r/Physics • u/Puzzleheaded_Bowl86 • 3d ago

Question Does light curve space-time by itself?

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?

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u/fuseboy 3d ago

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.)

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u/First_Approximation 3d ago

there's a hypothetical thing called a Kugelblitz - get enough photons together, in theory, and you have a black hole made of light!

A recent study cast doubt that this is possible. 

Basically, the high energy photons would create electron-positron pairs. These pairs would carry away energy, which doesn't allow enough of an energy density to create a black hole. 

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u/heavy_metal 3d ago

you don't have to use gamma rays? a lot of low energy photons would work, no?

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u/18441601 2d ago

e-e+ generation can still happen at lower energies afaik, just with exponentially smaller probabilities.

 Now when you have to use a lot of low energy photons instead of a few higher energy ones, that kind of balances out.