On 05-07-2021 09:00, Bruce Kellett wrote:
On Mon, Jul 5, 2021 at 2:23 PM smitra <[email protected]> wrote:
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.
The only interference pattern of relevance is that on the screen hit
by the buckyballs. Of course, for there to be such a pattern, many
balls have to be sent through the slits. But that is the case for any
interference experiment. Otherwise, I do not understand your comment.
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.
What on earth are you talking about? There is no screen for the
photons; and only one photon needs to be emitted by each ball sent
through the slits. Observing that photon on a screen is like firing it
into the wall -- such an operation does not recover the which way
information for the balls. You would have to use a microscope suitable
for IR photons (if such exists?). That would enable one to overcome
diffraction limits in order to determine where the photon came from --
from a ball through the left or right slit, for example.
I don't think this is actually done in the experiment. What is
observed is the presence or absence of the interference pattern on the
screen where the balls hit. The photons are not detected. But if, in
principle, they are of suitable wavelength to resolve the slit
difference, then the interference pattern vanishes. The experiment is
convincing in that they start wil cold buckyballs which show a clear
interference pattern. They then gradually heat the balls so that the
typical wavelength of the photons decreases. This gradually washes out
the interference pattern. (Because at lower temperatures, the
wavelength distribution of the IR photons is such that a few of them
have shorter wavelengths.) As the temperature is increased so that
most IR photons have short enough wavelengths, the interference
pattern disappears completely. The paper by Hornberger et al. is at
arXiv:quant-ph/0412003v2
This is then what I said previously, what you denied, i.e. that you are
only considering part of the system which is defined by the reduced
density matrix. The complete system of buckyball plus photons will show
interference, even if the wavelength is small enough to resolve the
slits provided you perform the right sort of measurement on the balls
and photons.
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.
This is not what happens. Read the paper referenced above.
It's not what happens in that experiment, but you can in principle
demostrate an interference pattern also when photons are emitted by the
balls.
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.
You have failed to understand what the experiment is about.
But that's irrelevant for the main issue here, i.e. that interference
can in principle always be denonstrated, no matter how far the system
has decohered.
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.
This is all totally irrelevant to the actual experiment in question.
And that experiment is in turn irrelevant to the question of whether or
not a real superposition actually exist. You can always perform a
measurement involving more particles where an interference has vanished,
that only demonstrates that the reduced density matrix described a mixed
state, the entire system is still in a pure state.
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.
These considerations do apply to each and every case. I mentioned the
buckyball experiment because it makes things obvious. But the general
principle is always true. Experiments that produced recorded results
are not reversible. Because, for example, they are not thermally
isolated, and IR photons can always escape to infinity and be
irretrievable.
Even if IR photons always escape to infinity, the complete quantum state
of the entire system is still a pure state. There is still a
superposition between the balls going through one and the other slit.
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.
What difference would that make. The IR photons are still faster than
any material object sent after them to capture them. They will always
escape. And because the universe is expanding, they will eventually
pass over the Hubble horizon and be forever lost from sight!
But the observations on the balls will be completed long before that, so
how is this relevant for the existence of parallel worlds?
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.
The fact that it is not physically relevant means that your physics
has failed to capture some important fact about the universe.
Experiments with recorded results are generally irreversible. If your
theory does not accommodate this fact, then your theory is deficient.
Whether this means that the off-diagonal terms of the density matrix
(in the appropriate basis) do actually vanish, or if this is achieved
by some other means, your theory has to adapt to the reality of
irreversibility or your theory does not describe the real world. It is
clear that for many reasons, pure Everettian QM, based solely on the
Schrodinger equation, fails to explain many important features of the
world we observe.
Which would mean that QM cannot be correct as a fundamental theory.
Saibal
Bruce
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