I know a photon has zero rest mass, but it does have plenty of energy. Since energy and mass are equivalent does this mean that a photon (or more practically, a light beam) exerts a gravitational pull on other objects? If so, does it depend on the frequency of the photon?
Yes, in fact one of the comments made to all matter has a mass but does all matter have a gravitational pull? mentions this.
If you stick to Newtonian gravity it's not obvious how a photon acts as a source of gravity, but then photons are inherently relativistic so it's not surprising a non-relativistic approximation doesn't describe them well. If you use General Relativity instead you'll find that photons make a contribution to the stress energy tensor, and therefore to the curvature of space.
See http://en.wikipedia.org/wiki/Electromagnetic_stress-energy_tensor for info on the photon contribution to the stress energy tensor, though I don't think that's a terribly well written article.
You can show via conservation of energy arguments that photons confined within a volume (for the sake of argument, the inside of a sealed box with totally reflective surfaces) must produce the same gravitational effect as an amount of matter in the same volume which would have a mass equivalent to the energy of the photons.
protected by Qmechanic♦ Feb 2 at 17:11
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