Why does gravity bend time?

Answer 1

A quick way out would be to tell you that it does so because Einstein's theory of general relativity says it does and that we can check that it does and that the theory makes correct predictions, however a more deep explanation might be as follows:

It seems to be the case that the laws of physics are the same in every reference frame; this means that you can have your laboratory on the Moon, in deep space, accelerating or even rotating and all your experiments should leave you to deduce the same laws of reality. This is called coordinate invariance.

Now if you want to have this coordinate invariance, gravity MUST bend time. It is a direct consequence although a bit mathematical to show (but it can be done).

I might make it a little more clear by using special relativity instead, which basically is the consequence of coordinate invariance except that it does not allow for accelerations (therefore the general theory of relativity is the more general theory). We shall call this inertial frame invariance (an inertial frame is a frame that does not accelerate, a frame is basically a coordinate system).

Now, one law of physics is the constancy of the speed of light. According to inertial frame invariance one MUST arrive at this law always, regardless of which inertial frame you happen to be located in. But this causes problems, because this would mean that someone shooting a laser on a train must measure c (the speed of light) but someone standing still on the station must also measure c even though the train itself is moving! The only way this can be true is if the train shrinks in the direction of motion and time slows down for the person on the train. These are the time dilatation and length contraction predictions of special relativity. These effects occur because the laws of physics are the same in every inertial frame. It must be true or the speed of light would not be constant for everybody.

In the same way gravity must bend time. In fact you can even show that gravity itself is a consequence of coordinate invariance. If you want coordinate invariance you must have gravity.

Answer 2

Gravity bends time because of the way it warps spacetime according to Einstein's theory of general relativity. Massive objects like stars and planets create a gravitational field that curves spacetime, causing time to pass more slowly in stronger gravitational fields. This effect, known as gravitational time dilation, has been observed and confirmed through experiments and astronomical observations.