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Wave/Particle Double Slit Experiment Questions

  1. Jun 12, 2007 #1
    I was watching a video about Quantum Mechanics and the double slit experiment which shows electrons to be both a wave and a particle, though not at the same time. I had learned this earlier in high school, though at that time did not understand the importance of this. Now I do and would like to discuss some aspects of this as well as ask some questions to further my understanding of this experiment.

    First I'd like to start off with my current understanding of the experiment and how the electron particles behave in this experiment to make sure I have my information correct.

    When electrons are shot through a single slit it produces a single line of concentration where the electrons hit the backdrop behind the slit resulting in something that looks like this: |

    This indicated that they are acting like particles or tiny balls of matter.

    When electrons are shot through a double slit it was thought that it should act as a particle and produce two lines of concentration where the electrons hit the backdrop to look something like this: | |

    However they discovered that instead of producing that result it produced something like this: | | ||| | |

    This indicated that the electrons were acting like a wave and that after exiting the slits the waves hitting each other caused cancelation of some of the waves and concentration of others resulting in the figure above.

    They also observed that sometimes the electron went through both slits. and sometimes through only one slit and sometimes through none. This seemed to imply that it had a potentiality of positions. I know that isn't a very good explanation for what I mean but I will assume for now that you know what I mean.

    Scientists thought they could check which slit it actually went through by setting up a device to measure or observe the electrons as they went through the slits. In doing this they found that trying to measure or observe which slit the electrons went it caused the electrons to act like particles again resulting in the backdrop looking like this again: | |

    So this seemed to mean that the very act of measuring or observing the electrons caused them to change from a wave of potentiality to a particle.

    So here are some of my questions.

    1. I understand that if a wave passes through a single slit it results in a concentration behind the slit and then fades outward to the left and right. Does this ever occur when firing electrons through a single slit? Or rather do electrons ever act like a wave when shot through a single slit?

    2. How exactly does the change in the electron, from measuring or observing it, work? I know that is quite a question so let me be a little more specific. If an electron can be a particle or a wave, though not both at the same time, what causes it to change from one to the other? For instance lets look back at the experiment where the electrons were measured and observed by a device to determine which slit they really go through. This experiment resulted in the electrons changing from their previous wave of potentiality to a particle. Now if the backdrop was much much farther behind the slit, would the particles, which are no longer being observed because they have passed by the measuring device and through the slit, eventually change back to a wave of potentiality?

    3. What determines the type of measurement or observation which causes this change in the electrons? In the experiment this measuring device... speaking of which, what measuring device did they use exactly and how does that measuring device work? Anyway, in the experiment this measuring device resulted in the electrons changing. It is my understanding that scientists have been able to take pictures of electrons showing them in multiple positions at the same time. These photos show colored specs next to each other which make it appear as if there are multiple objects when really it is only a single electron. So are these pictures showing part of the wave of potentiality of an electron? If so, how is it that the measuring and observing of the device taking the picture does not result in changing the electron from a wave of potentiality to a particle? Shouldn't the very act of trying to take a picture of the electron result in the electron becoming a particle and thus the picture should show only one spec?

    I have more questions but I would like to get some feedback concerning my understanding of the double slit experiment as well as some answers to my questions first.

    Thank you all for your time. It is immensely helpful to be able to converse online like this with people who have knowledge about quantum mechanics.
  2. jcsd
  3. Jun 12, 2007 #2
    Your understanding of the double slit experiment seems fine. I'll try to answer your questions as best I can, but I'm no expert at quantum mechanics.
    1. As long as there is the possibility of an electron taking more than one path through a single slit, then yes, it will act like a wave. However, if there is only one possible path for it, it will behave like a particle.

    2. No one really knows what causes a particle to change between a wave and a particle; there are many different theories about it. According to the standard theory of quantum mechanics, an observation of a particle/wave will cause it to act like a particle, but while it is not being observed, it will act like a wave.
    3.The "picture" you saw of an electron in two places at once (in a quantum superposition) was probably just a computer animation, not really a picture. You are right about it being impossible to take a picture of a particle being in two places at once. About what classifies as an observation: according to most modern theories, an "observation" is when a particle interacts with something else, e.g a photon.

    I hope that my answers helped your understanding of the double slit experiment.

  4. Jun 12, 2007 #3


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    Also, no expert but I'll try to help you as best I can. First off, and electron is neither a wave nor a particle. It is considered something else, with the properties of both of the previous entities.

    1)I'm not sure, but I believe this will happen for electrons.

    2&3)The question of observation is a good one. The electron is described by what is called a wavefunction. The square of the wavefunction's amplitude at a point describes the probability of an electron being at that point. Hence, when you pass electrons through a double slit, the wavefunction(which is spread out in space) will hit both slits and interfere with itself afterwards, and you get destructive interference at certain spots, which means there will be zero probability of the electron being found there. Thus, you get a fringe pattern. Now, enter a measurement device. Like you said, let's measure the position of an electron before it goes through the double slit. This measurement forces the electron to take a definite position, effectively collapsing the wavefunction and causing the wavefunction to be zero everywhere except where the electron is found. Now, since the wavefunction only exists at one point, it can't go through both slits and interfere with itself, since it won't spread back out after it has gone through them. Thus, the wavefunction wont be able to interfere with itself, and the interference pattern disappears, leaving the particle-like pattern.

    Some crackpots like to say that the electron "knows" it is being watched. In fact, I know of this cartoon video about this experiment of YouTube that says something similar. That is nonsense. The electron is an inanimate object. The reason that observation, i.e. hitting the electron with photons etc., changes the pattern is because the photons change the state of the electron so drastically that it's wavefunction can no longer go through both slits and interfere.

    I hope this helped somewhat!
    Last edited: Jun 12, 2007
  5. Jun 12, 2007 #4
  6. Jun 13, 2007 #5


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    Yes to some extent there are diffraction effects if the slit is thin enough (of the same order as the deBroglie wavelength of the electron which in turn is a function of its speed/momentum.). The outline of the slit through which the electrons travel will not be sharp but fuzzes out and indeed you may in principle produce secondary peaks, just as with single slit experiment with coherent light. Note that the "particle" nature of the electron still manifests in part even in the double slit experiment. Even as the interference pattern builds up as you shoot electrons through the double slits one at a time you still see one "flash" on the target screen for each electron you fire at the slits.
    Understand that "wave vs particle" is not a quality of the objective state of the electron but a description of its behavior. Behavior here implies circumstance so there is a relativity principle involved. Understand that waves are classical phenomena as are particles in the usual sense.

    Electrons are quantum phenomena but we may resolve their behavior in certain classical paradigms. Electrons are not waves but we can observe interference patterns analogous to classical waves. Electrons are not point particles but we can see localized interactions (e.g. cloud chamber tracks or florescent screen flashes) analogous to classical particles.

    To understand what electrons are you must understand that this must in physics be defined as a system of behaviors when you execute certain experimental setups in a laboratory. The observed behaviors are more complex than can be expressed by treating the electron as a classical object.

    For the specifics you must study QM and QED. But you can understand it in general via analogy to the say people. People are not monolithically "sinners" or "saints" yet under certain circumstances you may observer "sinful" behavior and under other circumstances you may observe "saintly" behavior. You then upon consideration realize that "saint vs sinner" is not an objective property of a person but rather a description of that person's behavior. You can at best define for people certain tendencies of behavior relative to the circumstances in which this behavior is to be observed.

    Likewise if and when you study quantum theory understand that the "state vectors" are not descriptions of the quantum's "state" but rather descriptions of its behavior.

    This question assumes it is the electron changes between "wave state" and "particle state" so re-read my earlier comments and try to rephrase if you still have a question.

    But let me make the following comments. In quantum theory you must take into account that any observation of a physical system requires you interact with that system and all interactions are two-way. The system must affect the measuring device in such a way as to register the measured quantity and so too the measuring device must affect the system. Think of it as a vast generalization of Newton's "equal and opposite" law. (You can express it in terms of conservation of information.)

    To make a "proper measurement" your device must not affect the quantity being measured (e.g. momentum), one assumes and must in the lab verify that a given observable can be repeatedly measured and that the measured value doesn't change between two immediate acts of measurement. This is necessary in principle but in practice we need only show that a given device measures an observable identically to such an idealized repeatable device.

    This having been said consider also that there is a thermodynamic element to quantum measurement. One must amplify very small signals to large effects. This requires you have a "heat sink" or "entropy dump" and due to the two-way interaction between measuring device and system being measured you necessarily have loss of prior information about some observables distinct from those actually being measured.

    This then means that to predict all future behavior of the system you must back-track not only the system history but also trace the effects on the environment. This however is impossible by the nature of an entropy dump.

    Hence measuring one quantity fundamentally introduces uncertainties into other system variables. The Heisenberg uncertainty principle expresses the limit of the extent to which this uncertainty can be minimized.

    Since it is impossible to measure simultaneously all observables of a system then it ceases to be meaningful to assign the system an "objective state" (and thus specify the values of all possible observables).

    What is more any objective description of the system even if said description is implicit (e.g. in the derivation of Bell's inequality) will logically contradict the implicit and explicit assumptions that any measurement was, is, or will be made. You must abandon the classical objective description all together and revert to the more pragmatic phenomenological description of system behavior.

    I think it goes even further than this. I think that the very idea of "an electron" refers to phenomena where we have sliced the universe into two parts "electron" and "its environment" where this cut is dependent on which properties you are measuring. Hence when you choose to describe "an electron with defined momentum" then "its position" is fundamentally non-existent even beyond pragmatic limits in the measurement process. This is an open question delving deeply into the metaphysics of nature and is usually argued in the discussions of "interpretation" of QM beyond the operational interpretation of the theory's predictions. Beyond opinions of elegance and aesthetics I don't think these questions can be resolved within the context of physical science.

    James Baugh
  7. Jun 13, 2007 #6


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    I just want to say thank you to James Baugh for a truly excellent 'postage stamp' exposition.
  8. Jul 3, 2007 #7
    This is a quote from a physicist in the video I saw...

    "You now can see in numerous labs around the United States... objects that are large enough to be seen by the naked eye... and they are in two places simultaneously. You can actually take a photograph of that. Now, I suppose if you showed a photograph, they'd say, ""Oh. Great. Here's this nice blob of colored light, and I see there's... a bit of it over here and another bit-- So you've got a picture of two dots. What's the big deal?''" You say, ""Look right in the chamber. You can see it right there."" ""I see two things there.''
    ""No, no. That's not two things-- That's one thing. It's the same thing in two places.''

    So now in order to be able to see the object being in two places at once some amount of light must hit and reflect off of the object(s) and hit your eyes. Same with a picture of this phenomena. So if photons of light are hitting this object why does it remain in two places at once instead of collapsing to the behavior of a particle as happened with the measuring device in the double slit experiment?
    Last edited: Jul 3, 2007
  9. Jul 3, 2007 #8
    Can you give us a link to this video?
    As far as I know, we can never see an electron or any other particle at two places at once.

    Mr V
  10. Jul 3, 2007 #9


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    If the object that was in two places at once comprised supercooled atoms or ions in a cavity, then they are usually seen by their own light - they are stimulated to emit and reveal themselves. I don't know if that has any relevance to your question, but I thought you might like to know. You didn't say what this video was so we can only guess.
  11. Jul 3, 2007 #10
    Regarding the quantum system(s) that the physicist above is talking about --
    this is not altogether unlike the way the different parts of a single chair occupy many different locations simultaneously. It can be said matter of factly that the chair is in many different places at once.

    What is actually seen, at the same time, in the presentations that the physicist is talking about (which presentations are amplifications and representations of single quantum systems) are two different things in two different places.

    Google Tonomura (he and others did a great double slit experiment using accumulations of single electron detections -- it's fascinating how they engineered the experiment). Below is a video of a cool talk he gave about observing magnetic lines of force within magnets using electron waves.

  12. Aug 9, 2008 #11
    The Uncertainty Principle says that an electrons location or momentum can be measured but not both. Is this perhaps what is ultimately affecting what we see in the double split experiment?
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