Also: on the most fundamental level (of quantum mechanics) there is absolutely no difference between past and future. We know the equations for what happens when two particles collide. You can run the equations backwards and it looks exactly the same.
Picture a movie of gas molecules bouncing around in a box. Now run the movie backwards. They will look basically the same; you won’t be able to tell which movie is going forwards in time, and which movie is going backwards.
Antiparticles actually move backwards in time. A positron—the antiparticle of an electron, with a postive charge instead of a negative charge—moves backwards in time, just as electrons move forwards in time. “Backwards” and “forwards” are from our perspective here, of course.
Even weirder, photons don’t move through time at all—from their perspective. A photon travels at the speed of light. From our perspective as slowed-down matter, it does take time for a photon to get from A to B. But if you were a photon, you would experience both points A and B simultaneously. No time would take place on your journey.
So the subjective experience of time must have something to do with matter and the accumulation of mass, along with entropy. There’s been a lot of work connecting entropy to mass and gravity, too (the three laws of entropy—thermodynamics—are basically identical to three laws concerning the mass of a black hole).