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Superposition / Probability Wave - frame rate problem?

  1. Aug 5, 2015 #1
    Okay, so this is my first post, and I'm probably going to embarrass myself, but here goes. I have a bunch of questions.

    So we have an atom, consisting of a proton and electrons, and these electrons spin either clock wise or anti clock wise but in fact appear to do both... until measured...Is that what is called super position?

    These electrons spin so fast that they create the effect of a shell around the proton, is that correct,?
    Giving the atom its volume in terms of how much it repels and attracts other atoms and how it occupies its own space separate from other atoms.?

    Now clock wise or anti clock wise.... doesn't that depend upon from where we observe the atom and its electrons? As in if you look from behind a see-through clock, the hands spins anti clockwise, don't they? If the whole atom is rotating itself in space.... what is up or down, and what is front or back?

    So in terms of super position, it is both clockwise and anti clockwise anyway depending on from where the observation is made relative to the orientation of the atom right?

    Also, so I know this is probably an idiotic question, in terms of probability waves, if we look at say, a knitting needle being waved back and forth, left and right in front of a flickering television screen, we observe multiple silhouettes of the knitting needle, if the rate of the swinging movement is constant, they will shift relative to the frame rate and appear to travel left or right... Lets say the frame rate is analogous to the peaks of the probability wave... if we take a picture, then it is likely that one of these observed silhouettes will be the position that the snapshot finds the needle at ......especially if it is our source of light, analogous to our ability to observe and measure and at what frame rate.... How finely we can measure in slices of time....

    Or if we take a spinning top and it has a wobble, it can appear to be in more than one place at one time, due to the frame rate of our observations, the limitations of the naked eye and our brains ability to process images...

    Could that be an analog for super position? If that's so then is it just that we can't carve time up into slices small enough in a continuous way, to follow the trajectory of the electron, it is like the spokes on a wheel, appearing as a disk, perhaps we can take a snap shot, like a very high speed still camera, and there the electron is, (or a particular spoke on the wheel) but we can't measure it in transit because we don't have a fast enough film camera, so to speak... I mean film as in capturing motion, as opposed to a stills camera, capturing one snap shot in time.

    What I'm asking is, probably in a really clumsy way, is to what degree are some of these confusing phenomena just a measurement problem?


    With the double slit experiment. The electrons are fired one at a time, yet, still they set up an interference pattern.
    What do we mean by probability wave? Does this have anything to do with the nature of the way the electrons themselves travel? Are they interacting with the barrier to some extent? What do we mean by the observer effect? Is it true that when "observed" the electrons just form two stripes like we expect? Mechanically, how do we observe the paths of the electrons with out affecting them...?

    Or are the electrons interacting with each other in a way that travels backwards in time? As if they "knew" the paths that the other electrons had travelled already? Or that the electrons leave some sort of trace of their paths that effect the following electrons... and why does this break down when observed?

    Also... how does an electron travel? What affects it's trajectory? Why do they not all hit the same spot like a well sighted, accurate rifle?

    I probably should have just called this post a bunch of newb questions....

    Thanks in advance to anyone who takes the time to answer...

    Last edited: Aug 5, 2015
  2. jcsd
  3. Aug 5, 2015 #2


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    I'm not sure how useful this is. You'll probably learn more if you read a book about quantum mechanics. Anyway, here are answers to several questions. I think the others got answered with them as well, or don't make sense.

    They don't spin around a nucleus. They are more like a "cloud" around it, without motion in the classical sense.
    Electrons have an intrinsic spin, but this is not an actual rotation. It does not have a classical equivalent. Electrons can be in a superposition of two spin states, but they don't have to.
    The probability to find an electron somewhere (if you look) is constant in time apart from very carefully prepared systems.
    It cannot. Quantum mechanical superposition has effects without any classical analog.
    It is not a measurement problem, it is intrinsic to the state of an electron. There are experiments that rule out alternative explanations.
    If you expect two stripes then you'll have a hard time seeing interference. Those two stripes (if only one slit is open) need significant overlap to make the experiment work.
    You cannot.
    No, nothing travels back in time.
    No they don't.
  4. Aug 5, 2015 #3


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    Sort of. A superposition means that the individual electrons actually have spin both cw and ccw, and when you measure the spin of the electron, the wave function of the electron, which was a superposition prior to measurement, collapses to be a spin eigenstate. Look up the Projection Postulate to get a little insight towards this.

    Yes, direction is relative. There is no up, down, front, or back. That is arbitrarily determined prior to experiment.

    No, that is not what superposition means. In quantum mechanics, everything can be described by its wave function. A superposition is a when a particle actually does have spin both cw and ccw (relative to the same reference frame). This is logically impossible according to classical mechanics, which is why its so hard to wrap your head around. It has to do with the limitations about the information one can know about a system, and how measurements play a role in that.

    I'm kind of copping out of this one and I'm not going to answer your individual questions further down (I'm simply out of time). Rather, I'm going to recommend the book "The Quantum Challenge" by Greenstein and Zajonc. It is cheap (~$15 for paperback I think), well written, short, and appropriate for your level of mastery and inquisitiveness. It is absolutely a must read when it comes to QM, and will give you a much better understanding of these exact phenomena.

  5. Aug 5, 2015 #4
    The problem is that your whole way of thinking is that you are assuming that subatomic particles are like classical macroscopic objects, and you are trying to imagine how a particle can be two places at once and you're guessing, "Well, maybe it's moving so fast you can't see it", and all of that is WRONG! You are totally missing the point completely. Your whole way of thinking is wrong. A subatomic particle is totally unlike anything you are used to. Its position is undetermined until you try to localize it in space. The quantum number called spin has nothing to do with rotation in the sense that a macroscopic object might be rotating. It certainly has nothing to do with an electron rotating around a nucleus so fast it was blur and that's what you guessed an electron orbital was. It would be hilarious if it wasn't sad at the same time. When I try to introduce people to the concepts of quantum mechanics, I make an analogy between the Stern-Gerlach experiment, and the polarization of light, which I got from the opening chapter of J.J. Sakuri's book. I learned quantum mechanics from J.J. Sakuri, where he talks all about bra-ket notation, and operators, and all of that. I learned quantum field theory from Peskin and Schroeder. However, these good introduction still assume the reader at least has an undergraduate understanding of classical mechanics, which you don't seem to. All I can tell you is that in quantum mechanics particles are not localized in space until you attempt to localize them, and the wavefunction is related to the probability of detecting at particle at a specific point, if you were to try to locate it. The oribitals represent surfaces of maximum probability of detecting an electron.
  6. Aug 5, 2015 #5
    I don't see anything in my questions which runs counter to classical mechanics. If so please point them out. In fact it's all based on classical mechanics, which appears to be the problem, asking frame rate of perception questions in order to ask whether any of the phenomena we see could be explained as simple measurement problems.
    I should have guessed not, other wise you wouldn't have world class quantum physicists scratching their heads over the last hundred years or so.
    But hubris is a well known human foible, witness the recent obliteration of the treasured belief of many physicists that quantum coherence could only be achieved in complex, heavily shielded apparatus at near zero K. Hello photosynthesis. Hello quantum vibrations in microtubules.
    But I have seen heard some people attempt to give classical world explanations in order to claim that there is nothing weird or "spooky" about quantum physics.
    I actually didn't really think there would be, I just wanted to see if there were any macro world physics type explanations for any of the enigmas of quantum physics, whether or not it was in part or in any way just a measurement problem.

    I learnt something at least, from all the responses so far, electrons have their own "spin" regardless of their orbits around protons, and electrons aren't understood to orbit the nucleus in a classical way...

    I still have a question as to whether an electrons' 'spin" characteristics affect it's trajectory.

    [Mentor's note - some off-topic personal discussion has been removed]
    Last edited by a moderator: Aug 6, 2015
  7. Aug 5, 2015 #6


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    Electrons do not have properties like spin or trajectory until observed to have them. QM is a theory about observations that appear here in a common-sense classical world. What's going on when not observed the theory is silent about.

    But when observed it is found that spin and trajectory are independent, meaning one has no effect on the other.

    One of the problems with QM is that popularisations and even beginning textbooks can confuse what QM is really about. The following may help in making things clearer:

  8. Aug 5, 2015 #7
    That is very confusing thanks for the reply.... So we are not assuming that because an electron shows certain characteristics when observed that it has those characteristics when not observed...? In fact do we think other wise... that it doesn't have those characteristics until observed?
    Or do we just have nothing to say about that pre observation state?
  9. Aug 5, 2015 #8
    Interesting that you brought up frame rates. If you find a universal screen that runs naturally at a certain frame rate, lets say one frame per second, and if you record that screen with a video camera that runs at a higher frame rate, will you be able to see more of whats going on on the screen with the help of what that video camera recorded ?
    Last edited: Aug 5, 2015
  10. Aug 5, 2015 #9


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    There are two aspects to this question. The first is what the formalism of QM says. That formalism is basically what the link I gave is about. In the formalism nothing is said apart from what's observed. It doesn't say anything is going on - or not - its literally silent on it. The second aspect is we have various interpretations that try to fill that in, or say there is nothing to fill in - nature is simply like that. The trouble is no one has been able to figure out how to experimentally decide which of the myriad of interpretations is correct. This means the question is unanswerable at this stage.

  11. Aug 6, 2015 #10
    I second the recommendation of the book 'The Quantum Challenge'; and further suggest 'Sneaking a Look at God's Cards' by GianCarlo Ghirardi -- https://www.amazon.com/Sneaking-Look-Gods-Cards-Unraveling/dp/069113037X/

    [Mentor's note: Edited to remove some off-topic personal comments]
    Last edited by a moderator: Aug 6, 2015
  12. Aug 6, 2015 #11
    Yeah I guess the thing is that if you inhabit a classical world, there may be a time problem or frame rate problem with your observations if what you were observing operated at a different time scale.

    Human perception runs at a certain frame rate... Perhaps classical physics has a frame rate, and the quantum world has a frame rate, in order to operate at the frame rate of the quantum world we'd probably have to create a "quantum camera"

    I guess thats why photons are used to measure particles?

    I just watched what looks like some sort of introductory lecture at a London university that seems to ask a similar question...

    (towards the end)

    But I have to admit all of this is quite beyond me....

    It seems some people feel that matter itself spreads out as a wave at sub atomic scales (?) , and others feel it is a measurement problem, and the probability wave only describes our ability to predict the behaviour of matter, not matter itself...
    Or that fundamentally matter only has probability peaks in relation to our macro scale.... We can't build a machine with a high enough frame rate to "catch the particles out"

    That doesn't explain how a single electron seems to pass through a barrier in the double slit experiment though does it?

    Or why they appear to behave themselves when they are being observed....

    In relation to your thought experiment... I think you possibly could capture the beginning and tail end of each frame coming into existence? As it fades in and out that might tell you something... But I'm only thinking of the way a screen renders from top to bottom left to right etc... If you had a camera faster than that screen, you would be able to see that happening...

    Of course you'd have to play back the camera at standard screen frame rates, in slow motion to be able to see that....
    Last edited: Aug 6, 2015
  13. Aug 6, 2015 #12
    I never said anything about rendering or anything else. If you have a universal screen that shows you one coloured frame of white and then black per second, you just wont get any other colour showing up on your fast recording camera.
  14. Aug 6, 2015 #13
    True, if there is in fact no transition between the two colours... which is what quantum theory states about electron orbits right?
  15. Aug 6, 2015 #14


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    In QM time is continuous. The frame rate analogy is not valid.

  16. Aug 6, 2015 #15
    I was more into exemplifying the information content of the screen having a limit, the limit not being in the number of frames the camera can record what the screen outputs.
  17. Aug 6, 2015 #16
    Well I really hope time isnt continuous...
  18. Aug 6, 2015 #17


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    To OP:

    When I first began learning QM, my professor put it very simply. It went something like this, although I'm going to paraphrase.

    "Even those who think they understand quantum mechanics don't understand quantum mechanics. The best way to start is to simply do quantum mechanics, grind the gears over and over, until you begin to understand the basics. Quantum mechanics correctly predicts outcomes very well, but it is inherently bad at explaining why. Before you can start asking questions about why certain things are the way they are, you have to be comfortable with basic principles of quantum mechanics (being able to comfortably talk in the language of wave functions, probabilities, superpositions vs mixtures, etc.) and accepting of quantum phenomena when it directly violates classical mechanics. The first part of this comes about with reading and practice - the second part is harder. You must learn to accept things about quantum mechanics that seem to be insane, and investigate what sorts of of outcomes these phenomena lead to. The outstanding thing about quantum mechanics is that, although it doesn't offer explanations for quantum phenomena, the predictions which the theory makes that have been tested have agreed 100% with what quantum mechanics tells us will happen - it just doesn't explain why."

    My advice: Read the books suggested in this thread. Keep an extremely open mind. Lots of things you read will likely make little to no sense at first. The most important thing to do at the introductory level is to accept what quantum mechanics tells you - no matter how insane. Once you become comfortable with the basics of QM (and perhaps not even then), it will be the appropriate time to revisit questions like these. Measurement seems like a trivial process, but when you dive deeply into quantum mechanics you find out that this is not the case.
  19. Aug 6, 2015 #18
    Thanks to everyone for all the replies... I will continue to study....

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