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Double slit experiment

  1. Feb 27, 2006 #1
    I stumbled upon this, basically what i want to know is. Does a single electron shot through two slots produce an interference pattern?
    When you observe which slot the electron goes through does the act of observing prevent it from producing an interference pattern?
  2. jcsd
  3. Feb 27, 2006 #2

    Doc Al

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    A single electron would not make much of a pattern! :smile: But if electrons are shot through the double slit one at a time, an interference pattern will develop. (This was demonstrated by Tonomura in 1989.)

    Correct. If your observation is sufficient to unambiguously identify which slit the electron passed through, then no interference pattern will be observed.
  4. Feb 27, 2006 #3
    something to add here is that quantum mechanics is satistacial in nature. so for example if you could only roll a dice once, you would not be able to see much of the probilistic nature of the die of having 6 equally differnt outcomes.
  5. Mar 7, 2006 #4
    ive been pondering this also. and how the observer addition affects results. But what im confused about. Did the experiment change at all?

    Take this for example. I'll try to be articulate.

    Since it is a double slit. There is some sort of paper or whatever for the slits to be put in.

    Now if you look way down to electron size. The paper would have a width that is much larger then the electron. Such that the electron could be reacting with the sides of the slits. If not colliding with the edge of the slit. The proton/electrons that make up the paper's edge. Their charge effects the electron's path and makes that sort of "effect" of interference.

    Perhaps i dont understand the other explanation of how the electron affects itself. Like Ive done the quantum physics which explain where the electrons are depending on the valance shell and such. Which seems like they are using the same principle. But it doesnt make sense. Like regardless of all the different positions its likely in. Its not in 4 pieces. Its just one electron-1piece.

    Anyway im betting its more simple of a solution and its the experiment equipment which made bad results.

  6. Mar 7, 2006 #5


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    Then you have lost your bet. You are ignoring the fact that I could get the same type of interference pattern using a superconducting loop, such as those observed using SQUIDs. There are no "slits" to "scatter" off here, since by definition, the supercurrent maintain long-range coherence. All I have is two separate paths for the supercurrent to flow through. Not only that, I can adjust the phase difference between the two path simply by threading a magnetic field flux around the loop of the two paths!

    And I haven't even commented yet on why this scattering scenario would be inconsistent with the observation that the interference pattern changes with slit width and slit separation.

  7. Mar 7, 2006 #6
    ive never heard of this squid thing before. Ill have to go look it up after my vacation.

    But my question is. Why would an observer make it act like a particle. When it acts like a wave with no observer. What does the observer bring to the table that would cause this.

    I ask this because i dont take the electron interferes with itself explanation. It makes absolutely no sense. Like even if the electron has 4 possible spots to be in. If you take one planck time interval. It is only in 1 of those spots. The math being not complicated enough to say which. But it should only have 1 position in space.
  8. Mar 7, 2006 #7


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    Read the FAQ on wave-particle "duality". You are ALREADY assuming that your classical description of a "particle" for an electron is valid. If such a thing is true, we would not have chemistry!


  9. Mar 7, 2006 #8
    im not assuming the classical is right. I either have an incorrect understanding of QM and wave/particle duality; as there has been an even newer understanding than what i have learnt(textbook was ancient)

    Lets scale things up. I am considered a wave in a way.

    If I walk i have speed. I bounce up and down when i walk. which gives me the wavelength and period. So in a way I am a wave. But just because I act as a wave. Doesnt make me immaterial. I'm still solid as ever.

    Same thing could be said for the wave which you might see at a hockey game or some sport. The people(electrons) exhibit the properties of a wave. While clearly are particle.
  10. Mar 7, 2006 #9


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    And you think you can do this with an electron? Can you please show me how you are able to track down the trajectory of an electron the very same way you are following the trajectory of your bouncing self? Can you show me your equivalence of a long-range phase coherence that an electron can condense to? Can you show me your bonding-antibonding band similar to the one that an electron in a valence band can form?

    You seem to be forgetting that for every analogy that you can come up with, I can come up with several in which there are NO equivalence whatsoever with your classical experience. YOU are not a quantum object. You will have an impossible time to convince me that every single part of your body is in a phase coherence so much so that you can exhibit quantum properties. If we can see quantum properties THAT easily, it wouldn't have been so difficult to accept!

    And again, you think this is the SAME thing? Horrors!

  11. Mar 7, 2006 #10
    so your basically saying that my analogy is wrong because the size im using isnt quantum. But thats what i dont get. How can the rules change when going quantum.

    I dont know specifics. but here is my understanding of this.

    They did the double slit and found that it had an interference pattern. But then they added an observer and it then followed the particle pattern.

    What im asking. How or in what way does the observer affect the results? Im betting that it doesnt have an effect(as observer's definition means non-interference) and that equipment simply changed for the better. So in the experiment with the observer they had a better electron gun that didnt spread or something. Or as i explained poorly above with the picture.

    But quantum mechanics if its the same. would say that each electron goes thru both slits, none, or just 1. and when the both slits occurs thats when its screwed up and becomes interfered, which it would interfere with itself. but when they use an observer this option doesnt happen. Sooooo. Usually when that would happen. It would probably mean the observer is interfering somehow. Or. the experiment with the observer is the true correct result. and the non-observer experiment was flawed.
  12. Mar 8, 2006 #11
    A single electron would not make much of a pattern!
    What do you mean not much? A single electron leaves a single mark at the backdrop. The inference pattern is the pattern of marks left by many electrons.
  13. Mar 8, 2006 #12
    Youre using newtonian ideals, which should be crushed, stabbed and burned before going into QM. A book which might help you understand QM better is "In search of Schrodingers cat" by John gribbon. Really simplifies stuff. As much as QM can make sense or be simplified.
  14. Mar 8, 2006 #13

    Doc Al

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    Exactly my point. A single electron leaves a single mark--not much of a pattern in a single mark. It takes many electrons to display a pattern.
  15. Mar 8, 2006 #14
    You'd be able to tell though whether the photon hit the the screen without interference though or had been interfered with even with a single photon wouldn't you? So you'd know if a single photon had "interfered with itself" wouldn't you? Which I believe they say it does unless you have detected which hole it goes through then the single photon is not interfered with or have I got this wrong?
  16. Mar 8, 2006 #15


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    Caveat: While I'll grant this as an analogy for the moment, it is critical that you realize that the subatomic world has no macroscopic equivalent. These analogies will ALWAYS fall apart.

    Now. You are a tangible object, bobbing up and down as you walk. You come upon two doorways, separated by a gap. Demonstrate how you pass through both of them.
  17. Mar 8, 2006 #16

    Doc Al

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    I'm not clear on what you are saying. If your experiment consists of passing a single photon through a double slit, you will get a single mark on the screen, whether or not you observed the slit that it passed through. Can't tell much from a single mark.

    But it's certainly true that the interference that takes place as photons pass through the double slits is not due to photon-photon interactions. You can set up the experiment such that at any given time only a single photon is passing through the slits. (You can also do this with electrons, as I mentioned earlier.) After many photons have passed, the pattern displayed will demonstrate the typical double-slit interference. But if you conspire to observe through which slit the photons pass (essentially localizing it), the interference pattern will be lost.
  18. Mar 8, 2006 #17
    if it's not photon - photon interaction then it must be the photon itself which causes the interference, it interferes with itself,at least I'm lead to believe(I have been wrong in the past and could so easily be so again) is it just me or are you unhappy with this. I can't help thinking that there's something simple there but because of the nature of light itself it wont let us see it, maybe I just need to do a bit more digging or research?

    Could I not as easlily say if it behaves like a wave unless we view it directly then it behaves more like a particle and the interference is lost is merely just the act of the detector itself causing or making the wave travel through that slit and not both(affecting it in some way, that forces the path, it is still a wave but it has been diverted repolarized or whatever you want to call it by the detector into that path and that path alone. If this is so then maybe it is simply an effect of the interaction and not some mysterious inderterminency? Am I way off here, I usually am?:biggrin:

    Since wherever you place the detector the light hits the screen in the same place because of the same effective interaction? I think I need to dig deeper maybe:confused:
  19. Mar 8, 2006 #18
    amazon.ca doesnt have it. and i dont have the time to goto the bookstore. Im goin to mexico in a couple days.

    Im using a sort of mix.

    Im sure you've done the valance shell stuff with QM and the atoms. With the probably places which 1 electron might be. Where you have a couple little balloon type areas where the electron will most likely be. I agree with this. But the sense im getting is that your trying to say that the electron isnt in just 1 of these spots. but rather all of them.

    Say I were omnipotent(im not saying im not) and i made us quantum size and froze time. We went up to the atom. What would we see? Im guessing that the electron is in one of those areas that QM would suggest. But not in all of them.

    Unlesssss. when they say there is 2 electrons in the 1st valance shell. its really just 1 thats split up in a sort of wierd type thing trying to be claimed here.
  20. Mar 8, 2006 #19
    Be careful there. Try and retain a lot of the Hamiltonian formalism (and the Lagrangian stuff too for later work) and QM won't appear as if it's been pulled out of thin air when you first come to it.
  21. Mar 8, 2006 #20


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    But here's the problem with what you are saying. IF the electron is just at one place at any given time, and all those "balloon" types are simply where the electron are whizzing around, you will get RADIATION coming out of an atom in the ground state. An electron whizzing around THAT fast in such a way that it could maintain chemical bonds would radiate SO MUCH ENERGY, you'd have cancer already by now.

    And we haven't even tackle yet how you propose to explain that the s-orbital, for example, has ZERO angular momentum. Is this typical of something whizzing around in a spherically symmetry volume?

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