a light beam is tugged by a mass (or its space is curved!) Is the effect mutual, like it is between masses? Would two light beams pull toward each other? if so, then gravity is not at all something that can be conjured as another photon-like thing. We would have photons emitting gravitons, and both get along at the same speed. This whole notion is only temporarily convenient, and however much I am prepared to accommodate mathematically useful models, I feel we do not have to pander to the grossly bizarre.
  14. Jul 15, 2008 #13
    Yes, Jesse that was pretty much my point, said much more artfully. :)

    Antenna Guy, what do you mean regarding a 0 Hz photon? Obviously there is no such thing. Are you suggesting there might be such a thing as a 0 Hz graviton?
  15. Jul 15, 2008 #14


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    Sure, in general relativity all forms of energy contribute to the curvature of spacetime, not just rest mass energy. And since light produces its own gravity in GR, this is probably going to be true in any theory of quantum gravity as well (whether or not the theory involves gravitons).
    Why do you think this means "gravity is not at all something that can be conjured as another photon-like thing"? Gluons are the carriers of the strong force just like photons are the carriers of the electromagnetic force, and gluons are also massless and move at the speed of light; but as mentioned here, gluons can emit other gluons (at least in virtual particle diagrams), so there's nothing unprecedented in quantum field theory about particles which move at c emitting other particles which move at c.
  16. Jul 15, 2008 #15
    @ peter : You have nicely ignored my request for a definition, despite your bold statement :
    So I'll add another question for you
    You accept the idea of graviton but admit that you have no clue what quantum gravity is. Do you understand that the graviton is needed only in quantum gravity ?
  17. Jul 15, 2008 #16
  18. Jul 15, 2008 #17
    Re: Straying off-topic

    What is the limit of red-shift? I thought it was 0Hz, but I may be wrong.


  19. Jul 15, 2008 #18
    Re: Straying off-topic

    Red shift is what occurs when the frequency if shifted towards lower values, wavelength towards larger values, pushed near the infrared if you will. In the mathematical limit of 0 Hz as I told you I agree, yes, there is no coupling to the graviton, because there is no energy anymore and truth is : there is no photon anymore.
  20. Jul 15, 2008 #19
    Re: Straying off-topic

    Where did it go?


  21. Jul 15, 2008 #20
    Re: Straying off-topic

    You want to take the limit, you tell me where all the energy went !!? I don't understand your confusion.

    Look, tell me what you think about the following imaginary conversation :
    Q: Would an apple with 0 volume bend the Roberval Balance ?
    A: No, it would not, since the mass would go to zero in this mathematical limit
    Q: But isn't it the massless limit ?
    A: Yes indeed, that's why it does not bend the Roberval Balance, and frankly there is no apple in this mathematical limit
    Q: Where did it go !!?
    A: You tell me !!!

    Sorry, I can't really help much better...

    The energy of the photon is related to its frequency via [tex]E=h\nu[/tex]
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