The Bending of Starlight as Predicted by Einstein

Light falls in a gravitational field, for instance, a laser beam fired horizontally across a room will be closer to the ground on the side where it hits the far wall than it was when it left the laser. Likewise, a ray of light that is passing over a massive object like the sun will fall a bit toward the sun.  The net effect is that this light ray is bent.

Let us assume that the ray of light comes from  a star, what that implies in the end is that, when the sun is close to the line connecting us with the star, the star appears to be in a slightly different place than when the sun is not close to that light ray. For a light ray that just passes over the surface of the sun, the effect is about 0.875 seconds of arc.

There is also another effect, which causes the ray to bend. This is due to the effect of the curvature of space on the light ray. This effect turns out to be exactly the same size as the first effect and with the same sign. Thus, Einstein predicted a total bending angle of 1.75 seconds of arc-twice what would come just from the observation that light falls in a gravitational field.

Also Read: What is a Black Hole?

Effect of Gravity on Frequency of Light

Radar time delay was the most accurate test of Einstein’s theory. It is based on the idea that the gravitational field effects not only the path through space taken by a light ray, but that it also influences the time the light ray takes to trace out the path. The concept behind this experiment is that you send a microwave (radar) signal over to the other side of the sun and back. You can either bounce it off a planet e.g. Venus or a space probe that you have to send over there for this task. If you measure the time it takes for the signal to go over and then return, this time is always longer than it would have been in flat spacetime. Thus you can test Einstein’s theory.

Related: Cosmology, a very Short Introduction