Even if there were as many positrons as electrons in the universe, I don't get the time-reversed character at a macro level. When leaving antimatter out of the picture, I dimly understand that the arrow of the arises from entropy. If time reversed particles are antiparticles, shouldn't we sometimes see a diffuse cloud of antiparticles in a lab spontaneously condense into a very low entropy concentrated configuration?
_We_ are still traveling forwards in time. If you followed a cloud of antiparticles backwards in time along its direction of travel, then you would see entropy decrease yes, though regular particles would do the same if we followed them back in time.
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