On Tuesday, May 30, 2017 at 9:14:07 AM UTC+10, Bruce wrote: > > On 29/05/2017 11:21 pm, Telmo Menezes wrote: > > On Mon, May 29, 2017 at 3:06 PM, Bruce Kellett > > <bhke...@optusnet.com.au <javascript:>> wrote: > >> On 29/05/2017 10:42 pm, Telmo Menezes wrote: > >>> On Mon, May 29, 2017 at 2:26 PM, Bruce Kellett > >>> <bhke...@optusnet.com.au <javascript:>> wrote: > >>>> On 29/05/2017 6:26 pm, Telmo Menezes wrote: > >>>>> On Mon, May 29, 2017 at 3:26 AM, Bruce Kellett > >>>>> <bhke...@optusnet.com.au <javascript:>> wrote: > >>>>>> I would say that there is only one history leading to our present > >>>>>> state. > >>>>>> Whether you take an MWI view or a collapse view, the wave function > >>>>>> branches > >>>>>> deterministically at every point, so if you follow your current > twig > >>>>>> back > >>>>>> down to the main trunk etc, there will be a unique path. > >>>>> I don't think we can say we are in a specific twig. Many things > about > >>>>> out present state are unknown/undefined. I can imagine that there > are > >>>>> many well-defined present states that are compatible with my current > >>>>> subjective state. > >>>> > >>>> Sure, but we are talking about wave functions, not subjective states. > >>> Replace "subjective" with "incomplete knowledge". > >> > >> Doesn't help. Of course our knowledge is incomplete, the wave function > isn't > >> completely known either -- but the result of specific measurements are > what > >> is at stake here, and they are known quantum states. > > In my view, what's at stake here is the possibility of latent > > variables having some degree of freedom leading to the same macro > > states (provided that there is incomplete information about these > > macro states, as is the case for humans). > > I am still not clear about what you are trying to say. It is certainly > true that many internal details of your microscopic organization could > be changed and you wouldn't know the difference. For that matter, an > almost infinite number of details about the rest of the universe could > be different and you would still be the same. But that is not what we > were talking about. I understood the issue to be whether there was a > unique past, or whether several (or many) different paths lead to our > current state. The determinism of the Schrödinger equation would suggest > that your unique state has a unique history. Variations in microscopic > details of with your body, or the universe, would correspond to > different decohered worlds with no overlap with our world. So while such > variations are possible, they do not amount to multiple histories > leading to our current state. We don't need to know that state in detail > to be able to argue the consequences of determinism. > > >>>>> In fact you can perform a quantum erasure experiment, and be sure > that > >>>>> your current state goes through at least two different shortest > paths > >>>>> to the root, and it becomes nonsensical to say that one is the > >>>>> "correct" one. I don't think anyone knows how far this can go into > the > >>>>> macroscopic world, but I don't see any reason to believe that it > >>>>> doesn't. > >>>> > >>>> I don't understand what you think you are getting in a quantum > erasure > >>>> experiment. If the "which way" information that was gathered is > erased, > >>>> normal interference patterns are seen in the double slit situation. > The > >>>> two > >>>> paths (through the separate slits) are in unresolved superposition > until > >>>> they hit the detector, when decoherence takes over. There are not two > >>>> separate worlds, and your state is the result of the superposed > paths, > >>>> not > >>>> of either path separately. There is no ambiguity about which the the > >>>> "correct" path -- neither is, both contribute equally. > >>> I would say that the delayed choice version of the experiment makes it > >>> clear that there are two possible pasts that lead to the same present > >>> state -- they differ by one bit of information. > >> > >> That is not what is implied by delayed choice quantum erasure. Whether > an > >> interference pattern is seen or not is determined by whether the "which > way" > >> information is erased or not. But whether it is or not, the > interference is > >> only seen when coincidence measurements tell one which photons to > count. And > >> the timing information necessary for coincidence determination is > available > >> only *after* all decisions about erasure or not have been made, whether > that > >> decision is made before or after the other photon of the entangled pair > has > >> reached its detector. > >> > >> "Delayed choice" is perhaps a misleading phrase in this context, and it > does > >> not lead to an ambiguity of path -- it merely tells whether there was > an > >> intact superposition or not. > > I know, this is not what I am trying to say. I'll choose something much > simpler: > > > > Suppose there is a computer running in an empty room. This computer is > > connected to a random number generator. At some point it uses the > > random value to decide if it's going to show a screen that is all > > green or all red. Nobody witnesses it. > > If the random number generator is based on quantum randomness, then in > principle you will get a superposition of red and green screens, but > this is like the question as to whether we see a superposition of live > and dead cats in Schrödinger's thought experiment. Even if there is an > underlying quantum event that would give a superposition, decoherence > steps in and resolves the outcome into separate worlds long before we > reach the macro level of live/dead cats or red/green screens. > Decoherence does not require anyone to witness it.... >
I believe Telmo is subscribing to a view of QM, suggested by Bruno's "comp" theory, that the observer's subjective state - down to some unknown level of resolution (the "substitution level") - is what determines the possible future states of that observer (probabilistically, by the measure on subjective continuations in the UD trace). So you are talking different languages. I don't know if Telmo is aware or not of the conventional view of decoherence - that it is a matter of the spread of information into the environment by means of physical interactions between particles. Telmo's musings about the effect of destroying memory (could it change the measure of different futures?) clearly expresses this subjectivist view. FWIW, you are expressing my own understanding of the situation: there can be no superposition of red and green screens or dinosaurs, or dead and live cats, because there can be no quantum superposition of macroscopic objects. Superpositions of wave functions are only possible for systems isolated from interaction with their environment, which is why quantum computers are so fricking hard to make: keeping aggregates of particles isolated from interactions with the surrounding environment is exponentially more difficult as the system grows in size. > > 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. To post to this group, send email to everything-list@googlegroups.com. Visit this group at https://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.
