skewzme said:
Who exactly does understand quantum mechanics? While I concede my inability to have that discussion on a purely mathematical basis, it would appear discussions among those who do are still unable to reach a consensus of understanding either:
https://www.technologyreview.com/s/...sts-disagreement-about-the-nature-of-reality/If as you say my quesion in #1 doesn't make sense to start with because it is based on a self-contradictory misconception, allow me to propose the question in a slightly different way. Does the experiment demonstrate that the appearance, or lack thereof, of a wave-like pattern is based on knowledge of the outcome, as opposed to any physical interactions between the "particles" and a measuring device ?
If my rephrased question is still based on my own misconceptions, I would appreciate any attempt you care to offer to explain why in laymen's terms & concepts.
Thank you.
Quantum mechanics is understood by every physicist with a university degree (at least that should be the case since passing the exams on several quantum-theory related lectures is necessary to obtain that degree).
Of course, if you ask for opinions on the socalled "interpretation of quantum theory" which deals with philosophical rather than scientific issues you get a plethora of opinions, and I guess on this level there are as many "interpretations" of the "meaning" of quantum theory as there are physicists (maybe sometimes even more since on such sunsolid ground your opionion easily fluctuates between various possibilities, and I'm not excluding myself from this sin).
Now, as far as the scientific issues are concerned, there's no such uncertainty at all. The natural sciences are about objective observable and quantifiable phenomena of nature, and this nice experiment is no exception. What's done is very clearly described in the scientific paper: entangled two-photon states are prepared using two berefringent BBO crystals at two places ##A## and ##B##. One photon of each pair is registered by a detector ##D_0## at an adjustable position ##x_0## and the other photon of the pair is registered in one of 4 detectors ##D_1,\cdots,D_4##. Now due to the entanglement of the photons and the setup with beam splitters you have the following
If there is a coincident registration of photon 1 in ##D_0## and
(a) photon 2 in ##D_1## or ##D_2## there's no way to know which path photon 1 has taken (i.e., whether photon 1 came from region A or B of the BBO crystal).
Now consider ensembles of such prepared photon pairs: Then registering all those photons 1 as a function of ##x_0##, whose partner photon 2 has hit ##D_1## gives an interference pattern.
Registering all photons 1 as a function of ##x_0##, whose partner photon 2 has hit ##D_2## gives the same interference pattern with some shift of the fringes, which is well understood from the unitarity of the transfer matrices describing the workings of the beam splitters.
(b) and photon 2 in ##D_3## or ##D_4##. Then due to the entanglement of the photons in each pair there's which-way information about photon 1.
Thus registering all those photons 1 as a function of ##x_0##, whose partner photon 2 has hit ##D_3## leads to no double-slit inteference pattern, because then it's known that both photon 1 and photon 2 must have come from the BBO at position B, i.e., there's clear which-way information and thus there's only a single-slit interference pattern.
Thus registering all those photons 1 as a function of ##x_0##, whose partner photon 2 has hit ##D_4## also leads to no double-slit inteference pattern, because then it's known that both photon 1 and photon 2 must have come from the BBO at position A, i.e., there's clear which-way information and thus there's only a single-slit interference pattern.
The choice of whether you want to have two-slit interference of which-way information is delayed by the fact that the detectors ##D_1,\ldots,D_4## are much farther away from the BBO than detector ##D_0##, i.e., photon 1 is always much earlier detected than photon 2, but then choosing to consider only those photons 1 as a function of position of ##D_0##, whose partner photon 2 has been detected by ##D_1## (or ##D_2##) leads to the loss of which-way information about both photon 1 and 2 and thus the appearance of a (shifted) two-slit interference pattern, while looking only at photons 1, whose partner photon has been registered at ##D_3## (or ##D_4##) implies that we know which paths both photon 1 and photon 2 have taken, and thus there's no double-slit interference pattern (but only a single-slit interference pattern) left.
Note also that just registering all photons 1 leads to a completely flat distribution.
As is shown in the paper, the experiment agrees with this predictions of QED. The only interpretation needed here is the unanimous meaning of the quantum formalism for real-world experiments as is known as the minimal statistical interpretation of the states (aka Born's rule).
