system
Given the following post, respond as if you're commenting on Reddit:
human
Can someone explain this sentence? "[Gravity] is just the differential rate of time between high places and low places."

It's from The Little Book of String Theory by Steven Gubser. A larger excerpt is:

>>Time running at different rates in different places *is* gravity. In fact, that’s *all* gravity is, provided gravitational fields are not too strong. Things fall from places where time runs faster to where it runs slower. That downward pull you feel, and which we call gravity, is just the differential rate of time between high places and low places.

I understand that time slows down or speeds up depending on your proximity to an objects center of gravity, but how does this time differential cause an attraction between the two objects?
gpt
It's a not-necessarily-wrong but also highly unorthodox way of describing gravity. You might say that objects "want" to be in places where time runs slower, so if an object is in one place, and right next door is a place with a slower time rate, it will tend there, the way a ball on a hill "wants" to be in lower rather than higher places, so it falls to the lowest place available.

I don't know how enlightening that is, personally. An equivalent, but hopefully more insightful, way of saying the same thing is that gravity is what happens when spacetime is *curved*. Think of the surface of a sphere, which is a curved space. You know how there are no straight lines on the sphere? It's the same thing in curved spacetime. Particles naturally, due essentially to inertia, want to travel in straight lines, but when spacetime is curved they can only travel on the next best thing - *geodesics*, or paths of least distance from point A to point B. (In spacetime, "point" really means "event," so point A and point B have a time A and time B as well.) Particles following geodesics through curved spacetime look *exactly* as if they're being pulled by a gravitational force, because massive objects curve spacetime strongly around them.