How can light be affected by gravity, if light isn't matter, but energy?

Answer 1

Because space-time is curved. The light travels in the straightest possible line in curved space-time. We just happen to perceive the curvature of space-time as a gravitational force.

Answer 2

If you don't really want to delve too much into General Relativity, you can think of it thus:
a photon has a momentum due to it's energy (analogy with a particle with mass, p = 2Ekinetic/v). As we know, a force causes a change in momentum, the attractive gravitational force causes a change in momentum of the photon . The hard bit is getting that a massless photon actually has momentum, which does require a little bit of Relativity (sorry), which isn't immediately obvious because in everyday parlance we always take of momentum of a body with a mass. As it turns out the relationship:

E^2 - (pc)^2 = (mc^2)^2
holds for all bodies at all speeds.( E is the total energy, not merely the rest mass energy, the formula for which is commonly known, E=mc^2). This is beause a particle has energy due to its mass AND its motion (long story). As you can see, for a massless particle this becomes:

p = E/c.

I hope that is enough detail for you.

Answer 3

Light is not affected by gravity. The medium through which light travels is distorted by the presence of massive objects like stars. Light travels in straight lines through curved spacetime. This is the standard Einstein type answer. For a much more interesting answer you might want to check out this guy`s half finished theory. He has a whole new slant on things and has managed to resolve some tricky problems without resorting to strings, branes, double differentiated quadratic equations or any of the other hocus pocus that has to be invoked to prop up current models.
http://www.box.net/shared/y2fxsxmcqo

Answer 4

Einstein answered that question for us.

"Gravitation" does not merely act on matter. It has the effect of bending, or curving, space. Light near an intense gravitational source is bent not because the light contains a sort of matter that is attracted by gravity, but because the gravitational effect curves space itself. The light is following a straight line, in the light's "eye," but to us it appears that the path of the light is bent along the curvature of space.

Answer 5

Gravity is the result of mass warping the fabric of space in it's vicinity. Like a bowling ball depressing a taught rubber sheet. As light passes near the large mass, the warp of space causes it to change direction.

Answer 6

Einstein figured out that space is actually warped by massive objects in it. It is this warping of space time that affect light. Light is caused to change directions by the warp.

Answer 7

Light can be bent just like when you put a pole in a tank full of water, it seems like the pole has been shifted left or right... same thing with planets sizes and gravity.

Answer 8

http://arXiv.org/abs/gr-qc/9909014
Amer. J. Phys. 71 770 (2003)
Phys. Rev. Lett. 92 121101 (2004)
falling light

Answer 9

Gravity bends space, and slows down time.

Answer 10

Light has material inertia, not mass.