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up vote 6 down vote accepted

The arrow is related to the flow of charge, not the direction of motion. So for antimatter they go the opposite way.

Or as some are fond of saying, the positron is like an electron moving backwards in time.

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Thanks! But moving backwards in time? This is a joke right? –  kame Feb 21 '11 at 15:06
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Not a joke at all. This isn't a joke either: en.wikipedia.org/wiki/One-electron_universe –  Keenan Pepper Feb 21 '11 at 17:00
    
@Keenan: ''moving backward in time'' is a relic from the times of the dirac sea. But in quantum field theory, all particles move forward in time (i.e., the momentum vector has a positive 0-component). –  Arnold Neumaier Mar 25 '12 at 19:43

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