Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Is Everything Predictable?

  1. Jul 25, 2010 #1
    Alright, so I am new to the whole "Quantum Physics" thing, so please don't jump down my throat. I have seen this question asked elsewhere, however a lingering question persists in my mind.

    Let's assume that we know all variables involved in a particular situation. Some will assert that:

    a) This is just not possible. We can never know all variables in a given situation (it would take too much time!).
    b) This is just not possible. There are situations in which random factors affect the situation, making it impossible to predict (at a subatomic level) what will really happen.
    c) Even if we know all variables, it is still not possible to predict the random factors associated with it.

    Now, I may not have produced the most eloquent set of circumstances, but the question I have for you folk is, what if those "random factors" that I hear so much about, are still not so random? I mean, when you pose the question of: will two identical unstable nuclei will not decay at the same identical time?; how do we know that at an even deeper level, these are really identical? Or that came into existence at exactly the same frame in time?

    I assert that for this particular question of randomness, if you will, that in exactly the same closed environment, if both unstable nuclei were created at the exact same time in the exact same coordinates within their closed environment, and have the exact same makeup, that they WILL, indeed, decay at the exact same time.

    Please tell me if I'm foolish.
  2. jcsd
  3. Jul 25, 2010 #2
    You may want to look up Bell's inequalities and Hidden Variables
  4. Jul 25, 2010 #3


    User Avatar
    Science Advisor

    If you have a state vector, e.g. |neutron>, this is all you can say and know about a neutron. Anyway, you can't say when it will decay.
  5. Jul 25, 2010 #4
    What exactly do you mean by "predictable"?

    I don't know the finer details of QM, but I do know that in Classical Mechanics everything is deterministic (i.e. given the current state + laws of motion, it fixes the universe at all other times), but most classical systems are not predictable. This might seem confusing, but in some systems the solution can't be solved analytically and can only be calculated numerically (i.e. to a certain allowed error -- and in chaotic systems small initial errors eventually result in a considerable difference), so even though the future is fixed in Classical Mechanics, it's not really possible to know it.
  6. Jul 26, 2010 #5


    User Avatar
    Science Advisor

    No not everything in qm is predictable. You can predict, that in a certain time 50% of a given amount of nuclei will decay. But you can't say which individual nucleus will deacy when.
  7. Sep 1, 2010 #6
    yeh i was new not long ago :)
    but no you cannot predict every future according to quantum physics
    you have an idea of what will happen, according to the liikelyhoods, but that only goes up to 50% of what you may know
    this happens becouse of the uncertanty principle, to predict what will happen in the future than you need to know the speed of a particle, and its exact position. To know these youo need to shine a photon onto the particle, but to find out the position of a particle, shining a photon on it changes its speed.
    the same goes for position
    The more you know of one thing the less you know of the other.
    So we cannot know the future
  8. Sep 1, 2010 #7
    according to QM nothing is predictable except probability distributions, which evolve determistically according to the Schrodinger Eqn.

    Isn't it possible (but very unlikely) that no nuclei will decay?
  9. Sep 1, 2010 #8


    User Avatar
    Science Advisor

    Of course 50% are valid only for a large number of nuclei - strictly speaking in the limit of infinitly many nuclei - according to the "law of large numbers". I am sorry for the sloppiness.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook