On Saturday, May 19, 2018 at 5:14:10 AM UTC, [email protected] wrote: > > > > On Friday, May 4, 2018 at 4:22:47 AM UTC, Bruce wrote: >> >> From: <[email protected]> >> >> >> On Thursday, May 3, 2018 at 11:52:00 PM UTC, Bruce wrote: >>> >>> From: Brent Meeker <[email protected]> >>> >>> >>> On 5/3/2018 4:03 PM, Bruce Kellett wrote: >>> >>> The problem, of course, is that this unitary operator is formed in the >>> multiverse, so to form its inverse we have to have access to the other >>> worlds of the multiverse. And this is impossible because of the linearity >>> of the SE. So although the mathematics of unitary transformations is >>> perfectly reversible, measurements are not reversible in principle in the >>> one world we find ourselves to inhabit. >>> >>> >>> I think we need a more precise term than "in principle" which could >>> confuesed with "mathematically". You really mean reversal is >>> *nomologically* impossible even though it's *mathematically* >>> reversible. It's more impossible that *FAPP* or *statistically* but >>> not *logically* impossible. :-) >>> >>> >>> Not doable "in principle" just means that there is no conceivable way in >>> which it could be done. It is not just a matter of difficulty, or that it >>> would take longer than the lifetime of the universe. It is actually >>> impossible. Quantum mechanics does not imply that all things that are >>> logically possible are nomologically possible, or could be achieved in >>> practice. That is why Saibal's claim that there exists a unitary operator >>> that does what he wants is rather empty -- there are an infinite number of >>> unitary operators that are not realizable in practice. And this limitation >>> is a limitation "in principle". >>> >>> Bruce >>> >> >> *If you take the view that quantum reality is irreducibly random, it >> MEANS that there is no process in nature that can explain how a random >> event could occur, for if such a process existed, it would contradict >> "irreducibly random". Bruce seems to take the view that all measurements >> are irreversible in principle. That might not be true. For example, suppose >> the temperature of a system decreases. Isn't it hypothetically possible to >> imagine a time reversal of all the IR photons which caused the cooling, to >> reunite with the original system and restore the previous higher >> temperature? If so, the cooling process in this example is reversible >> albeit hugely improbable -- which I refer to as statistically reversible, >> or irreversible FAPP. I think Bruce can give an example of a measurement >> which is time irreversible in principle, that is, impossible to time >> reverse. AG* >> >> >> Classical situations involving the second law of thermodynamics >> (increasing entropy) are reversible, though reversal is improbable because >> the second law is statistical. The situation in quantum mechanics is >> different when we have a measurement with several different possible >> outcomes. In MWI these outcomes are in different branches, and we cannot >> reach into these worlds to reverse things there. Decoherence in this branch >> is certainly statistical, and so it is in all branches, >> > > *So why don't you draw the obvious inference? If those other worlds don't > exist -- which if I can read English has been your passionate position all > along -- then quantum measurements in this world, the only world, are > statistical and hence NOT reversible in principle. AG* > > >> but it is different in each branch of the wave function, so reversing >> this branch does nothing for the others, and does not restore the original >> superposition. Thus the process is irreversible in principle (nomologically >> irreversible -- to reverse violates the laws of physics). >> > > > *But if those other worlds don't exist, it makes no sense whatever to rely > on them to establish irreversible in principle in this world (as > distinguished from statistically irreversible or irreversible FAPP). It > seems you want to have it both ways; that many worlds really don't exist. > but quantum measurements in this world are irreversible in principle due > the existence of many worlds. AG* >
*I suppose you could adopt the view that the other "branches" aren't other worlds, but are similarly inaccessible once a measurement occurs. But then you still have the unsolved problem of explaining what exactly is lost, and how, when a measurement occurs. Appealing to the properties of the projection operator is not enough IMO since it might just be, and probably is, a bookkeeping device in the CI to deal with the apparent collapse of the wf. That's what I meant earlier when I wrote that appealing to the properties of the projection operator is not a strong (or indeed any) argument for irreversibility in principle, insofar as it's really just a restatement of what you believe. AG * > >> 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 post to this group, send email to [email protected]. Visit this group at https://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.
