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Quantum uncertainty on macroscopic scale

  1. Aug 26, 2009 #1
    I don't quite understand why macroscopic objects we encounter in everyday life don't exhibit quantum probabilistic behavior. For example, the car standing in front of my house has a definite position and definite boundaries rather than being dissolved in some cloud where it's not clear whether it stands in front of my house or on the other side of the street.

    I have heard that the reason is because the quantum wave lengths of particles are very small relative to the size of macroscopic objects. If this is true, how large is the wave length of an electron? Doesn't the wave spread out across all universe? Or is the probability of the electron's position distributed in such a way that significant probabilities are concentrated in a very small area, perhaps a fraction of a milimeter, while the rest of the wave has negligible probabilities?

    Another reason I have heard is decoherence - that quantum waves are somehow suppressed because of interactions of particles. I'm not sure this answers my question though, because it is claimed that decoherence has been avoided for example in superconductors, but still I suppose that a macroscopic piece of superconductor has a definite position and boundaries and is not jumping unpredictably from one corner of the laboratory to another?
  2. jcsd
  3. Aug 26, 2009 #2
    "Decoherence" is indeed the key word you're looking for. There's a technical way of describing how particles go from obeying "quantum statistics" to "classical statistics" that can occur when they are interacting in large numbers. The word "coherence" shows up in any wave dynamics; e.g. a laser involves chromatically coherent light waves, while a light bulb involves a chromatically incoherent spectrum. In the quantum context, the coherence/decoherence refers to the probability waves. Light is wave-like because it is a coherent system of photons (in the quantum sense). In your example of getting coherent systems of electrons in matter, it is not the atoms of matter that are in a coherent state, just the electrons. Therefore, the electrical signals can follow quantum rules (which is important in quantum computer research) while the material the electrons are traveling through remains classical.
  4. Aug 26, 2009 #3


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    Well that's pushing the definition a bit. There isn't much about the car that is definite, and it certainly acts like a cloud rather than a bunch of point particles. If it didn't, you wouldn't be able to see it or touch it. A car is nearly a perfect vacuum, as is nearly everything on the planet. (Of course, this is in relative terms.) If you could see the center of gravity of the car, however, that would be relatively stable as the individual movement of the constituent atoms would tend to cancel out.

    So the point is that you should probably start with a better understanding of quantum particles before you consider what they will look like when you think about groups of them. In many situations their dynamics will tend to cancel out, while in other areas their dynamics creates reinforcement and discernible patterns.
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