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
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 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 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.
>> 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.
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!
>>>
> >>> 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.
Bruce
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