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Question about electron diffraction

  1. May 10, 2005 #1
    I have been reading lately about experiments done in the 30s and 40s regarding electron diffraction patterns. Apparently when electrons are fired through what effectively amounts to a double slit, then detected on phosphorous, they create a diffraction similar to that of coherent light. This is because, according to QM, electrons are only defined by their probability functions until there is an attempt to detect them, and these waves interfere with each other in the same way as photons might.

    I am ok with all of that. What gets me is that when the electron firing rate is slowed to a mere electron every 10 seconds you still get the same pattern. What causes photon diffraction is the path difference between two photons travelling and interfering at the same time. When you slow the rate to one electron every 10 seconds, though, you no longer have interference between two probability waves at the same time, so how on earth could they continue interfering which each other when one is detected and the other hasn't even left the emitter yet?

    I have a theory about why I think it happens, and maybe I'm right; I am probably not, though. I think that perhaps, since all electrons have an infinite, but defined, probability function, the interference between any two electrons in the universe is always decided at any given point in space/time. More clearly: if you were to freeze time, and check out (impossibly) the current distribution of probabilities for every electron, assuming you could do such a thing, then you could find the level of interference between any group of electrons at any point in space. Essentially, the interference pattern between any two electrons is already determined, and you only get to see it when the electrons are detected on the phosphorous screen....

    I am probably way off here, but to me it makes no sense that two waves could interfere with each other even if they're separated by 10 second intervals....can someone shed some light on this for me? (No pun intended, I swear.)
     
  2. jcsd
  3. May 11, 2005 #2
    Here's where you went wrong. If the photon was a classical wave this would be true but it's not. The photon sort of "interferes with itself" when "choosing" it's path.
     
  4. May 11, 2005 #3
    Ah, then it's the trickiest and weirdest part of the basic experiment you need to know. :smile:

    A single photon (or electron or any particle) acts like it went through both slits at the same time(!) if nothing measures which slit the particle went through. This is what causes the interference effect.

    The particle acts like it went through both slits and bumped into itself and the interference pattern builds up on the screen particle-by-particle as each particle somehow bumps into itself after it passes the slits.

    However, if we do try and measure which slit the particle goes through, this destroys this interference effect for the particle. It can be viewed that this forces the particle to decide it's going through a particular slit.

    It's weird, but there we go.
     
    Last edited: May 11, 2005
  5. May 11, 2005 #4

    ZapperZ

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    You will need to understand a lot more to figure out why this statement is wrong. Till then, you may want to hold off on forming a "theory". I suggest you do a search on PF for thread that include a discussion on why the interference pattern that you observe is a "1-photon" interference, and that 2-photon, 3-photon, etc. interference a extremely seldom seen (Dirac even said 2-photon interference doesn't occur - we know it does now but with very small probability)

    Zz.
     
  6. May 11, 2005 #5
    When my physics professor explained photon diffraction, he explained that what caused it was the phase shift from "two rays of light" - this is obviously wrong, as inha and caribou informed me (thank you), and I'll have to take that up with him. I naturally assumed that since electrons are expressed as probability waves until detection, that their collective interferences were already "finalized" before detection even began. Now it makes much more sense to me. Thank you.

    As I said, I only came up with a theory to explain it to myself. It's not as if I'm trying to overthrow the entire scientific community by posting a theory on a topic I know little about. I simply did not know the QM reasoning behind electron diffraction (and apparently photon diffraction was explained very poorly by my physics professor), developed what I thought could possibly be an explanation, and then asked about it. I'm only trying to learn, just like all of us are.
     
  7. May 11, 2005 #6
    Saying it's "two rays of light" is okay as long as it's made clear that one photon is made up of two parts, with one part in each ray. The usual view is that measurement at the slits causes the two parts in their rays to collapse into one part in one ray. :smile:
     
  8. May 11, 2005 #7

    ZapperZ

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    Your professor is not wrong. That explanation is using the WAVE picture of light. There is absolutely nothing wrong with that and we still teach that in optics classes. The problems comes in when you try to use that concept and apply it to photons. They simply do not mix. Once you have adopt the photon description of light, then the rules are now different. The QM description kicks in and HOW you perform your experiment is as important as WHAT you are performing on.

    .. and all I adviced was to hold off on forming a "theory", but start off with the "trying to learn" part. One usually tries to learn first, and forms theory after, not the other way around.

    Zz.
     
  9. May 11, 2005 #8
    Ah, don't worry about it. Even the greatest physicists and mathematicians aren't perfect. At a conference, Albert Einstein gave a lecture to show that quantum theory had a major problem and so was obviously wrong. Neils Bohr and others spent a few days desperately trying to find the flaw in Einstein's argument and, amazingly, discovered that Einstein had forgotten about the effects of general relativity, his own great theory he'd worked on for a decade! Ouch. :blushing:

    There are other examples involving more than a few big names. Nobody's perfect.

    Anyhow, I had a kind of similar idea to yours about the randomness of what a particle decides to do being the result not of a fundamental randomness in the universe but due to a combination of the effects of every particle in the universe interacting with it and all the tiny effects adding up to make it seem random, when in fact it isn't.

    I'm not sure where I'm going with that idea yet but the basic idea both of us have is certainly worth exploring, although someone else has very probably thought about it before. :smile:
     
    Last edited: May 11, 2005
  10. May 11, 2005 #9

    ZapperZ

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    If you are refering to the EPR paradox that Einstein brought up, let's make sure something is very clear. Einstein never claim that QM is wrong. Even he knew of the amazing agreement QM had with experimental observations. It would be silly for him, or anyone to claim that when faced with such evidence. What he claimed was that QM was incomplete! The EPR paradox (a paradox at that time) appears to indicate that QM is missing the so-called "hidden" variables that isn't included in QM formalism. Thus, it is incomplete in his point of view.

    We need to be very careful in how we describe historical events and physics ideas, because miconception and misunderstanding such as this can perpetuate or get taken out of context.

    Zz.
     
  11. May 11, 2005 #10
    No, it wasn't EPR, it was the "Clock in the Box" experiment.
     
    Last edited: May 11, 2005
  12. May 11, 2005 #11
    I don't see what's wrong about trying to flex my mind a little bit to try and see if I can figure out why something is happening while waiting for someone to answer. If I were actually forming a true "theory" about something I wouldn't have bothered asking the question in the first place. I simply stated what I thought it might have been, and asked if I was right. The proper way to handle it is to explain to me what's happening, not tell me I shouldn't be talking about stuff I don't know, which honestly amounts to little more than condescension.

    You are obviously very informed about the topics you help with here, and for all I know you teach by profession. However, I think telling someone they shouldn't postulate something on their own is a bad thing to do, imho. Part of learning is figuring things out for yourself... I may have been wrong in this case, but forcing myself to think about it helped me to think outside the box a little bit. It would have been nice to have the proper tools under my belt to answer the question for myself, but I didn't, and so I made an educated guess as to what the cause could have been while I waited for someone to explain it to me. Frankly, I don't see anything wrong with that.

    Caribou: I just read about the clock in the box experiment a few days ago, incidentally. Funny you mentioned it.
     
  13. May 11, 2005 #12

    ZapperZ

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    Not to belabor this any further, but if you noticed, I just didn't tell you not to do it. I also explained what concept you had wrong and why you had it wrong. In fact, I accompanied each of my replies to you with what I thought was an explanation why you were applying things incorrectly. You made it appear as if I asked you to stop without any kind of explanation.

    Zz.
     
  14. May 11, 2005 #13
    It is a classic example that even Einstein wasn't infallible. Bohr himself wasn't infallible either. Quantum physics is as a conceptually difficult subject as there is and they both learned that the hard way.

    By the way, something to know is that the word "theory" is used in a slightly different way in science to everyday usage, with "hypothesis" meaning rough ideas being considered and investigated and with "theory" meaning ideas which are much more developed and investigated than a hypothesis has. Google will bring up much better definitions than mine. :smile:
     
    Last edited: May 11, 2005
  15. May 11, 2005 #14
    Fair enough. I knew that, but I'm not really used to making a distinction between them :wink:
     
  16. May 11, 2005 #15

    ZapperZ

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    I tried to check up on what exactly is the "Clock in the Box" thought experiment that you brought up. If you are implying the "conference" as in the 1927 Solvay Conference, then the thought experiment that he brought up was what is now known as "Einstein's Boxes" experiment.[1]

    If this is correct, then my original response still stands, that Einstein did not claim that QM was wrong. He tried, in vain, to insist that QM was incomplete.

    Zz.

    [1] http://arxiv.org/abs/quant-ph/0404016
     
  17. May 11, 2005 #16
    I know. Okay, I'll add then that Einstein's overall aim in his objections was to show that quantum mechanics was obviously wrong about what was fundamental. And yes, it was "Einstein's Boxes". :smile:

    This, by the way, has reminded me of Einstein admitting in 1949 that quantum mechanics was "complete" (the underline is his):

    "This theory is until now the only one which unites the corpuscular and ondulatory character of matter in a logically satisying fashion; and the (testable) relations, which are contained in it, are, within the natural limits fixed by the uncertainty relations complete. The formal relations, which are given in this theory -- i.e., its entire mathematical formalism -- will probably have to be contained, in the form of logical inferences, in every useful future theory."

    Just thought I'd throw that in. :smile:
     
  18. May 11, 2005 #17
    Ah, it's one of those little details we're supposed to know even though it makes no difference in practice. :biggrin:
     
    Last edited: May 11, 2005
  19. May 12, 2005 #18
    Actually you’ve taken Einstein a little out of context with that one sentence quote.
    But even thou he was in failing health in 49, note the careful “qualifiers” he had in that sentence (within the natural limits & will probably). Not that he couldn’t be direct as well, note just two sentences prior the above Quote he was very direct in how he still thought QM was incomplete, when Einstein said:
    “In what follows I wish to adduce reasons which keep me from falling in line with the opinion of almost all contemporary theoretical physicists. I am, in fact, firmly convinced that the essentially statistical character of contemporary quantum theory is solely to be ascribed to the fact that this [theory] operates with an incomplete description of physical systems.”

    To the end Einstein still held, even if remote, that there may still be some chance that a description of our physical reality may yet be more complete than QM. I'm just one of the few that agree with him.
     
  20. May 12, 2005 #19
    This is almost a meaningless statement. I think any reasonable physicist would agree that any theoretical model MAY be incomplete, even if experimental evidence has not necessarily demonstrated so.
     
  21. May 12, 2005 #20
    And yet so many and the foundation of QM disagrees with Einstein on it.
     
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