# Dirac's equation and positron

## Main Question or Discussion Point

When Dirac solved his equation, he found out there are negative energy levels and he did not know how to make of it. Then Feynman figured out if electron travel backward in time, then those negative energy levels will become positive energy level, and it will show up as a positron travel forward in time. Is this just a math trick or is it real?

BTY, Feynman recalled the first time he met Dirac, Dirac asked him, "I have an equation, do you have one too?"

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When Dirac solved his equation, he found out there are negative energy levels and he did not know how to make of it. Then Feynman figured out if electron travel backward in time, then those negative energy levels will become positive energy level, and it will show up as a positron travel forward in time. Is this just a math trick or is it real?

BTY, Feynman recalled the first time he met Dirac, Dirac asked him, "I have an equation, do you have one too?"
Can we really interpret positron as electron travel backward in time?
Anti-particle as particle travel backward in time?

The traveling back in time observation is purely conventional, because the time evolution always goes like E*t, so if you flip the sign on E or t it's not actually distinguishable. But then positrons don't travel back in time because of causality, so really what Feynman observed was more novelty than anything else.