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How is gravity universal?

  1. Jun 10, 2012 #1
    If I understand correctly (Which I probably don't.) gravity is caused by hypothetical gravitons moving between two masses, but if this understanding is correct, these particles have got to travelling well past the speed of light, and at a extremely high speed but how can they?

    I don't understand. (By the way I'm 14 so I probably wouldn't.)
     
  2. jcsd
  3. Jun 10, 2012 #2

    Dale

    Staff: Mentor

    We don't currently have a successful theory of quantum gravity, so I wouldn't worry too much about gravitons. It is a concern not just about a hypothetical particle, but about a hypothetical particle predicted by a hypothetical theory.
     
  4. Jun 10, 2012 #3
    But if this kind of quantum gravity is confirmed, will we have to re-write alot of physics?
     
  5. Jun 10, 2012 #4

    Dale

    Staff: Mentor

    No, we shouldn't have to. That is the whole goal, to make a theory of gravity which doesn't require any re-writing of our other successful theories.
     
  6. Jun 10, 2012 #5
    To the best of my understanding, if the hypothetical gravitons did exist then they would not travel faster than the speed of light. The reason gravity "seems" instantaneous is because the magnitude of it's sources is unchanging. If the source of a gravitational field did change then it's wave front would not propagate faster than c. This has not been experimentally proven, but it is generally accepted in main stream physics.
     
  7. Jun 10, 2012 #6

    Dale

    Staff: Mentor

    It is impossible to know at this point whether, in fact, a hypothetical theory of quantum gravity will have FTL gravitons and if so whether they will be in any way in conflict with other theories. Your question simply assumes the behavior of an unknown particle in an unknown theory.

    Btw, virtual photons also travel FTL in QED without conflicting with other theories.
     
  8. Jun 10, 2012 #7
    No gravitons do not have to be travelling at any faster than light speed.

    I read somewhere that a typical electron might not yet have exchanged a graviton as the age of the universe is not old enough!!! In other words, the gravitational force is a lot weaker than other forces and in fact so far appears to be fundamentally different from them. So electrons, for example, do not exchange trillions of gravitons every nanosecond!!

    The kind of quantum gravity Dalespam refers to would most likely apply at a few points of extreme gravitational curvature, very strong gravity, like the center of a black hole. Its points of extreme gravity like that where neither general relativity nor our current attempts at quantum gravity give sensible answers. So far general relativity works just fine onlarge scales and moderate gravity, like the universe and solar systems, and gravity is so weak among particles, like electrons and protons, that the Standard Model of particle physics gives us most of the behaviors we observe.

    BUT: I know scientists have sent up some experiments aboard the US space shuttle to see how things behave in even lower gravity. Maybe they were growing crystals?/ I forget. Anyway, with a superior theory of small scale gravity, we will likely learn things, perhaps things we haven't even thought about yet.
     
  9. Jun 10, 2012 #8

    Nabeshin

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    Science Advisor

    I disagree with this. We know that a quantum theory of gravity needs to reduce to GR in the appropriate limit, so I think there are some definite things we can say about gravitons from their correspondence with gravitational waves. In particular, they are massless, and hence travel at c, spin 2 particles.
     
  10. Jun 10, 2012 #9

    Dale

    Staff: Mentor

    Yes, but the hypothetical theory could hypothetically also have hypothetical virtual gravitons which hypothetically could be off shell.
     
  11. Jun 10, 2012 #10
    Is there not a theory, within M-Theory, that our three dimensional universe is in fact a 3D brane, and that the strings can either be open or closed - Based on their features. All messenger particles, are open strings and are thus, attached explicitly to the branes three dimensions of space. However, the graviton would have the components of a closed string, and be able to interact beyond just the three dimensions, possibly explaining why it is such a weak force within the confides of our three boundaries of spatial awareness.
     
  12. Jun 10, 2012 #11
    Yes, but that's all just empty speculation unless there's any evidence for or against this theory.
     
  13. Jun 10, 2012 #12

    Nabeshin

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    I'm sure it does and I'm sure they are. It would be a rather strange theory if it did not!

    But the situation is analogous to the following: Suppose we understood classical E&M, maxwell's equations, perfectly but didn't have QED yet. Nevertheless, there would be some things we could say about the photon from what we know about classical E&M.
     
  14. Jun 10, 2012 #13

    K^2

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    That's not entirely correct. We have a field theory for gravity which has local gauge invariances. So we do have a gauge field. It's not generally quantizable, but it is quantizable at low excitation levels. (Because ANY field can be approximated as linear at low energies, and any linear field quantizes.) So we can talk about gravitons in certain contexts, and we can talk about some of their properties.

    It doesn't really matter if this falls apart at some energy levels. There is no guarantee that standard model doesn't either. We don't know if quantum mechanics universally holds, or if we simply never dealt with energy levels at which non-linearities in the field manifest. In fact, my money would be on the later, because there doesn't seem to be any good reason for particle fields to be fundamentally linear.
     
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