If photons of light have no mass, how can space be bent by gravity?

Why does light appear to curve in the universe?

Light bending around a galaxy

Here light is seen bending around a galaxy in a gravitational lens, an effect known as an Einstein Ring

We have seen from observations of light coming from behind objects of high mass, that the light is ‘lensed’ by the gravitational field of massive objects. However, light itself has no mass, so how is it affected by the gravity of these objects?

The first point to make is that while photons (little packets of light energy) do not have mass, they do have momentum, and a change in momentum yields a force, so in actual fact light is able to physically interact with matter. However, the key to this question came when Einstein developed his theory of general relativity. Photons of light are not technically affected by large gravitational fields; instead space and time become distorted around incredibly massive objects and the light simply follows this distorted curvature of space.

Answered by Sophie Allan from the National Space Centre

Image credit ESA, Hubble and NASA

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