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My layman's intuition tells me that wave packets normally spread out in space and disperse, except in special circumstances. Photons don't behave like that.
In The Principles of Quantum Mechanics, pp 124-125, Dirac discusses the equations of motions of a photon wave packet. He says:
Thus Dirac doesn't show that the packets don't disperse, he imposes it as a condition on the solution. I presume that the thing he left unsaid was "because that agrees with experiment," which is a compelling argument.
However, on the very next page he seems to waffle:
"the wave packet undergoing a spreading" So what's the deal? Photon wave packets do or do not spread?
In The Principles of Quantum Mechanics, pp 124-125, Dirac discusses the equations of motions of a photon wave packet. He says:
Dirac said:For a given S, let us take a solution of (42) for which at some definite time, the density A[itex]^{2}[/itex] vanishes everywhere outside a certain small region
Thus Dirac doesn't show that the packets don't disperse, he imposes it as a condition on the solution. I presume that the thing he left unsaid was "because that agrees with experiment," which is a compelling argument.
However, on the very next page he seems to waffle:
Dirac said:By a more accurate solution of the wave equation one can show that the accuracy with which the coordinates and momenta simultaneously have numerical values cannot remain permanently as favourable as the limit allowed by Heisenberg's principle of uncertianty, equation (56) of 24, but if it is initially so it will become less favourable, the wave packet undergoing a spreading. [see Kennary, Z.f. Physik, 44(1927), 344; Darwin, Proc. Roy. Soc. A. 117(1927)258]
"the wave packet undergoing a spreading" So what's the deal? Photon wave packets do or do not spread?