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Old 13th March 2019, 03:16 PM   #45
Robin
Philosopher
 
Join Date: Apr 2004
Posts: 9,940
Anyway, I think I understand better now. It is best to think of it in terms of a single particle in a single dimension.

If I have the quantum state of a single particle in one dimension and the expectation value for position then I could evolve it forward and get the quantum state at a future date with the expectation value of where the particle would be then.

Then I could start with the future quantum state and evolve it backwards and get my original quantum state with the same expectation value.

So Laplace's Demon doesn't need information about a particular momentum and position, rather he needs the quantum state - the probability distribution.

Similarly, in order to get back to the encyclopedia Laplace's Demon would not need a particular state of ashes and light, but rather a probability distribution of all the ways that the ashes and light could be as a result of the encyclopedia burning.
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The non-theoretical character of metaphysics would not be in itself a defect; all arts have this non-theoretical character without thereby losing their high value for personal as well as for social life. The danger lies in the deceptive character of metaphysics; it gives the illusion of knowledge without actually giving any knowledge. This is the reason why we reject it. - Rudolf Carnap "Philosophy and Logical Syntax"
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