I have recently read Richard Feynman's "QED" and in it Feynman describes positrons as 'how we view electrons when they are going back in time and we are stuck traveling forwards in time'. I was wondering, are all positrons electrons traveling back in time? Or can you have an electron with a positive charge (positron) that is not traveling backwards? Furthermore, if all positrons are (from a quantum dynamics standpoint) traveling backwards in time, then why (again from a quantum dynamics standpoint) would an element, such as Fludeoxyglucose (18F) emit these positrons so much more than other elements?

Thanks a bunch! I hope I was clear with my question, if not please let me know.


  • $\begingroup$ well Feynman indeed made such statement (based on other notions of time) but i would say no. But one can say a positron is an electron with negative frequency $\endgroup$
    – Nikos M.
    Nov 3, 2014 at 16:40
  • $\begingroup$ however by CPT theorem since all these parameters (time, charge, energy, frequency) are related in the lagrangian one can also say that $\endgroup$
    – Nikos M.
    Nov 3, 2014 at 16:43
  • $\begingroup$ Oh I see what you mean, so then what I was getting at is why do you think certain elements produce more of these seemingly random electron-positron pairs than others? $\endgroup$
    – Eric
    Nov 3, 2014 at 16:45
  • $\begingroup$ that is another question related to broken CP symmetry and weak interactions $\endgroup$
    – Nikos M.
    Nov 3, 2014 at 16:46
  • $\begingroup$ Ok, so now this may just be me misunderstanding Feynman or Quantum dynamics in general, but if we take the notion that a positron can be thought of as an electron going back in time, then wouldn't a PET scan have to be creating these "electrons" much later then their appearance as positrons and then they are simple canceled out at the point in time when the positron first appears? $\endgroup$
    – Eric
    Nov 3, 2014 at 16:49

1 Answer 1


It is an analogy made for the wide audience. The actual picture provided by QFT is far more technical.

Configurations of electrons and positrons are states of the quantum system, called the electron field. This states can be obtained from the vacuum state by acting with creation operators.

This creation operators evolve in time (in Heisenberg picture), and the their frequency is positive for electrons and negative for positrons (or vice versa, it doesn't matter).

The whole thing is even more complicated in QED, when the interaction affects properties like mass, charge or field normalization factor via the renormalization.

  • 1
    $\begingroup$ Thanks for the explanation, I'm still in high school so I suppose I just need to wait until I've taken all the formal classes before I start thinking too abstractly. $\endgroup$
    – Eric
    Nov 3, 2014 at 16:55
  • $\begingroup$ It sounds depressing, but I think you are right :) Good luck! $\endgroup$ Nov 3, 2014 at 16:58

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