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Thread: Quantum mechanics and Probabilities

  1. #1
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    Quantum mechanics and Probabilities

    In quantum mechanics, probability is wonderful. Do Experiment A and you receive a very good series of events that can be described by probability and statistics.

    Is it ever possible to run an experiment that has "Effect A" happen at x% and "Effect A" has a chance of causing "Secondary Effect B", y% of the time (and so on)?

    (Edit) Basically, I was wondering if quantum mechanics ever could build a chain of probabilities for a chain of events where you could say with some certainty that if A happens, B has a good chance of happening and if A and B happens then C is also very likely to happen and so on.

    I can't formulate a reasonable real word example, but is that reasonable?
    Solfe

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    Well, most particle physics is a bit like that? Collision products and decay products are generally probabilistic and when you are searching for a new particle you have to use these probabilities to model the expected decays and look for any differences from it.

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    Probabilistically additive (or multiplicative) behavior can only be assumed if the events do not interfere with each other, i.e., if they decohere.
    Everyone is entitled to his own opinion, but not his own facts.

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    Readers of New Scientist (5th Jan) will recall the article about the paper by Andreas Albrecht and Daniel Phillips who have argued that probability is not real but every time we use it we are are calling up quantum mechanics. I feel their argument is rather circular having derived the chance of the coin toss from quantum sequencing (including apparently neurotransmitter uncertainty), to be 50%. It seems they want to intrude on the multiverse idea. However it may be that they are on to something, random, that dangerous word, would mean according to them, "the result of a chain of quantum events". Well Yes.

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    Quote Originally Posted by Cougar View Post
    Probabilistically additive (or multiplicative) behavior can only be assumed if the events do not interfere with each other, i.e., if they decohere.
    So, if you stimulate an atom, you get a change in the electrons that perhaps cause the emission of a photon of a particular energy and the system has decoherence. But putting hydrogen in a bomb calorimeter (assume standard temp and atomo) and burn it, the system has lost coherence. Either one of the systems should progress the same way under the same conditions, but even if they are related to one another (IE introduced hydrogen to a heat source) you must look at them differently when quantifying them. The single atom may have a probability of a different series of events due to the assumption that this is a shirt sleeve environment, you are merely selecting one of many atoms in that environment.

    Quantum mechanics is so strange, I am pretty sure I should have used question marks after every sentence.
    Solfe

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    Quote Originally Posted by profloater View Post
    Readers of New Scientist (5th Jan) will recall the article about the paper by Andreas Albrecht and Daniel Phillips who have argued that probability is not real but every time we use it we are are calling up quantum mechanics. I feel their argument is rather circular having derived the chance of the coin toss from quantum sequencing (including apparently neurotransmitter uncertainty), to be 50%. It seems they want to intrude on the multiverse idea. However it may be that they are on to something, random, that dangerous word, would mean according to them, "the result of a chain of quantum events". Well Yes.
    To me, that kind of language represents a fundamental error in understanding what quantum mechanics, and indeed physics, is in the first place. To say that randomness in the world is a result of quantum mechanics strikes me as backward logic-- the world does not behave the way it does because of our ways of thinking about it, our ways of thinking about it are conditioned by what happens in the world. So the concept of "randomness" is not something the world does, it is something we use when we try to understand what the world does. Then we discover that this same concept of randomness is also helpful in understanding the predictions of quantum mechanics. Does that mean quantum mechanics causes randomness? Does the theory of general relativity cause gravity? Did Newton's?

    Of course quantum mechanics has to be consistent with the 50% coin probability, if it weren't it would have been junked long ago. That simply doesn't mean that the coin behaves as it does because randomness comes from quantum mechanics. In actuality, both randomness, and quantum mechanics, come from the same place: our efforts to understand our world.

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    Ivars Peterson, in his book The Jungles of Randomness, shows how randomness arises in classical mechanics. You don't need quantum mechanics to identify that a classical system whose evolution is highly sensitive to uncertainties in its starting position way, way beyond any practical level of precision - precision to zillions of places of decimals for example, is, in a truly real world sense, random.

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    What's more, classical mechanics, even without chaos, requires that we invoke probability concepts all the time. We must invoke probabilities whenever we do not have complete information about a system. When do we ever have complete information? It was always just pure faith on our part that complete information was possible in principle, or that the universe somehow had complete information about itself. The theory of classical mechanics never required that the universe contains any such thing as complete information, unless we confuse the attributes of our theories with the attributes of our universe. We should have learned to stop doing that a long long time ago!

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