system
Given the following post, respond as if you're commenting on Reddit:
human
How does gravity slow time?
gpt
There are a number of ways to think about this, but here's one.  This is basically a variant of the twin paradox.  Suppose there are two twins and one gets in a spaceship and travels to Alpha Centauri at very close to the speed of light.  The other stays home.  Due to time dilation, the one that stays home will have normally aged ~8 years whereas the one that went to Alpha Centauri will have hardly aged at all.  This is just your standard special relativity time dilation.

But remember that everything is relative, so according to the twin in the spaceship, the twin on Earth was the one that was traveling close to the speed of light.  In the reference frame of the twin in the spaceship, he was standing still!  So he should have aged ~8 years and the twin on Earth should hardly have aged at all.  

Why does this not happen?  Well, the twin in the spaceship had to turn around when he got to Alpha Centauri.  When he does this, he is subjected to enormous accelerations.  These accelerations basically forced the time of the twin on Earth to "catch up" relative to the twin on the spaceship.  In other words, just prior to turning around, the twin on the spaceship would have thought that the twin on the Earth had hardly aged, but in order for the twin on Earth to have aged ~8 years by the time he got back, all this time had to "catch up" during the acceleration phase.  So the twin on the spaceship would notice that time was moving much more rapidly for the Earth twin during this acceleration phase. 

But according to the general theory of relativity, you cannot distinguish between an acceleration and a gravitational field.  So, for all the twin in the spaceship knew, someone just turned on a really strong gravitational field.  But if time for the Earth twin moved more quickly during the acceleration phase, then time for the Earth twin would also have to move more quickly if he was outside of the gravitational field.  Hence, time must move more slowly for someone inside a gravitational field.