Dalo

I have been trying very hard to avoid this discussion because I do not think it changes anything as to whether a particle (or particle-wave, or wave-particle) comes through one slit or the other. All authors I have consulted agree that observing what is happening shows unambiguously that the particle (whatever its properties), goes either through one slit or the other. The big mystery is the role of observation in the disappearance of the interference pattern. That is what I have tried to tackle in my first post.

I have in fact been planning a separate thread on Bell's Theorem and non-locality. But then, strictly from the logical/epistemological/philosophical perspectives. I cannot say it often enough, I am not a physicist and will gladly make use of your expertise. But the Bell's thread will have to wait until I have completed my readings.

I am still looking for where I read about the double slit experiment and the vapor chamber. Meanwhile, here is another equivalent setting. This time from: Bohmian Mechanics : The Physics and Mathematics of Quantum Theory by Detlef Dürr & Stefan Teufel, 2009, p.8 " A Red Herring: The Double Slit Experiment This is a quantum mechanical experiment which is often cited as conflicting with the idea that there can be particles with trajectories. One sends a particle (i.e., a wave packet ψ) through a double slit. Behind the slit at some distance is a photographic plate. When the particle arrives at the plate it leaves a black spot at its place of arrival. Nothing yet speaks against the idea that the particle moves on a trajectory. But now repeat the experiment. The next particle marks a different spot of the photographic plate. Repeating this a great many times the spots begin to show a pattern. They trace out the points of constructive interference of the wave packet ψ which, when passing the two slits, shows the typical Huygens interference of two spherical waves emerging from each slit. Suppose the wave packet reaches the photographic plate after a time T. Then the spots show the |ψ(T)|2 distribution,4 in the sense that this is their empirical distribution. Analyzing this using Bohmian mechanics, i.e., analyzing Schr¨odinger’s equation and the guiding equation (1.4), one immediately understands why the experiment produces the result it does. It is clear that in each run the particle goes either through the upper or through the lower slit." The authors continue: "So is logic false? Is the particle idea nonsense? No, the argument is a red herring, since [Close slit 1 and open slit 2.], [close slit 2 and open slit 1.], and [Both slits are open.] are physically distinct." [original emphasis] In other words, it is a non-problem. Which is not very helpful. What is important here is that the experiment is another logical equivalent of the version with the vapor chamber.

You say it like it is a fact. The debate is still ongoing. ************************************* We find a very interesting strategy in Claude Cohen-Tannoudji (et al),Quantum Mechanics, Volume 1, 1991, ch.1. It consists in declaring impossible the knowledge of which slit the photon went through: "we can imagine placing detectors (photomultipliers) behind [each slit]... But, obviously, the photons detected in this way are absorbed and do not reach the screen. " (p.13) So, it is better to use only one detector behind one slit, which leaves what happens with the other one in a theoretical limbo.

Following the previous quote is also a very interesting variation on the double slit experiment: "Finally, some readers may find it instructively disturbing to consider a further variation of the two-slit experiment which was pointed out by Wheeler in 1978. In this variation, we imagine a situation in which the choice of the experimental arrangement in Fig. 1.7 is delayed until after the particle has passed the slits. We could, for example, insert the pin and fix the position of the screen just before each particle arrives at the screen. In this case an interference pattern builds up, which is characteristic of wave-like particles which pass through both slits. Alternatively, just before each particle arrives at the screen, we could withdraw the pin so as to allow the screen to recoil and determine the slit from which the particle comes. In this case, no interference pattern builds up. Thus, a delayed choice of the experimental arrangement seems to influence the behaviour of the particle at an earlier time. " (Phillips, op.cit, my emphasis) ************************** There is also a reference to a "real experiment" [sic], instead of a thought experiment. I will give you the abstract here, but I am afraid that it won't tell you much. At least, I did not feel any wiser after reading it. If anyone on this forum does have access to the full article and would be willing to write a short review, it would be very much appreciated. "The neutron interferometer as a device for illustrating the strange behavior of quantum systems Daniel M. Greenberger Rev. Mod. Phys. 55, 875 (1983) – Published 1 October 1983 The neutron interferometer is a unique instrument that allows one to construct a neutron wave packet of macroscopic size, divide it into two components separated by centimeters, and then coherently recombine them. A number of experiments clearly showing the difference between quantum and classical theory have been performed with it, which are suitable for presentation in elementary quantum courses. This article presents a simple mathematical model of the interferometer, which can be used to illustrate clearly many of the surprising features of quantum systems. For example, one can describe an experiment to determine which component beam the neutron takes (an analog of the two-slit electron experiment). One can then trace in detail the loss of coherence of the wave function, rather than merely invoke the usual "handwaving" uncertainty arguments. The author discusses the effect of gravity on the neutron beam [the classic COW (Colella, Overhauser, and Werner) experiment], including a simple analysis in an accelerated reference frame, and its relation to the equivalence principle, the red shift, and the twin paradox. Also described are the effect of rotation of the neutron by 360° to change its phase, the effect on the wave function of measuring the absence of the particle from a beam ("Dicke's paradox"), and a realizable version of Wheeler's "delayed-choice" experiments, as well as their relation to the problem of "Schrödinger's cat." The treatment is suitable for bright undergraduates and first-year graduate students."

I think I am Socrates! "When the pin is withdrawn, the screen becomes a mobile detection system which is sensitive to the momentum p = h/\ of the particles hitting the screen. It recoils when a particle arrives and, by measuring this recoil accurately, we can measure the vertical momentum of the particle detected at the screen and hence identify the slit from which the particle came. For example, near the centre of the screen, a particle from the upper slit has a downward momentum of pd/2D and a particle from the lower slit has an upward momentum of pd/2D. In general, the difference in vertical momenta of particles from the two slits is approximately Ap ps pd/D. Thus, if the momentum of the recoiling screen is measured with an accuracy of pd ' D ' A.17) we can identify the slit from which each particle emerges. When this is the case, a wave-like passage through both slits is not possible and an interference pattern should not build up. " A.C. Phillips Introduction to Quantum Mechanics, 2014, p.14 [the equations are jumbled, but are irrelevant here. Also this concerns another device than a vapor chamber, but the principle is the same.]

I am afraid I would not know. But I would very much like to.

I know about the bucky-balls. I am also wondering if I should have used atoms instead of electrons. But I want to make sure. "There was never any drawn-out controversy about whether electrons or any other constituents of matter were other than particle-like. Individual electrons produce scintillations on a phosphor screen—that is how TV works. But electrons also exhibit diffraction effects, which indicates that they too have wavelike attributes. An analog of the double-slit experiment using electrons instead of light is technically difficult, but has been done. " Sly Blinder "Introduction to Quantum Mechanics in Chemistry, Materials Science and Biology", 2004, p.23 Hopefully more references to come.

There are two important points I need to investigate again: - the use of a vapor chamber; - the use of electrons instead of photons. As I said, I will look into it again and come back to the thread.

I am taking a time out to review my sources and my notes. I will therefore not be answering to objections right away. That does not mean that others cannot come up with new points.

I would be interested in your description of the double slit experiment. Maybe then I will know what to answer you.

The issue of momentum is correct. Concerning the use of a vapor chamber I am surprised you have never heard of it. This is most definitely also a fundamental philosophical issue.

A perfect example of the caricatural (re)presentation of the double slit experiment is given in https://youtu.be/A9tKncAdlHQ To his defense I must point out that Jim Al Khalili, whose reputation as a scientist and a popularizer of science is deservedly beyond reproach, is here only presenting a universally accepted interpretation of the phenomenon.

The double slit experiment and Superposition The double slit experiment takes a special position in physics in that it is used as a "crucial experiment" both by classical and quantum Physics. While the first, classical physics, shows an understandable historical preference for light, quantum Physicists, also very understandably, very often use electrons to get their point across. When electrons or photons are ejected in the direction of the two slits, some strange things happen: 1) with one slit open we see the expected pattern of a single agglomerate of bright points on the screen at a location facing the slit. 2) when both slits are open we get the interference pattern discovered by Young in the 19th century. The same pattern is created whether we shoot electrons/photons continuously, or one by one. This has always been interpreted as the duality of the nature of light and matter. Instead of waves or particles, physicists prefer to speak of wave-like and particle-like properties of light or electrons. Still, the strangeness really starts when we want to know through which slit electrons go through each time. This is the point where myths are created. Apparently, observing the precise trajectory of electrons destroys the interference pattern. When we do that, when we can indicate through which slit an electron (or photon) has gone, it appears unambiguously that the electron has taken either one or the other slit on its way to the screen. The trouble is that, when unobserved, electrons seem to go through both slits at the same time. Since the only factor that has been changed is the fact or absence of observation, it seems logical to conclude that electrons/photons behave differently depending on whether an observer is present or not. I present here an explanation which is I think much more down to earth. This experiment has often, for obvious pedagogical reasons, been presented with diagrams or video images that, almost without exception, ignore the factual dimensions of the phenomena concerned. For instance, observation becomes a camera placed between the slits and the screen, and directed at one of the slits to catch the moment where an electron goes through it, without taking into account that such a camera could not possibly exist, for now, or find a place in the minuscule space taken by the experiment. When we know the dimensions of an electron we quickly realize that such a setup must be understood as an allegory without any real signification. I will therefore use a much more realistic model to explain the role of observation in double slit experiments. Instead of a camera, a vapor chamber is usually used, through which electrons can leave a clear path showing where they started and where they ended, eliminating any ambiguity as to which slit they went through. The known results of such a setting have always been interpreted as the confirmation of the mystery: the use of a vapor chamber as observation instrument destroys the interference pattern, which comes back with a vengeance as soon as only a screen is used to welcome the impacting electrons. What happens then when an electron goes through a slit? Obviously, in the case where there is only one slit, the question is trivial. The electron will either bounce off the separating wall, or it will go through the slit. leaving its mark through the vapor chamber. The question that needs now to be answered is why the interference pattern disappears when "observed", meaning when there is a device in place that can record through which slit the electrons have gone through. Here again we must take leave of the pedagogical caricatures of the double slit experiment, and look at how it can effectively be performed. To record the movement of the electrons, we will use again a vapor chamber. The results will be as can be expected from classical physics: each electron will go through one slit and one only. There can be no ambiguity over so called superposition. The tracks left by the electrons are clear enough. The great mystery is how we can get a simple distributed pattern with two distinct groups of spots each representing the slit through the electrons went, while when we are not using the vapor chamber, and therefore not observing the electrons, the same interference pattern reappears. My answer will seem rather unorthodox: the simple pattern is in all cases an extrapolation of the tracks left in the vapor chamber. It never really appears on the screen as such, simply because a vapor chamber is itself the screen, and offers no place for an external screen placed as in other experiments. The myth of the double slit experiment is therefore in many aspects the result of the modes of presentation of the experiment. We have either the interference pattern as it appears on a screen standing directly in the path of the electrons. Or a simple classic pattern showing the paths taken by each electron. When these patterns are extrapolated to another image, we get the classic pattern all the videos and diagrams are so fond of. You could look a long time for a video showing in real time the creation of the classical pattern, while the real images of photons or electrons creating the interference pattern can be seen overall.

A favorite police interrogation technique is to keep asking the same thing over and over again, and grasp any change in formulation, however innocuous, in shaking the suspect's confidence. The aim is not so much the search of truth as intimidation. It is a technique often used in this forum. Combined with coordinated personal attacks from different fronts they are very effective in at least destroying the credibility of the poster. These are means that are also used in "dirty" political campaigns. But then, as Phi said somewhere, scientists (and would-be scientists) are just normal people, so we should not be surprised if they react just like anybody else. I will not keep defending myself and my views when it is obvious that everything has been said. There is no way for me to ever satisfy the objectors unless I express my deepest apologies and culpability. To the objectors I say: Don't hold your breath.

That is the whole point of the discussion and the result that Strange and I had arrived at. He agrees with you. I do not, that is the reason for my experiment. You think that it is superfluous, I do not. We could go on forever, you will not change your mind and I will not change mine. Let us wait until I am in a position to perform the experiment myself, or until somebody else does it.

NO! THE LASERS ARE THE ONLY SOURCE OF LIGHT WHEN THEY ARE TURNED ON! MINI SUNS ARE AN ANALOGY!

You are right that turning the lasers off and using an extra source of light needlessly complicates the whole picture. I really felt that Strange had expressed all possible objections to my experiment, and I did not feel like starting the whole discussion again. So I looked for variation to make the difference clear between the sources of light as illuminated objects, and the beams that come out of them, just like the analogy of the mini suns that shine all in one and the same direction. Because of a new setup, I have to be more careful in the description of the different steps. 1) same diagram as before. Diaphragm also at same position. 2) extra light. Lasers off. 3) all lasers, which could as well be roasters, are visible from screen. 4) extra light off. Lasers on. Only three beams go through diaphragms 5) five bright spots visible from screen. The picture of the sun through a filter is used as an analogy for the laser lamps that are visible even though their beams are blocked.

you are right of course. It was a typo in step 4.

nope. I do mean off. but in step 4 it should be on.

You agree with Strange that only three light source would be visible, just like only three beams pass the diaphragm. You probably also would find an empirical confirmation superfluous. I do not. For arguments see the whole discussion with Strange which I thought was finished. +1 1) Take my diagram but this time it is not important whether the filter is present or not. 2) Let us turn all lasers off. 3) I suppose that everybody will agree with me that, even if the diaphragm is put at the same position as in in my diagram, all five lamps will be visible from the screen position, whatever the aperture? 4) Now turn all lasers off. Only three beams will make it through and two will be blocked if the diaphragm is closed. Let me know where I went wrong.

I don't think so either.

I am now reading Andrew Whitaker's "John Stewart Bell and Twentieth-Century Physics", and I came across a very interesting quote which has to do with observers: "both Einstein and John [were] for an 'observer-free realm', that is, that physics should exist without observers, without the necessity for measurements" (p.55) My first reaction is to say: why should physics be observer-free, or without the necessity of measurements? It seems to me that Einstein and Bell are making an error which is the exact opposite of that of Bohr, but still an error. Bohr seems to deny the independence of reality from the observer, and to counter that, Einstein and Bell want to annihilate the observer altogether to ensure the independence of reality. But while Bohr is indeed talking about reality, the other two have somehow shifted the focus to science. That is I think a very grave philosophical mistake, an observer-free reality is quite different from an observer-free science! I find the first very plausible while the second sounds as a metaphysical impossibility to me.

that is where our paths diverge.

Probability is not a property of matter but of knowledge. You second assertion confirms this. If you accept this then we are saying the same thing.