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Yet another double slit experiment thread

  1. Jan 7, 2005 #1
    I just have some narrow questions and got a headache searching through the other threads to see if they have been answered.

    I just want to understand more specifically what is actually going on with the detectors at the slits. 99% of the explanations and descriptions I read about just talk about a "magic" detector without getting into details.

    So here are my questions.

    First, how do you detect whether or not a photon has gone through a slit?

    Second, and this is the more important question, I would like to know more details on the constraints using these detectors for when the photon is being detected and having it's position "known", and when it isn't being detected.

    Third, in QED Feynman said (if I am remembering correctly) that you could put these detectors on the slits, but you didn't have to "turn them on". What exactly happens when they are on vs. off?

    Fourth, and here is the one that I'm really curious on, though it seems dependent on all the others. What constitutes "detection" in a greater sense. For example, if the detectors are turned on, but the set up is arranged so no human could possibly read the results, does this have any bearing whatsoever on the experiment?

    And related, how do you rule out that the detectors aren't "causing" the change just by being on as opposed to the idea that it is the information being detected that is causing the change?

    Thanks in advance.
  2. jcsd
  3. Jan 7, 2005 #2


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    If I remember, when Feynman discusses a double-slit experiment with detectors, he's no longer talking about "Young's double-slit experiment" involving photons, instead he's talking about the "Feynman double-slit experiment" involving electrons. See this page, which talks about the difference between the photon version of the experiment and the electron version. In the Feynman double-slit experiment, you can measure the position of the electrons by shining light of a sufficiently short wavelength on them. As discussed on this page (in the section 'Watching Electrons in the Double-Slit Experiment), in the Feynman double-slit experiment, the fact that we only see an interference pattern on the screen if we don't know which slit the electron went through can be understood as a consequence of the uncertainty principle. Shining light on an electron can give you an image of the electron with varying degrees of fuzziness, and the smaller the wavelength of the light used, the greater the resolution of the image, so the less uncertainty in the electron's position. But it turns out that the interference pattern is destroyed if the uncertainty in each electron's momentum is too large, and since less position uncertainty = larger momentum uncertainty, this means there's a minimum wavelength of light you can shine on the electrons and still get an interference pattern. When you actually calculate this minimum wavelength, it turns out to be exactly equal to the distance between the slits...but to actually know which of the two slits it went through, you'd need a wavelength smaller than the distance between the two slits!
    As I understand it, the issue is basically whether there was some time when you could have determined which slit it went through in principle, even if you didn't actually do so and even if it would have been very difficult to do so even at the time. For example, the electron double-slit experiment has to be done in a pretty good vacuum; if you do it in ordinary air, you won't see an interference pattern, and I think this is because the electron is interacting with air molecules in such a way that it would in principle be possible to tell which slit it went through by measuring the positions of all the air molecules at the right moment.
    If you assume there is a real truth about which slit the electron went through (a hidden-variables theory), then that would mean that with no detectors present, the electrons going through a given slit can't be forming the same "single hump" pattern that they do when there are detectors at each slit, since the sum of those two patterns wouldn't be an interference pattern, but you do see an interference pattern when both slits are open with no detectors. If you assume the only thing that causes the electrons going through a given slit to form a "hump" pattern as opposed to half of an interference pattern is the local interaction with the photons from the detector, then you should expect that if you close off one of the slits but continue to have no detector at the other slit, the electrons going through that slit won't know the difference, so they'll continue to behave like they do when both slits are open with no detector, forming half an interference pattern (again, this is only if you believe there's a definite truth about which slit the electron went through). But in fact, if you close off one slit, then the electrons going through the open slit will form a single-hump pattern, just like when there's a detector present. This shows it can't just be the local interaction with the detector that explains the change from half-an-interference-pattern to the single-hump pattern; it's as if the electron also "knows" whether the other slit is open or not. Of course, if you don't believe in hidden variables, and thus don't believe there's any definite truth about which slit the electron went through unless you measured it, then you wouldn't reason this way at all--you wouldn't assume that the interference pattern you see is actually a sum of two half-an-interference-patterns, for example.
  4. Jan 7, 2005 #3
    Classical wave explanations

    This may not be what you want to hear, but the fact is that there is no difficulty at all in explaining what happens in an ordinary optical double-slit experiment if you stick to a wave model of light. The apparent need for a "quantum" model only arises when you come to look at electrons, neutrons or other actual "particles". These are another matter, though I do have some (classical-style) ideas if you're interested.

    You can detect the light in a number of different ways, using a photographic plate in which it induces ionisation of molecules, for instance, or a photodetector that employs the photoelectric effect to produce a "photocurrent".

    The latter method is the one that interests me most, being the one employed in the Bell test experiments that I've studied. My picture involves the light wave arriving at a substantial area of the "photocathode" where its oscillating electric field combines with the field of the material and induces oscillations. The oscillations build up, sometimes interfering constructively, sometimes becoming augmented by local electromagnetic noise. Often they never build up sufficiently to cause any irreversible change, but when they do an electric pulse is triggered. [In the particular detectors I'm interested in, a forwards voltage bias is applied, so it is not the light energy alone that is responsible.] If the pulse exceeds a certain threshold value (set by the experimenter) then a "photon" is recorded. The criteria used to set that threshold are not cut and dried, though of course the experimenters try to be as objective as possible.

    It seems to me that what is being done is, effectively, to manipulate natural processes so as to simulate quantum-mechanical behaviour. Clearly the "photon counts" will be related to the beam intensity, but the beam never was made of photons. The idea that when you split the beam you only ever detect one photon or the other is not convincingly proved in any experiment. It is, though, probably true that if the beam is very low intensity you can find a time interval such that virtually no pairs of detections are made within that interval, but that would be just because of the way the detectors work. It takes a random time (from almost zero to the duration of the light pulses) for the oscillations to build up.

    If the above has seemed reasonable to you, I think you will find your other questions don't need answering. Obviously the presence of a detector is irrelevant. The beam intensity is a definite parameter anyway.

    Feynman's QED may provide (approximately) correct answers but gives a completely false picture. He deceived himself. His model fails not only for the double slit experiment but also for those critical "Bell tests" [See my web site or the pages I added to wikipedia, linking from http://en.wikipedia.org/wiki/Bell's_Theorem ]

  5. Jan 8, 2005 #4
    Thanks to both of you for rather different perspectives on the issue. I've been doing some more reading on it and thinking about what you both wrote.

    I think I'm more interested in the electron experiment, and I understand that the position is determined by hitting the electron with a photon.

    If I understand this correctly, then if a photon hits an electron during this experiment, then the electron will "behave like a particle" because it's postion could theoretically be known (even if there is no method set up for detecting the collision).

    If no photon hit the electron during the experiment, then it will "behave like a wave". Is more or less correct? I assume then that the experiment is conducted in some sort of light protected chamber so no stray photons can interefere?

    So, another question, how is it ruled out that the photon striking the electron changes some fundamental property of the electron? I understand the logic that if you use only one slit you see the same kind of pattern as when being detected, and one slit with a detector is no different than one slit without a detector, but I'm wondering if that is the only evidence, or if there is more to it than that, if there is another reason to assume that the photon isn't altering the electron in some major way?

    And then this seems to reaise another question. Since it seems to me most electrons are not shielded from photon interaction, does this mean most electrons are "acting like particles" most of the time, since they are constantly interacting with photons and thus their position could be theoretically determined? And only electrons that are somehow shielded from photon interaction can "behave like waves"? I assume there are other ways to determine an electrons position (don't they produce magnetic fields when they move? or am I mixing stuff up? could you detect this field to see which slit it went through? using an induced current or something?).

    So it seems like most electrons would be interacting with other "things" in such a way that their position could theoretically be determined, if you understand what I'm saying (I'm not even sure I do...)
  6. Jan 8, 2005 #5
    My explanation of actual "single-electron interference" experiments, in particular Tonomura's of 1989 (American Journal of Physics 57, 117 (1989)) is quite different and very simple: there may not have been any actual "electron particles" in the beam they used.

    If the beam was effectively a pure wave to start with, it would split and suffer interference in just the same way as light. In the actual experiment the interference pattern was detected by making the beam fall on a fluorescent medium. I imagine the picture of what actually happens there to be almost exactly the same as my picture (see previous message) of the detection of light. The fluorescence occurs in proportion to the intensity of the beam at that point. Just where the apparent "quantisation" comes into the story I'm not sure. I'd need to know more about the apparatus. It is possible, though, that the fluorescence is in fact continuous and the quantisation is produced by the method by which this is detected, so that we are really just seeing the same thing as with light. Natural phenomena are being manipulated so as to simulate quantum theoretical predictions.

  7. Jan 8, 2005 #6
    I would like to thank you Caroline for taking your time to answer me. I have now had a chance to "look into" you a little bit, starting with your web page. Unfortunately, I do not have the background or experience to decide if you are an unrecognized genius or just a mad scientist off on your own tangent. Because of this, I feel it more prudent of me to listen to the "conventional" wisdom for now instead. The future may prove you right, but until that time I have nothing else to go on except a greater trust of people I am more familiar with, such as Feynman. Thanks again for your input.
  8. Jan 8, 2005 #7
    Fair enough, and you are not alone in this decision. I'm glad you looked at my site though. Perhaps it will help you to maintain a balanced outlook, never forgetting the possibility that theorists may have forged ahead without paying sufficient attention to the realities behind what they accept as "experimental facts". Quantum theorists, after all, only concern themselves with the results of "measurements", and how those measurements are made in practice is of little interest to them. As for the writers of popular books, even when they have (as Aczel did for his recent book "Entanglement") interviewed considerable numbers of the right people, the chances of them coming across information on the real weaknesses of the experiments are almost nil! When it comes to the Bell tests (which, as you will have gathered, are somewhat of speciality of mine) most, whether amateur or "expert", don't seem to even know that most actual experiments have been done using plane polarised light, not the spin of electrons. Very few indeed are aware of more than one or two of the relevant "loopholes".

  9. Jan 8, 2005 #8
    I've been reading up on what the experts are saying so I hope my perspective is helpful. My own understanding is far from perfect but some things I know fairly well. :smile:

    Yeah, we get wave-like behaviour when nothing interacts with the electron (or photon or neutron or whatever we are sending through) and progressively more particle-like behaviour as more and more interactions occur. This is true for all particles in nature but some things like electrons interact easily become particle-like quickly while other things like photons don't interact easily and stay quite wave-like.

    The photon and electron become entangled so the electron is changed as it is then entangled -- has a quantum correlation -- with the photon and whatever else the photon is entangled with. This could include most of the rest of the universe! How entangled the electron is affects how wave-like or particle-like it will be. More interactions and thus more entanglement mean more particle-like behaviour.

    There is more about this in my reply to the next quote:

    Theoretically, an electron which doesn't interact with anything doesn't go through a definite slit. The electron acts like it went through both slits at the same time and the wave-behaviour of quantum theory is the interference effect of the electron bumping into itself! :bugeye:

    Actually, theory gives us a choice for when there is nothing interacting with the electron. We can describe the electron being detected at a definite place but that means we can't describe which slit it went through... or we can describe the electron going through a definite slit but we can't describe it being detected at a definite place!

    So, it's theoretically not allowed to describe the electron both as going through a definite slit and arriving at a definite place if the electron is not interacting with something.

    It's not just a case of not being able to tell which slit the electron went through experimentally (when we choose to use the description involving a definite place of detection) as according to theory no electron then existed which passed through a definite slit. The electron went through both slits but we can't detect that either, only the interference that results.

    I spent a fair amount of time last year trying to get my head around this. :wink:

    Interactions destroy the effect of the electron bumping into itself and allow both a definite slit and definite place of detection to be assigned to the electron. This is another way in which the electron is changed by its interaction with a photon.

    So, the short version of all this is, theoretically, there is no electron in a definite slit to detect when the electron is not interacting with a photon or anything else. The electron is going through both slits at the same time and interacting with itself, an effect which is broken down by interactions with other particles. :smile:
    Last edited: Jan 8, 2005
  10. Jan 13, 2005 #9
    Excellent Question

    This is my first post. Hello everyone. I was also wondering about the slit experiment I saw in a science magazine. I would like to read the other threads but do not see them. Is there an archive section? I am interested in the slit theory and its relationship to parallel universe theories. I read an article about this. I do not have the article :grumpy: I think it was at a doctor's office. :rolleyes: Has anyone else seen such an argument?

    I am also interested in quantum physics, parallel universes, and space travel to help with a debate I have with a friend over the existent of extra terrestials. He says "they" are too far away to travel here. I am a Ufo skeptic but qp seems to over some options for a Mr. Gray to get here. :wink:
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