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The double slit experiment

  1. Jan 5, 2010 #1
    Hi all,
    I'm not trained in physics beyond high school but have a healthy interest and understand a little quantum physics and special (still grappling with general!) relativity.
    Here's a quick question on a pop.science book I'm reading at the moment. The book is by Marcus Chown, it's called 'Quantum Theory Cannot Hurt You.'
    In the book he makes an example of the double slit experiment we all did in high school and says that if we fire one photon at the slitted screen, we'll get an interference pattern on the rear screen. So far, so good. He then goes on to say that if we can detect the photon going thru one slit rather than the other(i.e. detect which slit the photon passed thru), that we can't possibly pick up any interference pattern at the 2nd screen due to wave/particle duality.
    Is he serious? Are there experimental results that show this?
     
  2. jcsd
  3. Jan 5, 2010 #2

    Fredrik

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    Yes there are tons of experiments that support that claim. (I don't have any references for that, but I'm sure Google can help you find some).

    The simplest explanation by far is that QM doesn't tell you what actually happens in an experiment. It's just a set of rules that you can use to calculate the probabilities of possible results of experiments. (There are plenty of suggestions about how QM might be interpreted as a description of what actually happens, but all of them have problems).
     
  4. Jan 5, 2010 #3

    DrChinese

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    To quote from a physics lecture by Professor Henry Greenside:

    "Consider Young’s double-slit interference experiment but now put a linear polarizing filter in front of each slit. Then as transmission axis of one polarizer is slowly rotated about its axis while transmission axis of other polarizer is kept fixed, one finds experimentally that the bright regions become dimmer, the dark regions become brighter, until the fringes disappear into a smoothly varying region that is brightest behind the slits and gets dimmer off to the sides."

    In other words: when the polarization of the light tells you which slit the photon went through, you get NO interference. That occurs when the polarizers are crossed (90 degrees offset). When they are parallel, you have no idea which way the photon went - and there IS interference on the screen.
     
  5. Jan 5, 2010 #4
    Dr. Chinese- the only way that I could see to demonstrate that the photons 'stopped' going through the one hole is to somehow attach a photon counter to the rotated polarized slit and see if photons stopped flowing there. I also read that if one simply puts photon counters on the backside of the slits, the whole double slit image on the screen collapses- i.e.; the photons don't ' like being watched '.
     
  6. Jan 5, 2010 #5
    J12345 - are you attributing consciousness to particles? Bohr would just say that the measurement is the collapsing of the wave function, and that's the easy way out that physicists have followed for decades. You miss the subtle questions that we need to start asking again.
     
  7. Jan 5, 2010 #6

    DrChinese

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    Well, the photons don't know that you aren't checking their polarization when they hit the screen... you could check that in principle. As a result, the inteference disappears. (When the polarizers are crossed, a photon can only go through one slit or the other - so detecting the polarization would tell you which.)
     
  8. Jan 5, 2010 #7

    Mentz114

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    After such a sweeping and maybe slightly arrogant statement you should share your secrets and tell them where they've been going wrong.

    Ask away, don't be shy !
     
  9. Jan 5, 2010 #8

    Fredrik

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    Another interesting experiment is to do this with C60 molecules, and repeat the experiments many times with a different air pressure around the setup each time. You get a "wave" interference pattern (as if both slits are open at the same time) when you do it in a vacuum, and a "particle" interference pattern (as if only one is open at a time) when the air pressure is high. The interesting part is that you get an "intermediate" pattern when the air pressure is neither high nor very low.

    The reason is that interactions with the environment (the air) changes the statistical properties of the molecules, as more and more information about the paths taken can (in principle) be inferred by performing measurements on the state of the air. This proves that "wavefunction collapse" isn't a sudden and discontinuous physical process, and that "wave-particle duality" isn't about the system being either a wave or a particle. Apparently it's a little bit of both. For more information about this, see a book or a review article about decoherence.
     
  10. Jan 5, 2010 #9
    To me the arrogance is the way the transactional interpretation and bohms interpretation have been so ignored, and the defeatist Copenhagen and rediculous Many Worlds interpretations have been so embraced. It's as if modern scientists have the curiosity of old men and the dilligence of children. Upside-down in my opinion.

    How does the supposed Higgs particle give mass to itself? Why does dark matter seem like such a childish solution to a serious question ? Why does the moon spin at exactly the right speed so we only ever see one side, and why is it exactly the right size and distance from us for eclipses? Why is so much of science's funding spend on irrelevant issues where the answers are always sensational, but contradict each other year to year?

    Why does physics try so hard to refute the anthropic principle that it invents billions of unseen whole universes PURELY to refute the 'stain' in the mind that the universe (and all it's amazingly consistent laws and mathematical consistency, such that a pencil and paper can predict events billions of miles away), is more than randomness from nowhere.

    Until science frees itself from from post modernism and all the crap that the current education system feeds students, despite the efforts of brilliant teachers and tutors, students will continue to learn "facts", which will be proven wrong in years if not decades or even centuries. What they should learn is questions, but sadly science has become too specialised for that to be feasable anymore.
     
  11. Jan 6, 2010 #10

    Mentz114

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    I don't see the world in the same way at all. You are are generalizing too much. Physics is done by a very varied and often disparate bunch of people who certainly don't agree amongst each other about everything.

    How else could it be ? Scientific theories are hypotheses that more or less agree with observation. If they can be replaced with improved theories then so be it.
     
  12. Jan 6, 2010 #11

    DrChinese

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    Next time, please consider using the tags [RANT] and [/RANT]. :smile:
     
  13. Jan 6, 2010 #12
    Dr. Chinese? how do you 'register' on your web site- there seems to be no spot to hit to register thanks
     
  14. Jan 6, 2010 #13
    @SimonA: We get it, the elegance of the universe takes your breath away, and why not, it's all for you. *rolleyes* Anyway, you might want to consider that "counterintuitive" doesn't equal "wrong". As for your concepts of teaching, you would... what... throw the very latest theories at kids and hope they can cope? LOL Oh boy. Everyone needs a platform from which to begin their learning, and first we have to be taught HOW to learn. That's what school is about before graduate work frankly, but it seems that notion has been drowned in a torrent of your rightous indignation.

    I do have a solution for you however. If you want people to learn facts that haven't been discovered yet, build a time machine. Good luck with those CTCs buddy.
     
  15. Jan 6, 2010 #14
    Wow
    i did not not know that nice part about the PRESSURE
    Thank you
     
  16. Jan 7, 2010 #15
    I admit I'm biased, so I apologize if I offend anybody. It seems to me at least, that most of these crazy QM experiments can be pretty easily and logically explained. When we speak of an "observer", we don't literally mean just a mechanical eyeball or something like that. In order to gain a quantum measurement, the object being measured has to physically interact with a classical object, which will the measurements obtained. Quantum information changes when information is transformed into a classical "measurement". For example, with the double slit experiment, the "observer" is a piece of film in which the particles travel through. The object behaves differently when "observed", but people seem to ignore that physical interactions are taking place.

    The fact that we so willingly accept that a particle spins in both directions at the same time until measured, is a little bit absurd. If you're in space viewing Earth from the North Pole, the planet is rotating in the opposite direction compared to anybody viewing it from the South Pole. How can we tell which "pole" we're viewing when we "look" at a particle? Quantum Physics aren't as magical as they can be when we stop making assumptions.
     
  17. Jan 7, 2010 #16
    You do realize that the term "spin" is a term of art and does not literally refer to a particle spinning about an axis? The term was chosen because of a mathematical similarity, just as "colour" and "quark" were rather whimsical. Beyond that, explain the following: You take a single buckyball, or a Rubidium atom, and pass it through the double slit. It STILL forms an interfernece pattern as would be expected from quantum behaviour. Again... counterintuitive does not mean "wrong".
     
  18. Jan 7, 2010 #17
    In other words, when you cross polarity by 90 degrees you get no interference. This proves nothing at the quantum level
     
  19. Jan 7, 2010 #18

    DrChinese

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    Welcome to PhysicsForums, Dunce!

    We all have bias, and a lot of that comes from assumptions. It actually turns out that your view - which is considered a "classical" one - has more assumptions than the standard quantum mechanical perspective. So it is not the seasoned scientists making assumptions, it is you.

    It turns out that in the double slit experiment, for example, you would expect that the particle either goes through one slit or the other. Yet as mentioned above: all I have to do is put polarizers on each slit and set them certain ways, and I can make the interference disappear. This corresponds to knowing which-slit, and cannot be explained classically because it requires you to abandon your initial assumption that the particle (or whatever is it) only went through one slit or the other. Obviously, the interference effect is evidence to the contrary.

    To make this clear, consider the following two setups A and B, which are identical except for the settings of the polarizers in front of each slit. In each case, the source beam is polarized to 0 degress and the L (left) slit has a +45 degree polarizer. The R slit has a ++45 degree polarizer in the A setup but has a -45 degree polarizer in the B setup. Sorry for the crude drawing...

    A. Inteference IS seen
    ....======
    .... Source
    ....== | ==
    ........ |
    ........ V
    == /L/ = /R/ ==

    .===Screen===


    B. NO Interference seen
    ....======
    .... Source
    ....== | ==
    ........ |
    ........ V
    == /L/ = \R\ ==

    ===Screen===

    In both cases A and B above, the amount of light that is detected on the screen is the same. And that amount is half of the light that would go through if there were no polarizers in front of the L and R slits. In other words, the polarizers filter out half the light. In the classical view, where does the interference originate? And why does it disappear if we cross the polarizers?

    Regardless of whether you advocate the particle or wave perspective, your answers will not be consistent. On the other hand, quantum mechanics explains the A results based on a superposition of states which gives rise to interference terms. But there are no interference terms when the polarizers are crossed. So QM relies on a mathematical formalism rather than an "intuitive" description.
     
    Last edited: Jan 7, 2010
  20. Jan 7, 2010 #19

    DrChinese

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    Sure it does. See my A/B example in the above post. You can see that the only difference between the A and B setups is that the R slit polarizer is rotated 90 degrees. This provides the answer to the "which slit" question - at least in principle (you don't need to actually know this information, it is enough that you *could* know it). That is enough to eliminate the interference. There is NO chance that a particular photon polarized at 0 degrees could pass through both a +45 polarizer and a -45 degree polarizer in the quantum mechanical view. The operative formula is the COS^2(L-R) rule, where L-R=90 degrees so that the result is 0 and there is no interference.

    On the other hand, in the classical perspective, there IS a chance that any particular photon polarized at 0 degrees could pass through both a +45 polarizer and a -45 degree polarizer. Do you see why? The rule is different because the probability is resolved independently for each slit, unlike in the quantum view in which it is the relative angle of the L and R slits is important. So now you get COS^(L-0)*COS^(R-0) and there should be some interference because the result is .25 which is >0.

    In other words, the quantum view considers both the L and R slits as a system, while the classical view considers each slit separately. You see the same kind of things with entangled photons, which is a much better example than the double slit. But either way, it is a quantum effect although the traditional Young setup itself (not the example I gave which is modified) is sometimes considered a classical wave example too.
     
  21. Jan 7, 2010 #20
    @DrChinese: You have no idea how happy I am that you showed up.
     
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