Feynman's idea states that matter going backwards in time seems like antimatter.

But, since nature is $CPT$ symmetric, reversing time ($T$) is equivalent to $CP$ operation. So, reversing time gives parity reversed antimatter, not just antimatter.

What is happening here? Why does nobody mention this parity thing when talking about reversing time? What am I missing?

  • $\begingroup$ You're right, never mind. $\endgroup$ – probably_someone Mar 20 '18 at 0:45

Positrons have equal and opposite charge and parity to electrons. Hence when combined, they can produce a neutral gamma ray with no parity.



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