On 5/21/2018 11:21 AM, [email protected] wrote:
On Monday, May 21, 2018 at 5:28:38 PM UTC, Brent wrote: On 5/20/2018 6:25 PM, [email protected] <javascript:> wrote:On Monday, May 21, 2018 at 12:22:24 AM UTC, Brent wrote: On 5/20/2018 4:56 PM, [email protected] wrote:On Sunday, May 20, 2018 at 10:35:26 PM UTC, Brent wrote: On 5/20/2018 2:54 PM, [email protected] wrote:On Sunday, May 20, 2018 at 9:13:42 PM UTC, Brent wrote: On 5/20/2018 1:44 PM, [email protected] wrote:On Sunday, May 20, 2018 at 7:29:42 PM UTC, Brent wrote: On 5/20/2018 3:47 AM, [email protected] wrote:On Sunday, May 20, 2018 at 6:52:41 AM UTC, Brent wrote: On 5/19/2018 8:19 PM, [email protected] wrote:On Saturday, May 19, 2018 at 3:59:03 PM UTC, Brent wrote: On 5/18/2018 10:53 PM, [email protected] wrote:On Saturday, May 19, 2018 at 5:29:33 AM UTC, Brent wrote: On 5/18/2018 10:14 PM, [email protected] wrote:*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*You don't handle uncertainty well, do you. Brent You know, it's not a perfect analogy, but I don't believe that when I pull the one arm bandit with 64 million possible outcomes, that 64 million (minus one) worlds are created, each with an identical copy of me, getting those other outcomes. What do you believe? AGI believe I'll wait for a better theory. One that includes gravity and spacetime and consciousness. Brent I see. But you seem too ready to defend the MWI when it appears to imply irreversible in principle. Or do you accept Bruce's claim that the projection operator implies irreversible in principle? AGEither of them implies irreversiblity. Whether it is "in principle" depends on what principle you invoke, mathematics, practice, ...? MWI puts information in orthogonal subspaces where we exist in copies such that each copy can act only in one subspace and hence cannot put together the information from other subspaces. A projection operator is just a mathematical model of this confinement to one subspace. Brent Please; no evasive games. We can forget about the MWI, given its absurdity. Does, or does not the projection operator imply irreversibility in principle as Bruce claims? AGSure, a projection operator throws away information so its action is irreversible. If I project a star onto the celestial sphere I preserve its longitude and latitude, but it doesn't show how far away it is. That's why the MWI advocates say projection conflicts with all the rest of fundamental physics which is described by evolution that is at least reversible in the mathematical sense. Brent But how do you know that the projection operator represents an actual physical process? It could be just a bookkeeping device to describe the fact that when many possible outcomes are possible, we get a particular outcome.Exactly. The quantum Bayesian take this view How does "Baysian" fit into this picture? Can't one interpret the SWE as a representation of what we know about a system, without being a Baysian? AG and consider Schroedinger's equation also as a personal book keeping device of what one knows about a system and then the Born rule and projection operators fit neatly into the scheme of updating one's personal knowledge. I would delete "personal" from your comment. We're referring to the knowledge of any observer. AGISTM, that to argue for "irreversible in principle", it is insufficient to appeal solely to the properties of the projection operator. AGExactly what led to Everett, MWI, and decoherence theory. But at the price of having multiple, orthogonal "worlds" to explain the appearance of randomness. Of course some people hate randomness and are quite happy to have multiple worlds instead. Looks like I am in the decoherence camp; namely, that when a quantum measurement occurs, entanglements with reservoir states somehow suppresses all outcomes but oneBut that "somehow" is the magic of the Copenhagen interpretation. Decoherence is the process of making subspaces (worlds) orthogonal, but it doesn't choose one and "suppress" (vanish?) the others. The all continue to exist and to be orthogonal,. There's no magic; just an unknown process that allows one result and not others.Right. And there have been some serious proposals for that process, like Penrose's gravitational metric difference.That the other subspaces continue to exist and are orthogonal and inaccessible, is the result of imposing the projection operator.No. The SE causes them to evolve into orthogonal subspaces, no projection operator required. That's what Everett showed to be a consequence of treating the instruments and the observer all as quantum systems. How can the SE do that without a measurement? IIUC, the probability amplitudes just keep evolving in time absent a measurement. No measurement; no orthogonal subspaces. I think they come about due to the projection operator, a housekeeping device. AGThat's the problem decoherence solved. It doesn't have to be a "measurement" in the sense of someone seeing the result. That's the lesson of the buckyball two-slit experiment.I'm OK with getting rid of the projection operator. Are you now claiming information is lost or inaccessible in these orthogonal subspaces and therefore quantum measurements cannot be reversed?
They are inaccessible to the people of any one world of the MWI.
I'm confused as to where we are in this discussion. AGIOW, I conjecture that the founders of quantum theory made it an irreversible in principle theory without sufficient reasons. AGWell, they thought the fact that they observed their lab notes didn't reverse themselves was a sufficient reason. The lab notes showed they got a measurement; a single measurement. So they added the projection operator to account for that. Nothing firm about irreversible in principle AFAICT. AGAgain you casually use the pharse "in principle" with saying what principle.I don't have to since "irreversible in principle" means there is no physical process which allows the measurement to be reversed.
OK, then you've qualified it to mean nomologically irreversible...but still logically (mathematically) reversible.
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