From: <[email protected] <mailto:[email protected]>>
On Friday, May 4, 2018 at 5:50:04 AM UTC, Bruce wrote: From: <[email protected]>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, 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). Bruce *Can you give an example of an irreversible in principle measurement using CI, not MWI? I understand your MWI analysis, but if there is only one world, and decoherence is used in an attempt to explain the measurement process, and if decoherence is statistical in this world, is there a clear example of an irreversible in principle measurement if we only have one world, this world? AG*If there is collapse, as in the CI, then the irreversibility is even clearer: the other branches simply do not exist, so their contribution to the superposition no longer exists, so clearly cannot be reversed.all Bruce*Don't you think that decoherence, which is a reversible statistical process, is responsible for the disappearance of those other branches, and thus, in principle, recoverable, allowing the entire superposition to be recovered in principle in the CI, that is reversed? This where I am having difficulty in arguing that all measurements in CI are in principle irreversible. AG*
Decoherence is unitary, but unitary evolution via the Schrödinger equation produces a separate branch for each possible outcome. The CI throws all these extra branches away, retaining only the branch corresponding to what we see. This projection on to the observed eigenstate throws information away. That is why it is irreversible -- information is missing from the final state, so the inititial state cannot be reconstructed.
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.
