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Wha is quantum physics claiming?

  1. May 15, 2006 #1
    I was wondering if modern physics considers quantum mechanics that follows from the hysenberg principle to be actually the way the universe functions, or simply that it is the extent of what we can possibly know with observation.

    I mean it seems to me that most of quantum mechanics is saying that there is a limit on what we can know through observation, and this is a science which can make predictions about what we can know. The actual rules which govern the universe are too small. Or is it considered that when we arent looking at a particle, for example, it phyisically is in multiple places at once.

    If it is making physical descriptions of the world, then how did it go from the idea that we can never really know everything about a system because our observations are limited, to this being an actual physical phenomenon and not just a limitation of ourselves?
  2. jcsd
  3. May 15, 2006 #2
    The uncertainty principle is a deep fundamental of nature and has nothing to do with limited observations.
  4. May 15, 2006 #3
    WARNING: Hand-waving argument to follow:

    In some ways I see the uncertainty principle as a modern re-statement of Zeno's paradox. He claimed that it was self-contradictory to specify the position of a moving object. Newton used the calculus of continuous functions to show how you could do this for a continuously observable point particle, thus "resolving" the paradox. But modern QM is built around discrete observations of finite extent. For example, one moment you have a single particle and the next it has decayed. Even if you still get a single particle the next moment, you have no way to know that it didn't disappear (e.g. in an interaction with some unobservable part of the universe) and then re-appear as a new instance of the same type of particle before you make the next observation. This singular property of observation contradicts continuity. The imprecision in the simultaneous measurement of position and velocity is another example of this and Planck's constant is effectively a measure of the discreteness of observations.

    Whether you consider the uncertainty principle as more fundamental than discreteness of observations or vice versa is largely a matter of taste, I suspect.
  5. May 15, 2006 #4


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    This conclusion was suspected almost immediately after quantum theory was introduced. However, there are 3 primary stepping stones (published papers) that get us to where we are today:

    1. EPR Paradox (1935)
    2. Bell's Theorem (1965)
    3. Experiments of Aspect et al (1981).

    The generally accepted conclusion is that the limits of the Heisenberg Uncertainty Principle are very much real and physical.
  6. May 15, 2006 #5
    Hold it right there! QM gives us the rules for answering most reasonable questions one can ask about any physical system in the universe. Do not lessen the power and grandeur of QM.

    Whether or not we can properly follow those rules, or work out what those rules imply about the universe is another matter.
  7. May 15, 2006 #6


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    All this is interpretation-related. Quantum theory is a mathematical theory (as are all physical theories) that allows you to predict the probabilities of outcomes of experiment (if you use also some basic intuition). Up to this point, I guess that all physicists are in agreement, and it is essentially the thing which is scientifically falsifiable.
    Some say we should stop at this point. People even don't agree on the domain of applicability of quantum theory: does it apply to an apple too, or only to microscopic objects ? Point is, up to now, where it has been applied, it was in agreement with measurements.

    And then we enter into the dispute area: people think about what this formalism can MEAN. There are several views on that, and adherents of one view think that the views held by others are of course crazy and hilarious. Each view has its attractive, and its weird points. My point is that such a view should help you devellop some intuition for the formalism, but there are other people that hold other views. All this is rather personal, and philosophically inclined, and according to different views, the answer to your questions is radically different.
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