Do photons interact with spacetime?

[platosuniverse]

The way I understand this is that Relativity says space-time is like a field that's affected by the way mass moves through it. Photons are massless so is this why the speed of light is the same in all reference frames?

 

[mfb]

Spacetime is affected by everything with energy, this includes photons.

Photons are massless so is this why the speed of light is the same in all reference frames?

This is completely unrelated to what you wrote before.
The laws of physics are the same in all reference frames. The same speed for massless particles it the only option, everything else would make different reference frames different.

 

[Nugatory]

You have to be careful about mentioning relativity and photons in the same breath. There aren't any photons in relativity; it's a classical theory motivated by the behavior of light considered as classical electromagnetic waves. Thus, this thread might be off to a better start if the title were "Does light interact with spacetime?"

You also must be careful when you say that spacetime "is like" anything. Those words "is like" imply that you're making an analogy, and an analogy isn't the real thing. An analogy may help you form a mental picture, but conclusions drawn from the analogy are suspect.

But with that said... the curvature of spacetime is determined by the stress-energy tensor. Electrical and magnetic fields do contribute to the stress-energy tensor, so in that sense light does interact with spacetime.

 

[Ibix]

In addition to what's been said above, it's a fundamental property of spacetime that there is an invariant speed. It turns out to follow fairly simply from that fact that "massless" and "travels at the invariant speed" are the same statement in different words. The first massless thing we discovered (before anyone understood my last sentence) was light, so the invariant speed is often called the speed of light. Arguably, that is slightly conceptually misleading.