> On 9 Aug 2019, at 03:58, Bruce Kellett <bhkell...@optusnet.com.au> wrote: > > >> From: Bruno Marchal <marc...@ulb.ac.be <mailto:marc...@ulb.ac.be>> >>> On 8 Aug 2019, at 13:59, Bruce Kellett <bhkellet...@gmail.com >>> <mailto:bhkellet...@gmail.com>> wrote: >>> >>> On Thu, Aug 8, 2019 at 8:51 PM Bruno Marchal <marc...@ulb.ac.be >>> <mailto:marc...@ulb.ac.be>> wrote: >>> On 8 Aug 2019, at 11:56, Bruce Kellett <bhkellet...@gmail.com >>> <mailto:bhkellet...@gmail.com>> wrote: >>>> On Thu, Aug 8, 2019 at 7:21 PM Bruno Marchal <marc...@ulb.ac.be >>>> <mailto:marc...@ulb.ac.be>> wrote: >>>> >>>> What I use is the fact that when we have orthogonal states, like I0> and >>>> I1>, I can prepare a state like (like I0> + I1>), and then I am myself in >>>> the superposition state Ime>( I0> + I1>), Now, in that state, I have the >>>> choice between measuring in the base {I0>, I1>} or in the base {I0> + I1>, >>>> I0> - I1>). In the first case, the “parallel” history becomes >>>> indetectoble, but not in the second case, so we have to take the >>>> superposition into account to get the prediction right in all situations. >>>> >>>> I don't think this is actually correct. Take a concrete example that we >>>> all understand. If we prepare a silver atom with spin 'up' in the >>>> x-direction, then a measurement in the x direction does not produce a >>>> superposition -- the answer is 'up' with 100% certainty. But is we measure >>>> this state in the transverse, y-direction, the result is either 'up-y' or >>>> 'down-y' with equal probabilities. This is because the initial state >>>> 'up-x' is already a superposition of 'up-y' and 'down-y'. When we measure >>>> this in the x-direction, there is no parallel history. When we measure in >>>> the y-direction, we get either 'up-y' or 'down-y'. MWI says that for >>>> either result, the alternative occurs in some other world. And that >>>> alternative result is just as undetectable as the 'down-x' result for the >>>> x-measurement. >>> >>> >>> The pure state up-x is the same state as the superposition of up-y and >>> down-y. >>> Me in front of up-x and Me in front of up-y + down-y are only different >>> description of the same state. When measuring that state in the >>> x-direction, I don’t made that y-superposition disappears. >>> >>> All you are saying here is that if you measure the up-x state in the x >>> direction, the state does not change -- it is still a >>> superposition of up-y and down-y. Of course, if the state is not changed >>> it does not change. Tautologies are not very useful. >> >> OK, but you are saying that “ When we measure this in the x-direction, there >> is no parallel history”, like if the superposition did disappear, that is >> why I remind the tautology. They did not. > The state can still be represented as a superposition in some other basis, > true. But this fact is of no practical significance for the operation in > question -- measurement of the x-polarization. > > I think there is a basic confusion in your thinking between basis states and > "other worlds". You want to maintain the fiction that descriptions in terms > of alternative basis states are somehow "real". But descriptions are not > physical states, relative or otherwise. >
But they participate in the personal histories of the superposed state of the observers. And they do provide many relative states. > >> Let us use “superposition of state” instead. The word “world” has too much >> metaphysical implicit connotations. > You object to the use of the word "world" in order to cover this confusion > between basis states and worlds. A parallel world is a well-defined concept. > We can make attempt to make it precise, like the transitive closure of interaction. In that case, if I look at the Schroedinger cat, I just entangle myself with it, and I end up in the superposition state “seeing the cat dead + seeing the cat alive”. Then if I interact with you, you end up in the superposition state “listening to me saying that the cat is alive” + "listening to me saying that the cat is dead”, and the entire universe, splits or differentiate locally through those (stepped light limited) interaction. > I have defined it several times, and the basic characteristics are > orthogonality and non-interaction, inaccessibility. Expressing a state in > terms of some alternative set of basis vectors is, of course, always > possible, and that changes the superposition, but it does not change the > original state. In particular, it does not create additional "relative > states" or "worlds". The description of a state is not the state: changing > the description does not change the state. > > No problem with this. > It is like your oft-repeated assertion that "2+2=4" is not the same as two > plus two equals four. But they are the same, > ? No, they are not. “2+2=4” is a syntactical sequence of symbol. 2+2 = 4 is a fact, that you might consider true or false. > they are both just descriptions of the physical operation of adding two > objects to two other objects and getting the result as four. Descriptions are > not the things themselves. > In you materialist, dualist and non computational (but unknown) theory. Yes, perhaps. > >> In that case, if the computer run a superposition similar to the initial >> calculation in Shor algorithm (before taking the final Fourier transform on >> all superposed results), decoherence means that all computations are done on >> the superposed state. > That is not decoherence. > Indeed, it is only entanglement (in the larger 3p description), but it is lived as decoherence by the guy doing the final measurement on the quantum computer (with or without the FT being done). Decoherence is self-entanglement, or self-superposition, in the absence of collapse postulate. > >> That is the massive parallelism, than we can exploit through the final >> Fourier Transform and measurement. > And all this parallelism occurs by simple rotations of the single state > vector in Hilbert space. > > Indeed. 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 everything-list+unsubscr...@googlegroups.com > <mailto:everything-list+unsubscr...@googlegroups.com>. > To view this discussion on the web visit > https://groups.google.com/d/msgid/everything-list/fdefa5c6-9ab5-5e3d-c188-4badafcb6cca%40optusnet.com.au > > <https://groups.google.com/d/msgid/everything-list/fdefa5c6-9ab5-5e3d-c188-4badafcb6cca%40optusnet.com.au?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 everything-list+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/everything-list/A4FAC314-9AEC-4092-B502-FDF3692D09E0%40ulb.ac.be.
