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I know photon is an electromagnetic wave and are generally massless and therefore cannot exert a gravitational force, however I know energy can wrap the space around it so I'm confused. I think I'm mistaken somewhere so please help, thanks.

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marked as duplicate by Alfred Centauri, rob Aug 24 '17 at 2:44

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According to General Relativity, a gravitational force is created by energy (including, but not limited to mass). Therefore the answer is yes, photons gravitationally attract each other in General Relativity. The energy of the photons would bend the spacetime in such a way that the photons would get closer to each other. The fact that light deflects in a curved spacetime has been observed. However, the energy of photons is way too small to curve the spacetime in a measurable way. Therefore the fact that photons attract each other remains a theoretical conclusion of General Relativity and has not been observed in experiment.

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Fundamental massless particles (including photons) do not interact through gravity with each other but they do interact with the gravitational field of the environment through gravitational lensing. So the answer is "No". Just because they have energy, it means that that energy can be borrowed for a small period of time, according to Heisenberg's uncertainty principle, an hence converted to mass in the form of creation of fundamental massive particles which only then they can interact. So long as this creation has not happened, photons themselves won't feel each other gravitationally; they only feel each other electromagnetically.

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  • $\begingroup$ I think this is wrong. General relativity works with the stress energy tensor, and since photons have energy, it affects the curvature. $\endgroup$ – anna v Aug 24 '17 at 3:27
  • $\begingroup$ Dear anna, it's true that they contribute to the stress energy tensor but they feel the presence of each other only through electromagnetic fundamental interaction. I am sure you would agree that photons are mediators of electromagnetic interaction and not of gravity. Gravity is the distortion of the fabric of space-time and photons only follow this distortion regardless of their contribution to the distortion. So any two photons subject to this distortion will not be subject to each other. So maybe we can call this association an indirect one. Imagine a Universe only made of photons! $\endgroup$ – Allan Aug 24 '17 at 4:26
  • $\begingroup$ I think you are wrong. You assume that they are only subject to electromagnetic interactions, but in a quantized gravity unified field theory they will also exchange gravitons due to the fact that they have energy, thus affect the curvature. In the Big Bang model the universe is partially made out by the gauge particle of the unified Guts exchanged in the soup of zero mass particles. No distinction between photon gluon etc, all interactions are one at very hight energies, as far as present theory goes. $\endgroup$ – anna v Aug 24 '17 at 8:09
  • $\begingroup$ Anna is cirrect that photons are assumed to interact gravitationally in the yet to be developed theory of quantum gravity in order for this theory to be compatible with General Relativity. If light did not bend the spacetime, we would have to make an adjustment to GR. Personally I also believe that light does not bend the spacetime and that GR does need an adjustment, but my personal opinion is worth zilch on this forum, because the official consensus is that light does bend the spacetime, period. $\endgroup$ – safesphere Aug 25 '17 at 21:20
  • $\begingroup$ If there is such a thing as quantized gravity then of course the more you go back in time in the history of Universe the more possibility of finding gravity embedded in the GUT felt by ALL particles present. So, I would correct my statement by saying that at any time after 20 pico seconds after Big Bang, any two photons would not interact gravitationally but I will not rule out their memory of the FUNDAMENTAL interaction they used to have when energy was high enough. $\endgroup$ – Allan Aug 26 '17 at 2:25

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