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Layman's definition of CM and QM?

  1. Dec 3, 2005 #1
    As a layman's definition, would it be sufficient to define CM and QM differences as follows?

    CM: Macroscopic observations (theories) closely resembling the more accurate results that can be derived through QM.

    QM: Probability-based observations (theory) that more accurately describe the underlying reality, more applicable when applied to microscopic (or quantum) levels.

    Also, what of other approaches like Stochastic Electrodynamics (SED) that seems to combine the both disciplines? How realistic are such theories and do they hold any promise to supercede CM and QM?

    Thanks in advance for your replies.
  2. jcsd
  3. Dec 4, 2005 #2


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    It is not so much the probabilistic aspect or accuracy that tell us what is a quantum theory. After all, one can construct totally WRONG quantum theories too.

    I would say that a classical theory has as its basic idea the fact that the "state of the world" is given by a real structure which determines - in principle - uniquely all possible results of measurements one could imagine. This state of the world can then evolve according to a deterministic, or a stochastic evolution equation, but at any instant, there *IS* a unique state of the world determining all outcomes.

    A quantum theory ALSO has as a basic idea that the "state of the world" is given by a (complex) structure, but it DOES NOT DETERMINE all possible results of measurements one could imagine. This state of the world can evolve deterministically (Schroedinger equation) - one could modify it eventually into a stochastic evolution, but that's not standard QM.
    Moreover, in a quantum theory, there is the superposition principle: if state A of the world is possible, and state B of the world is possible, then there exists a state a A + b B of the world.
    This superposition principle is what makes it impossible for the state of the system (in general) to determine the outcome of an experiment.
    If state A contains "the ball is here" and state B is "the ball is at my home", then the state aA + bB has no precise answer to the question: where is my ball ? While a classical theory ALWAYS has a unique answer to that question.
  4. Dec 4, 2005 #3
    Thats for your reply, vanesch. That certainly helps me better grasp the difference between the two.
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