Why does light bend under gravity?

Light is supposed to be massless so how is it that gravity makes a beam bend?
24 January 2011
Presented by Diana O'Carroll


Making photons identical. A multi-photon down-conversion source in Bristol.


Light is supposed to be massless so how is it that gravity makes a beam bend? Plus, we ask what the hand wash adverts mean when they claim to "kill 99.9% of known germs."

In this episode

A simulated Black Hole of ten solar masses as seen from a distance of 600km with the Milky Way in the background (horizontal camera opening angle: 90°)

00:00 - How can light bend?

If light has no mass, and gravity is a property of mass - then how is light bent by gravity?

How can light bend?

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...


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