In his layman's guide to QED Feynman defines a particle propagator as a function that gives you the amplitude that a particle, that was initially at spacetime event ##x##, will be found at spacetime event ##y##.(adsbygoogle = window.adsbygoogle || []).push({});

But does this definition assume that the particle is unique so that if you find it at the spatial coordinates of ##y## then you are automatically guaranteed that it is no longer at the spatial coordinates of ##x##?

As particles are indistinguishable maybe the propagator only specifies the amplitude that, given a particle is at event ##x##, then an *additional* particle of the same type will be found at ##y##.

In that case maybe one also needs to apply a reverse propagator that gives the amplitude that an antiparticle will be found at ##x## given that a particle was found at ##y##?

Perhaps this would destroy the original particle at ##x## and so ensure that we are only left with a particle at ##y##.

Does this make sense?

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# Feynman propagator and particle uniqueness

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