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Q & A: does determinism work backward?

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Most recent answer: 06/26/2015
Q:
It is well-known that determinism is a philosophical theory that has is scientifically incorrect, because of the uncertainty principle. That means that, even if we knew all the laws of physics and had enough processing power, we could not create a computer simulation of the universe that could apply all the laws and predict the future. But is that the same case for the past? Our brains contain memories of the past (not the future), and what we see around us contains information about the past. So is it theoretically possible to create a computer simulation on which we can take all the steps backwards and spy Da Vinci while he is painting the Mona Lisa?
- Alex (age 25)
Corfu, Greece
A:

Great deep question!

So far as we know, the equations for how quantum states (which are, so far as we know, everything) change in time work the same forward and backward in time. So you might think that the forward-going lack of determinism would imply backward-going lack of determinism. Some historical questions would not only be difficult or practically impossible to answer but unanswerable in principle, like most quantum predictions. Yet no such effects have ever been found. This asymmetry between the past and the future is closely related to the asymmetry of the second law of thermodynamics, that entropy always increases. In the context of our current understanding, these asymmetries arise from a special feature of the particular conditions under which our universe started: low entropy, low quantum entanglement between remote objects.

Perhaps cosmologists will develop an understanding of this asymmetry. (The future is uncertain!) Meanwhile we believe that our past, at least as far back as just after the Big Bang, is definite.

That doesn't mean that a Laplacian perfect calculator could actualy make those past calculations. You might read Scott Aronson's book Quantum Computing Since Democritus, for discussions of the limitations on the computing power of any device which is itself a subset of the physical world about which it is making computations.

Mike W.


(published on 06/26/2015)

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