On 21-04-2022 14:13, Bruce Kellett wrote:
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.
While that's how we do computations in practice, superpositions of
observers having made different observations will continue to exist.
That one can trace out the unobserved environmental degrees of freedom
does not cause the superposition to vanish.
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.
The information content describing what she is aware of can be factored
out of the superposition.
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.
There is no collapse of the wavefunction. What matters is that Alice
considered as an algorithm run by her brain can be factored out of the
global superposition as long as that algorithm does not process the
information from Bob's result.
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.)
As long as she doesn't succeed in doing that and doesn't know the
result, she is identical in the different sectors.
-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!).
What matters is that he won't get that information without doing an
observation and therefore his mind (considered as an algorithm) won't be
processing that data and will therefore be the same in both sectors.
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.
What matters is that because Wigner cannot perform an observation
without actually looking, he will be the same person in both sectors
before looking at the result.
Saibal
Bruce
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