On Friday, May 4, 2018 at 1:21:52 PM UTC, Bruce wrote: > > From: smitra < <javascript:>smi...@zonnet.nl <javascript:>> > > > On 03-05-2018 03:22, Brent Meeker wrote: > >> On 5/2/2018 6:02 PM, smitra wrote: >> >>> On 02-05-2018 03:21, Brent Meeker wrote: >>> >>>> On 5/1/2018 4:43 PM, smitra wrote: >>>> >>>>> On 01-05-2018 20:47, Brent Meeker wrote: >>>>> >>>>>> On 5/1/2018 9:01 AM, Bruno Marchal wrote: >>>>>> >>>>>> On 29 Apr 2018, at 19:59, Brent Meeker < <javascript:> >>>>>>> meek...@verizon.net <javascript:>> wrote: >>>>>>> >>>>>>> >>>>>>> On 4/29/2018 8:53 AM, Bruno Marchal wrote: >>>>>>> >>>>>>> But that's my question: Why isn't it the same? And even if it's not >>>>>>> how would be know? The "conscious" quantum computer assures us that >>>>>>> it not only detected that there was a welcher weg photon but that >>>>>>> it's weg was known to the "consciousness" of the quantum computer, >>>>>>> before it was erased. But why would we believe it? We already have >>>>>>> these experiments in which we know the weg was available and could >>>>>>> have been recorded, but was erased. So what is the "consciousness" >>>>>>> that adds a secret-sauce to the experiment? >>>>>>> Good question. I doubt that you can fool quantum mechanics by >>>>>>> calling it "consciousness". I think in this case the interaction >>>>>>> with the welcher weg photon would amount to sufficient decoherence >>>>>>> -- basically information was extracted that was not restored. Also, >>>>>>> of course, if the QC "forgets" what it did, how can it report on the >>>>>>> fact that it did anything. How can we believe that it actually knew >>>>>>> which slit at some point? >>>>>>> >>>>>> >>>>>> Because in Deutsch experiment, not everything has been erased, notably >>>>>> the memory that he has known the result. He would say something like: >>>>>> I remember doing the measurement and writing it in the enveloppe. Now >>>>>> the envelop has been erased, and I can’t remember its content, but I >>>>>> definitely remember having known the content. >>>>>> But two questions remain. First, the empirical question of whether >>>>>> this erasure is enough to restore interference. >>>>>> >>>>>> I do not see why it would not been enough … in theory. You need only >>>>>> a computer able to forget a memory, but not some meta-memory that it >>>>>> has recorded a definite result. It isa bit like remembering we have >>>>>> done a dream, without being able to remember any of its content. >>>>>> >>>>>> In practice, that might be very difficult, if not impossible. I am not >>>>>> sure. >>>>>> >>>>>> Second, why should we believe the quantum computer. >>>>>>> >>>>>> >>>>>> In Deutsch proposal, it is a human. >>>>>> No, it's a conscious quantum computer. It it were a human or other >>>>>> (quasi-) classical instrument decoherence would happen when there was >>>>>> a detection of welcher weg and erasure would be impossible. >>>>>> >>>>>> Brent >>>>>> >>>>> Yes, but note that you can make that quantum computing simulation of >>>>> the observer in that thought experiment as precise as you like. You can >>>>> in >>>>> principle include a simulation of the entire Earth >>>>> >>>> >>>> And the outgoing EM and neutrino waves and their interaction with >>>> interstellar atoms. I'm suspicious of these fantasy thought >>>> experiments. But however detailed it may be doesn't answer my >>>> question as to what it would mean to erase the welcher weg but not the >>>> memory that the weg was detected. I noted that this is not like a >>>> classical erasure of a memory because in this case the coherence is >>>> maintained, so when the welcher weg is erased there is no long any >>>> fact-of-the-matter as to which way it went. There is no >>>> fact-of-the-matter that it was detected to go left or right. So the >>>> "memory" if it exists, is a false memory. >>>> >>>> with billions of other people and a lot of decoherence implemented by >>>>> qubits that simulate e.g. soft photons and other environmental degrees of >>>>> freedom (and all that decoherence will end up getting reversed by the way >>>>> the computation is set up ) The point is that if computation generates >>>>> consciousness, you can in principle let any given person do the >>>>> experimental verification of the existence of multiple branches by >>>>> uploading the brain to a quantum computer and letting it be subject to >>>>> such >>>>> a computation. >>>>> >>>> >>>> How will the person verify it? Reversing the computation will reverse >>>> the person and erase their memory. >>>> >>>> Brent >>>> >>> >>> It's a simple two step measurement process where you (as a virtual >>> person simulated by the QC) perform a measurement that tells you that the >>> spin (represented by a qubit) has been measured without giving you the >>> result. And then you perform the next measurement where you actually >>> measure the value of the spin component. It can then be shown that there >>> exists a unitary transform that will restore the original spin state that >>> will preserve the record of the first measurement. >>> >> >> But you're speaking poetically. I, as a classical being cannot >> perform such measurements. First, how can the simulated QC person >> perform a measurement that tells you that the spin has been measured >> without giving anyone the result? In what sense is this a measurement >> of the spin, not merely a measurement of some proxy that is >> independent of the spin value? Second, what is the point of the >> second measurement "where you actually measure the spin component"; >> are you saying the first measurement did not actually measure the spin >> component even though it is supposed to tell us that it was measured? >> Third, all the techniques I've heard of for quantum erasing a >> measurement and restoring the WF are like making it so it never >> happened. You seem implicitly to take this view since you're >> concerned to preserve the record of the first measurement (which >> didn't actually measure the spin value) but not the second (which >> makes no record). >> >> Brent >> > > Yes, it's a measurement of a proxy, analogous to letting someone else > measure the spin and then that person reporting to you that the spin has > been successfully measured, without disclosing the result to you. It's not > difficult to write down a QC program to see how this works in detail. You > can take a CNOT gate a very simple observer, the control qubit is the spin > that is going to be measured (using a Hadamrd transform it can be put in a > superposition of |0> and |1>), the other bit is initialized to be in the > |0> state. We then add another qubit that changes from |0> to |1> when the > gate is applied. One can then return qubits of the CNOT gate to the > original state while leaving that extra qubit in the state it was after the > measurement. > > So, the record of the measurement having taking place will be kept, while > the original spin state of, say, |0> has been restored, and that can be > verified by repeatedly carrying out this process and then also measuring > the spin in the final restored state. That final measurement always yields > the same result, proving that the qubit is indeed always restored to the > |0> state. But if the measurement at the time of the superposition were to > collapse the wavefunction, eliminating one of the two branches then the > original state would not be restored. > > What all of this proves, is that an observer implemented by a quantum > computer can experimentally falsify the Copenhagen Interpretation. > > > Unfortunately, it is not the case that you can implement absolutely any > unitary transformation in this way. For instance, you cannot implement the > unitary transformation that would reverse a totally decohered event. >
*If the decoherence was unitary, why can't the process be reversed statistically, analogous to the case of the classical cooling gas where we imagine the hugely improbable incoming and absorption of the previously outgoing IR photons? AG* > Your quantum computer ceases to function if there is any decoherence! For > example, you cannot implement a unitary transformation that would resurrect > my dead grandfather, even though his life and death were entirely unitary. > *ISTM, that the dead are not dead "in principle", but statistically; that is reversibly dead with a hugely low probability of being resurrected. What is the argument that this isn't the case? AG * > So you cannot reverse a recorded measurement. > > 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.
