Re: A thought on MWI and its alternative(s)
6/13/2017 4:11 AM, Telmo Menezes wrote: The reason why it would follow is precisely the point of my rhetorical question above. If you take the wave function seriously, then you take seriously that qubits really do exist in a superposition of states, and this explains the exponential increase in computational power as you add qubits to the systems in certain configurations. I guess you can accept superposition and deny many worlds, but I would say that it is quite an awkward move. Being in a superposition is just a matter choosing the basis. If it's a pure state then there's some basis in which it is not a superposition. And if it's not in a superposition, then you can choose another basis in which it is. Brent -- 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.
Re: A thought on MWI and its alternative(s)
You seem to be taking the older view of many worlds that is favoured by David Deutsch. This approach has serious problems with the notorious basis problem, and there does not seem to be any principled way from within the theory to select unambiguosly the basis in which all of these worlds form. More recent understandings of MWI take decoherence into account. Decoherence provides a principled dynamical way to solve the basis problem, but it means the worlds do not actually form until there is decoherence -- worlds cannot form until they know what basis is relevant! I recommend the paper I suggested to Telmo: Michael Cuffaro, http://arxiv.org/abs/1110.2514v2 Cuffaro discusses the problems with the older form of MWI and suggests that although many worlds might be a useful heuristic in quantum computing, decoherence is required before worlds could be considered to have any ontological basis. The exponential speedup with a quantum computer is then seen in the fact that the QC manipulates the phases inherent in the entanglement of qbits, and doesn't have to actually calculate the function in question for all possible inputs, as the older many worlds view requires. Bruce On 14/06/2017 4:09 am, John Clark wrote: On Mon, Jun 12, 2017 at 8:32 PM, Bruce Kellett mailto:bhkell...@optusnet.com.au>>wrote: >> I agree Interference must take place in a single world, but where did all the information that produced the interference come from, where did the computations that produced all those wrong answers (and a few correct ones) come from? > What calculations are performed in these parallel worlds? Whatever algorithm you and your doppelgangers decided to run on their quantum computer. > And what performs those calculations? Computers made of matter that obey the laws of physics. > You are the one who insists that calculation is possible only on a physical computer. Yes, but you almost make that sound as if it were a contradiction of some sort. > Who constructed all these physical computers in the parallel worlds? If the MWI is correct and if you're a computer engineer then you and your doppelganger s made the quantum computer, made lots and lots of them actually. >> Even the 2-slit experiment will not produce interference if you remove the photographic plate and just allow the photons to continue into infinite space after they pass the slits because then the world splits but the two never recombine again so no interference. > Of course the interference continues -- for ever if necessary. The screen or photographic plate is only there to enable you to see it. No, in the Many World's theory it doesn't matter if anybody sees the results, in fact a brick wall would work just as well as a screen or a photographic plate, the only thing the MWI is interested in is that all of those things destroy the photon. After the photon passed the slits that photon was the only difference between those 2 universes, when it is destroyed in both universes by a screen or photographic plate or brick wall there is no longer a difference between universes so they merge back together, but indications it went through slot A and indications it went through slot B remain. And that produces the interference pattern. We don't usually see this weird quantum effect in our everyday macro-world because when a large change is made between universes it's hard to arrange things so they evolve together toward the same point, become the identical again, and thus merge back together. That's why making a quantum computer is hard. > Of course the interference continues -- for ever if necessary. [...] Try moving the position of the screen, what happens? What happens is if you remove the screen or photographic plate or brick wall and just let the photon continue on into infinite space then *NON*-interference will continue forever because the 2 universes will always remain different and thus never recombine. In the MWI the rules are crystal clear about when things split and when things merge back together. And In MWI everything that can happen does happen, so when a photon approaches 2 slits the universe splits and one photon goes through the right slit and one goes through the left slit. After those 2 photon s hit a photographic plate or screen or a brick wall the photons no longer exist in either universe and so they merge back together into one universe , and this merger causes the interference lines. If instead , after passing the slits there is no photographic plate or screen or brick wall and the photons are allowed to continue on into infinite space then the 2 universes remain different and remain separated forever. And so no inte
Re: A thought on MWI and its alternative(s)
On 11 Jun 2017, at 19:07, David Nyman wrote: On 11 Jun 2017 16:44, "Bruno Marchal" wrote: On 11 Jun 2017, at 12:24, David Nyman wrote: On 11 June 2017 at 10:14, Bruno Marchal wrote: On 09 Jun 2017, at 20:21, David Nyman wrote: On 9 June 2017 at 12:34, Bruno Marchal wrote: On 08 Jun 2017, at 02:05, Bruce Kellett wrote: On 7/06/2017 10:38 pm, Bruno Marchal wrote: On 07 Jun 2017, at 11:42, Bruce Kellett wrote: On 7/06/2017 7:09 pm, Bruno Marchal wrote: On 06 Jun 2017, at 01:23, Bruce Kellett wrote: I have been through this before. I looked at Price again this morning and was frankly appalled at the stupidity of what I saw. Let me summarize briefly what he did. He has a very cumbersome notation, but I will attempt to simplify as far as is possible. I will use '+' and '-' as spin states, rather than his 'left', 'right'. He write the initial wave function as for the case when you and I agree in advance to have aligned polarizers: |psi_1> = }me, electrons,you> = |me>(|+-> - |-+>)|you> = |me, +,-,you> - |me,-,+,you> He says that at this point no measurements have been made, and neither observer is split. But his fundamental mistake is already present. A little test for you: what is wrong with the above set of equations from a no-collapse pov? skipping some tedium, he then gets |psi_3> = |me[+],+,-,you[-]> - |me[-],-,+,you[+]> where the notation me[+] etc means I have measured '+', you[-] means you have measured '-'. He then claims that the QM results of perfect anticorrelation in the case of parallel polarizers has been recovered without any non- local interaction! Spoiler -- in order to write the final line for |psi_1> he has already assumed collapse, when I measure '+', you are presented *only* with '-', so of course you get the right result -- he has built that non-locality in from the start. ? From the start shows that it is local. Your failure to see the problem here is symptomatic of your complete failure to understand EPR in the MWI. I could say the same, but emphatic statements are not helping. My feeling is that you interpret the singlet state above like if it prepares Alice and Bob particles in the respective + and - states, but that is not the case. The singlet state describe a multiverse where Alice and Bob have all possible states, yet correlated. The singlet state is rotationally invariant, yes, and can be expanded in any basis of the 2-d complex Hilbert space. That has never been in doubt. OK. Then in absence of collapse, all interactions, and results are obtained locally, and does not need to be correlated until they spread at low speed up their partners. That does not follow. Although there are an infinity of possible bases for the singlet state, these are potential only, I don't understand this. Potential? That is no more the MW. and do not exist in any operative sense until the state interacts with something that sets a direction. That looks more like Bohr than Everett. You appear to claim that A and B exist in separate worlds corresponding to each of this infinity of bases. Yes. It is the rotaional invariance of the singlet states "taken seriously" when we drop the idea of collapse, or of special dualism between observer and the observed. But that is a misunderstanding. They are in superpositions in every base, sure, but that does not mean that there are 'worlds' corresponding to each possible base until some external interaction occurs. This is even more fuzzy than the collapse. It looks like consciousness not only reduce the wave, but create the physical reality. That is correct in Mechanism, but that is another story. As you yourself have said, a world is something that is closed to interaction. But superpositions are not closed to interaction, they can interfere -- as in the two slit experiment, and essentially every other application of QM. Right. So there are no separate worlds corresponding to every possible orientation of the polarizers. Worlds can arise only after interaction and decoherence has progressed so that the overlap between the branches of the superposition is zero (FAPP if you like). It is only then that the branches can no longer interfere (interact) and are closed to interaction, and thus constitute different worlds. We will have to disagree with this. I use the Y=II rules, like Deutsch. In this case the reading of the singlet state gives 2^aleph_zero constantly spreading histories figuring Bob and Alice. With mechanism, those worlds/histories are more like dreams. They will be epistemological personal (and plural in the spreading interaction based spheres). The standard procedure in quantum mechanics when one is faced with a superposition that interacts with something external, is to expand the superposition in a base that corresponds to the external context. OK. In this case, A
Re: A thought on MWI and its alternative(s)
On 13/06/2017 9:11 pm, Telmo Menezes wrote: On Mon, Jun 12, 2017 at 3:43 AM, Bruce Kellett wrote: On 11/06/2017 1:31 am, Telmo Menezes wrote: I think you built a straw man and now you're attacking it. When I heard Deutsch make the argument, he was referring explicitly to Shor's algorithm. This is sufficient to demonstrate an increase in computational power that would be impossible in the classical world. No one is denying that Shor's algorithm on a quantum computer would factorize numbers exponentially faster that a classical Turing machine could do it. But that does not mean that a quantum computer is just lot of classical Turing machines acting in parallel. No, but it does mean that a quantum computer can have the computational power of a lot of Turing machines acting in parallel, and it is normal to ask "why?", and be unsatisfied with a theory that does not answer this question. I have come across an interesting paper that discusses these questions, and comes to the conclusion that it is problematic to see quantum computing as accessing the computing power of other worlds. Michael Cuffaro, http://arxiv.org/abs/1110.2514v2 The explanation for exponential speedup is: "On this view, quantum computers are faster than classical computers because they perform /fewer/, not more, computations. By means of entanglement, quantum computers make it possible to manipulate the correlations present between the logical elements of a computation without representing these elements themselves.far from computing all of the values of a function simultaneously, quantum computers are faster because they avoid the calculation of any values of the function whatsoever, this time by exploiting the difference between classical and quantum logic." I find that to be a quite satisfying explanation of quantum computer speedup. I don't see how that could follow. The wave function exists in complex configuration space -- that is not the "real world". Well, I'm no sure about that, but classical mechanics exists in R^3 configuration space that is demonstrably not the real world (although it is the model that most closely matches our day-to-day perception of reality). Actually, it is more usual to say that classical physics exists in 6N-dimensional phase space for N particles. But that relates directly to positions and momenta in ordinary 3-space. The reason why it would follow is precisely the point of my rhetorical question above. If you take the wave function seriously, then you take seriously that qubits really do exist in a superposition of states, and this explains the exponential increase in computational power as you add qubits to the systems in certain configurations. I guess you can accept superposition and deny many worlds, but I would say that it is quite an awkward move. Cuffaro argues that many worlds can be a useful heuristic for certain types of quantum algorithms, but that reifying the elements of superpositons as defining different 'worlds' runs into difficulty with the basis problem. In other posts you alluded to a purely probabilistic interpretation of quantum mechanics. In that case, I would say that it also becomes awkward to explain the exponential increase in computational power for the quantum Fourier transform. These are all just intuitions, of course. We all have ours. Not really difficult to explain if you look at in the right way. See above. Another problem for me with the purely probabilistic interpretation is that it gives base-level reality to true randomness, and that would also be quite mysterious in my view. My point being: you argue as if probabilistic interpretations remove weirdness from the explanation, but for me true randomness is weirder than many worlds. Well, we all have our intuitions, as you say. If it is the case that there is an objective collapse mechanism, as in Bohm's theory, Ghirardi et al, or Penrose and others, then there is base level randomness and we just have to get used to it. Intuitions developed in a deterministic classical world do not necessarily carry across into the quantum realm. 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.
Re: A thought on MWI and its alternative(s)
On Mon, Jun 12, 2017 at 8:32 PM, Bruce Kellett wrote: >> >> I agree Interference must take place in a single world, but where did >> all the information that produced the interference come from, where did the >> computations that produced all those wrong answers (and a few correct ones) >> come from? > > > > > What calculations are performed in these parallel worlds? > Whatever algorithm you and your doppelgangers decided to run on their quantum computer. > > > And what performs those calculations? > Computers made of matter that obey the laws of physics. > > > You are the one who insists that calculation is possible only on a > physical computer. > Yes, but you almost make that sound as if it were a contradiction of some sort. > > > Who constructed all these physical computers in the parallel worlds? > If the MWI is correct and if you're a computer engineer then you and your doppelganger s made the quantum computer, made lots and lots of them actually. > >> >> Even the 2-slit experiment will not produce interference if you remove >> the photographic plate and just allow the photons to continue into infinite >> space after they pass the slits because then the world splits but the two >> never recombine again so no interference. > > > > > Of course the interference continues -- for ever if necessary. The screen > or photographic plate is only there to enable you to see it. > No, in the Many World's theory it doesn't matter if anybody sees the results, in fact a brick wall would work just as well as a screen or a photographic plate, the only thing the MWI is interested in is that all of those things destroy the photon. After the photon passed the slits that photon was the only difference between those 2 universes, when it is destroyed in both universes by a screen or photographic plate or brick wall there is no longer a difference between universes so they merge back together, but indications it went through slot A and indications it went through slot B remain. And that produces the interference pattern. We don't usually see this weird quantum effect in our everyday macro-world because when a large change is made between universes it's hard to arrange things so they evolve together toward the same point, become the identical again, and thus merge back together. That's why making a quantum computer is hard. > > > Of course the interference continues -- for ever if necessary. > [...] > Try moving the position of the screen, what happens? What happens is if you remove the screen or photographic plate or brick wall and just let the photon continue on into infinite space then *NON*-interference will continue forever because the 2 universes will always remain different and thus never recombine. In the MWI the rules are crystal clear about when things split and when things merge back together. And In MWI everything that can happen does happen, so when a photon approaches 2 slits the universe splits and one photon goes through the right slit and one goes through the left slit. After those 2 photon s hit a photographic plate or screen or a brick wall the photons no longer exist in either universe and so they merge back together into one universe , and this merger causes the interference lines. If instead , after passing the slits there is no photographic plate or screen or brick wall and the photons are allowed to continue on into infinite space then the 2 universes remain different and remain separated forever. And so no interference between them ever occurs. > > Superpositions occur everywhere, and no new worlds are split off until > there is decoherence. But we don't ALWAYS see a superposition of states. You can place a detector next to one of the slits so you you know which slit each photon went through, but if you do that the interference pattern disappears because interference needs at least 2 different things to interferer with and with this modified experiment the universe doesn't split so it can't recombine so you see no interference pattern. In the unmodified experiment after the photon makes its decision on which of the 2 slits to go through i t then hits a screen or photographic plate or brick wall. When that happens the photons in both universes are destroyed and thus there is no longer any difference between the two, so the universes will merge back together. T hen you will see a superposition of states. Then you will see indications that you live in a universe where the photon went through slot A only and indications you live in a universe where the photon went through slot B only, and that is why you see an interference effect even if you only send one photon at a time at the slits. If you got rid of the film (or the brick wall) and let the photon head out into infinite space after it passed the slits then the unive
Re: A thought on MWI and its alternative(s)
On Mon, Jun 12, 2017 at 3:43 AM, Bruce Kellett wrote: > On 11/06/2017 1:31 am, Telmo Menezes wrote: >> >> On Sat, Jun 10, 2017 at 1:11 AM, Bruce Kellett >> wrote: >>> >>> On 10/06/2017 2:36 am, Telmo Menezes wrote: On Fri, Jun 9, 2017 at 12:37 AM, Bruce Kellett > The idea that the explanation is epistemological rather that > ontological > has > been my preferred position for a long time. If the wave-function is > merely > an epistemological device for calculating probabilities and not a > really > existing object, all worries about collapse and action-at-a-distance > vanish. > Of course, multi worlds also vanish, but in my opinion that is no bad > thing. So what's your position on Deutsch's argument about quantum computers? Where does the extra computing power come from? >>> >>> >>> It has long been understood that Deutsch is out to lunch on this. >>> >>> He appears >>> to assume that a quantum computer is just using the same algorithms that >>> a >>> classical computer would use, only executing them in a massively parallel >>> manner. >> >> I find it very hard to believe that David Deutsch does not have a good >> understanding of quantum computers. >> >>> This is manifestly false. Quantum computers operate in a completely >>> different way -- that is why there are so few actual algorithms for >>> quantum >>> computers to execute that gain massive speed improvements. >> >> I think you built a straw man and now you're attacking it. When I >> heard Deutsch make the argument, he was referring explicitly to Shor's >> algorithm. This is sufficient to demonstrate an increase in >> computational power that would be impossible in the classical world. > > > No one is denying that Shor's algorithm on a quantum computer would > factorize numbers exponentially faster that a classical Turing machine could > do it. But that does not mean that a quantum computer is just lot of > classical Turing machines acting in parallel. No, but it does mean that a quantum computer can have the computational power of a lot of Turing machines acting in parallel, and it is normal to ask "why?", and be unsatisfied with a theory that does not answer this question. >> As for more general speed improvements, there is for example Grover's >> algorithm, that offers a quadratic improvement in searching unsorted >> lists. This has wide applicability in software engineering. >> >> Of course, building more complex quantum computers is still beyond our >> technical abilities. I don't think that's news for anyone... >> >>> As Brent says in his recent post, Scott Aaronson points out: >>> "The way a quantum algorithms work is that they arrange for wrong answers >>> to >>> destructively interfere while the desired answer interferes >>> constructively. >>> Interference requires that they take place in the same world." >> >> Yes, but this is not classical interference, it's interference between >> superpositions of states. So how can this computation happen in the >> physical world? > > > No one is suggesting that this is classical interference. Interference > between superpositions of states does happen in the physical world! Yes, this was a rhetorical question. > Are you > suggesting that two-slit interference does not happen in the physical world? > Interference between qbits happens in the physical world just as much as > two-slit interference. If you define a "world" as something closed to > outside interactions, then interference can only take place in the one > world. > >> For me, that gives more credence to the claim that the >> wave function describes a real object. > > > I don't see how that could follow. The wave function exists in complex > configuration space -- that is not the "real world". Well, I'm no sure about that, but classical mechanics exists in R^3 configuration space that is demonstrably not the real world (although it is the model that most closely matches our day-to-day perception of reality). The reason why it would follow is precisely the point of my rhetorical question above. If you take the wave function seriously, then you take seriously that qubits really do exist in a superposition of states, and this explains the exponential increase in computational power as you add qubits to the systems in certain configurations. I guess you can accept superposition and deny many worlds, but I would say that it is quite an awkward move. In other posts you alluded to a purely probabilistic interpretation of quantum mechanics. In that case, I would say that it also becomes awkward to explain the exponential increase in computational power for the quantum Fourier transform. These are all just intuitions, of course. We all have ours. Another problem for me with the purely probabilistic interpretation is that it gives base-level reality to true randomness, and that would also be quite mysterious in my view. My point being: you argue as if probabilistic i
