Why does light bend under gravity?

We posed this question to Andrew Pontzen from the University of Cambridge...

Andrew - Regardless of the mass of the object, the acceleration caused by gravitational pull is the same for any object. Now, Newton came along and gave a mathematical explanation of this and the maths essentially is that mass appears on both sides of the equation which governs this behaviour. So it actually cancels out. But if the mass is actually zero, then it's no longer really mathematically valid to do that cancellation.

Nonetheless, it's certainly true experimentally and mathematically that as you go to smaller and smaller masses, these things are still deflected in the same way by gravity. But since this sort of mathematical paradox of trying to divide by zero, that isn't conclusive.

To get the full mathematical answer actually requires coupling a description of what we call electromagnetic waves, that's the kind of physics underlying the wave light travels, to Einstein's theory of gravity which is general relativity. Only then do we get rid of this paradox of dividing by zero and end up with a conclusive answer that shows that just as objects of any mass are affected by gravity. So light, which has no mass, is also affected by gravity.

Diana - So, what is it that relativity tells us about gravity that can help us solve the problem?

Andrew - So in the end, Einstein's description of gravity which is general relativity tells us that the effect of gravity is caused by distortions in space and time itself. Now if you do something as fundamental as distorting space and time, and reshaping it, anything that lives inside space and time will be affected. That includes waves, and so, waves can be bent and can follow different paths if you change the geometric properties of the space they live in.

Diana - Gravity can effectively bend space and time. Meaning that anything in its field is also distorted and that includes light...