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Double Slit experiment

  1. Feb 22, 2009 #1
    Lets say that I were to perform the double slit experiment so that at any one time only a single photon passes through the double slit at a time. Now lets say that I were now to place a polaroid film over each of the slits so that each piece of the film was orthogonal to the other. Now in front of the newly polarized double slit I will place a second piece of polaroid film which is oriented in parallel with the left slit.

    At this point I become slightly confused so I will tell you what I think is happening and I would really appreciate if someone can tell me if I am right or wrong.

    1) As a photon passes through the polarized slits it interferes normally with itself as if the films were not in place.
    2) Then, as the photon strikes the second piece of polaroid film the wave function that was a result of the double slit interference pattern collapses because we measure the polarization of the photon and we are left with the wave function of a photon passing through a single slit.

    Is my reasoning correct?

    Thanks guys!!!
  2. jcsd
  3. Feb 23, 2009 #2


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    Hi maggicmike660! :smile:
    Nooo … you can't split the observation …

    you set up the apparatus, and then you describe the wave function over the whole apparatus, not in parts. :smile:
  4. Feb 23, 2009 #3
    I have always seen the double slit experiment as where physics lost its way.

    If you imagine a child's swimming pool, in fact lets make it a large one with rigid sides, 3 feet deep. 30 feet in diameter.

    Now lets place some Styrofoam blocks floating in that pool, and on them, lets place some bowling pins. (somewhat equally distributed around the pool)

    Now we will use a machine, to create waves on the surface of this pool.

    The waves we will say are dark energy. Magnetism, background radiation, black body radiation etc.

    The Styrofoam is wobbling the pins are waving but none have tipped over.

    Now lets gradually increase the wave energy, until one of the pins topples.

    When that happens we will now point and say, "Look!, There is an electron!"

    And without changing the strength of the waves, we will see other pins topple, and they will topple in an interference pattern over time.

    So what does that tell us about how this all works?

    Well it tells us, that there is always, energy around everything, and only when enough energy, is localized, will it do something, like light up a dot, on a screen, which everyone knows, can only be lit up, when sufficient energy, EQUAL, to a bowling pin toppling, or EQUAL to an electron, occurs in that place at that time.

    Do you see what I am saying? Brian Greene would be able to see this.

    If, you are using a detection method, that says, x will happen, only when a sufficient amount of energy equal to an electron, is in one place at one time, then you are in effect, making that electron, at that place and that time, and you, are deciding at what point it IS an electron, when in fact, it is merely energy, in the form of dark energy, which just now, has sufficient energy localized, to trigger your detection mechanism.

    If you look at the old Encyclopedia Britannica, from the days of Einstein, and Niels Bohr you will see a totally different description of what a particle is, from today.

    At that time it was an instantaneous point particle, something that was an imaginary construct, had no substance whatsoever, and was just a record of qualities at one point at one time.

    Like the pin which topples over, only when the conditions to accomplish that are satisfied.

    And the same wave function can be used, to look at the entire pool, and say, yes, pins will topple, and statistics can be used to say, yes, pins will topple, but no pins, at any time, wafted around the pool on their way to the detector. They were semi stationary, the whole time. In fact in this thought experiment, there is fishing line, in a grid, holding the Styrofoam in place.

    That would be akin to an Einsteinian view, as opposed to one Niels Bohr would have taken.

    Bohr would have said, who cares what it really looks like, statistically the pins will topple.

    Why does it make a difference? Well we can look at that set up and know, that at no time, a pin will topple on the far side of the moon, in all possible universes.
  5. Feb 23, 2009 #4


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    Generally speaking, I believe the interference disappears at this point. If you are able to gain which path information, the interference will definitely disappear.
  6. Feb 23, 2009 #5


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    Your comments are off topic, and besides they don't even make sense. The double-slit is an experimental fact, and there are many versions of it including http://grad.physics.sunysb.edu/~amarch/ [Broken] that are in direct opposition to your view. Particles are discrete even when they exhibit wave-like behavior such as interference.
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  7. Feb 23, 2009 #6
    You see the problem is you have too much equipment muddying your waters here.

    Let me continue just a bit with this thought experiment and lets examine what I am saying.

    We have pins, which we are saying detect, a level of energy that we are saying is equivalent e, when they topple. We can adjust the wave strength, until, just one topples or none topple.

    As we look at the surface of the pool, we see waves, yet we do not see a pin topple.

    Now lets increase the wave strength, and then we see one, then another topple, and they will topple in accordance with an interference pattern.


    They will do so because of the geometry.

    The wave strength, is increased in an interference pattern, as a result of the underlying geometry.

    Geometry as you know, has a set of axioms, and is not chaotic, not random, it is an ordered system, whereby things occurs in a regular predictable way, that can be plotted.

    Now you see that it is the underlying geometry, which is causing the interference pattern, and this idea of probability, does not affect that geometry.

    Hence your observation of the quantum realm as being somewhat obscure, and fuzzy, and half dead and half alive, is not in accordance with the reality of the stage itself, which is in an ordered state.

    So the geometry is in an ordered state, you are sending waves through this, and it is only because there are so many different types of waves in that area, of so many varied strengths, that it appears to be random and chaotic.

    It is only when they combine at x, with the value of e, do they release a photon which you can detect.

    And from that, people then fashion all sorts of notions, that are not necessary to explain what is actually going on.
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  8. Feb 23, 2009 #7


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    You obviously did not follow the earlier link, and here is a specific paper that explicitly rejects your concept of "wave strength" (which is wrong).

    http://marcus.whitman.edu/~beckmk/QM/grangier/Thorn_ajp.pdf [Broken]

    "While the classical, wavelike behavior of light (interference and diffraction) has been easily observed in undergraduate laboratories for many years, explicit observation of the quantum nature of light (i.e., photons) is much more difficult. For example, while well-known phenomena such as the photoelectric effect and Compton scattering strongly suggest the existence of photons, they are not definitive proof of their existence. Here we present an experiment, suitable for an undergraduate laboratory, that unequivocally demonstrates the quantum nature of light. Spontaneously downconverted light is incident on a beamsplitter and the outputs are monitored with single-photon counting detectors. We observe a near absence of coincidence counts between the two detectors—a result inconsistent with a classical wave model of light, but consistent with a quantum description in which individual photons are incident on the beamsplitter. More explicitly, we measured the degree of second-order coherence between the outputs to be g(2)(0)=0.0177+/-0.0026, which violates the classical inequality g(2)(0)>1 by 377 standard deviations."

    You might want to look at prior research (such as the above, 2003/2004) BEFORE you start telling folks "where physics lost its way". Also Mandel's theoretical treatment (1976) and Aspect's earlier experiments (1986). All of which shows that your (wrong) idea has been thoroughly considered already... and rejected by experiment as untenable. Turns out that PDC crystals have a number of useful scientific applications.

    So I would rate your bowling pin idea as a strike...out. :biggrin:
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  9. Feb 23, 2009 #8

    I am not going to try to overturn years and years of physics, and established physics, here.

    I will simply say, that the detector, is determining the result, and you can make detectors, to give you results that you look for, and that is the physics of today.

    I will however show you an article, which shows quite clearly, that all these assumptions in the defunct standard model regarding free floating particles are erroneous, and that is with regards to BEC experiments, where you can cool a group of atoms, until they become one large atom, and further cool that large conglomerate atom, until it disappears into the background.

    Let me clarify that just a bit, when you cool this BEC, using lasers, at one point, you get the Bosenova. An implosion, then a small explosion, which mimics what happens on the grand scale, of a supernova.

    So you see there is some consistency, from the very large, to the very small.

    And on this very small scale, some of that mass disappears completely during the Bosenova.
    It becomes one with the background.

    But the point is, that the quality of what we think of as matter, is not like little pieces of dust particles at all. It behaves quite predictably as wave energy, which can be cooled, until it simply disappears.

    You see the idea of a particle on a trajectory, would merely imply, that if you were to try and detect it, along a trajectory, you can plot points along that trajectory. That does not mean those points are moving.

    ref:http://en.wikipedia.org/wiki/Bosenova" [Broken]

    http://www.weizmann.ac.il/home/davidson/tomography.pdf" [Broken]

    In case some are not able to read between the lines in the above article, because the authors do not wish to overturn established physics either, they are merely stating the results of their work, and the results themselves are self evident, but what it shows in a nutshell, is that if you cool BEC, until it becomes one with the background, it will not always Bosenova, it will at times, merely perform a conversion of mass to energy, in accordance with relativity, such that the mass will be converted to kinetic energy, or in this case what is referred to as phonon energy.
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  10. Feb 23, 2009 #9


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    Wow, I wonder what you are doing hanging around here then. May I suggest if you want to discuss the state of "physics today", you start a thread on the subject? Your comments are completely misplaced in this thread.

    And of course, your reference - as might be expected - has nothing whatsoever to do with either this thread or your completely wrong assertion that the electromagnetic field is not quantized in units of photons.
  11. Feb 23, 2009 #10


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    This thread went south very quickly. Let's get back to the OP's question after that unwanted diversion.

  12. Feb 23, 2009 #11


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    south or north?

    i hesitate to point out the obvious …

    but until you observed that it went south, nobody knew whether it had gone south or north :wink:
  13. Feb 23, 2009 #12
    Thanks guys. I actually performed this experiment a little while ago and I found that as the final polaroid film was introduced the double slit interference pattern vanished; however, I still observed a single slit diffraction pattern. If the wave function collapses completely once the measurement is made, why do I still observe the single slit diffraction pattern? Could my equipment perhaps be messed up, or am I missing something obvious?
  14. Feb 24, 2009 #13


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    Hi maggicmike660! Thanks for the PM. :smile:

    Collapse of the wave-function means that the wave-function was a superposition of eigenstates, but is now only one eigenstate …

    it can collapse in respect of one measurable, which still remaining a superposition in respect of others …

    for example, if you measure the momentum of an electron, it can still be in a superposition of spin states.

    In the double-slit experiment "with no peeking", the wave-function is a superposition of states from the two slits, resulting in an interference pattern when it hits the screen.

    But "with peeking", it is only a superposition of states from one slit, resulting in a diffraction pattern when it hits the screen …

    but I don't think that really counts as a collapse, since the wave-function in that set-up was never a superposition of states from both slits … the fact that a diffferent, previous, photon was in a set-up which had superposition of states from both slits is irrelevant.

    Anyway, the basic answer to your question is that the diffraction pattern arises from a superposition of states from one slit. :smile:
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