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Double Slit Experiment: one at a time

  1. May 26, 2015 #1
    Hi Folks,

    I am not sure I can appreciate the quantum weirdness that supposedly exist because I do not fully understand the experimental set-up of the double slit experiment. I watched the following which left me with some unanswered questions



    I have some questions about the experimental setup.

    In the case of the "particle" being fired one at a time at the board with the slits.
    1) Is the firing gun placed directly in the middle of the 2 slits? If this is the case, how can you expect the particle to pass through the slits, they will just bounce of the board....none will go through the slits?

    In the case of the particle being fired one at a time while there is a camera watching one of the slits.
    Despite the fact that I dont expect any particles to arrive on the back screen as I have mentioned above, I will proceed with this question anyway:
    2)If the gun is firing 1 particle per 10 seconds say, then we expect 6 particles to have left the gun in 60 seconds. How many particles will we count on the back screen?

    In the case of no cameras being switched on, we get the so called interference pattern when we fire one particle at a time.
    3)Does the number of particles that leave the gun match the number of particles that arrive on the back screen?

    It is claimed that the particle acts like a wave when we are not looking and so an interference pattern appears on the back screen.
    4)Has each particle split into 2 or more particles so as to form this pattern, if so wouldn't that violate the conservation of energy because we have created more particles?

    I just find reading various articles online to be conflicting and ultimately confusing....
    Any information will be appreciated
    Regards
     
  2. jcsd
  3. May 26, 2015 #2
    The gun isn't like a rifle it's more like a shot gun spraying the electrons out. As long as they get through the screen they will register. Particles this small are associated with having some wave form like properties. If you think of a small toy boat heading towards a slit with its own bow wave traveling slightly ahead of the boat you can imagine the boat being deflected slightly by the waves if they bounce off the side of the slit and even another slit if the bow wave travels like through that one. I doubt this entirely matches the formal description of the wave like character of the electron, but it makes it comprehensible as to the kind of thing that could be happening. No one knows precisely what it going on.
     
  4. May 26, 2015 #3
    Ok, its a shotgun. In that case, we will never see the total number of particles on the backscreen.
    How on earth can a particle which is being fired one at a time end up on the back screen in a location that is not in line with the slit? It cannot bounce off itself?!

    Is this the mystery?
     
  5. May 26, 2015 #4
    1) The gun has a certain spread to it.

    2) It depends on how large the slits are. If the slits are small, then only a few shots will reach the back screen. Most will hit the slit plate.

    3) No. Some hit the slit plate. Some pass through the slit plate but miss the screen. And some miss the slit plate entirely.

    4) No. You could informally say that the photon splits and takes both paths, but you can't call each path a particle. Particle refers to the whole thing, which includes contributions from all paths together. If you add together the contribution over all paths, the total amount of photons hitting the plate or the slit or missing will equal the total fired.

    post below) Particles don't move in straight lines. Shotgun shot moves in almost straight lines, but is bent by gravity and aerodynamics and stuff. Light gets bent by diffraction.
     
  6. May 26, 2015 #5

    bhobba

    Staff: Mentor

    It's because the slit acts as a position measurement hence it has an unknown momentum after the slit. However kinetic energy is unaffected, hence the magnitude of velocity remains the same so only direction is affected.

    Here is a correct analysis of the double slit:
    http://arxiv.org/ftp/quant-ph/papers/0703/0703126.pdf

    Thanks
    Bill
     
    Last edited: May 27, 2015
  7. May 27, 2015 #6
    The problem is in the questions asked, such as "how can it be like that", the first answer to which is to be found in the detector pattern obtained - that its not a question of how it can be like that but, given that it actually is like that, how might one rationalise it (amongst other things one might do).

    Its quite difficult to do.

    Describing the pattern is the first step. But this isn't strictly necessary since the pattern is already (in a sense) a description. But like a shimmer on the horizon in the desert, such a description ("shimmer on the horizon") can be misleading. Without further investigation one might jump to the conclusion that the shimmer means there is water to be found yonder, and while sometimes that might actually be the case, it is more often not so. When one doesn't find what one expected to find (such as water) one shouldn't assume the universe is an illusion playing silly buggers with an otherwise perfectly rational expectation of water, but rather one should question the logic that led to the expectation of water in the first place.

    If a rifle model can't reproduce something that looks like the pattern on the screen then it's not the pattern on the screen which is the problem. It's the rifle model, since it can't reproduce that pattern.

    Eventually one realises that the problem isn't so much the required gun, but the bullets. One needs some analogy better than bullets.

    The mystery is not in the way the world presents itself (that's just the way it does present itself) but in how we might rationalise it, or otherwise describe it, or otherwise theorise it, or otherwise manipulate it to evil ends. :)

    Carl
     
    Last edited: May 27, 2015
  8. May 27, 2015 #7
    I think there was some experiment that managed to detect some perturbation at either slit as the electron passed and showed it does indeed only go through one slit.
     
  9. Jun 3, 2015 #8
    Just a layman here but I will try to contribute. When we fire a shower of particles (say electrons) without detectors on the slits, some may indeed hit the slits plate and get absorbed, that's fine and not really relevant. Of those which don't (which pass through the slits), some of them because of the diffraction they suffer will interfere with others passing through the other slit and cancel out, so they will not make any impression on the back screen. Others will reinforce each other making a stronger impression on the back screen, and still others will neither cancel out nor reinforce completely leaving a middle intensity impression on the back screen.

    Now, when the electrons are fired one at a time, some will leave no impression at all, some will leave a strong impression, and some will leave a middle impression. After many many electrons fired we will observe the same pattern as when we fired them all at once in a shower.

    You could interpret this in two ways. Either the electrons 'know' that another electron will be fired in the near future and interfere with it 'even before it is fired', or they interfere with themselves. The mainstream view is that they interfere with themselves, meaning that they did not behave as what we intuitively think of as a 'particle' but that they behaved as a wave, passing through both slits at once. Because of probability, some get wave shapes that interfere and cancel out (leaving no impression), some get wave shapes which reinforce (leaving a strong impression) and some get wave shapes which do neither (leaving a middle impression), exactly with the same distribution as when firing many electrons at once in a shower.

    This however makes us reconsider the first statement where we said that when fired as a shower, interference occurs because electrons passing one slit interfere with others passing the other slit, due to their diffraction. When we know that they can interfere with themselves there is no need to invoke the other electrons to cause the interference.
     
  10. Jun 3, 2015 #9

    jtbell

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    No. Each electron produces the same "strength" impression on the detector plate. The impressions are distributed in such a way that as they build up on the detector plate, the diffraction pattern becomes apparent. The minima of the pattern have fewer impressions per cm2, and the maxima have more.
     
  11. Jun 3, 2015 #10
    Sorry but I don't get that. When fired as a shower, some of them interfere with each other cancelling out and therefore leaving no impression on the detector plate (dark area).

    When firing them one at a time, there's no way an electron passing through the slits could leave no impression at all (i.e. it could result in a completely dark area), unless it interfered with itself.
     
  12. Jun 3, 2015 #11

    jtbell

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    Each electron that goes through the slits eventually arrives at the detector screen and produces an impression with the same "strength". The effect of the interference or diffraction is to make the pattern of impressions different from a uniform one. It "rearranges" the pattern of impressions from what you would get if there were no interference.

    In terms of classical interference, the portion of the wave that passes through one part of the slit(s) interferes with portions that pass through other parts of the slit(s). Each infinitesimal portion of the slit(s) acts as if it were a separate source of waves, and all the waves thus produced combine to form the final interference pattern.

    http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/sinint.html#c1
     
  13. Jun 3, 2015 #12
    So you say that there are not truly dark areas due to cancelling out therefore leaving no impression, only less bright ones due to partial cancelling out. Right? I do not see a radical difference but it's worth making this clear if I was wrong.
     
  14. Jun 3, 2015 #13
    I think what is relevant here is the fact that if observation is happening after the slit you still get a wave pattern. If there is no observation you end up with a club pattern. So the very act of observing even after the photons pass through the slit changes how they behave when passing through the slit. So observing changes the past behavior.
     
  15. Jun 3, 2015 #14

    jtbell

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    There are some places where the intensity is zero, in the classical Fraunhofer single-slit and double-slit (and multiple-slit) diffraction patterns. An electron will never arrive at those places.
     
  16. Jun 3, 2015 #15

    ZapperZ

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    This is actually incorrect and why you are having a severe misunderstanding of this phenomenon.

    The interference effect that you see is a single-particle interference. It is the particle causing an interference with itself since, in QM, its wavefunction covers both slits. 2-particle interference looks DIFFERENT than this, and it is also not very common at all.

    It is why, when you accumulate many more particles, it begins to look like the well-known interference pattern. It is ALL the aggregate of the effect produced by each individual particles that went through those slits.

    Zz.
     
  17. Jun 3, 2015 #16

    Nugatory

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    No, that's not right. When we say that the electron interferes with itself, what's being cancelled is not the electron itself but the probability amplitude of the electron landing at a particular point on the film. The electron always lands somewhere on the screen, but that somewhere is one of the areas where the amplitudes interfere constructively rather than destructively.
     
  18. Jun 6, 2015 #17
    I read articles where researchers claim to manipulate groups of atoms to be in 2 different places at the same time (MIT 1997 bose einstein condensate sodium atoms).

    How does this relate to the double slit experiment in terms of the same particle being in 2 different places?
     
  19. Jun 6, 2015 #18

    Nugatory

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    Stuff like this is the reason we have the rule requiring that sources be properly cited. If it was talking about atoms being in two places at once, the odds are that the article was a popular-press summary that's misdescribing the research.... But without a citation, we have no way of knowing.

    We can't say because we don't what the research in question actually is.... But you should not be thinking of the double-slit experiment as if the particle is in two different places at once. The particle doesn't have any position except when it's interacting with something that can detect its position, and if there are no detectors in the slits, it isn't interacting with anything until it gets to the screen.
     
  20. Jun 6, 2015 #19

    Nugatory

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    Stuff like this is the reason we have the rule requiring that sources be properly cited. If it was talking about atoms being in two places at once, the odds are that the article was a popular-press summary that's misdescribing the research.... But without a citation, we have no way of knowing.

    We can't say because we don't what the research in question actually is.... But you should not be thinking of the double-slit experiment as if the particle is in two different places at once. The particle doesn't have any position except when it's interacting with something that can detect its position, and if there are no detectors in the slits, it isn't interacting with anything until it gets to the screen.
     
  21. Jun 6, 2015 #20
    Ok, noted. The claim was mentioned by Bruce Rosenblum and Fred Kuttner in Quantum Enigma. It does not give a reference. Surely these physicists wouldnt sensationalise such phenomena?
     
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