system
Given the following post, respond as if you're commenting on Reddit:
human
Why are anti-particles shown as going backwards in time in Feynman diagrams?

Are they really going back in time, and if so would this explain the lack of antimatter, due to the anti particles essentially creating an identical universe, just going back in time, relative to us?
gpt
Really, it's just a way of determining which quantities in your perturbation series are negative. It's mathematical notation only - it doesn't mean particles are actually traveling backwards through time.

That being said, it is a necessary mathematical convention to preserve causality. Particles traveling a certain direction in time are indistinguishable from their anti-particles traveling the opposite direction in time, and this causes the probability amplitude over spacelike intervals to exactly cancel, giving zero probability of a particle taking a "faster than light" path.