Dalo

The idea that observation changes the object or phenomenon observed is a central tenet of the Copenhagen interpretation of quantum mechanics. I will not attempt to discuss the meaning of this principle for the whole domain of the theory, but will limit myself to the case I have presented in this thread. Double slit experiments show interference patterns when one or two slits are open, but those same patterns seem to disappear when the experimenter tries to determine through which slit the particle has passed. I have claimed that this disappearance is a mere theoretical phenomenon, based on the belief that it is the only sensible explanation when one takes into account the dual nature, or at least the wave, theory of light. I would like to make it clear that my claim is not incompatible with this theory. The interference patterns may well be the result of constructive and destructive interference of particles moving together through the slits. They may even be the result of single particle interference. What is essential is the reason why they seem to disappear when "observed". I have found no indication, neither in the literature, nor in the reactions received in this forum, of an empirical proof that interference patterns effectively disappear when observation is involved. Rather, I am evermore convinced that the disappearance is a theoretical assumption, or conclusion if one prefers, rather than an observation. No empirical proof of the disappearance of the interference patterns has ever been given, this, as far as I know. If that is the case we are confronted with philosophical consequences that need to be explicitly stated. One of them is: The idea that observation changes the result of an experiment, whatever the further merits of the idea, remains unproven in the case of double slit experiments.

Empirical by scientific criteria. I have nothing to prove, since the facts are recognized by everybody, including you. What I am asking is that you prove what you claim, that the theory, any theory you may choose, leads to the practical results I have mentioned. This is the whole point, isn't it? You claim that you have provided empirical proofs, but I cannot see how what you have presented relates to the questions posed. Since you are the one claiming that you have proofs, the least you can do is show them unambiguously. Maybe I have not understood them, so why don't you tell me exactly what your empirical proofs consist of?

Nope, all you have done is explain or refer to theories I am not attacking. What I am looking for is much more modest: show me an empirical proof of both points: - that a particle can go through both slits; or at least, - that observation destroys interference patterns in a double slit experiment. As you see, nothing complicated, and you can use any theory you want as long as you do not stop at the theory, but show the empirical results.

I know the theory, now show me the results in the particular case I have described.

You did not show at any time that - a particle can go through both slits at the same time, - that the interference pattern is destroyed by observation. Your experiments, however impressive, are irrelevant to this thread.

My claim is empirical as can be. It can be easily infirmed, all you need to do is show one single experiment in which observation destroys the interference patterns. Empirically, not theoretically. By the way, I am not quoting any theory to my defense. I am using examples from different books to show cases of double slit experiments. My claim does not need Bohm, Bohr or Einstein to make it empirical.

Those are theoretical arguments, now show me how they apply to the problem at hand. If you argue that observation in itself destroys the interference pattern, it would only mean that the theory is unprovable and must therefore be considered as a metaphysical position.

I am not sure what you are expecting from me. I have no opinion on QFT. I do not feel the need to attack it, nor to defend it. What I do not understand is how all you have been saying relates to the following: - in a double slit experiment, when one slit is open nobody doubts that all particles go through that slit, even if single particle interference may create interference patterns. - when two slits are open, there is a clear interference pattern, and the question is how that happens. The theory is constructive and destructive interference. Okay. - when the experimenter tries to find out through which slit the "particle" has gone through, it is said that the interference pattern disappears. - I say, that is not true, that it has never be shown in a real experiment. All we have are theoretical justifications. If you think that you can prove my claim wrong, empirically prove my claim wrong, and not just by appealing to theoretical considerations however respectable they may be, then I welcome your contribution and will change my mind accordingly.

It leaves me slightly indifferent. The debate between Copenhagen and Bohm is interesting, but certainly not life changing.

I can understand the wrong impression my references may have conveyed. But I am not beholden to the Bohmian theory, even though I find the fact that there is an alternative to the Copenhagen interpretation very reassuring. My claim does not depend on the validity of the Bohmian approach, even though it is incompatible with Bohr's convictions.

First reaction to the first article. The article is obviously way above my pay grade, and I would certainly not presume reviewing it. To be honest, I do not see how it relates to the subject of this thread. It concerns definitely not a double slit experiment, and, even if I disregard the use of a Mach-Zehnder interferometer (see my previous posts), there are, if I understood correctly, two sources of light involved, instead of one. It certainly shows the degree of expertise of all involved in the experiment, but, once again, I fail to see the relevance to this thread

Thank you. I will study them very carefully.

I have no reason to doubt you. This is after all your domain. I would be very curious about the setup of the experiment though. Maybe you could post a link or a reference?

No. I leave that to mathematicians, which I am not. I consider myself a philosopher (didn't finish my Phd through circumstances not relevant to the subject, and now I am too old to care.). did you ever observe a particle passing through two slits at the same time?

Maybe you will like this quote, written by a Bohmian author: "our most basic physical theory contains no account of the constitution and structure of matter, corresponding to the interacting particles and fields of classical physics. It is a means to compute the statistical results of macroobservations carried out on systems that are unspecified and, indeed, unspecifiable. The word 'electron' does not actually mean anything at all - it is simply shorthand for a mathematical function. Quantum mechanics is the subject where we never know what we are talking about. " Peter R. Holland, The Quantum Theory of Motion : An Account of the de Broglie-Bohm Causal Interpretation of Quantum Mechanics, 1993.

Why bizarre? They are really simple: - it must concern double slit experiments; You can start a thread on interferometers and I will gladly contribute by explaining why I think they cannot be considered as the equivalent of a double slit experiment. You won't agree with me and we will have to come back to this thread. - the disappearance of the interference pattern must be shown "live" and not as a theoretical assumption or conclusion.

link 1 is a google listing. Which article am I supposed to read? link 2 concerns interferometers. See above. link 3 shows interference patterns. They are not the issue. This is as far as I got. Could you please choose one link which you think proves me wrong? edit: link 4 is behind a pay wall same for link 5 same for link 6 same for link 7 that also concerns interferometers.

Show it to me.

I stopped as soon I came across "Mach-Zehnder interferometer". I am willing to speak about this kind of experiments in another thread. Specifically, whether they can be considered as equivalent to a double slit experiment. I am not so sure about that. I wrote about interferometers long ago in another forum and I would have to consult my notes. To make a long story short, I do not think it is an acceptable alternative for a two slit experiment. Moreover, I do not think that (Mach-Zehnder) interferometers actually show what the theory tells us they show. Just like with double slit experiments, interpretation of the data is crucial. But then interpretation is always crucial, with all scientific experiments. But I promise you that I would be willing to discuss them in their own thread.

It is not a matter of Physics, or Mathematics, since everybody, me included, agrees on what we can "see" happening at each double slit experiment. The question is how to interpret it. My argument, and as far as I know, it has never been presented by anyone else, is that the so-called disappearance of the interference pattern does not actually happen, but is a theoretical assumption. I am willing to change my mind and go with the flow on one simple condition: show me this disappearance of interference pattern in (the video of) a real life experiment. Not an animation, and not a diagram.

The two examples given above, that of the screen with the pin, and the photographic plate, both show that the "particle" goes either through one or the other slit. That is what matters. That and the fact that there is only an interference pattern when there is no observation. The problem then becomes: does the "particle" go through one or the other slit only when it is observed? Bohr says yes, because it in fact goes through both slits. But then, according to Bohr again, only observables "exist". Which observables? The fact that the particle goes through a single slit can also be called an observable. In fact, it is the fact that the particle goes through both slits that is never observed! *** Do the references I have given fall under this category?

I think I confused him with de Broglie who indeed changed twice of position concerning his pilot wave theory.

This is an assumption central to the Copenhagen interpretation of Quantum theory. It is not an empirical fact but an attempt to explain empirical observations. I understand if you consider it valid, but this is not a matter of expertise, only of "philosophical" conviction. Bohm later changed his position, and not everybody agrees with Bohr anymore. In my first post, I argue that observation does not need to change the outcome of the experiment, but rather that the explanations given are themselves problematic. That is the whole point of the matter: Does observation/measurement change the outcome? Bohr, von Neumann, Heisenberg and others are convinced that this is the case. My impression is that it is more a petitio principii. I am very glad that you have joined the discussion because you have finally brought it to the main point. I only do not think that the issue can be solved with technical arguments. It is a deeply philosophical/metaphysical issue.

Bohm also said on the same page: "an electron is neither a particle nor a wave, but is instead a third kind of object which has some, but not all, of the properties of both particles and waves." No problem. But It wouldn't resolve the issue of the disappearance of the interference pattern in case of observation.

David Bohm: Quantum Theory (1951; edition of 1985) "To show that this conclusion does not depend on the particular method used to find through which slit the electron goes, let us consider, for example, the possibility of setting up a cloud chamber at the detecting screen." p.119 I am afraid that it concerns a thought experiment, not a real one. Besides, it was used by Bohm to emphasize his point that it is impossible to know through which slit the electron went, due to the Uncertainty Principle. "We must remember, however, that the behavior of the electron in the cloud chamber is also limited by the uncertainty principle." This is a limitation that the other authors quoted previously did not recognize. I had used the vapor chamber example from memory, and my example certainly does not stroke with Bohm's interpretation. Still, I think that my argumentation still stands, even if we accept that the vapor chamber is not really a good example.