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Another misleading observer example, or not?

  1. May 12, 2007 #1
    In this example, which I found via Wikipedia on Delayed Choice,


    it makes the "observer" special, i.e. it's the observer who "chooses" whether the photon goes through one slit or the other, or through both as a wave, or more dramatically, whether it came from one side or the other of a black hole out in the nether regions of deep space.

    It's a standard double slit where two detectors are focused tightly, one at each slit. If you put a screen in front of them, you see an interference pattern on the screen, if you take away the screen, then you see the photons only in the two detectors. Yadda, yadda, yadda.

    Now, here's the thing I have trouble with. The web page finishes up with several statements like:

    "If you are serious about the idea that the result is determined only upon observation ..."

    this once again puts the emphasis on the Observer, with a capital O. You, the Observer are special; you are a fundamental part of the magical process called QM. Feel good about yourself.

    Here's where I start to get hazy, so please correct: if you move one of the two detectors slightly closer to the light source, that detector should get all the photons, since the wave function will reach it first, and collapse there. Similarly, if you add a third detector so all three are equidistant from the light source, then you should start to see a probabilistic distribution between the detectors. You should then be able to add detectors until you fill the space, and get the full interference pattern as collected by all of them.

    What happens if you put the screen over the two detectors, and cut two holes such that the detectors are now flush with the surface of the screen? Will we find that the interference pattern has vanished because now there are special spots on the surface which "know" about photons?

    Next, you can replace the detectors with bricks, and boy howdy, you get the same results. Now take away the bricks, and shine it right out into your living room, and look at that, the plastic Monty Python penguin on your Telly set has a little dot of light on its nose; it has "chosen" the result of your quantum experiment!

    Ok, so, am I all wet, or did this web page example just promulgate the notion that there is some spooky relationship between the observer/human and the experiment? It does so in a more subtle way than some, but in the end, it is a nice clear explanation which finishes up doing a disservice to the reader.

    Granted, the home page of the site is titled "The Notebook of Philosophy and Physics". Still, it is quoted on Wikipedia page which otherwise gives a pretty no-nonsense description of the DCQE. The Wikipedia page is a little dense, though, so I thought I'd go straight to some pictures, and where do I end up, but on the site that says, "To understand quantum mechanics is to understand that there is a central mystery in physics."

    They also say that

    "If the universe is a computer running along conventional programming lines, then who are we? And who programmed this virtual reality simulation? And why? That is philosophy, certainly. And it is religion. And it is still science. "

    which happens to be one of my favorite pet conspiracy QM interpretations, but nevertheless, it's a site with an agenda (it has some pages dedicated to the link between consciousness and QM), so is *not* appropriate for introductory material, it's more like indoctrination material.

    Oh well, I thought I'd check here about it all before I go off and rant to myself for a while.

    :grumpy: :yuck: :cry: :rofl: all apply tonight.
    Last edited: May 12, 2007
  2. jcsd
  3. May 13, 2007 #2
    Isn't that argument about possible outcomes of telescope observing incomplete? I think the most probable outcome is, that the photon is not seen anywhere, because it flied some where else :biggrin: No, seriously. I haven't try to calculate what happens to a wave packet there, but it looks like it has very small change of reaching the end of either telescope. I mean, the wave reaches the first lenses of the telescopes, but not the eye that is behind them. And if a flash is seen in either of the telescopes, isn't it simply incorrect to conclude that it had then travelled through only one slit?

    I see no reason to assume that a history of a wave function would depend on where it collapses at some instant. That claim doesn't make any sense.

    My interpretation is this. The wave does not necessarely collapse onto the first detector, but it can also collapse outside it, and continue.
  4. May 13, 2007 #3
    Why? Light waves pass through glass....
    Well, AFAIK, that's exactly what Young's experiment demonstrates. On the other hand, I don't know about "histories" of wave functions.
    "Outside" it? Waves collapse when a component becomes fixed, i.e. position, which happens when it hits anything, right?
    Last edited: May 13, 2007
  5. May 13, 2007 #4


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    Yes, but just putting a detector somewhere doesn't guarantee you'll get a "hit" at that position, it may be that you'll register no hit there and then later register a hit at a detector further away. The wavefunction determines the probability that you'll get a hit at any given location if you have a detector set up there.
  6. May 14, 2007 #5
    Oh, yeh. :redface:

    Then I need to rethink what I think about the original example. When the screen is removed from the front of the two detectors pointing at the slits, then there should really be an interference pattern painting across the wall behind the detectors, right?

    Presumably, the two wave fronts from the slits are filling the available space. This then makes the original example completely wrong. It shows that when the screen is removed, that the full count of emitted photons will be distributed evenly between the two detectors. I.e. it shows 14 photons in a interference pattern on the screen, and then 7 photons in each of the detectors after the screen is removed.
  7. May 14, 2007 #6


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    Ah, I get what you're asking now. You're right, given the setup on the page you linked to, it's probably true that if you put the screen behind the detectors instead of in front of them, a certain proportion of photons would hit the detectors but a certain proportion would hit the screen, and the ones that hit the screen would still form an interference pattern. But I suppose it's possible that the diagram is misleading in the way it draws the detectors, and that their lenses are actually supposed to be large enough so that almost every photon will hit one lens or the other.
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