> On 6 Aug 2019, at 14:46, Bruce Kellett <[email protected]> wrote: > > On Tue, Aug 6, 2019 at 10:13 PM Bruno Marchal <[email protected] > <mailto:[email protected]>> wrote: > On 6 Aug 2019, at 11:51, Bruce Kellett <[email protected] > <mailto:[email protected]>> wrote: >> On Tue, Aug 6, 2019 at 7:23 PM Bruno Marchal <[email protected] >> <mailto:[email protected]>> wrote: >> On 6 Aug 2019, at 10:28, Bruce Kellett <[email protected] >> <mailto:[email protected]>> wrote: >>> >>> No, you are just attempting to divert attention away from the fact that you >>> have no answer to my original argument that a quantum computer can quite >>> reasonably do the calculation by rotating the state vector in Hilbert >>> space, and consequently, there is no need to imagine a large number of >>> parallel worlds in which the calculations are performed by a series of >>> clunky linear processing Turing machines. The hypothetical observer is >>> entirely irrelevant. >>> >>> In that state, O has still the choice to look at this in the {a, b} base, >>> or in the {a+b, a-b} base. In the first, the universal ray will describe >>> ((O seeing a) a + (O seeing b) b) (well normalised), >>> >>> A change of base does not make the idea that there are parallel worlds any >>> more convincing. Again, this is just a diversionary tactic. >> >> You are a bit too much fuzzy for me. I don’t see how a rotating ray in an >> Hilbert space fail to described superposition states, and without wave >> collapse the local (partial trace description) of the situation above makes >> the superposition of the observer states not eliminable. >> >> I do not understand your objections here. They make no sense. All I am >> claiming is some basic facts about vector spaces. If you have a vector >> space, you can form an arbitrary number of sets of basis vectors that span >> the space. Any vector in the space can be described in terms of its >> projections onto these basis vectors. Correspondingly, any set of values >> along the basis vectors can be summed to give a single vector (or ray) in >> the space. Any change to either the basis vector components, or the vector >> itself, is reflected in the other representation. In other words, change the >> vector and you change the projections on to the basis. Or change these basis >> components and you change the vector. >> >> In the case of a quantum computer, description of the calculation in terms >> of some set of basis vectors is completely captured in the corresponding >> changes to the summation vector. Consequently, the description of the QC >> action in terms of some set of basis vectors is entirely unnecessary -- the >> same action of the QC is entirely captured by the unitary rotations of the >> summation vector in the Hilbert space. > > I have no problem with this, but if in some base we have some brain state > corresponding to some local measurement, that will be a superposition of > brain state in the base corresponding to those state, and that is what we > have to take into account, when, for example, an observer look at the > Schroedinger cat in the base {live, dead}. From outside, we can use any base > for the evolution of the system, but for the personal first person view, we > should use the base with the corresponding memories. > > > The problem here is that you are confusing the operation of the QC while it > is doing the calculation with the operation of reading the output.
Because I study the QM applied to the couple “observer + observed”. > The operation is completely independent of any observer and any brain state; > it corresponds to unitary rotations of the state vector. Yes, in my case too. > Reading the output corresponds to projecting this vector on to an output set > of basis states. In the relative way, or you are bringing some collapse of the wave in the picture. > If the QC does its task effectively, the output basis qbits will be put into > definite states, Relatively to the observer, but in the global state, the observer will inherit the superposition state, by linearity of the tensor products and of the evolution. > so the probability of the desired result approaches unity, and there are > effectively no "other Everettian worlds" in which different results would be > obtained. Then some branch of the superposition disappeared, and QM do no more apply to the couple “observer-observed”, and we are out of Everett’s Relative state theory. Note that I don’t really believe that the word “world” makes sense. I prefer to use “history” instead. Eventually, those will be computations, and they will not need any world at all (just a tiny segment of the arithmetical truth). > > All your talk about superpositions of observers is just a distraction from > the main issue, which you seem to accept (i.e., the basic vector analysis), > but the consequences of which you do not wish to acknowledge. There are no > "parallel worlds" in which the computations take place. A projection is not unitary. It does not represent some evolution “out-there”. The observation, as you seem to describe it, is not obeying to a quantum (unitary) transformation. > >> That is all that there is to it. The advantage of the vector description is >> that such a description is independent of the chosen basis -- what happens >> to the vector can be described in terms of any one of the infinite number of >> possible alternative bases. > > No problem. > >> This is the basis ambiguity, or problem of a preferred basis. > > The change of base does not change the relative state, accessible, by, say, > some machine with quasi-classical memories. > That is shown in Everett explicitly (also ver clearly in the book by > Hirvensalo on quantum computation). > > All of which is irrelevant to the issue under discussion. But, moreover, it > appears to show a lack of understanding of the basis problem as it affects > Everett. There is no serious base problem, once we understand that the observer has chosen the base corresponding to sharp measurement result that he can memorise. Whatever base is used, the relative state theory gives the sema results for the measurement made in a base in which we can describe the (mechanist) working of his brain and sensory apparatus. > >> To pick out one set of base vectors and claim that these vectors represent a >> set of parallel worlds in which the computations actually occur, is simply >> unnecessary -- description in terms of the single summation vector >> eliminates this stupidity. > > Unfortunately, we cannot eliminate the fact that an observer looking a the > cat will use some position base to get the memory of the result of its > observation, and without collapse, *whatever base* you are using to describe > the overall situation (including the cat and the observer and its memory) the > observation of the cat will lead to an observer seeing only the cat alive, > and one seeing only the cat dead, and I don’t see how you will make this > superposition disappearing. > > Still irrelevant. I don’t see why. > >> Maybe you can tell me what happens in that situation. Note that even after >> measurement, we can get back the interference effect by erasing the >> memorised outcome of the result. Without collapse pure state remains pure, >> and decoherence is relative to each “copies” of the observer in the terms of >> the (universal) >> >> >> These observations are entirely beside the point. You cannot erase the >> memory of the result because memory is intrinsically irreversible. > > Quantum mechanics is reversible. > > Hmmmm! Measurement results are generally not reversible. Zurek talks of the > environment as witness, and quantum Darwinism -- the many copies of the > result in the environment are not reversible, even in principle. Can you > pursue the escaping IR photons and turn them round? In principle. And that is enough to address the interpretation of QM problem. > >> Quantum erasure is a technical matter that occurs only in highly constrained >> situations. I think you should catch up on some recent work on quantum >> foundations, in which Everett does not necessarily require the continuing >> purity of the quantum state. Measurement changes the pure state into a >> mixture. Zureck has made considerable progress in this direction in recent >> years. Quantum foundations has moved on since 1957. > > Mixture are relative personal outcome for the observer in the superposition > state. There are no mixture in the global universal wave function, or you are > introducing a wave packet reduction somewhere. > > The global universal wave function is a non-operational entity. The question addressed here is conceptual. > > Measurement changes nothing, that is why Everett allows (and use) Mechanism. > Bohr and wave-reduction philosophy require the observer to disobey quantum > physics, and it uses a dualist (and unknown) theory of mind. > > All irrelevant to the issue at hand. I don’t see this at all. You are saying that the many terms of a superposition becomes one term after an observation, which I say would entail a theory where the observer is no more described by quantum mechanics. Nothing can make a superposition into a pure state, except relatively to the first person associated to the (mechanical) observer. Or some collapse of the wave is at play, but that is not a unitary evolution. Bruno > > Bruce > > -- > 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] > <mailto:[email protected]>. > To view this discussion on the web visit > https://groups.google.com/d/msgid/everything-list/CAFxXSLQ-Jf3dj60GTvC4h3TSrOg%2BVKQKNS8FP9qHtNDfxEzNVg%40mail.gmail.com > > <https://groups.google.com/d/msgid/everything-list/CAFxXSLQ-Jf3dj60GTvC4h3TSrOg%2BVKQKNS8FP9qHtNDfxEzNVg%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/30DC0EC8-217C-4F6E-8633-244D6B6B5F75%40ulb.ac.be.
