On Thu, Apr 21, 2022 at 6:32 PM smitra <smi...@zonnet.nl> wrote:

> On 21-04-2022 02:53, Bruce Kellett wrote:
> > On Thu, Apr 21, 2022 at 10:05 AM George Kahrimanis
> > <gekah...@gmail.com> wrote:
> >
> >> On Tue, Apr 19, 2022 at 11:09 AM Brent Meeker <meeke...@gmail.com>
> >> wrote:
> >>
> >> The only purpose of the box in Schroedinger's thought experiment was
> >> to put off the observers perception.  Really the thought experiment
> >> is over when the radioactive decay occurs.  That atom has
> >> transitioned to a different nuclear state which is entangled with
> >> and recorded in the environment.
> >
> > On Tuesday, April 19, 2022 at 4:20:49 AM UTC+3 Bruce wrote:
> >
> >> Yes. Schrodinger had the cat in a box to emphasize the idea that the
> >> cat was in a macro-superposition of alive/dead. This misled Wigner
> >> to the extent that he thought the state collapsed only when the box
> >> was opened. All of this was made redundant when it was realized that
> >> decoherence  rendered the state definite almost instantaneously.
> >> Saibal makes the same mistake when he claims that Alice, after her
> >> measurement, is still in a superposition until Bob sees her result.
> >> The idea that the superposition still exists since decoherence is
> >> only FAPP is something of a red herring -- in MWI, Alice has
> >> branched according to her result into up and down branches that no
> >> longer interfere. There is no macro-superposition.
> >
> This is wrong, because inability to demonstrate interference does not
> mean that there is no superposition.

As has been pointed out by others, superpositions are ubiquitous. It is
only superpositions of the basis vectors of pointer states that are
relevant. Pointer states are those that are robust under environmental
decoherence. Once you trace over the unobserved environmental degrees of
freedom, the superposition is gone, and the worlds no longer interfere --
there is no macro-superposition.

Alice does not branch due to decoherence.

So what does she branch due to? Of course Alice is subject to decoherence
as much as anyone else.

> It is true that there are two
> branches where the results of Bob are different due to rapid
> decoherence. But before Alice knows the result of Bob, the state of the
> algorithm that represent Alice's mind will be identical in both
> branches

The state of Alice's mind is of minor relevance. You are coming close to
Wigner's failed idea that consciousness collapses the wave function.

What matters is whether or not information about Bob's result
> can change Alice's subjective state. Only then can the two branches from
> Alice's point of view, diverge. If this were possible, then that means
> that Alice could obtain information about Bob's result  without even
> looking at his result. So, Alice would have psychic abilities.

If Alice's mind/brain is entangled with the results of Bob's measurement,
then if she can access that brain state directly, that will enable her to
ascertain Bob's result. There are multiple copies of the result entangled
with the environment. But people can seldom directly access the state of
their own brain. (It is all irrelevant, anyway. Physics is not
determined by subjective states.)

> -1- Decoherence (by a chaotic environment) turns an entangled
> > superposition into a non-coherent density matrix, only if we
> > subsequently omit the environment from the description of the system.
> > (Not if we keep the environment in the description.)
> >
> > FAPP is for a reason -- we automatically trace out unneeded
> > environmental variables.
> >
> >> -2- The "box" (in which Scroedinger's cat is enclosed, with the
> >> lethal apparatus) contains also its "environment", so a quantum
> >> descrition of this box describes the environment also. Therefore I
> >> do not agree that decoherence INSIDE THE BOX will ruin the
> >> superposition ASSESSED FROM OUTSIDE THE BOX. So, Wigner was right. I
> >> suppose that Saibal also is right, though I have not checked that
> >> message (sorry).
> >
> > Unfortunately for this idea, decoherence does not stop at the box. In
> > the time that Wigner thinks about this before he opens the box,
> > decoherence has enveloped essentially the whole world, so Wigner
> > himself has decohered into either a world with a dead cat or a world
> > with a live cat. He can't dissociate himself from the split that
> > occurs, so from his point of view outside the box, the superposition
> > is long gone, and he has to deal with a simple classical state of
> > either a dead cat or a live cat -- no superposition remains.
> >
> The superposition does remain, it's just that it cannot be probed in
> interference experiments.

No one is doing interference experiments on Schrodinger's cat....

Before Wigner knows the result, his subjective
> state of his mind is the same in both branches. It doesn't matter that
> everything in his environment  including the atoms in his brain is
> entangled with the state after the experiment. What matters is that
> Wigner cannot know the result without observing it.

If his brain is entangled with the result, he can know that result if he
can access his brain state (unlikely!).

> The bitstring that
> contains all the information he is aware of, is the same in both
> branches. If not and the bitstring would have diverged across the two
> branches and Wigner could then simply feel the result without having to
> do any observation.

The observation would be of his own brain state. Wigner (and the cat) are
in a world where the cat is dead, or in a world where the cat is alive.
What Wigner thinks about it is irrelevant.


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