# How is light affected by gravity?

Light is clearly affected by gravity, just think about a black hole, but light supposedly has no mass and gravity only affects objects with mass.

On the other hand, if light does have mass then doesn't mass become infinitely larger the closer to the speed of light an object travels. So this would result in light have an infinite mass which is impossible.

Any explanations?

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Possible closed duplicate: physics.stackexchange.com/q/18900/2451 –  Qmechanic Aug 16 '12 at 20:32

In general relativity, gravity effects anything with energy. While light doesn't have rest-mass, it still has energy---and is thus effected by gravity.

If you think of gravity as a distortion in space-time (a la general relativity), it doesn't matter what the secondary object is, as long as it exists, gravity effects it.

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The source of gravity in general relativity is an object called the stress-energy tensor, which includes energy density, momentum density, energy flux, momentum flux (which includes shear stress and pressure) etc. Obviously, light has energy, so it acts gravitationally in GR. Since $E = mc^2$, we see that mass contributes an enormous amount of energy - so, massive objects have very strong gravitational fields, so that the other terms are negligible, which is why Newton's law works so well. However, they are there - so, light does have a gravitational field, even though it has zero mass.

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