The Biggest Ideas in the Universe 1: Space, Time and Motion

by Sean Carroll

So the speed of time, if that concept were to make sense at all, would be the rate at which time passes with respect to time. To wit: one second per second. It literally cannot be anything else.

The problem is that in order for us to make sense of the claim that Bob’s clock ticks at a different rate from Alice’s, we would have to somehow compare them.

By the time we see what the distant clock reads, more time has passed on the nearby clock.

The right strategy is to give up on the idea of comparing clocks that are far away from each other.