> On 28 Jan 2021, at 12:02 pm, Bruce Kellett <[email protected]> wrote: > > On Thu, Jan 28, 2021 at 11:47 AM Pierz Newton-John <[email protected] > <mailto:[email protected]>> wrote: > On 28 Jan 2021, at 11:32 am, Bruce Kellett <[email protected] > <mailto:[email protected]>> wrote: >> On Thu, Jan 28, 2021 at 11:20 AM Pierz Newton-John <[email protected] >> <mailto:[email protected]>> wrote: >> On 28 Jan 2021, at 11:03 am, Bruce Kellett <[email protected] >> <mailto:[email protected]>> wrote: >>> On Thu, Jan 28, 2021 at 10:44 AM smitra <[email protected] >>> <mailto:[email protected]>> wrote: >>> >>> FAPP, therefore not well defined at all. Sticking to FAPP you could >>> never have discovered Special Relativity, General Relativity, found the >>> correct way to resolve Maxwell's Demon paradox, etc. etc. >>> >>> FAPP is well-defined for all practical purposes. That is all that you >>> require for special and general relativity, statistical mechanics, and the >>> rest of physics. You cannot point me to any physical result that is not >>> FAPP -- we have only limited measurement precision, after all. And that is >>> good enough for real-world physics. >>> >> Bruno’s point IIUC is that FAPP is OK for the physics you have now, but >> possibly not for the next physics. "Irreversible FAPP” means irreversible >> today. It’s true that there does come a point with decoherence where the >> state is irreversible, but that point is arbitrary and depends on the >> technology you have available. >> >> That is not true. Decoherence ultimately involves the emission of IR photons >> into outerspace -- through heat dissipated in the atmosphere if no other >> way. Such effects are truely irreversible, not just FAPP, because once you >> have lost photons to space there is no way to get them back -- you can't >> chase after them and turn them around..... >> >> The point about decoherence is that the irreversibility is ultimately a >> matter of the laws of physics. FAPP is just for laboratory purposes, but in >> the wider context, the irreversibility is absolute, not just FAPP. > > That’s true - decoherence by definition means something escaped the > experimental boundaries and then it’s game over. I should have said that > where decoherence begins is technologically determined. > > That is not really true. Decoherence begins with the interaction with > environmental degrees of freedom. If you limit this interaction > technologically, then you might be able to reverse things in particular > cases, but that does not really prove anything because, as I have pointed out > in the general case, the interaction with the environmentultimately produces > heat, and some of that escapes to outer space. You can't travel faster than > these IR photons rto turn them round, or harvest them. So the laws of physics > themselves, relativity and thermodynamics, mandate that quantum events are in > general irreversible. > > You can utilize technology to maintain quantum coherence over ever larger > domains (as in quantum computers), but that domain can never extend to the > whole universe; not even beyond the laboratory to the wider earth. This is > not a technological limit -- the limit is in the laws of physics themselves. > > This actually has very little to do with the question of whether quantum > physics is universal or not -- quantum mechanics can be the correct theory of > everything in the universe, but it would still be the case that decoherence > is irreversible in principle. This does not bear on the question whether > Everett is correct or not.
I’m not saying decoherence is reversible. I’ve corrected myself (or accepted your correction) on that point. But my understanding of proposals for disconfirming MWI involve extending quantum coherence to larger and larger scales. Deutsch has argued that if we can get enough qubits into a quantum computation, we’ve effectively “proved” MWI since “where did all that information come from?". Other proposals similarly involve reversibility at large scales. If QM is not universal, then at some scale that will prove impossible not merely due to technological limits, but limits of the laws of physics. If such a limit were found, that would certainly disconfirm MWI. > > Bruce > > > The point is that the bounds of what is reversible or not depend on what we > are technologically capable of. After all that’s the problem of quantum > computers - maintaining larger and larger superpositions in a controlled > state. In the future it’s to be hoped that we can extend those bounds to the > point where the question of whether QM is universal or not can be resolved. > Surely that is a meaningful question and surely the only way to answer it is > through something like what I am describing. If we can be confident that QM > is universal, then we can get closer to an answer on whether MWI is the right > interpretation. > > -- > You received this message because you are subscribed to a topic in the Google > Groups "Everything List" group. > To unsubscribe from this topic, visit > https://groups.google.com/d/topic/everything-list/xsl8cSDT4M8/unsubscribe > <https://groups.google.com/d/topic/everything-list/xsl8cSDT4M8/unsubscribe>. > To unsubscribe from this group and all its topics, send an email to > [email protected] > <mailto:[email protected]>. > To view this discussion on the web visit > https://groups.google.com/d/msgid/everything-list/CAFxXSLTmyeF-zMUAXKZsFW2NEJ667s7bmg4_2wOeVTq6jmmtgg%40mail.gmail.com > > <https://groups.google.com/d/msgid/everything-list/CAFxXSLTmyeF-zMUAXKZsFW2NEJ667s7bmg4_2wOeVTq6jmmtgg%40mail.gmail.com?utm_medium=email&utm_source=footer>. -- 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/4941FC3A-CB20-41D0-B6F1-70E8A63E41FF%40gmail.com.
