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Basic Question About Quantum Theory

  1. Oct 6, 2011 #1
    I want to preface this by stating that I have only a rudimentary knowledge of this subject. I've only began reading about quantum physics casually and on my own time. With that being said. Here's what I don't fully understand.

    Is the above quote actually stating that matter only exists when it is being observed? If so, I have a hard time grasping that.
  2. jcsd
  3. Oct 6, 2011 #2
    No. It says that the nature of the things you want to study depends on what kind of properties you want to analyse.

    In the realm of elementary particles (but sometimes also of molecules) everything has a dual nature: both particlelike and wavelike. It is the kind of experiment that decides what kind of properties will be shown.

    For example, talking of light, you can see it as a wave in all the phenomena involving interference and diffraction - and that's the characteristic behaviour of waves - but in some other experiments - like Compton scattering and the photoelectric effect - the only way to describe effectively light is thinking of it as a particle (the photon).

    It is the same for the electrons.

    Actually the quote says something more: it says that the effect of observing a system can (and will) affect your measurement. In fact, before the measurement, the electron is described as a wave (a wave of probability), while just after the measurement the electron collapses into a particle, meaning that you can "localize" it with a certain grade of precision.
  4. Oct 6, 2011 #3
    One other interpretation is that what we perceive as reality is actually a computation. As such only the bit of the universe we are actually engaged with is fully computed at any given time. The rest of the universe exists in a pluripotent wave of possibilites which can collapse to any one of an infinite number of possible realities when needed.
  5. Oct 6, 2011 #4

    Ken G

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    Also, note that the quote says "Quantum theory suggests..." That's not necessarily a good way to start out talking about quantum theory. The theory itself doesn't suggest anything, what it does is make predictions about what will happen in certain idealized conditions, which can then be checked with experiment under similar conditions, and these predictions work astoundingly well when suitably idealized conditions are obtainable. That's it, that's the end of the theory of quantum mechanics. It's best to learn what those predictions are, and even more importantly, what the experiments that they are intended to predict are, before you worry about what all that "suggests." The problem with "suggestions" is that they are not unique-- two people who both can apply quantum theory, and both can agree on the experimental outcomes, may still disagree on just what the theory is suggesting about those outcomes.
  6. Oct 6, 2011 #5

    I knew that I was misunderstanding the quote. It just seemed to read that way to me for some reason.

    Would it be something like the way that water can exist in different states? It's just that the electron changes states based on observation?
  7. Oct 6, 2011 #6
    So all possibilities already exist even before we "choose" them?
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