0
$\begingroup$

I'm an A level student and as i was reading QED by Richard Feynman, I came across something really interesting, that light can be reflected from all parts of the mirror according to the quantum theory. However, in normal conditions the probability amplitude of the event occurring at the edges of the mirror cancels out. Could it possible that photons with opposite spin cancel out the probability of an event?

$\endgroup$
1
$\begingroup$

Be careful with your use of the term 'spin' here: Feynman is talking about a sort of phase associated with a photon (the arrow which rotates in time). This is not analogous to the quantum concept of spin.

That said, it is true that two probability amplitudes with the same magnitude but opposite direction (phase) will cancel eachother out. Here's an example:

Suppose you have two tiny mirrors (1 and 2), some distance apart. Next, place a photon emitter (A) in front of them. Finally, add a photon detector (B). There is now a difference in path length between photons traveling from mirror 1 to B, and those traveling from mirror 2 to B. This results in a different 'phase' (angle of the probability amplitude) between the photons at B. If B is positioned such that the photons are exactly in 'anti-phase' (the probablity amplitudes have opposite directions), then the total amplitude of detecting a photon at B will be zero.

QED sketch

$\endgroup$

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.