Gravity affects light, bending it. Does light affect gravity

Interactions between physical things are generally not one-way, so I expect light to have some effect on gravity.

Yes light affects gravity. Gravity is nothing more then a effect of the curvature of spacetime.

This curve is caused by the presence of matter, any matter. But you need loads and loads of matter to cause even a small curve.

Light is a form of energy. So is mass, following E = mc^2 meaning that mass equals a massive amount of energy. It takes the mass of the entire earth to cause even a small curve. A beam of light contains nearly infinitely less energy and therefor causes barely any curve.

So yes. Light causes a curve. But it is so small you'd need a massive flash of light, probably brighter then anything in the known universe can generate to notice it.

Ralathon is correct. I was startled to learn it, myself, in a sophomore physics class at MIT, and I almost didn't learn it--the professor in my recitation section just mentioned it as an aside, and when I protested, he explained. Light is a "massless" particle, but that only means it has no "rest mass" which is trivial because it is never at rest. It turns out that not only mass but anything equivalent to mass---energy, pressure, even angular momentum---can bend spacetime. It's all part of what is called the stress-energy tensor, in general relativity.

Ralathon is also correct that the effect would be incredibly tiny.

Gravity is a force that attracts mass. Light is a wave, a wave does not have mass, only frequency, amplitude and speed, the particles it vibrates have mass which are affected by gravity, in turn bending the wave. Since gravity is strong enough to bend light then light cannot be stronger than gravity so that it could affect it back.

It's an interesting question, because light shouldn't have mass, but is affected by gravity, which should only affect things that have mass, so perhaps light does have mass, in which case it could alter a gravitational field, but the effect would be so small that it might actually be nothing.