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Macroscopic quantum mechanics

  1. Jun 21, 2008 #1
    I have heard many times thats quantum mechanics doesnt apply at the macroscopic level. But Im not sure if I fully understood what was meant. Surely quantum mechanics applies even to macroscopic objects, its just that the wavefunction of such objects is so highly localized that we in effect can never see any quantum effects?

    On the wikipedia page about MWI it says this:
    "Everett's theory just considers it (quantum superposition) a real phenomenon in nature and applies it to macroscopic systems in the same way as it is conventionally applied to microscopic systems."
    and
    "Tipler reports Hawking saying that MWI is "trivially true" (scientific jargon for "obviously true") if quantum theory applies to all reality"

    But as I said, isnt it true that quantum mechanics applies at all scales, and thus to all reality, but we are just unlikely to see weird quantum effects on the macroscale?
     
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  3. Jun 21, 2008 #2

    ZapperZ

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    I would suggest reading the article written by Phillip Ball in May 1st 2008 issue of Nature. For example, there is still an issue regarding the classical-quantum boundary.

    The leading candidate for possibly explaining this quantum-classical transition is decoherence. Explains why it isn't a matter of size, but rather the rate of interaction of a system with the environment.

    It explains why in the Stony Brook/Delft experiments, quantum effects can still be observed with an "object" consisting of 10^11 particles!

    This, however, is not the only explanation in town. Roger Penrose has an alternative idea that it is the coupling to gravity (or gravitons) that diminishes the quantum effects. It will be interesting to see if Dirk Bouwmeester manages to test that idea.

    Zz.
     
  4. Jun 21, 2008 #3
    An electron or a photon (with negligible mass) experiences superposition in the double slit experiment. However a bullet composed of quantum particles does experience superposition but due to its mass its severely affected by gravity which results in our deterministic classical physics. That's how I look at it.
     
  5. Jun 21, 2008 #4

    ZapperZ

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    The how come a buckyball, which presumably has a lot more "gravitational effects" than an electron or a photon, also exhibit the same QM behavior? Penrose's idea is a lot more complicated than this.

    You need to go easy on the guesswork and speculation here. Please read the PF Guidelines if you have forgotten it.

    Zz.
     
  6. Jun 21, 2008 #5
    But before it is observed, isnt the compass needles in a superposition of the states pointing in all possible directions, even though it is a macroscale object? It is just unlikely to be found to be pointing in all but one direction because the wave funciton is highly localized. As I understand there is no deep distinction between classical and quantum, just that classical objects have tiny de broglie wavelengths and so quantum effects are so small as to be pretty much unobservable.
     
  7. Jun 21, 2008 #6

    ZapperZ

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    No it's not. It has a definite direction, except that we don't know what it is. This is distinctly different than QM's superposition mainly because the effect of the superposition CAN be observed (example: bonding-antibonding in chemistry, the coherent energy gap in the Delft/Stony Brook SQUID experiments). So if your classical compass is in a superposition of state, there must be some experiment to indicate it is doing so. This, we do not have.

    This is why QM is different than classical. If not, QM would not have been THAT weird in the first place if it is nothing more than the same classical old thing.

    Zz.
     
  8. Jun 21, 2008 #7
    That sounds like an interesting test, but what was the result? (It's great seeing attempts to push the limit between quantum and macroscopic, regardless of Penrose's ideas - which seem here to involve nonexistance of gravitons incidentally, and for the mainstream this sounds like a measurement of gravitons through decoherence.)

    I've read the 2005 pop. article, and it looks like Penrose, Bouwmeester and others proposed this experiment (in PRL) in 2003, but I still haven't found any announcement of their results. Do you have any idea what progress they've made?
     
    Last edited: Jun 21, 2008
  9. Jun 22, 2008 #8

    ZapperZ

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    As far as I know, the "slight of hand with mirrors" experiment is still untested. So no, no new results so far.

    Zz.
     
  10. Jun 22, 2008 #9
    I thought it goes like this:

    If some preparement leaves a system in superposition so that there are interferences, then we can directly prove the existence of superposition by detecting the interferences with repeated experiments.

    If some preparement leaves a system in superposition so that there are not interferences (as can be the case when too macroscopic stuff is involved), then we cannot directly prove the existence of superposition, because interferences would be the only way.

    Surely it would not be correct to conclude, that if in some situation we cannot prove the existence of superposition, then the superposition does not exist?
     
  11. Jun 22, 2008 #10

    ZapperZ

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    I'm not sure how they're different.

    The signature of superposition are quite clear in quantum systems. These signatures are missing in classical systems. To me, that's the most obvious evidence. Now whether we can somehow find later on that quantum systems eventually merge with classical system via a smooth crossover is an entirely different matter, because that requires a lot of speculation at this moment. All that we can say for now is that classical systems show no sign of having any quantum superposition. A compass does not point in two opposite directions at once the same way the spin of an electron can because we have no signature of that.

    Zz.
     
  12. Jun 22, 2008 #11
    what about the "bucky ball" and the "superfluid with 10^11 particles" , aren't these classical systems? From what you are saying I conclude that a bullet in the double-slit exp. doesn't experience superposition yet the individual particles that make it up, does?!!

    I think the bullet does experience superposition, but the explanation of why it doesn't have an interference pattern, I can't tell.
     
  13. Jun 22, 2008 #12

    ZapperZ

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    I think you are missing a lot of the points that I've made.

    You first of all need to carefully look at WHY the "buckyball" and the 10^11 particles were able to behave as a quantum system. The whole point here is to demonstrate that SIZE isn't the issue preventing quantum behavior. If you have looked at the links that I gave, this was clearly mentioned. It is to counter your original argument of only things having little to no gravitational effects are the only ones exhibiting quantum behavior.

    Secondly, those two "large" systems can show quantum phenomena due to their ability to maintain coherence. As Carver Mead has mentioned in the article that I've highlighted several times already, a superconductor is the clearest example of quantum effects at the macroscopic scale, because of the ability of the superfluid to maintain coherence over large lengths. That is why the 10^11 particles (making up the superfluid in the SQUID experiments) have quantum behavior. So this argument is more in favor of the coherence as being the major reason that separates the quantum and classical world, not size. This is the quote that I highlighted.

    Zz.
     
  14. Jun 22, 2008 #13

    Andy Resnick

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    I agree that decohrence is a critical concept to understanding quantum mechanical behavior in meso- and macroscopic systems. I wonder though, if that's not what the OP was really getting at.

    Here's my take on the question: it's possible to treat some spatially extended systems like superfluids, superconductors, crystals, etc as quantum systems because they have ground states (and low excited states), and wavefunctions can be written down that correspond to elementary excitations: Bloch wavefunctions, etc. These wavefunctions rely on some abstraction of the macroscopic material that allows a wavefunction to be written down: flux tubes, excitons, holes, etc that are elementary excitations and spatially extended.

    It's no accident that these systems exist only at very low temperatures or at atomic perfection- that's the connection to coherence and decoherence- there needs to be cooperation amongst the players.

    "regular" macroscopic things (like a brick, or a 2x4, or a car...) don't have an obvious representation in terms of linear, orthogonal states. Yes, interference patterns can be generated by many buckyballs. But I imagine the wavefunction of a single buckyball at room temperature to be horrendously complicated, which leads to a mental abstraction of a classical object rather than a truely quantum object. The same for proteins, although there is some interesting work on modeling macromolecules.

    I haven't seen anyone try to write a wavefunction for a simple Newtonian fluid- a homogeneous fluid of uniform mass and charge density. That's different than a superfluid, which relies on cooperative behavior of a spatially extended system.

    It's not that quantum mechanics does not hold for macroscopic, messy things at room temperature (and higher). It's just that there's no obvious reason why someone would take the time to try and write out a wavefunction (or density matrix, or whatever else) because there's no obvious or compelling reason to- better to spend time on mesoscopic systems, like SQUIDS or quantum dots or macromolecules and the like that would result in something that is useful.

    Or did I miss the OP's intent?
     
  15. Jun 23, 2008 #14
    Andy I believe you answered my question, but Id like some confirmation that youre correct.

    To put my question in another way, I always hear people talking about the transition from quantum to classical as if it is something abrupt and discontinous. But it seems to me that all that is happening is the de broglie wavelength of the object in consideration is getting smaller, and the wavefunction becoming more localised - this is a continous change, nothing abrupt about it. I am trying to reconcile these two ideas.
     
  16. Jun 23, 2008 #15

    ZapperZ

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    But I'm not sure if people DO know if such a transition is abrupt. I have never read that at all and all the papers that I'm familiar with are discussing on how to get to such a boundary.

    Now, there are indications on how this boundary might be approached and what could possibly be there. I've already highlighted a few of them, such as this one here. It shows that for a single-particle system, even the interaction with just one other particle is sufficient to destroy the single-particle quantum behavior. This again seems to indicate that decoherence plays a big role in why quantum properties are destroyed upon interaction with other degrees of freedom.

    Now, the question on whether classical system can in fact still be described, in principle, quantum mechanically is something I can't answer, and I don't think anyone can. That's the same issue that we have with emergent phenomena in many-body systems where "More Is Different", according to Phil Anderson. There are plenty of obvious indications that they ARE different.

    Zz.
     
  17. Jun 23, 2008 #16
    Quantum mechanics hold even at large scales as you say, the classical mechanics is a special case of the quantum mechanics.

    1)Maby you have heard of so called Wiegner crystals? Lets say you have 3 electrons confined within a parabolic potential, then for strong confining potentials you would get atomic-like structure of the electrons, because the size of their wave functions are comparable to the distance between the localized wave-packages -i.e., they start to overlap and cause more complicated quantum phenomenas. For weak confinement the wave packages will look like classical point particles (but with a small spreading) separated in a triangle. The "transition" has some kink in terms of symmetry properties of the centers of these packages.

    2) Its difficult to talk about "the wavelength" for many particles, as in macroscopic objects. Consider a hydrogen atom travelling at a specific velocity. Then we have in fact two different de Broglie wavelengths (they have different mass)! Secondly, the composed many-particle wave function cannot be treated as a superposition like: Psi_electron+Psi_proton, but rather as a product of them. -> Its quite complicated to say what happens with an apple traveling through a "slit" -would you see interference patterns?

    3) Its better to talk about the energy spectra for small and large systems. The separation of energy levels of electrons in a quantum well of size 0.2 nm is of the order 1 eV, a very large energy, comparable to blue light. But for a quantum well of 100microns the energy levels are separated in the far infra red range, they are extremely close together , so you can hardly distinguish them from a continuum spectra of energy levels. Comparing classical calculation with quantum mechanic calculation gives no difference (I have tried)! Generally you could say that energy separation should be of the order of the thermal energy kB*T (=26meV for room temperature), to see quantum effects.

    /Per
     
  18. Jun 23, 2008 #17
    ah ok I think I 'understand' now. I thinkI thought that this area of physics was better understood than it actually is. It definitely something id like to look into more. thanks for your replies everyone, especially ZapperZ.
     
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