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FUNDAMENTALLY RANDOM occurrences in physics.

  1. Mar 22, 2010 #1
    Hello everyone
    I'm new to these forums,
    I'm not from an English speaking country
    so I'll try to explain my self as clearly as possible.
    I have a question that has been troubling me for quite a while.
    Are there any fundamentally random occurrences in physics?
    in other words,
    Is there a phenomenon that its behavior is not calculable?
    and I don't mean calculable with today's technology or knowledge,
    I mean a phenomenon that has been proven to be completely random,
    or is thought to be,
    because if everything is calculable then
    only one thing can happen in the universe,
  2. jcsd
  3. Mar 22, 2010 #2
    have you watched the feynman qed lectures regarding light reflecting from glass
    They are available on the internet if you know where to look although im sure its not alowed for me to link to copyrighted material on a bitt torrent site im sure you could find them "nuge nuge"
  4. Mar 22, 2010 #3


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    Radioactive decay is completely random. Hence the Schrodinger's Cat thought experiment.
  5. Mar 22, 2010 #4
    I see,
    I wish I could understand what that cat experiment is all about.
    So what areas of physics do I need to learn in order to stand a chance in
    comprehending such complex (for me) concepts?

    edit: I saw a youtube video and I think I kinda got it... so my next question is,
    how did we prove that radioactive decay is random? ( I know what radioactive decay is )
    Last edited: Mar 22, 2010
  6. Mar 22, 2010 #5


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    Just to be explicit, we can model the rate of radioactive decay of a large ensemble of particles, but there's no way of telling when a specific particle will decay.
  7. Mar 23, 2010 #6

    Andy Resnick

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    Brownian motion, or any diffusive process.

    Randomly polarized light and incoherent sources.

  8. Mar 23, 2010 #7
    I'd argue that, classically, neither turbulence nor brownian motion can be considered fundamentally random.

    In the classical argument, as long as we know exactly where every particle is at some point, and what it's velocity is at that time, we can predict exactly what will happen, even how turbulence will develop, or exactly how a bit of dust will be knocked about by the sea of water molecules over which it wanders.

    Sure, it is unrealistic to expect to be able to account for every one of the billions of particles in the systems described above, but classically, it is possible.

    However, it was only with the advent of quantum mechanics that we realised that this was not the case, that it was, in fact, actually impossible to know everything about a particle at some given time, so we can never be sure how a system will change over time, however perfect our experimental kit is.

    This may seem like a long winded answer, but I feel it's the only way to deal properly with the question.

    So the final answer is that in reality everything is fundamentally random, but the laws of physics give us a good idea of what happens on the average. So we can guess how many particles will decay over, say, a minute, but we can never say when, exactly the next decay will happen. This is not because our technology isn't good enough, it's just that current thinking suggests that it literally is impossible to know.

    However, before quantum mechanics, many people believed that nothing at all was random, that the whole of the Universe's future could be predicted if only we knew the positions and velocities of all the particles in it at some given time.
  9. Mar 25, 2010 #8


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    Even that is not really true. Although, mathematically, of course with given real elements as initial conditions, the equations of motion determine one, unique, solution, the point is that a real number is unknowable. You cannot write down most real numbers. So it is impossible to even write down general initial conditions of any classical system - we can at most approximate them with rational numbers.

    So it is impossible to *know* the initial conditions perfectly (and not even just due to finite precision of measurement - the initial conditions are not "downwritable"), and from there on, there is unavoidable "spread" of the spot of delimited initial conditions.

    A more philosophical question would be: "is" nature random, or is it just "unknowable". Does "nature know" what to do but it is in principle impossible to find out, or does "even nature doesn't know". The answer is that in such a case, it is always possible to take on both viewpoints. You can always make a model where the randomness is "fundamental" and another where the "knowledge is hidden" somehow. Some models might be more elegant and attractive than others of course, but there will in principle not be any way to distinguish them on experimental grounds, and hence we leave the realm of science.
  10. Mar 25, 2010 #9
    that !

    is nature truly random or just unknowable,
    nice way to put it !
  11. Mar 25, 2010 #10

    Andy Resnick

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    Exactly- and I would say it's worse than that, because even having a specific initial condition, it's possible to have an indeterminate final state: Hopf bifurcations. A typical example is the buckling of an axially loaded column: one cannot predict in what direction the column will buckle.
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