On 5/2/2018 3:56 PM, [email protected] wrote:
On Wednesday, May 2, 2018 at 10:01:38 PM UTC, Brent wrote: On 5/2/2018 2:44 PM, [email protected] <javascript:> wrote:On Wednesday, May 2, 2018 at 6:01:28 PM UTC, Brent wrote: On 5/2/2018 4:48 AM, [email protected] wrote:On Monday, April 30, 2018 at 3:33:23 AM UTC, [email protected] wrote: Implied by standard QM insofar as the theory is inherently irreversible, that is, irreversible in principle at the quantum level since the wf cannot be recovered by time reversal. AG I argued this conclusion on the Entanglement thread. Here I will add some additional considerations. When you think of time reversibility, say for an electron being measured by SG device, you naturally think of passing the measured electron backward along the same path, trying to recover the original wf by running time backward. Of course you can't run time backward during or even after a measurement because QM doesn't provide any time dependent equations for the measurement process. But even if you could do the thought experiment, according to QM, if the measurement was, say, spin UP, it remains spin UP by virtue of the measurement postulates of QM. Further, This MUST be the backward in time measurement result if you simply accept time symmetry, and not appeal to the measurement postulates of QM. Thus, it seems highly plausible that the original wf, a superposition, cannot be recovered after the measurement, and that QM is a time IRREVERSIBLE theory. AGIn MWI the is both a spin UP and a spin DOWN, as projections on orthogonal subspaces. The theory is mathematically reversible in the sense that if you reversed the evolution of the state vector it would reverse the projection in both subspaces. Brent In MWI and CI we have projection operators, aka in CI as collapse. Aren't they all non unitary regardless of the interpretation, implying IIUC, that they can't be time-reversed. AGYes, a projection operator is non-unitary. Maybe I didn't phrase it well, but that's why I avoided invoking projection operators. The subspaces become orthogonal in the approximation that we can average out the cross terms, but that approximation is only a good one when decoherence has taken place. BrentTo avoid confusion, please distinguish between the MWI and CI when making claims. I have been limiting my remarks to CI. Does decoherence occur in both interpretations? I see decoherence as a unitary process. Is this correct? I don't understand your comments about averaging out the cross terms. AG
I was discussing MWI. Decoherence, being a physical process, must occur in both. It's a FAPP explanation for why CI works, i.e. where the Heisenberg cut is and why a projection operator is a good model of a measurement. Decoherence is unitary which is why it's only FAPP, or "statistical", it ensures that in some variable (hopefully the one you want to measure) the pointer states for different values will belong to FAPP-orthogonal subspaces.
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