I just read https://en.wikipedia.org/wiki/Annihilation#Examples and there is the popular $ e^+e^- \to \gamma \gamma $ reaction described.

Is it always possible to produce a $ \gamma $? A photon does not have mass (but momentum and thus energy and then, somehow, in return, mass finally) so is there any energy threshold for a photon?


There is no threshold because photons can have arbitrarily small energy (and arbitrarily small momentum).

Their invariant mass is zero so thinking that “then, somehow, in return, mass finally” is a bad idea. If you learned about the obsolete concept of “relativistic mass”, it’s just the energy, up to factors of $c$, and does not warrant a confusing name.

  • $\begingroup$ But $ E=mc^2 $ is valid for photons as well? $\endgroup$ – Ben Aug 20 '19 at 16:38
  • 2
    $\begingroup$ No, it is not, because for a photon $E$ is nonzero but $m$ is zero. The general formula that applies to all particles is $E^2-(pc)^2=(mc^2)^2$. For a photon this becomes $E=pc$. Remember the general formula and forget about $E=mc^2$. $\endgroup$ – G. Smith Aug 20 '19 at 16:40

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.