On 21 Mar 2014, at 20:17, Jesse Mazer wrote:
On Fri, Mar 21, 2014 at 3:00 PM, John Clark <johnkcl...@gmail.com>
wrote:
On 18 Mar 2014, at 22:33, LizR wrote:
> Am I right in assuming that in a quantum mechanical universe you
can trace the history backwards?
Absolutely not because in Quantum mechanics 2 very different states
can evolve into the exact same state.
Not if you're just talking about the evolution of the quantum state
vector according to the Schroedinger equation, which is totally
deterministic.
Deterministic is compatible with the fact that 2 very different states
can evolve into the exact same state, making it non reversible.
But the solution of the SWE are more than deterministic, they are
reversible. In QM (without collapse) 2 different states evolves into
two different states.
But John was correct in thinking that determinism does not entail
reversibility. He gave the example of the game of life. But most
arithmetical operations are like that too. 2+3 gives 5, but from 5
you can't necessarily retrieve 2+3, it might be 1+ 4 or 101 - 96.
With the combinators, the reduction (Kxy --> x) is deterministic, but
not reversible, as KKS and KK(K K) will both gives K, and eliminates S
and (K K) respectively. You can still build universal combinator base
(set of combinators) which will make computation reversible. We can
simulate elimination of information with process which don't eliminate
information (the basic idea consist to put the information in some
trash, and never "empty" the trash.
I am rather confident that the comp+theaetetus "core physical laws"
will be deterministic and reversible (fro formal reasons related to
the "material views").
Bruno
As I said to Liz, non-reversibility only appears if you assume the
"collapse" of the wavefunction to a new quantum state on measurement
is a real physical phenomenon distinct from normal wavefunction
evolution, rather than an approximate description of something that
happens due to decoherence (as would be true in the many-worlds
interpretation where the universal state vector is all there is, and
also in Bohm's hidden variables interpretation which is
deterministic at all stages).
Jesse
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