On 02-07-2021 06:46, Bruce Kellett wrote:
On Fri, Jul 2, 2021 at 2:21 PM smitra <[email protected]> wrote:
On 02-07-2021 03:50, Bruce Kellett wrote:
On Fri, Jul 2, 2021 at 4:02 AM smitra <[email protected]> wrote:
This definition only works when you replace the real physical
world
by an approximation obtained by taking an appropriate infinite
scaling
limit that allows decoherence to involve an infinite number of
degrees of freedom.
This is not true. You can have decoherence with the involvement of
only a very small number of environmental degrees of freedom. The
buckyball experiments show precisely this -- it only takes the
escape
of one or two IR photons of an appropriate wavelength to cause
complete decoherence and the destruction of interference.
You are then considering the reduced density matrix by tracing out
some
of the degrees of freedom, in this case the IR photons. That's an
ad-hoc
way of defining the reduced system, not much better than
interpreting
part of a superposition as a world.
No, I am not tracing out anything. I am looking at whether an
interference pattern is formed or not. I don't have to detect the IR
photons in order for the interference to be destroyed.
You choose to look at an interference pattern involving only part of the
relevant degrees of freedom and then you find that there is no
interference pattern. That's equivalent to replacing the pure state by
tracing out the IR photons and considering the density matrix describing
the reduced state.
You can do this by e.g. letting hbar tend to zero. While we
as macroscopic observers are in some sense close to this limit,
the
world we actually live in only has a finite number of physical
degrees of freedom in a finite volume. And locality implies that
in a finite
time after some experiment, only a finite volume can be
physically
affected by the experiments, therefore the decoherence is in
reality
nothing more than an entanglement with a finite number of
environmental degrees of freedom.
The important point to notice is that decoherence always involves
the
escape of IR photons at the speed of light. These are never
recoverable, so the laws of physics ensure that the decoherence
is, in
general, irreversible. You have to take extreme care in very
controlled settings to have things reversible. And if they are
reversible, there can be no permanent environmental record of the
result of the experiment, so one could reasonably say that no
measurement has been made.
It's implausible that escaping IR photons should be relevant for the
question of what an observer is, what observations are etc.
How is it implausible? It is the inevitable existence of the IR
photons that ensures that the measurement process is irreversible. It
is the formation of permanent (irreversible) records in the
environment that determines the existence of a measurement. If no such
records are made then no measurement has been made.
While IR photons and permanent records are associated with macroscopic
observers making observations, these things cannot play a fundamental
role in the measurement process if we assume that QM is indeed a
fundamental theory that also describes observers. If QM is exactly true
then one cannot make an essential part of the theory dependent on a
degenerate limit of this theory that is in violation of this theory. QM
is reversible there are no such things as irreversible records, IR
photons escaping from a system don't cause the system to evolve from a
pure state to a mixed state.
How can it matter whether or not very far away all IR photons are
captured and
billions of years later the entire original state is restored?
Irreversibility means that this is not physically possible. The
escaping photons can never be restored with the correct phases.
It's the other way around. You need to demonstrate that it's physically
impossible before you can claim that the process is irreversible.
Irreversibility requires it to be a violation of the laws of physics for
any hypothetical process, no matter how impractical that leads to the
original state being restored. That's obviously not the case.
The exact physical state of the system plus environment therefore
does not become a mixed state. The fact that one cannot
demonstrate that
the state after measuring a superposition is still a
superposition using
an interference experiment does not mean that it isn't a
superposition.
The observer itself has become entangled with the measured
system, which
is the real reason why the observer cannot even in principle
detect the
superposition.
No. The real reason is that decoherence, and the recording of a
result, is irreversible.
These things are not irreversible in principle, only FAPP.
Says you. The laws of physics, principally the limitation of the speed
of light, means that the state cannot be restored, even in principle.
One can have a system locked up in a finite volume with the outer walled
cooled arbitrarily close to absolute zero and with many layers of inner
walls such that everything from the interior is absorbed or reflected
well before reaching the outer limits of the system.
Decoherence
only involves a finite number of degrees of freedom and can
therefore be
simulated by a large quantum computer. Observers implemented
virtually
in a quantum computer can perform measurements, the system will
decohere, but the entire setup can then be such that the original
state
gets restored. How can it matter for the measurements that much
later
the original state gets restored?
The trouble with this standard quantum computer response is that if no
permanent record of the result is made, no measurement has been
performed. And permanent records are, in principle, irreversible.
Permanent records cannot possibly be relevant to the measurement
process. This is only associated with it because we are macroscopic
systems and in the macroscopic realm QM becomes approximately classical.
And in case of our real universe, how can it be relevant that time
evolution is really irreversible? What if the universe is closed and
will end up evolving back in time according to exact time reversal
invariance in 10^40 years. How can that be relevant for measuring a
spin here and now?
This is just a fairy story. The universe is, in fact, open. Even if it
were closed, it would never evolve back in time.
It would undergo Poincare recurrences.
The practical obstacle that the massive entanglement
involves an astronomically large number of degrees of freedom is
of
course also true, but this cannot be physically relevant.
Of course the irreversibility, even without involving a large
number
of degrees of freedom, is physically relevant. Whereas, the
presumed
persistence of the superposition in the mythical "universal wave
function" is, indeed, physically irrelevant.
So, if you measure the z-component of a spin polarized in the
x-direction and I'm not aware of the measurement result, then my
mind will not have been entangled with the measurement result
(you can
also put me outside your light cone for argument's sake).
That does not always work -- consider entanglement and Bell pairs.
Locality is not always true.
Locality in the sense needed here is always valid, it's not violated
in
Bell-type experiments. In those experiments you have to create an
entangled pair using local interactions and then bring those some
distant away from each other. But then then what is demonstrated in
such
experiments is that local hidden variables don't exist in general.
So,
we can then make use of the fact that when you measure the spin, the
result is actually not determined if I'm not aware of what you
found,
which implies the existence of multiple worlds
The violation of the Bell inequalities implies non-locality; it does
not show the existence of multiple worlds. You cannot get a local
explanation of the Bell violations in MWI.
QM is a strictly local theory. What is nonlocal is any classical account
of QM.
The spin entangled
with you and a large but finite number of degrees of freedom will
therefore be in a superposition. The fact that hidden variables
don't exist means that it cannot be the case that you have made a
definite
observation that I'm unaware of.
Of course that can be the case. It is the formation of a permanent
record in the environment that is relevant to the existence of the
measurement, whether you are aware of the result or not. You can
be
entangled with the spin-up state without being aware of it.
The permanent record will then also be in a superposition (involving
all
the degrees of freedom, including escaping IR photons). Part of my
body
can be entangled with that superposition, but the relevant brain
parts
that implements my mind will not be entangled If it were entangled
then
that would imply that at least in principle, I could know the
measurement result, because the information about it it would be
present
in my brain.
But obviously if I ask what you've
measured I'll always get an answer that I can verify to be
correct.
So, the only way out of this problem is to assume that these
suppositions after measurements exist as different worlds where
different
experimental outcomes have been found.
That conclusion does not follow.
It follows from the assumption that QM is a fundamental theory, so
we
then don't invoke effective macroscopic physics that is only true
FAPP,
like escaping IR photons.
The irreversibility of measurement when there is a definite recorded
outcome is the fundamental theory. Whether or not there is still a
superposition in the mythical "wave function of the universe" is
physically irrelevant. Which means that such a thing could not affect
the outcome of any measurement now, or in the future.
A degenerate scaling limit of a theory where essential parts of the
theory are lost, cannot possibly be the basis of the fundamental
description of a physical process. At most this gives you an effective
account of that is really going on. Just like thermodynamics proves for
a practical but, in principle, inexact description of reality.
Saibal
Bruce
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