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I recently read R.P Feynman's QED:A Strange Theory of Light and Matter. It is believed that time travel to the past is not possible. Then why is particles going backward in time considered in the book while drawing Feynman diagrams?

Qmechanic
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Rajath Radhakrishnan
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  • I'm not an expert on the details, but as I understand things this idea has been since been ruled out. It was a very promising possibility in Feynman's day though. – David H Nov 17 '13 at 11:19
  • @DavidH IIRC The idea that all electrons are the same electron has been ruled out, however the idea that a positron can be viewed as a backward-in-time electron is still around. I think, but not used much. – Manishearth Nov 17 '13 at 11:31
  • Related: http://physics.stackexchange.com/q/391/2451 and links therein. – Qmechanic Nov 17 '13 at 11:44

1 Answers1

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Going back in time is "one way" to look at it. To be honest, time isn't well defined when you work in the frame of purturbation theory in QED, because it's subject to the Heisenberg uncertainty principle, and that's why, when you define your time ordering operator, you consider the sum of two situations, back in time and forward in time, which is exactly equivalent to particles and anti-particles go in opposite directions.

So to summarize:

1- Time isn't well defined due to uncertainty;

2- working with anti-particles is equivalent to time reversal (read more about CPT symmetry)

The Quantum Physicist
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