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Is there direct evidence photons attract gravitationally?

  1. Sep 16, 2009 #1
    Not sure where better to place this thread, here or gr.

    Is there direct evidence photons attract one another gravitationally? I am aware of the arguments that GR couples to the energy-stress tensor so photons should attract one another gravitationaly but I wonder if Gravity needs an inertial mass to act as a kind of charge analogous to electric charge in E&M.
    So just as photons do not have electric charge, they don't have "gravitational" charge in the form of mass?
    Last edited: Sep 16, 2009
  2. jcsd
  3. Sep 16, 2009 #2

    But there's evidence that light and massive objects gravitate (light if deflected by stars e.g.). If photons weren't "charged", how would this occur?

    Because the gravitational interaction is not part of QED. QED is the interaction of photons
    and particles that interact via photon exchange.
  4. Sep 16, 2009 #3
    That would be a vanishingly tiny effect that is impossible to detect at this level of technology (a 100 watt lightbulb emits 10-12 g worth of photons every second ...) We do know that photons experience gravitational pull from massive objects, just like everything else.

    Photons' gravitational self interaction is an important component of the Big Bang theory, the universe would evolve differently if they weren't attracting each other. And since the Big Bang theory seems to work well, that could be evidence in favor of this assertion. One would have to redo the math and see what predictions come out differently ...
  5. Sep 16, 2009 #4
    Can anyone explain this comment??
  6. Sep 16, 2009 #5
    Really simple ... for the first 50 000 years or so after the Big Bang, radiation density exceeded matter density. Expansion during this period is modelled using GR equations, which assume that photons gravitationally interact with each other. In a sense, photons are the driving force behind this expansion (which is a curious paradox - gravity is an attractive force, but universe nevertheless expands!) If photons don't REALLY interact, GR is not applicable and our expansion timeline is off.

    And expansion timeline affects numerous other things, from CMB temperature, to relative densities of basic isotopes, to large-scale galaxy structure ...
    Last edited: Sep 16, 2009
  7. Sep 16, 2009 #6
    Which quantum aspect of the radiation is necessary in this period of the Big Bang ? I mean, why can I not treat the radiation classically ? If I can, is it fair to say that "photons" have been indirectly tested to attract each other ?
  8. Sep 17, 2009 #7

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    I should point out that there is no direct evidence that marbles attract gravitationally. However, nobody doubts they do. Why should photons be different? We know that they respond to a gravitational force, and conservation of momentum requires that this force be balanced. Isn't that good enough?
  9. Sep 17, 2009 #8
    Gravitational attraction of macroscopic objects ("marbles") was directly observed in the lab as early as 1798 by Cavendish.

    It is a good point that photons respond to a gravitational force from macroscopic objects ... HOWEVER ... we could imagine a world where photons gravitate to baryons, and baryons gravitate to other baryons, but photons don't gravitate to other photons.

    Or, in GR language, maybe we could write a complicated version of Einstein's field equation that replaces the full stress-energy tensor with the part that only contains massive fields. There's an obvious problem that such a stress-energy tensor would no longer be divergenceless .. but maybe there are ways around it.

    But that would most likely be in vain due to the Big Bang argument above.
  10. Sep 17, 2009 #9

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    No, gravitational attraction of a marble by a very large mass has been directly observed in 1798. That tells you that the marble is attracted to the large mass, not the other way around. The marble-marble force is also unmeasured.

    Of course, I don't doubt that marbles have gravitational fields - it's the OP's requirement of direct evidence that I think is unreasonably strong. Isn't conservation of momentum good enough to close the loop? It is for marbles.
  11. Sep 17, 2009 #10
    An interesting question: but as others have implied gravity seems to be a unique "force"...it effects ALL known particles and energies (kinetic, potential,heat,etc) and even pressure. Photons on the other hand seem to mediate only the electromagnetic force between charged particles while the strong and weak forces also have their unique, less wide ranging effects....

    I don't believe anything is immune from gravity...not even spacetime!!!!
    Is the Higgs boson(s) immune...I'm not sure....there are lots of different theoretical models.
  12. Sep 26, 2010 #11
    i have some questions about this, maybe you folks can educate me.

    einstein says energy carries gravity, but he makes the exception for gravity itself, that the tensor has an entry of zero for gravity's gravity. since gravity does not have gravity itself and it can carry energy away (seems pretty well proven by binary star decay) then the gravity in the universe is not a conserved quantity, with significant cosmological implications since energy is more often emitted than absorbed.

    the fact that photons are deflected by gravity doesnt mean they generate gravity. the deflection is due to the shape of spacetime, the photons are following the geodesic. we would expect gravity waves to do the same thing, because if for no other reason, there is no straight line EXCEPT the geodesic.

    further, if a photon for example has gravity, it seems like a bit of a problem getting "gravitons" in some quantum version of gravity to be "entrained" by the photons.

    as i understand it the gravity of photons is still not established. we know for sure that the nuclear force causes gravity from decay weight differences in fission and fusion. not sure about electromagnetic static fields, whether chemical energy has been proven for example to have mass.

    perhaps massless particles do not carry gravity, just massive particles and static (nongravity?) fields. we already have an example of this with gravity waves themselves. if photons dont have gravity, then when they are absorbed for example as chemical energy, one would presume they must "reinstate" the gravity.

    seems like there are a lot of loose ends here.
  13. Sep 26, 2010 #12


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    So is the overall view in science that Photons have their own gravity, or that they do not but are attracted by it?
  14. Sep 26, 2010 #13
  15. Sep 26, 2010 #14
    Suppose we send two light pulses traveling parallel to each other towards the moon and they are reflected back. It they attract each other, the distance that separates them when they are detected back here on Earth should be smaller than the distance between the two sources.

    Can anyone calculate the change in separation from an existing "photon-photon gravity" theory?
  16. Sep 26, 2010 #15


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    Also, if light did attract itself, wouldn't light from far away stars and galaxies look...different after thousands, millions, or billions of light years?
  17. Sep 26, 2010 #16
    Doesn't attraction necessitate a change in velocity, something impossible for light?
  18. Sep 26, 2010 #17


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    Light most certainly can show a change in velocity (via change in direction) - just not a change in speed.
  19. Sep 27, 2010 #18
    i have a problem with the 1931 paper that it assumes what it is trying to prove, that is, that the mass energy tensor applies to photons. and really, it doesnt deal with photons, it deals with a "beam", which to say the least is not descriptive of what is really going on. the proof would have to be for an individual photon.

    photons are deflected by gravity but this doesnt men they generate gravity. they have to "deflect" because space is curved.

    so it seems that the question of photons gfenerating a grav field is an open question, and if they do it results in a number of problematic consequences.
  20. Sep 27, 2010 #19
    In relativity it says that mass energy or pressure bends space-time .
    And if i have an electron positron annihilation and it produces photons , It seems to me that the gravitational fields of the electron and the positron would be transferred to the photons .
  21. Sep 27, 2010 #20


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    What about N3? If a photon is being pulled by a large mass, shouldn't it be pulling back [strike](even if orders of magnitude lighter?)[/strike]

    EDIT: scratch "orders of magnitude lighter"
    Last edited: Sep 27, 2010
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