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B How to make the wave collapse in double-slit experiment

  1. Aug 24, 2016 #1
    I successfully created the fringe pattern at home with a simple laser light and a black plastic sheet with two thin cut as double-slits. I then used two mobile phone cameras at two sides in hope that the wave function of light will be collapsed. But nothing happened i.e. the fringe remained there. Is there any easy way to test both wave and particle behavior of light using some homemade detector ? What exactly is a detector used by scientists in such experiments which makes the wave collapsed?
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  3. Aug 24, 2016 #2


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    You are recreating Young's purely classical, no quantum mechanics needed, demonstration in 1801 that light is a wave. There's an interference pattern if both slits are open, no interference pattern if either slit is closed, and the presence or absence of a detector is irrelevant.

    To see any quantum effect, you need a single-particle source and a screen that records each individual impact so that the pattern builds up one dot at a time. That's well beyond the reach of any home experiment that I am aware of.
  4. Aug 24, 2016 #3


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    Here's a "one particle a time" video:

  5. Aug 25, 2016 #4


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    There was a nice article in the recent issue of AJP

  6. Aug 25, 2016 #5
  7. Aug 25, 2016 #6
    My kid son discovered that the ray path of the laser light are clearly visible when performing the double-slit experiment inside the refrigerator/deep freezer or if use talcom powder. I could not explain him properly why after going in a straight thin line, the moment ray hits the wall it creates fringe pattern although no visible rays are seen which created the bands on left and right sides . He knows the interference pattern are created by the two light waves created at the double-slit but wants to know why no other light rays are created/visible besides the central main thin ray?
  8. Aug 25, 2016 #7
    I might be wrong but you only "know" the position and not the momentum. If you would try to measure it then the detector will get entangled with each photon. In order for that to happen then you need to build a detector for the slits.

    You would need to do that on each individual photon. By just looking at the wall or whatever you'll see the overall position of photons in a stream of them (many). Not to be confused with actual knowledge of their momentum.

    If I replied in error then any member should feel free to correct me.
  9. Aug 25, 2016 #8
    seeing the light is a matter of dispersion of photons from the water vapor (freezer) or talc particles. After the light goes through the slits the intensity is diminished due to absorption/reflection by the black plastic, and thus there are fewer photons to be dispersed by the water/talc particles, and with fewer dispersed photons there is a lower chance of the naked eye being able to detect the weak rays between the wall and the slits (beam intensity increases with the number of photons present).

    the wave will only collapse if you can tell which slit the photon passed through (path information). because you are looking at more than one photon at a time, and your camera is unable to detect a single photon, you will not be able to get path information from your device, and the wave will not collapse.
  10. Aug 26, 2016 #9
    Nugatory replied above that "You are recreating Young's purely classical, no quantum mechanics needed,....."

    So I am in doubt whether the position and momentum uncertainty are applicable in this test
    I can see quite bright fringes and ray inside the freezer using my red-light laser pointer. I wonder if I can use a more powerful laser then can I view all the rays coming out from the slits like a cone and create the fringes?
  11. Aug 26, 2016 #10


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    A laser "ray" is not a single-photon state but a coherent state. At high intensities as provided by a laser pointer, it's to utmost accuracy describable by good old classical electrodynamics. The fascinating thing with lasers is, however, that they provide light with huge coherence lengths at high intensity in such sharp beams as your son can observe as described which is not achievable with "conventional" light sources. The thing that's quantum here is not the light itself, because it's very well described as a classical electromagnetic wave (in fact as a Gaussian beam), but the creation of such a coherent em. wave, which is due to stimulated emission from the active laser material:


    Of course, Nugatory's assessment above is completely right. To get a double-slit interference pattern with good contrast, the laser beam must be wide enough to shine over both slits. Thus a single photon "contained" in the coherent state, as a wide enough position uncertainty such as to make it impossible to say through which slit it went before hitting the screen where you observe the pattern. What you also immediately realize from this hands-on experience is that the position of a photon "contained" in the coherent state in direction of the beam is completely undetermined, it's not even definable as an observable in the first place.

    I think it's great that your son and you do such experiments with the laser pointer, because it provides direct insight in what's so very wrong with the picture many popular-science concerning photons and the electromagnetic field at the quantum level (but unfortunately also some introductory QM textbooks at the intro level, where they produce the most severe confusion about QT yound students have to suffer from such outdated textbooks, which are however still even written today by just copying the bad intro sections from the old textbooks). You only have to be aware of the many flaws of such books, which provide an outdated picture of "old quantum theory" with its incomprehensible ad-hoc assumptions like wave-particle duality (which is most severely wrong for photons).
  12. Aug 26, 2016 #11
    Why do people say there is no interference when one slit is closed when clearly you can measure the fringes that are mathematically well defined...and visible.
  13. Aug 26, 2016 #12


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    Sure, there is interference also for a single slit. You always have the superimposition of the single and the diffraction pattern. It's most easily seen in Fraunhofer observation (infinitely far away source and plane of observation), where the interference pattern (in the usual Krichhoff-Sommerfeld approximation of the diffraction problem) is given as the Fourier transform of the source.

    For a double slit with the distance of the slits ##d## with a width ##b## you get (with ##k=2 \pi/\lambda \sin \theta##, where ##\theta## is the angle between the apperture and the normal to the observational plane)
    $$A_{\text{double slit}}(k) \propto \int_{d-b/2}^{d+b/2} \mathrm{d} x \cos(k x) = 2 \cos (k d) \frac{1}{b} \sin \left (\frac{k b}{2} \right),$$
    while for the single slit you get
    $$A_{\text{single slit}}(k) \propto \int_{0}^{b/2} \mathrm{d} x \cos(k x)=\frac{1}{k} \sin \left (\frac{k b}{2} \right).$$
  14. Aug 26, 2016 #13


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    That's diffraction, also a wave phenomenon.
  15. Aug 26, 2016 #14
    I thought the wave particle duality is correct. It's a wave which collapses (particle) but never both.

    What makes you state otherwise?
  16. Aug 26, 2016 #15
    Interference must still occur in a single slit to cause fringes just like diffraction must occur in a double slit.
  17. Aug 26, 2016 #16


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    Yes, of course - that's how classical electromagnetic waves behave. But this thread started out being about the two-slit interference pattern, not the single-slit pattern. In any case, the appearance of a single-slit diffraction pattern in OP's test setup has nothing to do with any quantum phenomena; he's sending classical electromagnetic waves through one or two slits, and getting the behavior you'd expect from classical electromagnetic waves.
  18. Aug 26, 2016 #17

    David Lewis

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    Light can exhibit some behaviors associated with waves or particles. It could be argued, however, that light consists of neither waves nor particles, but rather something we can't picture, and don't have a word for.
  19. Aug 27, 2016 #18


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    I'm a proponent of the minimal statistical interpretation of quantum theory, i.e., the interpretation you need in physics without philosophical distortions of the scientific theory. There is no wave-particle duality but only the Hilbert-space formalism a la Dirac with the probabilistic interpration of states via Born's rule. The wave-function collapse is a short-hand concept of some flavors of the Copenhagen interpretation that's heuristic at best misleading at worst. It has to be handeled with care.

    In the double-slit experiment in this interpretation you have particles (I don't discuss photons here since they do not admit a proper particle interpretation in the literal sense, because they do not allow for a properly defined position observable, so take electrons, neutrons, or any other massive particle as an example) prepared at a pretty well defined momentum running towards a double slit, where many of them are absorbed (if you wish they "collapsed" in a quite literal sense, but there's no mystery, they are simply absorbed by the wall, and you dont' care about them anysmore) and some are going through the one or the other slits. If the momentum was defined precisely enough, due to the Heisenberg uncertainty principle this implies that the probability to go through the one or the other slit is quite similar (in the wave-mechanics language: the wave packet should be broad enough in transverse direction to cover both slits), you cannot decide through which slit an individual particle comes. Also you have no more information at which point the particle will hit the screen where it is detected than the probability given by the corresponding solution of the Schrödinger equation, applying Born's Rule. This can be verified by preparing a lot of particles in this way, getting the interference pattern on the observational screen from many events. What's called "collapse" is nothing else than the interaction of the particle with this screen, usually getting absorbed, and you don't care anymore about its further fate.
  20. Aug 27, 2016 #19


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    Well first you need to read a more advanced treatment of QM like Ballentine:

    But you can't build up to that cold. I recommend the following in the following order:

    Then you can undertake Ballentine - but don't hurry - its a long hard slog.

    At each stage you will find superseded ideas like the wave-particle duality replaced by more advanced ones.

    Eventually you will understand a more advanced explanation of the wave-particle duality motivation - the double slit:

    Be warned however, as Vanhees will correctly point out, even the above is not correct:

    To make matters worse even the corrected version above is not correct. Physics can be truly maddening like that. QFT in particular is badly prone to it but that is a whole new story. You might like to become acquainted with QM myths:

    Bottom line is the whole situation is quite subtle. But you have come to the place where you get the full story - warts and all - that will take some time to get accros. There is unfortunately no short-cut if you want the truth.

    For what its worth I MUCH prefer the following as a starting point to QM:

    But it's way non-standard - what I recommended before is usual and bog-standard - but takes quite a while to get to the 'meat' so to speak.

    Last edited by a moderator: May 8, 2017
  21. Aug 27, 2016 #20
    Time to shake off the ignorance. Thanks for taking the time to reply, I hope I will get through the maths. Quite discouraging when I'm having issues understanding very simple systems like force of friction in inclined planes etc.
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