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The casual nature of QM relatons

  1. Jun 17, 2008 #1
    One commonly hears that Quantum Mechanics refutes or disproves the principle of causality.

    and yet if this were the case, qm could not be used to build highly precise machines such as lasers.

    the truth is that qm makes of use of a modified causality.

    the normal causal relation is that event A necessarily determines event B

    in qm, this relaton gets modified such that: event A necessarily determines result B or C.

    thus event A necessarliy determines the the result will fall within the specified range.

    qm is determistic in this sense.
  2. jcsd
  3. Jun 17, 2008 #2


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    yeah, I'm not sure it would be science if it didn't have some determinism in it.

    On the other hand, it doesn't necissarily mean the universe is deterministic.
  4. Jun 17, 2008 #3
    The OP is equivocating what we mean by "deterministic" and "cause".
  5. Jun 17, 2008 #4
    what is the difference?
  6. Jun 17, 2008 #5
    Quantum mechanics (QM) is a deterministic theory. Let's compare to classical mechanics (CM) which everyone agrees is deterministic.

    The classical state of a system of n particles in 3 dimensions is a vector of length 2n that contains the 3d position and momentum coordinates of each of the particles. Classically, this is all the information that is possible to know about the system. If the state of the particle is known exactly at time t, then using any formulation of mechanics that is equivalent to Newton's 2nd law we can predict with certainty the classical state of the system at any given time.

    The only difference is that in QM the state of even a single particle is given by an infinite-dimensional vector. The Schroedinger time-evolution of such a state is perfectly deterministic.
  7. Jun 17, 2008 #6

    Main Entry: de·ter·min·ism
    Pronunciation: \di-ˈtər-mə-ˌni-zəm, dē-\
    Function: noun
    Date: 1846
    1 a: a theory or doctrine that acts of the will, occurrences in nature, or social or psychological phenomena are causally determined by preceding events or natural laws
  8. Jun 17, 2008 #7
    So, determinism does not require precise measurements?
    That's good, because there QM is unavoidably probabilistic.
    I'm happy it can be both deterministic and probabilistic.
    It lets me sleep at night.
  9. Jun 17, 2008 #8
    Certainly QM is deterministic in so far as an event determines a range of values that must fall within definite bounds.

    But even a purely determistic theory can have probable outcomes which would be the case where we didn't know enough about the system to predict with accuracy. In the determistic model this is commonly called a margin of error.

    The critical question is this: why are the results statistical?

    I can't understand why people are so sure the failure to discover a cause proves that causes exist.
  10. Jun 17, 2008 #9
    I strongly agree.
    However, I cannot shake the curiosity that maybe there is an explanation for the apparent randomness of the QM world.
    Einstein's god dice haunt me.
  11. Jun 17, 2008 #10
    I don't see why not. Why are people so sure that causes must not exist. I don't get it.
  12. Jun 17, 2008 #11
    Science attempts to explain how the world works.
    There seem to be limits on what we can and can't explain.
    It takes years of training to quickly distinguish between the two.
    The lay-person sometimes demands that science to explain every detail in a way they can immediately understand.
    When this cannot be done, the expectably common result is frustration and disbelief.
    The goal of a science educator is to present the explanation of nature in a way that is congenial to the public.
    This is a very hard job.
  13. Jun 17, 2008 #12
    i'm a little confused. are we talking about the same thing?

    i think i mistyped.

    my question was really: why are people so sure that causes don't exist for apparently random behavior?

    is that the question you were answering?
  14. Jun 19, 2008 #13
    That's not what we mean when we use the term "deterministic". Determinism here means "events could not have preceded differently if replayed".
  15. Jun 19, 2008 #14
    A good scientist will always look for patterns in the chaos.
    If, however, no explanations can be found after much effort, one looks for order elsewhere.
    The statistical behavior of quantum mechanics was not accepted easily.
    It has been made clear that there is no avoiding the "apparently random behavior" of nature.
    This was done by careful argument, and has been verified time and time again in experiment.
    This is no reason to stop looking and give up!
    Neither is it a reason for everyone to focus on this one illusive problem.
    A sensible distribution of effort would allow for the occasional brave physicist to research new explanations for the "apparently random behavior" of the quantum world.
    So far, no tied and proved theory has arisen from this line of research, to my knowledge.
    Personally, I wish someday this could all be explained, but I don't expect it will.
  16. Jun 19, 2008 #15
    that's fine, but it is determinstic insofar as an Event necessarily determines a result that must fall within a specifed range - if you replay the experiment, the result will always fall within the range. It's sort of a restricted casually determined relation
  17. Jun 19, 2008 #16
    What are you doing with measurement ? Are you advocating Rovelli's interpretation ?

    Basically, the OP describes t'Hooft deterministic QM. This does take into account measurements by equivalence classes.
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