Q: If time were reversed would things fall up?

Physicist: Reversing time seems to reverse how things work.  Instead of growing, plants shrink.  Instead of going forward, airplanes fly backward.  And, “intuitively”, instead of falling down, things fall up.  If you have a video of someone jumping into water, then they’ll always fall downward if the video is played normally, and they’ll always fly out of the water and upward if you play the video backward.

If you “rewind time” a couple of seconds you’ll find everything as it was a few seconds ago. So the problem here is that several seconds ago you were not falling from the sky.  If you’ve been walking around here on the surface of the Earth recently, then in reverse-time you’ll still be walking around on Earth.  Just backwards.

So, if you reverse time, things will not fall up.

Turns out that, in general, physical laws can’t tell the difference between time running forward or backward.  The one very big exception is entropy.

As long as things aren’t slamming into each other gravity is time-reversal-invariant.

The second (and most awesome) law of thermodynamics says that entropy increases in time (technically, it just doesn’t decrease).  Gravitation is a beautiful example of time-symmetry.  So long as the gravitational interaction doesn’t increase entropy, you’d never be able to tell whether or not time is running forward or backward.  A good way for gravity to increase entropy is to make things hit each other.  In that case you’ve got heat being generated, stuff breaking, things flying around, that sort of thing.

So, orbits (which don’t increase entropy) are the same forwards and backwards, but cannonballing into a pool (which increases entropy a lot) only makes sense forwards.

Reversing time does flip the direction that things are moving, but the universe couldn’t possibly care less about how what direction things are moving (this is a less elegant way of describing relativity).  But weirdly, reversing time does not change the direction of acceleration, which the universe does care about.  So an orbiting planet switches direction, but the force of gravity (the acceleration) still points inward.  Isn’t that cool?