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Laplace's demon

  1. Aug 12, 2013 #1
    Laplace’s demon knows all forces and “positions of all items of which nature is composed” and enjoyed the knowledge of the future just like the past to be “present before its eyes”.

    Acording to Wikipedia, also the source of the first quote, “Due to its canonical assumption of determinism, Laplace's demon is incompatible with interpretations of quantum mechanicsthat stipulate indeterminacy”.

    But is that really so?

    Isn’t it the case that, on the contrary, quantum physics assumes classical physics in every respect?

    Uncertainty of a particle’s position and energy assumes that such quantities exist (although they can’t be ascertained by non-demon entities).

    Is not then the conclusion that Laplace’s demon still has the same privileges, although the demon idea is of less practical value to us?
  2. jcsd
  3. Aug 12, 2013 #2
    No. Why do you expect this? QM is supposed to be fundamental, and we are supposed to be able to derive classical physics from QM, so it would be a bit backwards for QM to be based on classical physics.

    On the contrary, in the standard interpretations of QM particles *don't have* definite positions or energies. It's not merely that we lack sufficiently precise knowledge of these properties. It's not merely that we couldn't, even in principle, measure these quantities beyond a certain precision. Rather, these quantities simply don't have definite values. If you want to know more, you can read about Bell's theorem and the impossibility of "local hidden variable theories."
  4. Aug 13, 2013 #3


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    The answer depends on the interpretation of quantum mechanics you use. Some interpretations (e.g. Bohmian and many-world) are completely deterministic.
  5. Aug 13, 2013 #4
    There are plenty of "deterministic" interpretations of QM, but AFAIK none exist where the world is experimentally deterministic. i.e. we could never obtain enough knowledge to predict the future with certainty, because some things are inaccessible to us.

    For example, in MWI you could easily calculate all of the future worlds resulting from some initial system. However, there is no way of knowing which world you will end up in (of course you end up in all of them, but each version of you is only aware of that world and could not have predicted that specific outcome)
  6. Aug 13, 2013 #5


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    It is certainly not an assumption of the HUP or of QM in general. It is an assumption in some interpretations of QM, as Demystifier points out.

    I would say that few believe that non-commuting observables - such as position and momentum - have simultaneously well defined values independent of the act of observation.
  7. Aug 13, 2013 #6
    Thank you for your interest.

    I was thinking along the following lines:

    i) first I considered how things, as I speculate, may have looked to Heisenberg initially, i.e. particles indeed have locations, energy etc. but it seems that for some combinations we may not be able to find a set of these variables beyond a limit. Uncertainty of a parameter’s value, implies there is a value to be uncertain of. I realize that later models may do away with simultaneous position and momentum etc.

    ii) I then moved on to a philosophical issue, namely whether the relations of quantum mechanics empower us to refute a demon-type Newtonian mechanics, or whether they just allow us to declare it un-operative.

    iii) Let me - now - take the opportunity to add a third step, pointing put that both forms of mechanics are just theories, none are reality. Therefore, the demon’s privilege is to see “the future and the past /model of reality/ before its eyes”.
  8. Aug 13, 2013 #7


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    Certainly a possibility. And if she does, she will also have non-local vision with which to behold that future.
  9. Aug 14, 2013 #8


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    Historical sidenote: you have it backwards. It was Heisenberg's starting point to get rid of unobservable quantities like the position of the electron in the Bohr model. From this, he derived matrix mechanics and it took him about a year to realize that it implies an uncertainty relation.
  10. Aug 14, 2013 #9
    That sounds reasonable enough.
    Actually, I am mainly paraphrasing general explanations such a Wikipedia's (and many others):

    "In quantum mechanics, the uncertainty principle is any of a variety of mathematical inequalities asserting a fundamental limit to the precision with which certain pairs of physical properties of a particle known as complementary variables, such as position x and momentum p, can be known simultaneously".

    You have to admit that in this formulation it appears as if the particle indeed has a simultaneous x and momentum p (which cannot be ascertained).
  11. Aug 15, 2013 #10


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    I don't disagree, but it is quite difficult to phrase such statements in a way that they are understandable to laymen and are reasonable short. Even more so, if you want them to be interpretation-neutral.

    I think it is very hard to understand the implications of QM without learning the math or the help of someone who knows it. The best you can do is stick to experiments and check what assumptions are made in the explanations your sources give.
  12. Aug 18, 2013 #11
    Thank to all of you.

    I believe we may have come to the end of the road with this.
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