I already raised this point earlier and the author of the blog did not answer it, he just moved the goal post from “The statement is bold, unambiguous, often quoted — and wrong!” to “The problem as I see it is that Dirac’s statement is oversimplified, misleading, and almost meaningless in the context of modern quantum optics.”

The experiment reported here does not disprove Dirac’s statement (as I explained above two years ago), but that does not mean that Dirac’s statement is correct either. Actually, one of the most insightful conclusions to draw from this paper is that interpretation of experimental fact is extremely difficult and that even great physicists can get it wrong.

Thanks again,
Julian

” the modified statement would be that a 2-photon field would never interfere with a 3- or a 4-photon field”

That’s a good way to put it, especially the emphasis on “the modified statement”! Part of the point I tried to make with this post is that Dirac’s original statement was made in complete ignorance of later QFT developments. In a broad sense, one can reinterpret his statement as being “true”, but such reinterpretations go way beyond what he likely envisioned.

But I have a question: Is the reason for the interference that what maurizio said in his first post; namely that there is interference because you cannot distinguish between both sources, i.e. they are identical and thus the wavefunction of the events should be added before ‘squaring’?

If so then I wonder about the result of the following experiment:

If you take two lasers, where one has a slightly higher or lower frequency (of only a few hz-khz or so). If these interfere, this would give a time-dependent interference pattern right? in the order of the modified frequency. My question is, would this interfere?

What I just described is a modified case of laser doppler interferometer. In the normal setup (http://en.wikipedia.org/wiki/Laser_Doppler_vibrometer) a laser beam is split into a reference beam and a laser beam which frequency is modified because it is reflected of a moving object (doppler shift), both beams then show a time dependent interference pattern because of the frequency modification (the ‘frequency’ of the interference pattern is dependent on the speed of the moving object, so thats where it is used for). Now if you replace the reference beam by a second laser, instead of splitting the old laser. Do you still see an interference pattern? If so, is the previous description of identical particles or events incorrect?

I hope I made myself a bit clear

The situation to me is similar to the wave-particle duality controversy of the past three hundred years. First scientists thought that light was a particle, then a wave, then a particle (photon) with wavelike properties. Does this mean that the statements of the early particle proponents were correct? Only on a superficial level; the real nature of light is way more complicated than those early researchers envisioned.

I would say that according to the authors of [3], Dirac’s statement is bold, unambiguous, often quoted — and possibly correct.