On Fri, Oct 16, 2020 at 5:03 PM Russell Standish <[email protected]> wrote:
> On Fri, Oct 16, 2020 at 04:49:35PM +1100, Bruce Kellett wrote: > > On Fri, Oct 16, 2020 at 3:38 PM Russell Standish <[email protected]> > wrote: > > > > On Fri, Oct 16, 2020 at 10:07:32AM +1100, Bruce Kellett wrote: > > > > > > It is refuted by the idea of unitary evolution in QM. Unitary > evolution > > means > > > that everything is reversible, If new microstates are created as > the > > universe > > > expands, then this expansion cannot be reversed: the creation of > such > > > microstates gives an absolute arrow of time. This is generally > rejected, > > > because physicists tend to believe in unitary dynamics. If > dynamics are > > not > > > unitary, then the universe is not governed by the Schrodinger > equation, > > and > > > arguments for the multiverse collapse. > > > > I'm not sure the last point follows, perhaps you can expand on it. > But > > it is an interesting argument that the Layzer style "increase in > > microstates" > > should be enough to prevent a Hawking style "wavefunction of the > > universe". > > > > > > I was talking about the Everett-style quantum many worlds. Other types of > > multiverse (such as the existence of other cosmological Hubble volumes) > are not > > necessarily affected. Hawking's "wave function of the universe" is a > definite > > casualty if unitary evolution is denied. > > > > > > > > Could the ideas be made compatible by have the number of accessible > > microstates increasing over time, due to the expansion of the > > universe, but that the total number remains constant, or is even > > infinite? Or does that place us right back at the original problem of > > having a low entropy initial state. > > > > > > I don't really understand this. An infinite number of microstates makes > little > > sense in standard thermodynamics. > > > > Quite true. It would have to involve some sort of limiting process, > which would definitely be non-standard thermodynamics. But that's > never stopped anyone before :). > > I was more speculating along the lines of the usual way of reconciling > irreversible processes with a reversible multiverse. Where the > interesting stuff happens in a finite dimensional subspace of an > infinite dimensional Hilbert space, but that dimensionality grows in > time due to "splitting", or "decoherence" or what have you. > Decoherence does not increase the relevant number of dimensions of Hilbert space. If you do a spin measurement, the up and down branches entangle with the environment through decoherence, but there are still really only two primary bundles of states - the operative Hilbert space (as far as the original measurement is concerned) is still two-dimensional. The number of dimensions in the original problem does not magically increase. If you want to see decoherence as an indication of irreversibility, then that is fine. But that is not what Everett says. To get irreversibiliy into unitary QM you have to introduce a real collapse process, such as GRW -- or some of the branches resulting from decoherence have to be traced over (coarse-grained). Bruce -- 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 view this discussion on the web visit https://groups.google.com/d/msgid/everything-list/CAFxXSLScANNx%3DUNT_6jDU1D34WBsi4fTnOR5nHgMv7KGONqJQA%40mail.gmail.com.
