So in Feynman's QED book strange theory of light and matter, he mentioned as a photon travels, it spreads a little, thus the "arrow" shrinks inversely with distance, and that is the inverse square law. I'm having trouble $\bf{intuitively}$ understanding this. I understand the classical inverse square law with a point source emitting light in all directions. But for a single photon, going from source S to detector D, what does it mean that this photon will spread a little as it travels to D?
$\begingroup$
$\endgroup$
2
-
$\begingroup$ Related: physics.stackexchange.com/q/142159/2451 and links therein. $\endgroup$– Qmechanic ♦Commented Jun 7, 2023 at 0:25
-
$\begingroup$ This is exactly the reason why I don't tell people to read "QED" any longer. It's a book that makes you feel good by feeding you a number of grave technical mistakes as fundamental physics. I think Feynman meant really well, but he sets you up for a total misunderstanding of quantum mechanics, yet again. $\endgroup$– FlatterMannCommented Jun 7, 2023 at 0:26
Add a comment
|
1 Answer
$\begingroup$
$\endgroup$
5
Would you accept it as being intuitive if I tell you that the little arrows that Feynman was talking about, are actually the electric field vector in circular polarisation? i.e. they diminish as distance increases, in the same reason that classically electric fields should.
Otherwise, it is often difficult to expect there to be an intuitive argument for complicated things like these.
does it mean that this photon will spread a little as it travels to D?
No, the photon is still one entity; if you accept de Broglie-Bohm Pilot Wave Interpretation, then we can use the language of saying that it is the guiding wave that is spreading. (In this case, it is the electric field wave.)
answered Jun 7, 2023 at 0:11
-
$\begingroup$ You get it right... until you say that "the photon is still one entity". A photon is not an entity at all. It's a property change of two systems. The human mind simply likes to imagine that conserved quantities have to be represented by things. It did that for heat energy when we were considering the phlogiston. It did that, again, while we were pondering the aether. Einstein did it in 1905 when he falsely assigned location properties to small amounts of energy and a whole host of people on the internet are mistaking the function of money for the necessity to store large amounts of gold. $\endgroup$ Commented Jun 7, 2023 at 0:30
-
$\begingroup$ @naturallyInconsistent Huh interesting, I see what you mean. If treating it as E field, I understand it diverges as $1/r^2$ for a point charge, but in that case the field is in all directions. For a EM oscillation traveling in a single direction though, would the field diverge/weaken as well? (Why?) $\endgroup$– ABCCommented Jun 8, 2023 at 4:47
-
$\begingroup$ @ABC Yes, it will, just for the sake of needing to have the quantum, photon propagator lead back to the correct classical E field limits in behaviour. Also, if you read Feynman in more detail, he covers how all the propagators' behaviour get simpler if you relax the constraints. Note also that the diminishing is of the spherical wave type, which is physically meaningful. $\endgroup$ Commented Jun 8, 2023 at 5:03
-
$\begingroup$ @naturallyInconsistent "Note also that the diminishing is of the spherical wave type, which is physically meaningful", but the field of spherical wave is not in one direction, is it? $\endgroup$– ABCCommented Jun 9, 2023 at 4:20
-
$\begingroup$ @ABC I have been telling you multiple times, that you will NOT be able to get one direction if you keep insisting on that. You have to first accept that it is going to be spherically going outwards, and then the contribution from neighbouring points will be the reason why it only goes that direction. $\endgroup$ Commented Jun 9, 2023 at 4:22