On Fri, Mar 21, 2014 at 5:15 PM, Bruno Marchal <[email protected]> wrote:
> > On 21 Mar 2014, at 20:17, Jesse Mazer wrote: > > > > On Fri, Mar 21, 2014 at 3:00 PM, John Clark <[email protected]> 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). Jesse -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at http://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.
