interpretations-of-quantum-mechanics

"QBism explicitly takes the “subjective” or “judgmental” or “personalist” view of probability, which, though common among contemporary statisticians and economists, is still rare among physicists: probabilities are assigned to an event by an agent and are particular to that agent. The agent’s probability assignments express her own personal degrees of belief about the event."

"QBism, which combines quantum theory with probability theory, maintains that the wave function has no objective reality. Instead QBism portrays the wave function as a user’s manual, a mathematical tool that an observer uses to make wiser decisions about the surrounding world—the quantum world."

"Of course, quantum mechanics is tinged with this kind of solipsism, because there is a sense in which an electron doesn’t really have any properties until you measure it. There is a subjective aspect to quantum mechanics. Some experts like Eugene Wigner were convinced the universe wouldn’t exist if it didn’t have observers in it. He argued that, without a conscious mind observing the quantum events, the events don’t really exist, which I think is a crazy point of view. But it is defended by a number of quantum mechanics experts."

"In the consistent-histories approach, the classical limit can be studies by using appropriate subspaces of the quantum Hilbert space as a "coarse graining," analogous to dividing up phase space into nonoverlapping cells in classical statistical mechanics. This coarse graining can then be used to construct quantum histories. It is necessary to show that the resulting family of histories is consistent, so that the probabilities assigned by quantum dynamics make good quantum mechanical sense. Finally, one needs to show that the resulting quantum dynamics is well approximated by appropriate classical equations."

"Some months before Feynman's death in 1988, Gell-Mann described to a class at Caltech the status of our work on decoherent histories at that time. Feynman was in attendance, and at the end of the class, he stood up, and some of the students expected an exciting argument. But his comment was, "I agree with everything you said.""

"There is nothing absurd or inconsistent about the decoherent histories approach in particular, or about the general idea that the state vector serves only as a predictor of probabilities, not as a complete description of a physical system. Nevertheless, it would be disappointing if we had to give up the “realist” goal of finding complete descriptions of physical systems, and of using this description to derive the Born rule, rather than just assuming it. We can live with the idea that the state of a physical system is described by a vector in Hilbert space rather than by numerical values of the positions and momenta of all the particles in the system, but it is hard to live with no description of physical states at all, only an algorithm for calculating probabilities."

"If we did not have free will, we could never decide to test a scientific theory. We could live in a world where objects tend to fly up in the air but be programmed to look only when they are in the process of falling. I must admit, I have no proof that you have free will, but I certainly enjoy free will, and you will never be able to show otherwise. This kind of discussion can often go round in circles. It is logically possible, but totally uninteresting, rather like solipsism which asserts that I am the only person in the world and that everyone else is just an illusion inhabiting my own mind. This hypothesis of superdeterminism hardly deserves mention and appears here only to illustrate the extent to which many physicists, even among specialists in quantum physics, are driven almost to despair by the true randomness and nonlocality of quantum physics. But for me, the situation is very clear: not only does free will exist, but it is a prerequisite for science, philosophy, and our very ability to think rationally in a meaningful way. Without free will, there could be no rational thought. As a consequence, it is quite simply impossible for science and philosophy to deny free will."

"...One of the three commonsense assumptions (in Bell's theorem) will need to be abandoned. Which leads to the question: Which one? ... proponents of other interpretations might just claim that a violation of inequalities would invalidate one of the other two assumptions — freedom of choice or locality."

"There is the so-called idea of ‘superdeterminism’. Recall Schrödinger’s class of identically prepared students. We are told they can all answer any of a set of questions correctly, but each can only answer one, and then forgets the answers to the rest. It’s an odd idea, but we can still test it: we ask the questions at random, and find that we always get the right answer. Of course it is possible that each student only knows the answer to one question, which always happens to be the very one we ask! But that would require a massive coincidence, on a scale that would undercut the whole scientific method. Or else we are being manipulated: somehow we are led to ask a given question only of the rare student who knows the answer. So we switch our method of choice, handing it over to a random number generator, or the throw of dice, or to be determined by the amount of rainfall in Paraguay. But maybe all of these have been somehow rigged too! Of course, such a purely abstract proposal cannot be refuted, but besides being insane, it too would undercut scientific method. All scientific interpretations of our observations presuppose that they have not have been manipulated in such a way."

"I am absolutely convinced that one will eventually arrive at a theory in which the objects connected by laws are not probabilities, but conceived facts, as one took for granted only a short time ago. However, I cannot provide logical arguments for my conviction, but can only call on my little finger as a witness, which cannot claim any authority to be respected outside my own skin."

"In the non-Relativistic version you just postulate some point particles, and a single universal quantum state (represented by a mathematical wavefunction) and two simple dynamical equations: the Schrödinger equation for the wavefunction and the so-called guidance equation for the particle motions. You could have guessed both equations easily, and you get out all of the iconic quantum behavior: two-slit interference effects, violations of Bell’s inequality, decoherence due to observation or more generally due to coupling to the environment in the right way, etc., etc. What’s not to like? The only sticking point is the Relativistic version, but there I hold a minority view and would happily violate fundamental Lorentz invariance, explaining observational Lorentz invariance by appeal to what is called quantum equilibrium. There is a lot you just can’t do in complete thermal equilibrium, such as extract useful work from heat and send signals. Something you can’t do in quantum equilibrium is experimentally access a preferred “frame of reference”. C’est la vie."

"As far as I know the BB scheme reproduces all predictions of quantum mechanics. A decision can therefore be made only on aesthetic grounds. I must confess that the scheme, with both hidden variables and probability rules, seems to me exceedingly ugly, but of course one cannot argue about this."

"But why then had Born not told me of this 'pilot wave'? If only to point out what was wrong with it? Why did von Neumann not consider it? More extraordinarily, why did people go on producing 'impossibility' proofs, after 1952, and as recently as 1978? When even Pauli, Rosenfeld, and Heisenberg, could produce no more devastating criticism of Bohm's version than to brand it as 'metaphysical' and 'ideological'? Why is the pilot wave picture ignored in text books? Should it not be taught, not as the only way, but as an antidote to the prevailing complacency? To show that vagueness, subjectivity, and indeterminism, are not forced on us by experimental facts, but by deliberate theoretical choice?"

"So what is the disagreement in the end? Should it be locality or realism? Should it be quantum mechanics in minimal statistical interpretation, with an operational stance, or Bohmian Mechanics, or maybe something else? I guess Bohmians and I agree that the choice is not between equally viable positions. We only disagree about which one it is. To me one is a sound basis for doing physics, including theoretical and mathematical physics with a foundational interest, and the other has turned out to be fairly sterile. In 60 years the number of interesting new physical or even mathematical problems from the Bohmian and Neo-Bohmian community has been rather modest. The workshop certainly didn’t convince me otherwise, although the hope was what made me come. Bohmian Mechanics feels to me like a theologian explaining the origin of the universe. He could say: “With all your physics, which anyhow does not cover the singularity, you cannot explain Why it happens, but theology can”. I can see that many people would go for that sleeping pill. But it is a really lousy contribution to cosmology nonetheless."

"You’ve probably seen this picture of the potential that you have to use in the Bohm–de Broglie approach to describe, by hidden variables, the double-slit experiment. You see the electron coming in and doing this crazy thing. You may think that the Himalayas are wonderful, but this potential beats them by far. Yet they never tell you where the ‘screwdriver’ is—where in that morass of valleys and peaks the electron is going to start off. It just transforms the problem that eats them, back to square one. But, in the course of it, it encumbers the landscape with a lot of decoration."

"I'm not a student or a fan of Bohm's idea. It seems to me (1) their extremely limited in scope to particles […] (2) they seem to just give you additional elements like pilot wave, which didn't seem to me, […] give any satisfactory resolution of the problems."

"It is easy to find good reasons for disliking the de Broglie-Bohm picture. Neither de Broglie nor Bohm liked it very much; for both of them it was only a point of departure. Einstein also did not like it very much. He found it 'too cheap,' although, as Born remarked, 'it was quite in line with his own ideas'. But like it or lump it, it is perfectly conclusive as a counter example to the idea that vagueness, subjectivity, or indeterminism, are forced on us by the experimental facts covered by nonrelativistic quantum mechanics."

"Why were people so intolerant of de Broglie’s gropings and of Bohm? For twenty-five years people were saying that hidden-variable theories were impossible. After Bohm did it, some of the same people said that now it was trivial. They did a fantastic somersault. First they convinced themselves, in all sorts of ways, that it couldn’t be done. And then it becomes ‘trivial.’ I think Einstein thought that Bohm’s model was too glib—too simple. I think he was looking for a much more profound rediscovery of quantum phenomena. The idea that you could just add a few variables and the whole thing [quantum mechanics] would remain unchanged apart from the interpretation, which was a kind of trivial addition to ordinary quantum mechanics, must have been a disappointment to him. I can understand that—to see that that is all you need to do to make a hidden-variable theory. I am sure that Einstein, and most other people, would have liked to have seen some big principle emerging, like the principle of relativity, or the principle of the conservation of energy. In Bohm’s model one did not see anything like that."

"What we are proposing here is that this disparity between physical concepts (e.g. particle, wave, position, momentum) and the implications of the mathematical equations arises because the physical concepts are inseparably involved with the Cartesian notion of order, and this violates the essential content of quantum mechanics. What we need is a notion of order for all our concepts, both physical and mathematical, which coheres with this content."

"Have you noticed that Bohm believes (as de Broglie did, by the way, 25 years ago) that he is able to interpret the quantum theory in deterministic terms? That way seems too cheap to me. But you, of course, can judge this better than I."

"In the last few years several attempts have been made to complete quantum theory as you have also attempted. But it seems to me that we are still quite remote from a satisfactory solution to the problem. I myself have tried to approach this by generalising the law of gravitation. But I must confess that I was not able to find a way to explain the atomistic character of nature. My opinion is that if an objective description through the field as an elementary concept is not possible, than one has to find a possibility to avoid the continuum (together with space and time) altogether. But I have not the slightest idea what kind of elementary concepts could be used in such a theory."

"Bohm's Eastern metaphysics, even though it helped shape his interpretation of quantum mechanics, should not be held against the potential fruitfulness of his pilot wave theory. In a similar fashion Isaac Newton's Biblical fundamentalism and his alchemical research cast no shadows over his contributions to physics. Nor did Kepler's belief in astrology throw doubts on his great discoveries."

"Let me illustrate some of the ideas I believe Bohmian mechanics should have triggered. This list is obviously subjective—it is only important that it is not empty. Bohmian mechanics, like quantum theory, is in deep tension with relativity theory. I know of Bohmians who claim that it is obvious that any non-local theory, Bohmian or not, requires a privileged universal reference frame. I also know of Bohmians who claim that it is obvious that Bohmian mechanics can be generalized to a relativistic theory (though, admittedly, I never understood their model). However, I know of no Bohmians who are inspired by their theory and its tension with relativity to try to go beyond Bohmian mechanics, as illustrated in the next two paragraphs."

"Generally, position measurements sometimes reveal information about Bohmian positions, but never full information and sometimes none at all. Simple and handy criteria for determining when the Bohmian position measurements of a particle under test highly correlate with the position of the center of mass of some large pointer are still missing. Bohmian mechanics is attractive to philosophers because it provides a clear ontology. However, it is not as attractive to researchers in physics. This is unfortunate because it could inspire brave new ideas that challenge quantum physics."

"The first conference, Bohmian Mechanics 2000, was the total fiasco: two leading representatives of Bohmian school, Shelly Goldstein and Basil Hiley, presented two totally different interpretations of Bohmian mechanics. Finally, they accused each other in misunderstanding of Bohm’s views (both had very close connections to David Bohm). My students whom I invited to learn Bohmian mechanics from its creators were really confused. The only useful information which I extracted from Bohmian Mechanics 2000 was that Bohmian mechanics does not give new experimental predictions comparing to conventional QM."

"Bohr’s principle of complementarity – the heart of the Copenhagen philosophy – implies that quantum phenomena can only be described by pairs of partial, mutually exclusive, or ‘complementary’ perspectives. Though simultaneously inapplicable, both perspectives are necessary for the exhaustive description of phenomena. Bohr aspired to generalize complementarity into all fields of knowledge, maintaining that new epistemological insights are obtained by adjoining contrary, seemingly incompatible, viewpoints. [...] The value of Bohr’s philosophy for the advancement of physics is controversial. His followers consider complementarity a profound insight into the nature of the quantum realm. Others consider complementarity an illuminating but superfluous addendum to quantum theory. More severe is the opinion that Bohr’s philosophy is an obscure ‘web of words’ and mute on crucial foundational issues."

"In recent years the debate on these ideas has reopened, and there are some who question what they call "the Copenhagen interpretation of quantum mechanics"—as if there existed more than one possible interpretation of the theory."

"[I]n the Copenhagen interpretation of quantum theory we can indeed proceed without mentioning ourselves as individuals, but we cannot disregard the fact that natural science is formed by men. Natural science does not simply describe and explain nature; it is part of the interplay between nature and ourselves; it describes nature as exposed to our nature of questioning. This was a possibility of which Descartes could not have thought, but it makes a sharp separation between the world and the I impossible. If one follows the great difficulty which even eminent scientists like Einstein had in understanding and accepting the Copenhagen interpretation... one can trace the roots... to the Cartesian partition....it will take a long time for it [this partition] to be replaced by a really different attitude toward the problem of reality."

"Maxel, you know I love you and nothing can change that. But I do need to give you once a thorough head washing. So stand still. The impudence with which you assert time and again that the Copenhagen interpretation is practically universally accepted, assert it without reservation, even before an audience of the laity—who are completely at your mercy—it’s at the limit of the estimable […]. Have you no anxiety about the verdict of history? Are you so convinced that the human race will succumb before long to your own folly?"

"MWI is not some crazy speculative idea that runs afoul of Occam’s razor. On the contrary, MWI really is just the “obvious, straightforward” reading of quantum mechanics itself, if you take quantum mechanics literally as a description of the whole universe, and assume nothing new will ever be discovered that changes the picture."

"As Bohr acknowledged, in the Copenhagen interpretation a measurement changes the state of a system in a way that cannot itself be described by quantum mechanics. […] In quantum mechanics the evolution of the state vector described by the time-dependent Schrödinger equation is deterministic. If the time-dependent Schrödinger equation described the measurement process, then whatever the details of the process, the end result would be some definite state, not a number of possibilities with different probabilities. This is clearly unsatisfactory. If quantum mechanics applies to everything, then it must apply to a physicist’s measurement apparatus, and to physicists themselves. On the other hand, if quantum mechanics does not apply to everything, then we need to know where to draw the boundary of its area of validity. Does it apply only to systems that are not too large? Does it apply if a measurement is made by some automatic apparatus, and no human reads the result?"

"It works, is useful to understand our experiments, and makes no unnecessary assumptions."

"I have always felt bitter about the way how Bohr’s authority together with Pauli’s sarcasm killed any discussion about the fundamental problems of the quantum. [...] I expect that the Copenhagen interpretation will some time be called the greatest sophism in the history of science, but I would consider it a terrible injustice if—when some day a solution should be found—some people claim that ‘this is of course what Bohr always meant’, only because he was sufficiently vague."

"According to my attempts to understand them, reality is systematically denied in the Copenhagen interpretation in order to circumvent consistency problems (such as “Is the electron really a wave or a particle?”). If there is no reality, one does not need a consistent description!"

"... I think, because of the split in the Copenhagen Interpretation between the thing we study and the observer himself, that it can't survive in a fundamental sense."

"If the MWI were supported by some sound science, we would have to deal with it – and to do so with more seriousness than the merry invention of Doppelgängers to measure both quantum states of a photon. But it is not. It is grounded in a half-baked philosophical argument about a preference to simplify the axioms. Until Many Worlders can take seriously the philosophical implications of their vision, it’s not clear why their colleagues, or the rest of us, should demur from the judgment of the philosopher of science Robert Crease that the MWI is ‘one of the most implausible and unrealistic ideas in the history of science’."

"The “many worlds interpretation” seems to me an extravagant, and above all an extravagantly vague, hypothesis. I could almost dismiss it as silly. And yet…It may have something distinctive to say in connection with the “Einstein Podolsky Rosen puzzle,” and it would be worthwhile, I think, to formulate some precise version of it to see if this is really so. And the existence of all possible worlds may make us more comfortable about the existence of our own world…which seems to be in some ways a highly improbable one."

"Yes, I have strong feelings against it, but I have to qualify that by saying that in this particular Einstein-Podolsky-Rosen situation there is some merit in the many-universes interpretation, in tackling the problem of how something can apparently happen far away sooner than it could without faster-than-light signalling. If, in a sense, everything happens, all choices are realized (somewhere among all the parallel universes), and no selection is made between the possible results of the experiment until later (which is what one version of the many-universes hypothesis implies), then we get over this difficulty."

"It's extremely bizarre, and for me that would already be enough reason to dislike it. The idea that there are all those other universes which we can't see is hard to swallow. But there are also technical problems with it which people usually gloss over or don't even realize when they study it. The actual point at which a branching occurs is supposed to be the point at which a measurement is made. But the point at which the measurement is made is totally obscure. The experiments at CERN for example take months and months, and at which particular second on which particular day the measurement is made and the branching occurs is perfectly obscure. So I believe that the many-universes interpretation is a kind of heuristic, simplified theory, which people have done on the backs of envelopes but haven't really thought through. When you do try to think it through it is not coherent."

"He explains that the multiple worlds are not an add-on to the theory but instead are simply what happens naturally when you take the equations of the theory at face value. The standard (“Copenhagen”) interpretation is the one that needs to postulate an ad hoc extra rule. We should simply rename things:The “many-worlds interpretation of quantum mechanics” should henceforth be known as “quantum mechanics.” The Copenhagen interpretation should henceforth be known as “the disappearing-worlds interpretation of quantum mechanics.”"

"The conclusion, therefore, is that multiple worlds automatically occur in quantum mechanics. They are an inevitable part of the formalism. The only remaining question is: what are you going to do about it? There are three popular strategies on the market: anger, denial, and acceptance. The “anger” strategy says “I hate the idea of multiple worlds with such a white-hot passion that I will change the rules of quantum mechanics in order to avoid them.” And people do this! […] The “denial” strategy says “The idea of multiple worlds is so profoundly upsetting to me that I will deny the existence of reality in order to escape having to think about it.” Advocates of this approach don’t actually put it that way, but I’m being polemical rather than conciliatory in this particular post. And I don’t think it’s an unfair characterization. This is the quantum Bayesianism approach, or more generally “psi-epistemic” approaches. The idea is to simply deny that the quantum state represents anything about reality; it is merely a way of keeping track of the probability of future measurement outcomes. […] The final strategy is acceptance. That is the Everettian approach. The formalism of quantum mechanics, in this view, consists of quantum states as described above and nothing more, which evolve according to the usual Schrödinger equation and nothing more. The formalism predicts that there are many worlds, so we choose to accept that. This means that the part of reality we experience is an indescribably thin slice of the entire picture, but so be it. Our job as scientists is to formulate the best possible description of the world as it is, not to force the world to bend to our pre-conceptions."

"Fortunately, a minority of physicists, myself included, likewise side unequivocally with realism, by adopting Hugh Everett’s multiple-universes interpretation of quantum theory. According to this view, no particles exist where they have insufficient energy to be; it is simply that in some universes they have more energy than average, and in others, less. All alleged “paradoxes” of quantum theory are similarly resolved.So, while most accounts say that Bohr won the debate, my view is that Einstein, as usual, was seeking an explanation of reality, while his rivals were advocating nonsense. Everett’s interpretation doesn’t make Einstein a demigod. But it does make him right."

"To call MWI an interpretation is like talking about dinosaurs as an 'interpretation' of fossil records."

"In fact the physicists have no good point of view. Somebody mumbled something about a many-world picture, and that many-world picture says that the wave function ψ is what's real, and damn the torpedos if there are so many variables, NR. All these different worlds and every arrangement of configurations are all there just like our arrangement of configurations, we just happen to be sitting in this one. It's possible, but I'm not very happy with it."

"There is, I think, no sense at all to be made of the splitting of worlds-plus-agents in many worlds. Of course, one can repeat the words over and over until one becomes deaf to the nonsense, but it remains nonsense nevertheless. Curiously, those who favor this interpretation concentrate their defense on dealing with some obvious technical issues: preferred basis, getting the right probabilities via “measures of existence” (or the like), questions of identity and individuation across worlds, and so on. But the fundamental question is just to explain what it means to talk of splitting worlds, and why we should not just write it off, à la Wittgenstein, as language on holiday. (Einstein once described the writings of Hegel as “word-music.” Perhaps that would be a gentler way of dismissing many worlds.)"

"The greatest danger I see in the many-worlds/one-Hilbert-space point of view (beside the ridiculous silliness of it all) is the degree to which it is a dead end. The degree to which it is morally bankrupt. Charlie, by thinking that he has taken some of the anthropocentrism out of the picture, has actually emptied the world of all content. Beyond that though, I think, many-worlds empties the world of content in a way that’s even worse than classical determinism. Let me explain. In my mind, both completely deterministic ontologies and completely indeterministic ones are equally unpalatable. This is because, in both, all our consciousnesses, all our great works of literature, everything that we know, even the coffee maker in my kitchen, are but dangling appendages, illusions. In the first case, the only truth is the Great Initial Condition. In the second, it is the great “I Am That I Am.” But many-worlds compounds that trouble in a far worse fashion by stripping away even those small corners of mystery. It is a world in which anything goes, and everything does. What could be more empty than that?"

"It is true that the MWI, in this realist form, avoids some of the paradoxes of QM. The so-called “measurement problem,” for example, is no longer a problem because whenever a measurement occurs, there is no “collapse of the wave function” (or rotation of the state vector in a different terminology). All possible outcomes take place. Schrödinger’s notorious cat is never in a mixed state of alive and dead. It lives in one universe, dies in another. But what a fantastic price is paid for these seeming simplicities! It is hard to imagine a more radical violation of Occam’s razor, the law of parsimony which urges scientists to keep entities to a minimum."

"The many-worlds theory is incoherent for reasons which have been often pointed out: since there are no frequencies in the theory there is nothing for the numerical predictions of quantum theory to mean. This fact is often disguised by the choice of fortuitous examples. A typical Schrödinger-cat apparatus is designed to yield a 50 percent probability for each of two results, so the “splitting” of the universe in two seems to correspond to the probabilities. But the device could equally be designed to yield a 99 percent probability of one result and 1 percent probability of the other. Again the world “splits” in two; wherein lies the difference between this case and the last? Defenders of the theory sometimes try to alleviate this difficulty by demonstrating that in the long run (in the limit as one repeats experiments an infinite number of times) the quantum probability assigned to branches in which the observed frequencies match the quantum predictions approaches unity. But this is a manifest petitio principii. If the connection between frequency and quantum “probability” has not already been made, the fact that the assigned “probability” approaches unity cannot be interpreted as approach to certainty of an outcome. All of the branches in which the observed frequency diverges from the quantum predictions still exist, indeed they are certain to exist. It is not highly likely that I will experience one of the frequencies rather than another, it is rather certain that for each possible frequency some descendants of me (descendants through world-splitting) will see it. And in no sense will “more” of my descendants see the right frequency rather than the wrong one: just the opposite is true. So approach of some number to unity cannot help unless the number already has the right interpretation. It is also hard to see how such limiting cases help us: we never get to one since we always live in the short run. If the short-run case can be solved, the theorems about limits are unnecessary; if they can’t be then the theorems are irrelevant."

"I regard this last issue as a problem in the interpretation of quantum mechanics, even though I do not believe that consciousness (as a physical phenomenon) collapses (as a physical process) the wave packet (as an objective physical entity). But because I do believe that physics is a tool to help us find powerful and concise expressions of correlations among features of our experience, it makes no sense to apply quantum mechanics (or any other form of physics) to our very awareness of that experience. Adherents of the many-worlds interpretation make this mistake. So do those who believe that conscious awareness can ultimately be reduced to physics, unless they believe that the reduction will be to a novel form of physics that transcends our current understanding, in which case, as Rudolf Peierls remarked, whether such an explanation should count as “physical” is just a matter of terminology."

"As a philosopher, I am fascinated by the appearance of the Many-Worlds Interpretation as a cultural phenomenon. This is so similar to what we have seen over and over again in the history of metaphysics! A well-known poet (Derek Walcott) once riddled, "What is the difference between a philosopher and a ruler?" The answer was a pun: "A ruler will only stretch to one foot, but a philosopher will go to any length." But the pun contains a deep observation; it is part of our philosophical tradition that at least one kind of philosopher will go to any length to preserve what he regards as a central metaphysical principle, a principle that is "necessary" in the peculiar philosophical sense of "necessary." What is startling is to observe a metaphysical system as daring as any being born in the unexpected locus of a discussion among physicists about how to understand the deepest and the most accurate physical theory we possess."