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According to classical physics if we know space-time coordinates of every atom in the universe, we can predict the future. But quantum physics introduced probability throwing determinism out of question. But one of the famous explanations of Schrödinger's cat paradox, many worlds theory, says that at each instance the universe splits into many making all probabilities possible. In this sense, we can say that an event will have all the outputs possible in one or the other universe.Don't you think this is also deterministic in nature as we can exactly predict the future ( of course when all the universes taken together)?

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marked as duplicate by ACuriousMind, CuriousOne, user36790, Norbert Schuch, Kyle Kanos Jan 31 '16 at 20:11

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    $\begingroup$ Possible duplicate of Is the universe fundamentally deterministic? $\endgroup$ – ACuriousMind Jan 30 '16 at 17:51
  • $\begingroup$ The answer to that question also answers yours: In MWI, collapse is absent, so time evolution is unitary, and the universe is "deterministic". $\endgroup$ – ACuriousMind Jan 30 '16 at 17:53
  • $\begingroup$ One can't predict the future even in classical physics. As to MWI... stay away from that nonsense. It makes blatantly false statements about the universe based on a total misunderstanding of the measurement process. What really happens in measurement is well understood. The necessary "randomness" for the outcome of a measurement comes out of the thermalized phase space of the measurement device. $\endgroup$ – CuriousOne Jan 30 '16 at 20:12
  • $\begingroup$ According to Laplace and many other scientists, prediction is possible.Even Einstein believed in it.He once said that causality makes a person do a crime.In one sense he need not be jailed for nothing is in his hand.If we have a big computer enough to process every atom's position in the universe, since we know how atom's react, their future course can be predicted.This is what many believed until quantum revolution $\endgroup$ – siva phanindra Daggubati Jan 31 '16 at 0:26
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    $\begingroup$ Laplace was simply wrong and Einstein didn't pay attention to 19th century physics and mathematics if he believed that classical mechanics was predictable. Newton had already noticed that he couldn't solve the three body problem and by 1987 Bruns and Poincaré had proven the a general solution in form of algebraic expressions and integrals was not possible. Today we can prove that classical mechanics does not allow for long term prediction, except in a handful of cases. It doesn't matter what size computer you have, non-integrable systems are not long-term predictable. $\endgroup$ – CuriousOne Jan 31 '16 at 2:22
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As long as no measurements are performed, quantum mechanics is perfectly deterministic. That is what we mean when we say that 'time evolution is unitary'.

The wave function evolves in time according to Schrödinger's equation. Just as in classical physics, if you know it at a given time, you can compute it for any time in the future. The problem with measurement (in the standard, Copenhagen interpretation) is that it involves an external observer. The system evolves deterministically, but for reasons that are exterior to the system, someone performs a measurement. This is a sudden and violent event where the whole wave function of the system interacts with another incredibly complex system. This usually collapses the wave function onto eigenstates of observables in an unpredictable way.

Within the many-world interpretation, there is no external observer. Instead, our view of the system is restricted to one single outcome of the measurement. I would not call this determinism, because according to this interpretation, there is no possible way for us to interact with other 'worlds'. Most of the information necessary to predict the future is hidden from us. Most importantly, this is built into this theory and can not be avoided.

However, as CuriousOne says, the difference in between different interpretations of quantum mechanics is pure philosophy. Whatever the interpretation you subscribe to, the outcome of the calculations that you make will be the same. Choose the interpretation that you prefer, it makes no difference at the experimental level.

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  • $\begingroup$ I wouldn't say that MWI is pure philosophy. It clearly makes false assumptions about the measurement process. $\endgroup$ – CuriousOne Jan 30 '16 at 20:42
  • $\begingroup$ As I recall it the predictions of MWI are strictly identical to the predictions of other interpretations of QM. There is (by definition) no observable difference in between different interpretations of QM. If this were not true they would be different theories. $\endgroup$ – Steven Mathey Jan 31 '16 at 1:10
  • $\begingroup$ I wouldn't call the splitting of the universe as the result of a human experiment (how would nature even be able to tell the difference?) an identical outcome, but maybe if a blind man squints with both eyes while jumping on one leg it may just amount to "identical". $\endgroup$ – CuriousOne Jan 31 '16 at 2:11

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