Question: How do black holes affect time?

Jose Eliel Camargo Molina answered on 15 Mar 2019:

The main special thing about black holes is that they are so massive and small in comparison that they create a super strong gravitational pull.

And one of the things that we know, predicted by general relativity, is that gravity slows down time! So around black holes, time will run slower compared to how time runs far away from it.

This is something that happens even if it is not a black hole, but a planet. In fact, this is an effect we have to take into account for GPS satellites. When you add up all the effects (which we can calculate precisely using general relativity), the clocks in GPS satellites are running faster by 38 microseconds every day.

This is mainly because they are further away from Earth than us, so they feel the “gravity-slows-down-time” thing less than we do.

If we would not account for our time running slower in comparison, GPS satellites would be off by something like 11km per day at worst.