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B When are particles in a quantum state?

  1. Mar 6, 2016 #1
    These are just a few quick and simple questions. When are particles in a quantum state? Are they always in a quantum state in-between interactions, or once interacted with are they never in a quantum state again? At the instant of the big bang was everything in a quantum state? If so, what caused the first particle to stop being in a quantum state?

    If these questions are stupid, then basically how does a quantum state particle come into being, and when does it stop being in a quantum state.

    Oh, and perhaps hardest of all, can you put this in terms a non-physicist would understand.
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  3. Mar 6, 2016 #2


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    It doesn't make sense to talk about particles being "in" or "out" of a quantum state. The "state" of a particle is a what we know about and use to calculate its future behavior, so one way or another a particle always has a quantum state.

    In classical physics, the state of a particle is given by its position and momentum at some point in time; if we know that, we can calculate its position and momentum at any time in the future (and that's how we do the orbits of the planets, the trajectories of artillery shells, the speed with which dropped objects hit the ground). In quantum mechanics the "state" is an abstract mathematical object that we use to calculate the future behavior of a particle.... and the particle always has a quantum state, just as it always has a position and a momentum in classical physics.
    Last edited: Mar 9, 2016
  4. Mar 7, 2016 #3
    Thanks Nugatory. That helped. And I'm sorry if this sounds stupid, but how do things come to be in a "fixed" state? Like when one measures the position of a particle. It's my understanding that the act of measurement causes the particle to collapse into a "fixed" state. But how did the measuring device come to be in a fixed state? I assume because it was measured by something else. But how did that thing come to be in a fixed state. This whole chain of measuring devices must have had a beginning somewhere. What was the first thing that had the capacity to act as a measuring device for something else. Or will a sufficiently complex system of quantum particles just spontaneously collapse into one with a fixed state? I'm basically wondering what the first measuring device was, or if quantum systems just spontaneously collapse.
  5. Mar 7, 2016 #4


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    Basically everything is being 'measured' by everything else by a process called decoherence. In pretty much all cases it leads to everything having a definite position.

    Here is the book you should get:

  6. Mar 8, 2016 #5
    This is really a good book to start with. It's written well, and is understandable even to laypeople like myself. You'll still have questions, but it fills in most of the gaps.
  7. Mar 8, 2016 #6
    Basically the correct answer is don't think about this kind of stuff until you've acquainted yourself with the underlying mathematics. But the basic idea is that, like Nugatory said, every particle always has a state. The state is a mathematical object comprised of a weighted sum of what are called "energy eigenstates." Each eigenstate corresponds to a definite energy value (meaning an energy measurement in an eigenstate will yield a definite value). The coefficients of the eigenstates in the sum tell you the probability that a measurement of energy will yield a particular value, namely the value associated with the eigenstate. I don't know if this is making sense to you. Basically the "state" of a particle is a mathematical box containing information about the probability that measurement of some observable (like energy, position, momentum, spin) will yield a certain value. The act of measurement may indeed change the state of the particle, since the measuring apparatus might have some nontrivial interaction with the particle, but the act of measurement emphatically does not place the particle in a "definite" or "fixed" or eigenstate. What measurement does is precisely this: it gives the experimenter a number.

    P.S. The idea of "collapse" is an unfortunate artifact of popularizations. It simply points out the fact that the mathematical representation of the object is probabilistic, whereas the measurement is definite.
  8. Mar 8, 2016 #7
    But what any measurement does change is the state of the measuring apparatus (and of the experimenter, and of the whole universe), reducing the initial uncertainty.
  9. Mar 8, 2016 #8
    As I understand it, measurement does not "reduce uncertainty," whatever that means. What measurement does is it gives you a number. The number is definitely a number, and moreover it is a definite number. But the state is no less "uncertain" after the measurement than it was before. It is still a weighted sum of eigenstates, giving a probabilistic description of future behavior.

    The term "collapse" was designed by popularizers to mystify people and sell books. Nothing collapses or reduces or converges or actualizes. There's just a state, which is probabilistic. Then there's a measurement, which gives a definite value. And after that there's still a state, which is still probabilistic.
  10. Mar 8, 2016 #9
    But could we think this way...

    Suppose our measurement, repeated, gives one of several numbers, A or B or C etc.

    Suppose this time we got B, it's the final apparatus state of the first measurement.

    Now, let's imagine we can reverse the unitary evolution of the apparatus + particle system, back to the start. Then we get the modified start state (of particle + apparatus) which would certainly give the B outcome of our measurement. Therefore this modified state, compared to the first start state, is reduced - which proves that there was a reduction in the first measurement.
  11. Mar 8, 2016 #10
    Did the doctor who delivered the baby Einstein shout for joy?

    The term "state" refers to a particular mathematical object. This object is determined by certain inputs. These inputs include things like mass and potential energy. As a rule, they do not ever include information about the future. So no, as I understand it, information about the future can neither reduce, nor augment, nor in any way change a particle's state.

    If you want to use the word "state" loosely (i.e. inaccurately) to say that after the measurement we know something more about the particle than we did before the measurement, then that seems legitimate. But realize that you're just saying, "We started with a probabilistic description. Then we made a measurement. We still have the probabilistic description. In addition, we now also have the measurement." All of which is trivial.

    People probably disagree with this, but in my opinion it's a mistake to confuse our mathematical descriptions with reality. Quantum mechanics works by giving us probabilistic descriptions. Reality works by giving us definite values upon measurement. To say a whole lot more is metaphysics.
  12. Mar 8, 2016 #11
    Thanks, very interesting... I would like to ask, Do you think we are ourselves a part of that definite reality? - but, again, that's metaphysics.
  13. Mar 8, 2016 #12
    If the question is whether human beings are real, I guess the answer is yes.
  14. Mar 8, 2016 #13
    So, you actually saying this,
    Our quality of being real works by giving definite values upon measurement?
  15. Mar 8, 2016 #14
    That does not strike me as a productive question. I advise a strong dose of common sense and a textbook on QM before thinking too hard about what theoretical physics implies about "qualities of being real".
  16. Mar 8, 2016 #15


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    No, it would not (do the bolded part). If you start with EXACTLY the same setup, you do not get the same result. That is the essence of the Heisenberg Uncertainty Principle and is easily demonstrated by the single slit experiment.
  17. Mar 8, 2016 #16


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    That's not right. Only the state of what's being observed, what's doing the observing and in some cases the immediate environment is changed.

  18. Mar 8, 2016 #17


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    Hmmmm. Sort of. Because everything is interacting with eveything else that's why we get a classical world. But adjectives like real are best avoided since it has a lot of disputed philosophical baggage.

  19. Mar 9, 2016 #18
    It seems to me that you are implying a reversal in time here, by reversing the unitary evolution of the system. This would be as if the initial measurement was never made. Therefore, there would be no information regarding the system carried forward and the same "uncertainty" would remain with regard to the initial measurement.
    Last edited: Mar 9, 2016
  20. Mar 9, 2016 #19
    Returning to your initial question though, Fiziqs, I'm afraid that much of this discussion is simply going to confuse you further.
    You've seen several references to the "quantum state" being a MATHEMATICAL OBJECT, and that's what you've got to keep in mind. The mathematical formalism is pretty straight forward and unequivocal (though beyond my abilities). However, the confusion arises when you try to extrapolate what the mathematical formalism is saying about actual physical existence (often referred to as "reality" by the laymen class... which greatly frustrates the academics and professionals).
    What the formalism suggests about "reality" then becomes a question of interpretation, which many label as philosophy rather than science. So, not surprisingly, there's a great deal of controversy about interpretational issues. For example, you've seen it suggested here that "collapse" of the quantum state is something that doesn't really happen. Yet, there are many physicists that believe that it actually does happen. These interpretational arguments can be fascinating, but are ultimately unsatisfying until experimental evidence is successful in disproving any of them.
    Perhaps the general take away idea for you should be that the quantum state of a system describes all of the observable information regarding that system. So, everything is ALWAYS in a quantum state of some kind. The "precision" of a given system's quantum state will become progressively refined (reduced) as further observations, measurements and interactions occur within that system. Again, I would highly recommend the book that Bill suggested earlier. It should address many of your conceptual concerns, and is actually quite understandable.
    Last edited: Mar 9, 2016
  21. Mar 9, 2016 #20
    For some reason, a multitude of particles is not the same(is actually very different) as its single constituents. Take a look at the so called double slit experiment and see how the setup is split between a common sense always classical aparatus and the quantum particle which behaves in a quantum mechanical way.

    In nature, there is many a duality like this - electrons can manifest as radio waves travelling through walls or as walls(being made up of electrons among other particles).

    If qm seems incomplete as it stands, that's because it probably is. It is not a complete theory but a useful stepping stone.
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