> On 17 Aug 2018, at 13:34, Bruce Kellett <bhkell...@optusnet.com.au> wrote: > > From: Bruno Marchal <marc...@ulb.ac.be <mailto:marc...@ulb.ac.be>> >>> On 17 Aug 2018, at 01:05, Bruce Kellett <bhkell...@optusnet.com.au >>> <mailto:bhkell...@optusnet.com.au>> wrote: >>> >>> >>> I think this may be the origin of your problem. If we look at a position >>> measurement, we have some wave function describing a wave packet as a >>> superposition of position eigenstates. The Schrödinger equation for a >>> measurement interaction with this state describes the evolution according >>> to the interaction with each component of the original superposition, >>> leading to decoherence or entanglement with the environment, so that >>> multiple branches emerge, each corresponding to a different result for the >>> position measurement. >> >> Good. >> >>> We do not have an analogous situation with the singlet state. The only >>> superposition that is involved is the superposition of the two basis >>> vectors of the spin Hilbert space in any arbitrary direction. The crucial >>> point is that there is no superposition of different sets of basis vectors. >>> Such an idea makes no sense within the formalism of quantum theory. So when >>> I write the state as: >>> >>> |psi> = (|u>|d> - |d>|u>)/sqrt(2) >>> >>> that is the only superposition involved. I can certain write this in terms >>> of some other set of basis vectors, |u'> and |d'>, but these are >>> alternative representations of the state, and the alternatives are not >>> additive, so there is no superposition of all possible bases as there is >>> for all possible results of the measurement of position. >> >> I do not superpose the bases. I take only into consideration that those >> alternative representations describe the same singlet state. We need to do >> that, because Alice has the choice of which base to use when measuring her >> particle. > > The alternatives do not need to actually exist in order for Alice to make a > choice. Only possibilities need exist.
Then I understand why you still believe in physical influence, while rejecting FTL signals or information transfer. My point was just to show that with the MW we cannot use the violation of the Bell inequality (no-locality) to claim that the MW is still not local. We differ on the interpretation of the SWE’s solution. You can see my version as a variant of the “many-minds”, but with a quite different identity relations between a first person view and a brain state. > >> That will localise her in different branches: so they all have to exist >> prior to the measurement. > > No, they don't have to exist as separate branches. That is to assume a > superposition, It reflects different possibilities, which needs to be realise in some way, if only to avoid physical influence being instantaneous or giving more importance to a base on another. > and you appear to want to deny that there is such a superposition. You are > becoming incoherent. > >> A singlet state describes a collection of different superposition. > > You are interpreting it as a superposition of representations in different > bases. That is not part of the quantum formalism. It is just something you > have made up. You can deduce it from Everett: no base is more real than any other, + locality. > >> That is why both Alice and Bob are totally ignorant of the result that can >> get when choosing a direction for their spin measurement. > > They are ignorant of what they will get because for each of them the > probabilities of up and down are both equal to 0.5, regardless of the > direction in which they measure. You do not need to have all outcomes > pre-existing for this to be true. Then you will have instantaneous action at a distance, even when no information is transferred. I am skeptical this make sense, but my point was logical. It is false that MW entails non-locality with Bell’s inequality violation. > >>> You can only ever get one of two results for a spin measurement -- you >>> can't get an infinite number of different results. >> >> Sure, but you have a lot of choice (an infinity) for the direction chosen >> for the spin measurement. > > Of course, but that does not justify the idea that all branches pre-exist. But that is not exactly what I am saying. I am saying that some superposition can be interpreted as different superposition. When Alice choose her spin direction, she will chose a partition of the multiverse, which is a sort of multi-multiverse. Without this, not only we have FTL, but we get super-determinism. Alice is no more choosing anything. > >>> Another way of putting this is that you choose which measurement to make >>> (i.e., the direction of your magnets). You do not measure this direction. >> >> Indeed. But to get a coherent MW picture, that changes nothing … before the >> measurement is done. >> >>> If you want to go from your home to work there are several different routes >>> you can take. You can turn left at your door and go down the High Street >>> and catch a tram, >> >> I use a bike :) > > Ha, Ha! > >>> or you can turn right at your door and go to the station to catch a train. >>> Or you can go into your garage and drive your car to work. These are >>> alternatives, and it makes no sense to add them together. However, there is >>> a way in which you can turn this into a superposition: you make the choice >>> about which way to get to work according to the time of some radioactive >>> decay or other random quantum event. Then, since you are amplifying the >>> decay time by entangling it with a transport option, your journey routes >>> enter in a quantum superposition with the decay times. But this is not what >>> you generally do when you go to work. You usually have other reasons for >>> choosing between tram, train, or private car. Generally, these reasons are >>> complex, and you make a rational decision. You decision is not the result >>> of a random quantum event. >> >> I have no problem with this. Our difference concerns the MW interpretation >> of the entanglement state, like the singlet state. >> >>> If you can grasp the distinction I am making here, you will understand the >>> various attempts I have made to understand your idiosyncratic portrayal of >>> the quantum singlet state: I tried to get a superposition by considering >>> the effect of random quantum fluctuations in Alice's body; I suggested that >>> Alice choose her measurement angle according to the time of some >>> radioactive decay; and no doubt I could think of other possibilities. But >>> you assured me that this is not what you meant. So I can only conclude that >>> you do not know what you are talking about. > > The position has not changed. You are still in your own fantasy world -- you > use a superposition, and yet deny that you are using such a superposition. > That is incoherent. What I still do not understand in your view is how can you interpret |psi> = (|u>|d> - |d>|u>)/sqrt(2) as a unique superposition. It seems to me you can do that because Alice and Bob have prepared that state, but that state represents also another superposition, like |psi> = (|u'>|d'> - |d'>|u'>)/sqrt(2). Why would |psi> denotes a superposition of |u>|d> and |d>|u> and not |u'>|d'> and |d'>|u’>. It seems to me that you choose a particular base, when Everett makes clear that this would lead to nonsense. The physical state represent by |psi> must be the same whatever base is chosen. That leads to considering that psi describes not one superposition, but many superposition. That get worse with GHZ and n-particles state, and that is why I have often (in this list or on the FOR list of Deutsch) explained why the multiverse is a multi-multi-multi-… multi-verse. I don’t insist too much because more careful analysis would require a quantum theory of space-time, and the Everett theory will certainly needs some improvement. It is also why I prefer to describe the “many-worlds” as a many relative states, (or even many histories), and you are right, they are not all reflecting simply the superpositions, but different partitions of the multiverse. When Alice choses a direction for measuring her particle’s spin, she choose the partition, and enforced “her” Bob, that is the Bob she can meet in the future, to belong to that partition, wth the corresponding spin. But she could have used another direction, and they both would be described (before the measurement), by a different (locally) superposition, despite it describing the same state. (You were right that it is different from the position of the electron in the orbital). So if you can clarify your view of the MW-description of the equality between (|u>|d> - |d>|u>)/sqrt(2) and (|u'>|d'> - |d'>|u'>)/sqrt(2), it could be helpful. It seems to me that the “many-worlds” are not dependent of the choice of the base |u>,|d> or |u'>,|d’>. I mean, unlike Deustch (initially) and some many-worlders, the whole multiverse has to be the same physical object whatever base we are using. 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