Reading some basic explanations of the arrow of time, it seems like there is a lot of emphasis on entropy, and how things tend towards a disordered state. But I don’t see any mention of how the basic forces (except electric charge I suppose which is both attractive and repulsive) are all only attractive with respect to the forward direction of time and repulsive with respect to the backwards direction of time. Isn’t this fact at least as important to the direction of time as entropy?
As an object rolls downhill, the hill/object system is moving towards a lower energy state over time. Reverse entropy, and the object can roll uphill.
I’m not sure you can say gravity is attractive when viewed in forward time, and repulsive when viewed backward. For example, a movie of a body in orbit can be reversed without the viewer being able to tell which direction is which. A movie of a roller coaster in motion could also be reversed without it looking physically wrong, or improbable. (Assuming no other visual cues of course, like which way the people are facing.)
Gravity and electromagnetism do not, by themselves, suggest an arrow of time. All of those forces are reversible, at least in classical physics.
In ordinary daily human experience, objects falling in the vicinity of Earth’s surface won’t usually have reversible motion — but that’s because they’re leaking energy into the surrounding air as they move, and losing most or all of their energy on impact with the ground. It very rarely happens, for example, that a bunch of random heat in the dirt will approach a golf ball resting on the grass, form into a compression wave, and launch the ball up into the air. Then, coincidentally, air molecules all along the ball’s trajectory happen to bounce off it, mostly from the “rear”, in such a way that they keep contributing to the ball’s total energy, propelling the ball perfectly onto a golf tee hundreds of yards away.
It’s thermodynamics (and not gravity) which tells you that this sequence of events is practically impossible, and therefore that there’s an “arrow of time”.
Consider throwing a ball into the air. The ball starts off moving upwards quickly, but it slows down. Eventually, it reaches the highest point, at which it momentarily stops. Then it starts moving downwards, slowly at first, then faster. Finally, it lands back in your hand, moving quickly.
If we reverse the direction of time here, then we have the ball leaving your hand quickly, moving upwards. It slows down, until it reaches the highest point, where it momentarily stops. Then it moves downward again, slowly at first, but speeding up. Finally, it’s back in the hand you threw it from. If there were no air resistance or other frictional forces, this would be identical to the forward situation. And once you introduce frictional forces, you’re dealing with entropy.