From the September 2019 issue

How does gravity affect photons (that is, bend light) if photons have no mass?

Robert Arrington Butner, North Carolina
By | Published: September 12, 2019 | Last updated on May 18, 2023
All objects with mass warp space-time around them; the more massive an object, the more pronounced the warping it causes. When photons travel through the region near a massive object that has caused significant warping, they follow curved paths because the space-time through which they are moving is curved.
Astronomy: Roen Kelly
While it is true that photons have no mass, it is also true that we see light bend around sources with high mass due to gravity. This is not because the mass pulls on the photons directly, but instead because the mass warps the space-time through which the photons travel. 

Imagine a bowling ball on a mattress. The ball is a massive object — say, the Sun — and the mattress represents space-time, in which it sits. (Of course, space-time is four-dimensional, but it’s a bit harder to imagine that!) When you place the bowling ball on the mattress, it deforms the surface. If a grid were drawn on the mattress, you would see the grid deform, so the straight lines of the boxes were no longer straight. The same is true for a star sitting in space-time — the star deforms space-time around it, causing it to curve toward the star. 

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Now imagine a marble; this represents a photon. If you roll the marble in a straight line on the mattress and it comes too close to the bowling ball, the marble will curve because the mattress it’s traveling on dips and curves around the bowling ball. This is what happens to light traveling through space: When it comes too close to a massive object, it encounters warped space-time and curves not because it’s being pulled by gravity, but because the space-time it’s traveling through is curved, so its “straight” path becomes a curved, bent one.

Alison Klesman
Associate Editor