On 04-07-2021 08:38, Bruce Kellett wrote:
On Sun, Jul 4, 2021 at 2:59 PM smitra <[email protected]> wrote:
On 02-07-2021 06:46, Bruce Kellett wrote:
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 does not make sense.
You are not observing the IR photons, there is still an interference
pattern in the many particle state involving all the relevant particles.
That's equivalent to replacing the pure state by
tracing out the IR photons and considering the density matrix
describing
the reduced state.
No, that is not what is going on. I am not "tracing out" the IRphotons
-- I don't even know what that might mean. I can observe the photons,
or choose not to observe them, that will make no difference. It is the
existence of the IR photons with sufficient resolution to determine
'which way' information at the slits, that is relevant. If such
photons exist, whether or not they are ever observed, the interference
pattern vanishes. This is a simple matter of the fact that the
interference depends on coherence at the slits. If there is some way
that one could determine which slit the buckyballs went through, then
there is no interference - the determination has decohered the paths,
destroying the possibility of interference. This happens whether the
IR photons are observed or not -- it is merely a matter of their
existence: an 'in principle' determination of which way information.
So it is not a matter of 'tracing over' any degrees of freedom at all.
There is no reduced density matrix involved. I do not consider the
situation when the ball went through the left slit compared with the
situation in which the ball went through the right slit. There is no
"splitting into worlds according to paths" here. As stated, it is not
even necessary to use the IR photons to make a path determination --
their mere existence is all that is required to inhibit the
interference at the downstream screen.
You need to consider the many particle interference pattern where you
use different screens for each photon and a screen for the balls. You
consider the number of balls on the screen for the balls as a function
the position on this screen for fixed positions of dots on the screen
for the corresponding IR photons. For every such fixed position of the
dots made by the IR photons there will be a different interference
pattern of the balls. If you don't observe the IR photons in this way,
then the pattern for the balls will be the integral over all the
interference patters, which means that the interference pattern will be
washed out.
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.
The formation of permanent records is as much a fully quantum process
as anything else.
Permanent records only arise in the classical limit of QM which is a
degenerate limit, i.e. the classical limit is no longer consistent with
QM, which proves that permanent records are unphysical and cannot
therefore explain observations.
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.
The formation of records is not a violation of QM. It is not a
degenerate limit of the theory. It does not depend on the existence of
a separate classical realm, although the formation of permanent
records of experimental outcomes may be an important part of the
emergence of the classical from the quantum substrate. Nothing in
what I have said about the buckyball experiments depends on the
existence of a classical limit.
Interference in the buckyball still exists when doing an appropriate
multi-particle experiment. Of course, such experiments are extremely
difficult to do, but it would not violate the laws of physics to perform
such an experiment. The difficulty is, of course, that there are a very
large number of IR photons being emitted and you have to use a screen
for each photon. But nothing in the laws of physics says that this is
forbidden. This would only truly become impossible if there were an
infinite number of IR photons. Suppose we consider approaching this
impossibility as a limit where more and more IR photons are emitted.
Conservation of energy would then imply that the average energy of these
IR photons would have to tend to zero. The wavelength of the IR photons
then also increases, and if this becomes of the order of the separation
between the splits they don't reduce the visibility of the interference
pattern, so these photons then would not count. To get to an infinite
number of relevant IR photons, one then also has to increase the
distance between the slits as the number of IR photons is increased an
their energy is decreased.
Clearly, you are only going to get a truly invisible interference
pattern in the classical limit where the size of the system consisting
of the slits, the screen etc. tends to infinity, and an infinite number
of photons with infinitesimal energy are emitted. Only in this
degenerate limit does there exist no interference pattern at all, not
just one that one can measure in practice, but also none that in
principle exist involving any finite number of the emitted photons.
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.
Can you prove that? There certainly are irreversible records in the
environment. And the irreversibility is 'in principle' it does not
just depend on the involvement of an intractably large number of
degrees of freedom, which would just be FAPP irreversibility. The laws
of physics forbid the recovery of escaping photons. And the formation
of any record, even writing a result in a lab boo, inevitably involves
the escape of irrecoverable photons. According to the laws of
thermodynamics, any physical interaction will generate some heat. Heat
causes IR photons, and these easily escape to infinity. They are not
recoverable, so they lead to permanent irreversibility.
I suspect that this irreversibility actually leads from the pure state
to a mixed state. This is not covered by the Schrodinger equation,
which would suggest that since the evolution is unitary, there is
always a unitary matrix that will restore the original state. But this
misses the fact that no unitary process can avoid the limitations of
the speed of light. Your whole case relies on an inappropriate use of
pre-relativistic physics. This is one of the fundamental problems with
MWI: it does not reflect the actual situation in the physical world --
it relies on arbitrary simplifications that are simply not true. The
theory does not replicate the actual physical situation -- it does not
explain the observed world.
As I said before in a previous posting on this thread, you are seeking
to explain what should be fundamental concepts using certain messy
macroscopic aspects of a theory which are not universal. This cannot
possibly work well because the things you invoke like IR photons
escaping at the speed of light and being unrecoverable in principle
would have to be rigorously true and it would have to apply in each and
every case. For example, one may object by invoking that the universe is
filled with a plasma and that the IR photons travel at a speed slightly
below the true vacuum speed of light.
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.
No such system is ever perfectly isolated. And besides, that is not
the situation for the majority of laboratory experiments that do give
results, and for which permanent records are easily made.
Interference between the different records in the different sectors
involving all the particles never vanishes. That we in practice cannot
see this does not mean that it does not exist.
Saibal
Bruce+
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