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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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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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