On Thu, Jul 1, 2021 at 2:29 AM smitra <[email protected]> wrote: > On 29-06-2021 01:46, Bruce Kellett wrote: > > > > I think John's trouble here is that he still adheres to David > > Deutsch's concept of worlds. Deutch talks as though every component of > > a superposition is a separate world. This leaves Deutsch no language > > to talk about decohered worlds, pointer states, and all the other > > usual apparatus of quantum interpretations. The trouble with taking > > every component of a superposition as a separate world is that in > > Hilbert space (as in any vector space) you can define an infinite > > number of different sets of basis vectors, so any vector in the space > > is represented by an infinity of different worlds, and there is no way > > to distinguish between these. > > > > I think Bruno has flirted with this idea as well. Deutsch, through his > > popular writings, has done an immense amount of harm to the cause of > > quantum interpretations. > > > > Bruce > > There ids a large body of rigorous work in this field, it's not that you > have just a handful of advocates who are defending the MWI based on > dodgy nonrigorous arguments. Of course, you can't just take nay > component of a superposition as a separate world.
But that is precisely what people like John, Deutsch, and Bruno do. > But given that Worlds > do exist A world exists. That is all that we can be sure of. > and given that time evolution is given as a linear operator, it > follows that if QM is a fundamental theory that also describes > observers, that you inevitably end up with superpositions of entire > Worlds. > Worlds have to be carefully defined. According to decoherence theory (which is also a consequence of the linearity of the Schrodinger equation), decohered worlds are truly separate and do not recombine. Non-decohered elements of a superposition do not constitute separate worlds. > > This conclusion does not depend on any assumptions of how observers > should be defined rigorously, how experiments and ultimately > observations arise out of the physics. These issues that are not yet > 100% solved, are totally irrelevant provided QM is indeed a fundamental > theory. > > It's not any different from someone claiming that conservation of > momentum may not be true. How do we convince this person that it is > true? We can appeal to fundamental laws of physics and argue on the > basis of symmetries, Noether's theorem and then say that this rigorously > establishes conservation of momentum. But the skeptic can then take > issue with the assumption about the validity of the fundamental laws, > he will insist that it's still possible for momentum to get lost. If he > does an experiment involving many particles, then he'll say that unless > you measure the momentum of each particle to infinite accuracy, you > can't really tell that momentum is conserved. He'll then turn the logic > about the fundamental laws upside down by arguing that because you can't > really be sure about momentum conservation, you can't therefore say that > the fundamental laws have been all that well established. > > Of course, there is then a lot to argue about this reasoning suggesting > that there is room for momentum nonconservation. But the arguments > against MWI (regardless of whether or not you need to add Born's rule as > a postulate and if so, regardless about any discussions about this then > invalidating the original goals of some MWI advocates), are of the same > nature. Not really. You can accept the Schrodinger equation as fundamental without agreeing to MWI. The fact that you can't derive the Born rule from the Schrodinger equation in a non-circular fashion is quite telling. It means that the Schrodinger equation is more naturally seen as a way of calculating the time evolution of probabilities. QM is a probabilistic theory, so its fundamental laws give probabilities. And probabilities are not worlds. Bruce Here you have a supposedly fundamental theory, QM and it implies > in a rather straightforward way the existence of parallel Worlds, and > because people don't like that conclusion, you have arguments against it > that can only work if QM is not true as a fundamental theory. The > problem with those arguments is then that it's invoked as a standalone > argument against the MWI. If these arguments were well motivated on > their own merits, then they would form the basis of a lot of physics > research in many different fields ranging from condensed matter, > particle physics etc. But that's not the case. > > Saibal > -- 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/CAFxXSLTzv%2ByTamQNGGuz8Qki2iokXGQUetK1qm7sASObj1L3rg%40mail.gmail.com.
