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Is QM truly random and many world theory

  1. Jan 29, 2013 #1
    Hi everybody,

    i didn't wanted to create two separate threads so merged them into one.

    i got confused watching Brian Green explaining QM on one of his shows.
    He compared distribution in double slit experiments with throwing a ball on a roulette.
    He said that casino doesn't have to know where ball on roulette wheal is going to land, but casino knows that in the long run that he is going to get some kind of distribution. In other words, when we shoot electrons through double slit, we can't know for sure where it will go but we know that it will follow normal distribution. But in principle ball and roulette table follows Newtonian laws and if we had all the data we need, we could say with 100% certainty where will ball land. Does the same principle apply in QM i.e. if we had all the data we need we could say where the electron land? If that is the case then is QM truly random? On the other hand if we had all the data and we still couldn't predict where electron will land then it seems to me that QM is truly random. Also does qm behaving random drives stake through the heart of fatalism and predeterminism?

    Second question is about many worlds theory. Here is video where Sean Carroll explains a many world theory, and he uses example of a car choosing which why to go. Now, i hope that this many world theory has nothing to do with actual cars choosing which way to go i.e. when actual car chooses which way to go he doesn't create a new world, but rather all of this is on a microscopic level.

    Thank you.
  2. jcsd
  3. Jan 29, 2013 #2


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    QM does not have a mechanism for predicting the exact outcome of many quantum level events, regardless of the knowledge you have. In such cases, the results follow a probability distribution of some kind as mentioned. It is commonly accepted that those outcomes have a truly random element in this sense: there is nothing about the state of a system *here and now* that will determine the outcome of a future measurement independently of the act of observing it.

    There is a formulation of QM called Bohmian Mechanics that asserts that it is possible, in principle, to determine the results with complete certainty, thus restoring determinism. However, they say there are practical limitations that make it impossible to achieve this. This is not a generally accepted viewpoint, although it is considered an acceptable interpretation since the ultimate predictions of Bohmian Mechanics are the same as QM.
  4. Jan 29, 2013 #3


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    If you can predict an event or sequence of events with a certain level of accuracy, how can it be truly random?
  5. Jan 29, 2013 #4
    Exactly that kind of the system is not random, for me truly random system is system that you can't possibly predict, even if you have all the possible data and know all the laws of physics.
  6. Jan 29, 2013 #5
    If you change it to, "events or a series of events where the best you can do is to predict the probability of the outcomes", then it becomes pretty much an definition of randomness. If you then move from purely ideal theoretical statistics to the real world you also get the limits on accuracy from such things as measurement errors and sample size.

    That is however an entirely nonstandard definition of (statistical) randomness as used in QM. The definition you're both using comes much closer to non-causal than random.

    In the case of "truly random", which usually just means "not-pseudorandom", your definition is sort of the opposite of actual usage. Quantum effects are even used as school books examples of actual real world truly random processes like random number generators powered by radioactive decay.
  7. Jan 30, 2013 #6


    Staff: Mentor

    No one can say if QM is truly random or not because deterministic systems can mimic randomness. We have tests that can determine if something is pseudo-random (ie created by a deterministic process) and it has passed all those. But those tests are not 100% reliable - some very complex pseudo-random number generators pass it. So the best we can say today is as far as we can tell it's truely random but cant say for sure - nor do I think is it possible to ever do so.

    Regarding MWI - yes it occurs at the quantum level but since what happens at the quantum level determines the classical under that interpretation there would be a world where the car went a different way. Personally I think the MWI is mystical mumbo jumbo - but hey its a valid interpretation and all interpretations IMHO suck in their own unique way - even the one I hold to - so simply choose the one that sucks the least - its your choice.

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