On Fri, Aug 9, 2019 at 7:39 PM Bruno Marchal <[email protected]> wrote:
> On 9 Aug 2019, at 03:58, Bruce Kellett <[email protected]> wrote: > > From: Bruno Marchal <[email protected]> > > On 8 Aug 2019, at 13:59, Bruce Kellett <[email protected]> wrote: > > On Thu, Aug 8, 2019 at 8:51 PM Bruno Marchal <[email protected]> wrote: > >> On 8 Aug 2019, at 11:56, Bruce Kellett <[email protected]> wrote: >> >> On Thu, Aug 8, 2019 at 7:21 PM Bruno Marchal <[email protected]> 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. > There are no superposed states of the observer to which we have access. All other relative states are orthogonal and inaccessible. 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. > But, as I have just said, those other states of the superposition are orthogonal and not accessible. They are, in terms of the definition, other worlds. 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]. To view this discussion on the web visit https://groups.google.com/d/msgid/everything-list/CAFxXSLTU-C%2BKGVYXQt0L4TXWpTHK%2Bh-JsrGRnxV5Q0ge-ry5Og%40mail.gmail.com.
