See the quote below from Dirac's 1975 "An historical Perspective on Spin".
Question:
You seem to feel very strongly about the necessity to have a description of one electron by itself in the framework of quantum mechanics. What about the description of one photon by itself.
Answer: P. A. M. Dirac
That is provided by the Maxwell theory. There is the difficulty that we cannot talk about the probability of a photon being at a given place. We can give a definite meaning to a particle being at a given place only in the case of a particle with a spin of 1/2. For any other kind of particle, we cannot talk accurately about its position. We can talk about its momentum and this is sufficient for most purposes. The spin 1/2 particles are unique in that we can discuss accurately the probability of their positions.
Is Dirac correct that only a spin 1/2 particle can have a position, and why is that?
