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. 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. >> > >> 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. 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. 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. > > > > 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. Bruce+ -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/everything-list/CAFxXSLSRzPg8Hn3ZwRywbYALe93Q5GD7rnwyQP8bhckWXZ5M8g%40mail.gmail.com.
