> On 4 Jan 2020, at 13:11, Philip Thrift <[email protected]> wrote: > > > > Quantum theory cannot consistently describe the use of itself > Daniela Frauchiger & Renato Renner > https://www.nature.com/articles/s41467-018-05739-8 > > Quantum theory provides an extremely accurate description of fundamental > processes in physics. It thus seems likely that the theory is applicable > beyond the, mostly microscopic, domain in which it has been tested > experimentally. Here, we propose a Gedankenexperiment to investigate the > question whether quantum theory can, in principle, have universal validity. > The idea is that, if the answer was yes, it must be possible to employ > quantum theory to model complex systems that include agents who are > themselves using quantum theory. Analysing the experiment under this > presumption, we find that one agent, upon observing a particular measurement > outcome, must conclude that another agent has predicted the opposite outcome > with certainty. The agents’ conclusions, although all derived within quantum > theory, are thus inconsistent. This indicates that quantum theory cannot be > extrapolated to complex systems, at least not in a straightforward manner. > > from > https://www.quantamagazine.org/frauchiger-renner-paradox-clarifies-where-our-views-of-reality-go-wrong-20181203/
Classical Mechanics is Turing universal with only three bodies, and quantum mechanics is Turing universal with 0-bodies. And whatever is Turing universal can embed itself completely in itself, like RA and PA are described entirely in all models of the arithmetical reality (standard and non-standards included). I suspect the paper defends implicitly the many-worlds, by showing some difficulties if we make disappear some branch/states in the physical superpositions. As I have shown, quantum mechanics is a consequence of the consciousness theory that all universal machines discovers when introspecting themselves (in the modal shapes []p & <>t & p with p sigma_1, and with the modal operator interpreted respectively by the arithmetical realisation of “provable”, “consistent” and “true”. (Which can be justified by through experiences or by using the classical definition already proposed by the platonicians and the neoplatonicians. Many people seem to use the assumption that there *is* an (ontological) physical universe, but that requires a non computationalist theory of mind. Those who believe in both materialism (there is an ontological universe) and in Mechanism, have to explain how an ontological universe can make some computations more conscious than their emulation in arithmetic, despite being done at the right substitution level. That makes simply no sense. It is not that quantum mechanics is universally applicable. It is only applicable in the theory of the observable, it typically failed on qualia, although the universal machine discover QM as a special case of a more general theory of qualia. A quanta is a sharable, first person plural, quale. Bruno > > ... > > > The experiment has four agents: Alice, Alice’s friend, Bob, and Bob’s friend. > Alice’s friend is inside a lab making measurements on a quantum system, and > Alice is outside, monitoring both the lab and her friend. Bob’s friend is > similarly inside another lab, and Bob is observing his friend and the lab, > treating them both as one system. > > Inside the first lab, Alice’s friend makes a measurement on what is > effectively a coin toss designed to come up heads one-third of the time and > tails two-thirds of the time. If the toss comes up heads, Alice’s friend > prepares a particle with spin pointing down, but if the toss comes up tails, > she prepares the particle in a superposition of equal parts spin UP and spin > DOWN. > > Alice’s friend sends the particle to Bob’s friend, who measures the spin of > the particle. Based on the result, Bob’s friend can now make an assertion > about what Alice’s friend saw in her coin toss. If he finds the particle spin > to be UP, for example, he knows the coin came up tails. > > The experiment continues. Alice measures the state of her friend and her lab, > treating all of it as one quantum system, and uses quantum theory to make > predictions. Bob does the same with his friend and lab. Here comes the first > assumption: An agent can analyze another system, even a complex one including > other agents, using quantum mechanics. In other words, quantum theory is > universal, and everything in the universe, including entire laboratories (and > the scientists inside them), follows the rules of quantum mechanics. > > This assumption allows Alice to treat her friend and the lab as one system > and make a special type of measurement, which puts the entire lab, including > its contents, into a superposition of states. This is not a simple > measurement, and herein lies the thought experiment’s weirdness. > > The process is best understood by considering a single photon that’s in a > superposition of being polarized horizontally and vertically. Say you measure > the polarization and find it to be vertically polarized. Now, if you keep > checking to see if the photon is vertically polarized, you will always find > that it is. But if you measure the vertically polarized photon to see if it > is polarized in a different direction, say at a 45-degree angle to the > vertical, you’ll find that there’s a 50 percent chance that it is, and a 50 > percent chance that it isn’t. Now if you go back to measure what you thought > was a vertically polarized photon, you’ll find there’s a chance that it’s no > longer vertically polarized at all — rather, it’s become horizontally > polarized. The 45-degree measurement has put the photon back into a > superposition of being polarized horizontally and vertically. > > This is all very fine for a single particle, and such measurements have been > amply verified in actual experiments. But in the thought experiment, > Frauchiger and Renner want to do something similar with complex systems. > > As this stage in the experiment, Alice’s friend has already seen the coin > coming up either heads or tails. But Alice’s complex measurement puts the > lab, friend included, into a superposition of having seen heads and tails. > Given this weird state, it’s just as well that the experiment does not demand > anything further of Alice’s friend. > > Alice, however, is not done. Based on her complex measurement, which can come > out as either YES or NO, she can infer the result of the measurement made by > Bob’s friend. Say Alice got YES for an answer. She can deduce using quantum > mechanics that Bob’s friend must have found the particle’s spin to be UP, and > therefore that Alice’s friend got tails in her coin toss. > > This assertion by Alice necessitates another assumption about her use of > quantum theory. Not only does she reason about what she knows, but she > reasons about how Bob’s friend used quantum theory to arrive at his > conclusion about the result of the coin toss. Alice makes that conclusion her > own. This assumption of consistency argues that the predictions made by > different agents using quantum theory are not contradictory. > > Meanwhile, Bob can make a similarly complex measurement on his friend and his > lab, placing them in a quantum superposition. The answer can again be YES or > NO. If Bob gets YES, the measurement is designed to let him conclude that > Alice’s friend must have seen heads in her coin toss. > > It’s clear that Alice and Bob can make measurements and compare their > assertions about the result of the coin toss. But this involves another > assumption: If an agent’s measurement says that the coin toss came up heads, > then the opposite fact — that the coin toss came up tails — cannot be > simultaneously true. > > The setup is now ripe for a contradiction. When Alice gets a YES for her > measurement, she infers that the coin toss came up tails, and when Bob gets a > YES for his measurement, he infers the coin toss came up heads. Most of the > time, Alice and Bob will get opposite answers. But Frauchiger and Renner > showed that in 1/12 of the cases both Alice and Bob will get a YES in the > same run of the experiment, causing them to disagree about whether Alice’s > friend got a heads or a tails. “So, both of them are talking about the past > event, and they are both sure what it was, but their statements are exactly > opposite,” Renner said. “And that’s the contradiction. That shows something > must be wrong.” > > This led Frauchiger and Renner to claim that one of the three assumptions > that underpin the thought experiment must be incorrect. > > “The science stops there. We just know one of the three is wrong, and we > cannot really give a good argument [as to] which one is violated,” Renner > said. “This is now a matter of interpretation and taste.” > > Fortunately, there are a wealth of interpretations of quantum mechanics, and > almost all of them have to do with what happens to the wave function upon > measurement. Take a particle’s position. Before measurement, we can only talk > in terms of the probabilities of, say, finding the particle somewhere. Upon > measurement, the particle assumes a definite location. In the Copenhagen > interpretation, measurement causes the wave function to collapse, and we > cannot talk of properties, such as a particle’s position, before collapse. > Some physicists view the Copenhagen interpretation as an argument that > properties are not real until measured. > > This form of “anti-realism” was anathema to Einstein, as it is to some > quantum physicists today. And so is the notion of a measurement causing the > collapse of the wave function, particularly because the Copenhagen > interpretation is unclear about exactly what constitutes a measurement. > Alternative interpretations or theories mainly try to either advance a > realist view — that quantum systems have properties independent of observers > and measurements — or avoid a measurement-induced collapse, or both. > > For example, the many-worlds interpretation takes the evolution of the wave > function at face value and denies that it ever collapses. If a quantum coin > toss can be either heads or tails, then in the many-worlds scenario, both > outcomes happen, each in a different world. Given this, the assumption that > there is only one outcome for a measurement, and that if the coin toss is > heads, it cannot simultaneously be tails, becomes untenable. In many-worlds, > the result of the coin toss is both heads and tails, and thus the fact that > Alice and Bob can sometimes get opposite answers is not a contradiction. > > “I have to admit that if you had asked me two years ago, I’d have said [our > experiment] just shows that many-worlds is actually a good interpretation and > you should give up” the requirement that measurements have only a single > outcome, Renner said. > > This is also the view of the theoretical physicist David Deutsch of the > University of Oxford, who became aware of the Frauchiger-Renner paper when it > first appeared on arxiv.org. In that version of the paper, the authors > favored the many-worlds scenario. (The latest version of the paper, which was > peer reviewed and published in Nature Communications in September, takes a > more agnostic stance.) Deutsch thinks the thought experiment will continue to > support many-worlds. “My take is likely to be that it kills > wave-function-collapse or single-universe versions of quantum theory, but > they were already stone dead,” he said. “I’m not sure what purpose it serves > to attack them again with bigger weapons.” > > Renner, however, has changed his mind. He thinks the assumption most likely > to be invalid is the idea that quantum mechanics is universally applicable. > > This assumption is violated, for example, by so-called spontaneous collapse > theories that argue — as the name suggests — for a spontaneous and random > collapse of the wave function, but one that is independent of measurement. > These models ensure that small quantum systems, such as particles, can remain > in a superposition of states almost forever, but as systems get more massive, > it gets more and more likely that they will spontaneously collapse to a > classical state. Measurements merely discover the state of the collapsed > system. > > In spontaneous collapse theories, quantum mechanics can no longer to be > applied to systems larger than some threshold mass. And while these models > have yet to be empirically verified, they haven’t been ruled out either. > > Nicolas Gisin of the University of Geneva favors spontaneous collapse > theories as a way to resolve the contradiction in the Frauchiger-Renner > experiment. “My way out of their conundrum is clearly by saying, ‘No, at some > point the superposition principle no longer holds,’” he said. > > If you want to hold on to the assumption that quantum theory is universally > applicable, and that measurements have only a single outcome, then you’ve got > to let go of the remaining assumption, that of consistency: The predictions > made by different agents using quantum theory will not be contradictory. > > Using a slightly altered version of the Frauchiger-Renner experiment, Leifer > has shown that this final assumption, or a variant thereof, must go if > Copenhagen-style theories hold true. In Leifer’s analysis, these theories > share certain attributes, in that they are universally applicable, > anti-realistic (meaning that quantum systems don’t have well-defined > properties, such as position, before measurement) and complete (meaning that > there is no hidden reality that the theory is failing to capture). Given > these attributes, his work implies that there is no single outcome of a given > measurement that’s objectively true for all observers. So if a detector > clicked for Alice’s friend inside the lab, then it’s an objective fact for > her, but not so for Alice, who is outside the lab modeling the entire lab > using quantum theory. The results of measurements depend on the perspective > of the observer. > > “If you want to maintain the Copenhagen type of view, it seems the best move > is towards this perspectival version,” Leifer said. He points out that > certain interpretations, such as quantum Bayesianism, or QBism, have already > adopted the stance that measurement outcomes are subjective to an observer. > > Renner thinks that giving up this assumption entirely would destroy a > theory’s ability to be effective as a means for agents to know about each > other’s state of knowledge; such a theory could be dismissed as solipsistic. > So any theory that moves toward facts being subjective has to re-establish > some means of communicating knowledge that satisfies two opposing > constraints. First, it has to be weak enough that it doesn’t provoke the > paradox seen in the Frauchiger-Renner experiment. Yet it must also be strong > enough to avoid charges of solipsism. No one has yet formulated such a theory > to everyone’s satisfaction. > > The Frauchiger-Renner experiment generates contradictions among a set of > three seemingly sensible assumptions. The effort to explicate how various > interpretations of quantum theory violate the assumptions has been “an > extremely useful exercise,” said Rob Spekkens of the Perimeter Institute for > Theoretical Physics in Waterloo, Canada. > > “This thought experiment is a great lens through which to examine the > differences of opinions between different camps on the interpretation of > quantum theory,” Spekkens said. “I don’t think it’s really eliminated options > that people were endorsing prior to the work, but it has clarified precisely > what the different interpretational camps need to believe to avoid this > contradiction. It has served to clarify people’s position on some of these > issues.” > > Given that theoreticians cannot tell the interpretations apart, > experimentalists are thinking about how to implement the thought experiment, > in the hope of further illuminating the problem. But it will be a formidable > task, because the experiment makes some weird demands. For example, when > Alice makes a special measurement on her friend and her lab, it puts > everything, the friend’s brain included, into a superposition of states. > > Mathematically, this complicated measurement is the same as first reversing > the time evolution of the system — such that the memory of the agent is > erased and the quantum system (such as the particle the agent has measured) > is brought back to its original state — and then performing a simpler > measurement on just the particle, said Howard Wiseman of Griffith University > in Brisbane, Australia. The measurement may be simple, but as Gisin points > out rather diplomatically, “Reversing an agent, including the brain and the > memory of that agent, is the delicate part.” > > Nonetheless, Gisin is not averse to thinking that maybe, one day, the > experiment could be done using complex quantum computers as the agents inside > the labs (acting as Alice’s friend and Bob’s friend). In principle, the time > evolution of a quantum computer can be reversed. One possibility is that such > an experiment will replicate the predictions of standard quantum mechanics > even as quantum computers get more and more complex. But it may not. “Another > alternative is that at some point while we develop these quantum computers, > we hit the boundary of the superposition principle and [find] that actually > quantum mechanics is not universal,” Gisin said. > > Leifer, for his part, is holding out for something new. “I think the correct > interpretation of quantum mechanics is none of the above,” he said. > > He likens the current situation with quantum mechanics to the time before > Einstein came up with his special theory of relativity. Experimentalists had > found no sign of the “luminiferous ether” — the medium through which light > waves were thought to propagate in a Newtonian universe. Einstein argued that > there is no ether. Instead he showed that space and time are malleable. > “Pre-Einstein I couldn’t have told you that it was the structure of space and > time that was going to change,” Leifer said. > > Quantum mechanics is in a similar situation now, he thinks. “It’s likely that > we are making some implicit assumption about the way the world has to be that > just isn’t true,” he said. “Once we change that, once we modify that > assumption, everything would suddenly fall into place. That’s kind of the > hope. Anybody who is skeptical of all interpretations of quantum mechanics > must be thinking something like this. Can I tell you what’s a plausible > candidate for such an assumption? Well, if I could, I would just be working > on that theory.” > > @philipthrift > > > > -- > 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] > <mailto:[email protected]>. > To view this discussion on the web visit > https://groups.google.com/d/msgid/everything-list/44d55955-6cd5-4b99-9a91-617e5d43d4d9%40googlegroups.com > > <https://groups.google.com/d/msgid/everything-list/44d55955-6cd5-4b99-9a91-617e5d43d4d9%40googlegroups.com?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 [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/everything-list/04793BD5-D9E5-4F8A-92DB-758BF8368F30%40ulb.ac.be.
