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I Question about superposition and measurement

  1. Jun 28, 2016 #1
    Hello everyone. I've gotten interested in quantum mechanics after binge watching a ton of documentaries about it. I've grown particularly interested in superpositions and the measurement problem. I'm not very good at math, so I can't understand the math behind it, but I like to take the information from theories and experiments and use that information to come to conclusions. Out of all those documentaries and educational videos explaining quantum mechanics that I saw, I haven't seen anyone mention what I'm going to ask about.

    Please correct me if I get any of this wrong but it seems all of them are saying that a "thing" (a particle, atom, object, ect) can only be in one of two states: a complete superposition where no particular quality of it is collapsed and defined, OR a much more definite state (but not completely definite because of the uncertainty principle I think). And the collapse of that "thing" happens when something measures it. This made me think: "then why is it I trip over a rock, even though I did not see it there?" And: "why do, for example, new planets we discover have the specific particular and persistent qualities even when we stop measuring them?".

    The conclusion I've come to after asking these questions is what I want to ask about. Instead of a whole particle switching from wave of superposition into a particle with definite qualities when measured, is it possible that what actually switches from a superposition to definite position instead is merely the qualities of the particle rather than the whole particle itself? I think this would make a lot of sense. For example: before an electron is fired at slits in a double slit experiment, while the electron is contained in the reservoir and assuming the electron is not moving in the reservoir, instead of it being a wave, it would have a different form? Because it seems to me that waves, in order to exist, they have to have velocity. If that's true, and if an electron has no velocity, then its not possible for it to act like a wave. Rather than its velocity having a superposition, only its momentum and all other qualities that have not interacted with other objects are in superposition while its velocity is collapsed.

    If that's accurate then I think it means, as particles are sent into motion, they act like waves. And as they smack into other particle's waves in the void of space, the resulting interference pattern becomes collapsed and definite, while their position is still in superposition. This should mean even their velocity is collapsed and not in superposition. I remembered the documentaries I watched saying something about how the details we find in nature depend on the kinds of questions we ask. When they say this, do they mean that particles can have qualities that are collapsed, and qualities that are in superposition at the same time like I said above? Or has this idea never considered? Or has it been considered and been disproven? Please let me know what you think, thanks.
  2. jcsd
  3. Jun 28, 2016 #2


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    Staff: Mentor

    That's because most of them will be garbage and the ones that aren't haven't been structured as a logical progression from the beginning. If you're not good at math but want to understand how QM works anyway, get hold of Giancarlo Ghirardi's book "Sneaking a look at god's cards".

    Try to forget everything you've heard about things going from waves to particles when they're observed. All of that stuff dates from long ago, before the modern understanding of quantum mechanics was hammered out in the decades after the first world war.

    That is most emphatically not correct - it is one of the most wrong misunderstandings suggested by these popular sources you've been studying.

    That's a better picture than the one that you've been getting, but without a clear understanding of what superposition is and what wave-particle duality is not it's still not going to get you anywhere. Start with that Ghirardi book.

    [Edit - just noticed that the autocorrect ate the "h" in "Ghirardi" - maybe fixed it this time]
    Last edited: Jun 28, 2016
  4. Jun 28, 2016 #3


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    Gold Member

    Quantum particles have a number of observable properties. Certain pairs (such as position and momentum) of those are non-commuting; in that case, a definite value for one means a superposition for the other.

    It is possible to measure and thus change which is in a superposition. This can even be done without affecting other pairs of observables (if done properly). Further, it is possible to perform partial measurements on an observable so its state is not exactly known (and it is therefore in a partial superposition).

    Once you grasp the above, you will understand the significance of Nugatory's excellent comments. Learn more before attempting to form a picture. There are a lot of pesky details that will get in your way if you don't.
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