On 21 Mar 2014, at 22:30, Jesse Mazer wrote:
On Fri, Mar 21, 2014 at 5:15 PM, Bruno Marchal <marc...@ulb.ac.be>
wrote:
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.
True. I spoke too quickly, I guess my mind jumped to determinism
rather than reversibility (which is a type of reverse determinism)
because I figured John was thinking of quantum randomness, which
only enters in QM if you adopt the postulate of a random "collapse"
on measurement.
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.
I agree with what you say, but I was actually the one who brought up
the Game of Life in the discussion with John, because I was using it
to make the point that the second law of thermodynamics is more than
a tautology, that it actually depends on some specific properties of
the laws of physics such as satisfying Liouville's theorem. With the
appropriate choice of macrostates (namely, defining a macrostate by
the ratio of live to dead cells), in the Game of Life the odds can
favor a higher-entropy state evolving to a lower-entropy one (since
if you start with a random 50:50 mix of live and dead cells, after
enough time you are likely to end up in a state where most or all
the cells are dead).
OK. We agree. Thanks for the clarifications.
Bruno
Jesse
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