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There is a classic result by Tolman, Ehrenfest, and Podolsky that in classical GR, the interaction between pencil beams of light is zero in the case where the beams are parallel, nonzero otherwise. (The case where they're not parallel can always be reduced to a case where they're antiparallel by a change of frames.) The original paper is
Tolman, R.C., Ehrenfest, P., and Podolsky, B. Phys. Rev. (1931) 37, 602,
which I don't have access to, but there's a freely available presentation here:
Mitskievich, "Gravitational interaction for light-like motion in classical and quantum theory," http://arxiv.org/abs/1007.2589
This type of gravitational interaction of light with light was the dominant form of gravity in the early universe, which was radiation-dominated. In the lab, it's too small to measure, and the dominant mechanism for photon-photon interaction is represented by a Feynman diagram with a fermion box in it.
What do people here think of the following plausibility arguments for the vanishing of the interaction in the parallel case? If there are flaws in them, can the flaws be patched up?
1. If the interaction didn't vanish, then the two beams would oscillate back and forth across each other or twine in a double helix or something like that. That would be a clock made out of photons, which is impossible.
2. The three-force of a beam with energy E1 acting on a beam with energy E2 is proportional to E1E2 in the weak-field limit, where gravity is linear. By doing Lorentz boosts to chase the beams, you can Doppler shift them so as to make E1E2 shrink by a factor of D2, where D is the Doppler shift factor. The inertia of E2 only shrinks by D, so its acceleration shrinks by D, and the beams take longer to collide. The time to collision can be made as long as desired. Transforming back into the original frame, this becomes an even longer time. Since there is no limit on D, the time to collision must be infinite.
3. Since the system's total energy and momentum E and p are conserved, the equivalent rest-mass [itex]m=\sqrt{E^2-p^2}[/itex] of the whole system is conserved as well. Initially m=0. If the beams were deflected and became non-parallel, they would then have an equivalent rest-mass that was nonzero, violating conservation of energy-momentum.
Tolman, R.C., Ehrenfest, P., and Podolsky, B. Phys. Rev. (1931) 37, 602,
which I don't have access to, but there's a freely available presentation here:
Mitskievich, "Gravitational interaction for light-like motion in classical and quantum theory," http://arxiv.org/abs/1007.2589
This type of gravitational interaction of light with light was the dominant form of gravity in the early universe, which was radiation-dominated. In the lab, it's too small to measure, and the dominant mechanism for photon-photon interaction is represented by a Feynman diagram with a fermion box in it.
What do people here think of the following plausibility arguments for the vanishing of the interaction in the parallel case? If there are flaws in them, can the flaws be patched up?
1. If the interaction didn't vanish, then the two beams would oscillate back and forth across each other or twine in a double helix or something like that. That would be a clock made out of photons, which is impossible.
2. The three-force of a beam with energy E1 acting on a beam with energy E2 is proportional to E1E2 in the weak-field limit, where gravity is linear. By doing Lorentz boosts to chase the beams, you can Doppler shift them so as to make E1E2 shrink by a factor of D2, where D is the Doppler shift factor. The inertia of E2 only shrinks by D, so its acceleration shrinks by D, and the beams take longer to collide. The time to collision can be made as long as desired. Transforming back into the original frame, this becomes an even longer time. Since there is no limit on D, the time to collision must be infinite.
3. Since the system's total energy and momentum E and p are conserved, the equivalent rest-mass [itex]m=\sqrt{E^2-p^2}[/itex] of the whole system is conserved as well. Initially m=0. If the beams were deflected and became non-parallel, they would then have an equivalent rest-mass that was nonzero, violating conservation of energy-momentum.