On Sun, May 15, 2022 at 5:11 PM smitra <smi...@zonnet.nl> wrote:

> On 15-05-2022 00:55, Bruce Kellett wrote:
> > On Sun, May 15, 2022 at 1:17 AM smitra <smi...@zonnet.nl> wrote:
> >
> >> The big advantage is that decoherence is a well researched area of
> >> (mathematical) physics, results like the density matrix becoming
> >> approximately diagonal, and relations between decoherence to entropy
> >> increase making it effectively irreversible are all rigorous results
> >>
> >> that are uncontroversial. People may still have objections against the
> >> MWI, but they'll still accept these results on decoherence.
> >
> > Yes. And decoherence says that with time, the off-diagonal elements of
> > the density matrix become arbitrarily small. If there is a smallest
> > non-zero probability, then eventually these off-diagonal elements
> > become zero. This reduces the pure state to a mixture. Which is to say
> > that there is a collapse; unitary evolution ceases, and  we have
> > reached a classical world.
> >
> >> Non-zero minimum probability on its own, however, does not cause a
> >> system to evolve in a non-unitary way.
> >
> > It does when decoherence is taken into account. See the above
> > explanation. Bruce has not omitted anything.
> >
> > Bruce
> I see, but these sorts of models can already be ruled out. There are
> plenty of simple systems where one can make extremely accurate
> measurements on which can be kept totally isolated and quantum coherent
> for long enough where such effects would have become visible.

The effects are due to decoherent entanglement with the environment. So of
course they are not seen in isolated systems. Duh.


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