Do you think Laplace's Demon would be valid again if quantum mechanics became predictable?

Question

Hypothetically, lets say we find the Theory Of Everything that unites the macro and microscopic and also provides us a way to know everything about subatomic particles... ie. we know if and when the cat in the box is dead.

Answer 1

No, I don't think so.

The universe has a large number of particles (by which I mean any kind of information-bearing elementary object).  The behavior of each one of these particles affects the behavior of every other particle.

Now, in order to predict the future behavior of these particles requires a computation engine of some sort, which must consist of particles itself.  It must therefore be able to predict its own behavior.  Not only does this essentially constitute an example of the Halting Problem (known to be impossible to resolve generally--see link below), but its own operation would impact the behavior of particles outside the computation engine.

Note that this problem is not solved by moving the computation engine "out of the universe" somehow.  In order to make meaningful computations, information about the universe must be input into the engine.  The very fact that this information can enter the engine means that it must be considered part of the same system as the universe--that is, the same arguments above apply.

Answer 2

"We may regard the present state of the universe as the effect of its past and the cause of its future. An intellect which at any given moment knew all of the forces that animate nature and the mutual positions of the beings that compose it, if this intellect were vast enough to submit the data to analysis, could condense into a single formula the movement of the greatest bodies of the universe and that of the lightest atom; for such an intellect nothing could be uncertain and the future just like the past would be present before its eyes."

— thus spake 'Marquis Pierre Simon de Laplace'.

'Laplace's Demon' concerns the idea of determinism, namely the belief that the past completely determines the future. Clearly, one can see why determinism was so attractive to scientists.

Indeterminism! thy name is 'Quantum mechanics'.