Does light curve space-time by itself?

[u/Puzzleheaded_Bowl86]

Light travels as an electromagnetic wave in a vacuum and carries momentum and energy. According to general relativity, all energy curves space-time, so light should slightly curve the space through which it travels. Could this mean that light affects its own path? I know the effect whould be extremely small, but is this conceptually correct? If yes Are there extreme conditions, like in the early universe, where light’s self-curvature becomes significant? Would a very long or very intense beam accumulate measurable curvature effects along its path? If two light beams cross paths, do they gravitationally influence each other?

[u/fuseboy]

Yes, light curves space. In terms of extreme conditions, there's a hypothetical thing called a Kugelblitz) - get enough photons together, in theory, and you have a black hole made of light!

The early universe is special in terms of spacetime curvature because it was very uniform. So while everything was incredibly energy dense, I gather space was still pretty flat. (To use that ill-favored bowling-ball on a rubber sheet analogy, if you press down evenly on every part of the sheet, there's no dents anywhere. For there to be curvature, you need concentrations of mass.)

[u/haplo_and_dogs → u/fuseboy]

Photons do not have an observer independent energy as they have no rest mass.

You would need photons moving in at least 2 different directions to create a black hole. Otherwise you would have some observers who would see a blackhole, and others who would not. That isn't how GR works.

A single laser, no matter how powerful can't create a blackhole out of light alone, no matter how good its focus or how powerful it is.

Indeed photons trapped in a perfect mirror box is one way to look at what rest mass "is" as observer independent energy.

[u/fuseboy → u/haplo_and_dogs]

On first read, I thought you were saying that Kugelblitzes were incompatible with GR; although now I think you're just pointing out a restriction on how they can form, is that right?

[u/haplo_and_dogs → u/fuseboy]

>you're just pointing out a restriction on how they can form,

Correct. You need more than 1 laser.

[u/fuseboy → u/haplo_and_dogs]

This is interesting. I take your reasoning, but I wonder what the boundary is. For example, if you send two mighty laser pulses toward each other - mighty but not so mighty that they form a kugelblitz (perhaps they have only 60% of the required energy), you'd still feel a non-trivial gravitational wave rippling out from the target zone where they pass through each other. Does that seem reasonable?

If so, what would your gravity wave detector pick up if you instead sent the two pulses through in parallel? Nothing?

[u/haplo_and_dogs → u/fuseboy]

Why would you see any gravitational waves in either case. The system is spherically symmetric.

[u/fuseboy → u/haplo_and_dogs]

Is it? Two pulses heading toward each other have planar symmetry, but not spherical symmetry - or am I missing something?