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Is reality at quantum level just probablilistic?

  1. Jan 1, 2013 #1
    Hi. First of all English is not my native language and i am a layperson, so I hope i can explain my self.

    It is said that reality at the quantum level is probabilistic, not determined.

    My question is : That is really the case or a probabilistic knowledge is the best we can get as humans?

    And, if it is probabilistic, how do we know?
  2. jcsd
  3. Jan 1, 2013 #2
    Best to leave the word 'reality' out of it I think. That word has a lot of baggage associated with it.

    Within quantum theory 'observations' (I'll use this word instead of reality), observations are random. They are inherently random. The probability we will observe a particular value is the best we, and anyone or anything can know, its the best that can be known. We know this from many experiments, from the double slit experiment to tests of the Bell Inequalities. Understanding these experiments and what they entail can be confusing. But what they do entail is that observed quantities are random.
  4. Jan 1, 2013 #3
    Hi. Modus.

    The values observed are random but how do we know that there is not a deterministic proceses behind the aparent random value?. Like a random emulation in a computer program.

    Thanks for the answer.
  5. Jan 1, 2013 #4


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    Staff: Mentor

    When you have a chance, search the web for "Bell's Theorem" and "EPR Paradox".

    The EPR paper (written by Einstein, Podolsky, and Rosen in 1935) made a very strong argument that there should/must be such a deterministic process underlying QM; or as they phrased it, 1935-vintage QM is "incomplete".

    In 1964 John Bell discovered his theorem, which basically says that any theory that Einstein and company would have accepted as complete must necessarily disagree with the predictions of quantum mechanics under some circumstances. Over the next few decades, various experiments have tested these predictions under those circumstances, and the QM predictions have been confirmed.

    So, despite ongoing metaphysical debate about what quantum mechanics IS, there is general agreement that it IS NOT a deterministic process of the type that EPR had in mind and that (I think) you are thinking of.

    Your analogy with a computer program emulating the randomness is good - take a moment to google for "DrChinese Challenge" and "Quantum Randi Challenge".
  6. Jan 2, 2013 #5
    As humans, yes.

    If there is a deterministic set of laws for the universe, they are certainly outside the realm of science. If such laws do exist, even knowing them would not help us better predict experiments than we already can.
  7. Jan 3, 2013 #6


    Staff: Mentor

    There seems to be a very deep feature of nature that at a fundamental level it is probabilistic. Check out Gleasons Theorem:
    Gleason's theorem highlights a number of fundamental issues in quantum measurement theory. The fact that the logical structure of quantum events dictates the probability measure of the formalism is taken by some to demonstrate an inherent stochasticity in the very fabric of the world. To some researchers, such as Pitowski, the result is convincing enough to conclude that quantum mechanics represents a new theory of probability. Alternatively, such approaches as relational quantum mechanics make use of Gleason's theorem as an essential step in deriving the quantum formalism from information-theoretic postulates.
    The theorem is often taken to rule out the possibility of hidden variables in quantum mechanics. This is because the theorem implies that there can be no bivalent probability measures, i.e. probability measures having only the values 1 and 0. Because the mapping is continuous on the unit sphere of the Hilbert space for any density operator W. Since this unit sphere is connected, no continuous function on it can take only the value of 0 and 1. [2] But, a hidden variables theory which is deterministic implies that the probability of a given outcome is always either 0 or 1: either the electron's spin is up, or it isn't (which accords with classical intuitions). Gleason's theorem therefore seems to hint that quantum theory represents a deep and fundamental departure from the classical way of looking at the world, and that this departure is logical, not interpretational, in nature.

    Also check out:

    It seems QM is pretty much the only theory possible if you require the possible states to continuously change to other states - which if you think about it is basically what you want of physical systems.

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