Somewhere with no gravity, a windowless box accelerates upward. Open a small hole in one side wall and send light straight across, horizontally.
While the light crosses, the box moves up. The light strikes the far wall lower than the hole it came in by.
From outside the light goes straight, but to the person in the box it seems to follow a downward curve. Even for a box m wide accelerating at m/s², the light drops only m.
By the equivalence principle, an accelerating box and a box in gravity cannot be told apart. So light should bend in the same way in gravity.
Light passing near a heavy body bends toward it. Light has no mass, but the spacetime it travels through is curved, so its path bends too.
It is not pulled aside by a force; going straight through spacetime is what shows up as a curved path.
Starlight passing close to the Sun arrives bent, so the star should appear slightly shifted away from the Sun. Ordinarily the Sun's glare hides the stars near it.
In Eddington and his colleagues measured star positions during a total eclipse. Converted to the shift at the Sun's edge, they found arcseconds at Sobral in Brazil and arcseconds on the island of Príncipe off Africa.
These results, close to the prediction of general relativity, made the theory widely known. Today the bending of radio waves from distant sources confirms the prediction to about one part in ten thousand.
Treat light as a particle with mass and compute its bending with Newtonian gravity as it grazes the Sun, and you get arcseconds.
General relativity gives arcseconds, exactly twice as much. The part coming from the different rates of time and the part coming from the curvature of space contribute equal amounts.
A factor of two was large enough to tell apart by observation, and the values were closer to arcseconds than to .
When a heavy galaxy or galaxy cluster lies in front, light from a distant galaxy behind it bends around it and appears as several images. This is called gravitational lensing.
When the observer, the heavy body in front and the galaxy behind line up exactly, the image becomes a ring, a shape called an Einstein ring.
The way light bends tells how much mass lies in front. The distribution of dark matter, which gives off no light, is studied this way.