If Many-Worlds Had Come First

Not that I’m claiming I could have done better, if I’d been born into that time, instead of this one…

Macroscopic decoherence, a.k.a. many-worlds, was first proposed in a 1957 paper by Hugh Everett III. The paper was ignored. John Wheeler told Everett to see Niels Bohr. Bohr didn’t take him seriously.

Crushed, Everett left academic physics, invented the general use of Lagrange multipliers in optimization problems, and became a multimillionaire.

It wasn’t until 1970, when Bryce DeWitt (who coined the term “many-worlds”) wrote an article for Physics Today, that the general field was first informed of Everett’s ideas. Macroscopic decoherence has been gaining advocates ever since, and may now be the majority viewpoint (or not).

But suppose that decoherence and macroscopic decoherence had been realized immediately following the discovery of entanglement, in the 1920s. And suppose that no one had proposed collapse theories until 1957. Would decoherence now be steadily declining in popularity, while collapse theories were slowly gaining steam?

Imagine an alternate Earth, where the very first physicist to discover entanglement and superposition said, “Holy flaming monkeys, there’s a zillion other Earths out there!”

In the years since, many hypotheses have been proposed to explain the mysterious Born probabilities. But no one has yet suggested a collapse postulate. That possibility simply has not occurred to anyone.

One day, Huve Erett walks into the office of Biels Nohr…

“I just don’t understand,” Huve Erett said, “why no one in physics even seems interested in my hypothesis. Aren’t the Born statistics the greatest puzzle in modern quantum theory?”

Biels Nohr sighed. Ordinarily, he wouldn’t even bother, but something about the young man compelled him to try.

“Huve,” says Nohr, “every physicist meets dozens of people per year who think they’ve explained the Born statistics. If you go to a party and tell someone you’re a physicist, chances are at least one in ten they’ve got a new explanation for the Born statistics. It’s one of the most famous problems in modern science, and worse, it’s a problem that everyone thinks they can understand. To get attention, a new Born hypothesis has to be… pretty darn good.”

“And this,” Huve says, “this isn’t good?

Huve gestures to the paper he’d brought to Biels Nohr. It is a short paper. The title reads, “The Solution to the Born Problem.” The body of the paper reads:

When you perform a measurement on a quantum system, all parts of the wavefunction except one point vanish, with the survivor chosen non-deterministically in a way determined by the Born statistics.

“Let me make absolutely sure,” Nohr says carefully, “that I understand you. You’re saying that we’ve got this wavefunction—evolving according to the Wheeler-DeWitt equation—and, all of a sudden, the whole wavefunction, except for one part, just spontaneously goes to zero amplitude. Everywhere at once. This happens when, way up at the macroscopic level, we ‘measure’ something.”

“Right!” Huve says.

“So the wavefunction knows when we ‘measure’ it. What exactly is a ‘measurement’? How does the wavefunction know we’re here? What happened before humans were around to measure things?”

“Um…” Huve thinks for a moment. Then he reaches out for the paper, scratches out “When you perform a measurement on a quantum system,” and writes in, “When a quantum superposition gets too large.”

Huve looks up brightly. “Fixed!”

“I see,” says Nohr. “And how large is ‘too large’?”

“At the 50-micron level, maybe,” Huve says, “I hear they haven’t tested that yet.”

Suddenly a student sticks his head into the room. “Hey, did you hear? They just verified superposition at the 50-micron level.”

“Oh,” says Huve, “um, whichever level, then. Whatever makes the experimental results come out right.”

Nohr grimaces. “Look, young man, the truth here isn’t going to be comfortable. Can you hear me out on this?”

“Yes,” Huve says, “I just want to know why physicists won’t listen to me.”

“All right,” says Nohr. He sighs. “Look, if this theory of yours were actually true—if whole sections of the wavefunction just instantaneously vanished—it would be… let’s see. The only law in all of quantum mechanics that is non-linear, non-unitary, non-differentiable and discontinuous. It would prevent physics from evolving locally, with each piece only looking at its immediate neighbors. Your ‘collapse’ would be the only fundamental phenomenon in all of physics with a preferred basis and a preferred space of simultaneity. Collapse would be the only phenomenon in all of physics that violates CPT symmetry, Liouville’s Theorem, and Special Relativity. In your original version, collapse would also have been the only phenomenon in all of physics that was inherently mental. Have I left anything out?”

“Collapse is also the only acausal phenomenon,” Huve points out. “Doesn’t that make the theory more wonderful and amazing?”

“I think, Huve,” says Nohr, “that physicists may view the exceptionalism of your theory as a point not in its favor.”

“Oh,” said Huve, taken aback. “Well, I think I can fix that non-differentiability thing by postulating a second-order term in the—”

“Huve,” says Nohr, “I don’t think you’re getting my point, here. The reason physicists aren’t paying attention to you, is that your theory isn’t physics. It’s magic.”

“But the Born statistics are the greatest puzzle of modern physics, and this theory provides a mechanism for the Born statistics!” Huve protests.

“No, Huve, it doesn’t,” Nohr says wearily. “That’s like saying that you’ve ‘provided a mechanism’ for electromagnetism by saying that there are little angels pushing the charged particles around in accordance with Maxwell’s equations. Instead of saying, ‘Here are Maxwell’s equations, which tells the angels where to push the electrons,’ we just say, ‘Here are Maxwell’s equations’ and are left with a strictly simpler theory. Now, we don’t know why the Born statistics happen. But you haven’t given the slightest reason why your ‘collapse postulate’ should eliminate worlds in accordance with the Born statistics, rather than something else. You’re not even making use of the fact that quantum evolution is unitary—”

“That’s because it’s not,” interjects Huve.

“—which everyone pretty much knows has got to be the key to the Born statistics, somehow. Instead you’re merely saying, ‘Here are the Born statistics, which tell the collapser how to eliminate worlds,’ and it’s strictly simpler to just say ‘Here are the Born statistics.’ ”

“But—” says Huve.

Also,” says Nohr, raising his voice, “you’ve given no justification for why there’s only one surviving world left by the collapse, or why the collapse happens before any humans get superposed, which makes your theory really suspicious to a modern physicist. This is exactly the sort of untestable hypothesis that the ‘One Christ’ crowd uses to argue that we should ‘teach the controversy’ when we tell high school students about other Earths.”

“I’m not a One-Christer!” protests Huve.

“Fine,” Nohr says, “then why do you just assume there’s only one world left? And that’s not the only problem with your theory. Which part of the wavefunction gets eliminated, exactly? And in which basis? It’s clear that the whole wavefunction isn’t being compressed down to a delta, or ordinary quantum computers couldn’t stay in superposition when any collapse occurred anywhere—heck, ordinary molecular chemistry might start failing—”

Huve quickly crosses out “one point” on his paper, writes in “one part,” and then says, “Collapse doesn’t compress the wavefunction down to one point. It eliminates all the amplitude except one world, but leaves all the amplitude in that world.”

“Why?” says Nohr. “In principle, once you postulate ‘collapse,’ then ‘collapse’ could eliminate any part of the wavefunction, anywhere—why just one neat world left? Does the collapser know we’re in here?

Huve says, “It leaves one whole world because that’s what fits our experiments.”

“Huve,” Nohr says patiently, “the term for that is ‘post hoc.’ Furthermore, decoherence is a continuous process. If you partition by whole brains with distinct neurons firing, the partitions have almost zero mutual interference within the wavefunction. But plenty of other processes overlap a great deal. There’s no possible way you can point to ‘one world’ and eliminate everything else without making completely arbitrary choices, including an arbitrary choice of basis—”

“But—” Huve says.

“And above all,” Nohr says, “the reason you can’t tell me which part of the wavefunction vanishes, or exactly when it happens, or exactly what triggers it, is that if we did adopt this theory of yours, it would be the only informally specified, qualitative fundamental law taught in all of physics. Soon no two physicists anywhere would agree on the exact details! Why? Because it would be the only fundamental law in all of modern physics that was believed without experimental evidence to nail down exactly how it worked.”

“What, really?” says Huve. “I thought a lot of physics was more informal than that. I mean, weren’t you just talking about how it’s impossible to point to ‘one world’?”

“That’s because worlds aren’t fundamental, Huve! We have massive experimental evidence underpinning the fundamental law, the Wheeler-DeWitt equation, that we use to describe the evolution of the wavefunction. We just apply exactly the same equation to get our description of macroscopic decoherence. But for difficulties of calculation, the equation would, in principle, tell us exactly when macroscopic decoherence occurred. We don’t know where the Born statistics come from, but we have massive evidence for what the Born statistics are. But when I ask you when, or where, collapse occurs, you don’t know—because there’s no experimental evidence whatsoever to pin it down. Huve, even if this ‘collapse postulate’ worked the way you say it does, there’s no possible way you could know it! Why not a gazillion other equally magical possibilities?”

Huve raises his hands defensively. “I’m not saying my theory should be taught in the universities as accepted truth! I just want it experimentally tested! Is that so wrong?”

“You haven’t specified when collapse happens, so I can’t construct a test that falsifies your theory,” says Nohr. “Now with that said, we’re already looking experimentally for any part of the quantum laws that change at increasingly macroscopic levels. Both on general principles, in case there’s something in the 20th decimal point that only shows up in macroscopic systems, and also in the hopes we’ll discover something that sheds light on the Born statistics. We check decoherence times as a matter of course. But we keep a broad outlook on what might be different. Nobody’s going to privilege your non-linear, non-unitary, non-differentiable, non-local, non-CPT-symmetric, non-relativistic, a-frikkin’-causal, faster-than-light, in-bloody-formal ‘collapse’ when it comes to looking for clues. Not until they see absolutely unmistakable evidence. And believe me, Huve, it’s going to take a hell of a lot of evidence to unmistake this. Even if we did find anomalous decoherence times, and I don’t think we will, it wouldn’t force your ‘collapse’ as the explanation.”

“What?” says Huve. “Why not?”

“Because there’s got to be a billion more explanations that are more plausible than violating Special Relativity,” says Nohr. “Do you realize that if this really happened, there would only be a single outcome when you measured a photon’s polarization? Measuring one photon in an entangled pair would influence the other photon a light-year away. Einstein would have a heart attack.”

“It doesn’t really violate Special Relativity,” says Huve. “The collapse occurs in exactly the right way to prevent you from ever actually detecting the faster-than-light influence.”

“That’s not a point in your theory’s favor,” says Nohr. “Also, Einstein would still have a heart attack.”

“Oh,” says Huve. “Well, we’ll say that the relevant aspects of the particle don’t existuntil the collapse occurs. If something doesn’t exist, influencing it doesn’t violate Special Relativity—”

“You’re just digging yourself deeper. Look, Huve, as a general principle, theories that are actually correct don’t generate this level of confusion. But above all, there isn’t any evidence for it. You have no logical way of knowing that collapse occurs, and no reason to believe it. You made a mistake. Just say ‘oops’ and get on with your life.”

“But they could find the evidence someday,” says Huve.

“I can’t think of what evidence could determine this particular one-world hypothesis as an explanation, but in any case, right now we haven’t found any such evidence,” says Nohr. “We haven’t found anything even vaguely suggestive of it! You can’t update on evidence that could theoretically arrive someday but hasn’t arrived! Right now, today, there’s no reason to spend valuable time thinking about this rather than a billion other equally magical theories. There’s absolutely nothing that justifies your belief in ‘collapse theory’ any more than believing that someday we’ll learn to transmit faster-than-light messages by tapping into the acausal effects of praying to the Flying Spaghetti Monster!”

Huve draws himself up with wounded dignity. “You know, if my theory is wrong—and I do admit it might be wrong—”

If?” says Nohr. “Might?

“If, I say, my theory is wrong,” Huve continues, “then somewhere out there is another world where I am the famous physicist and you are the lone outcast!”

Nohr buries his head in his hands. “Oh, not this again. Haven’t you heard the saying, ‘Live in your own world’? And you of all people—”

“Somewhere out there is a world where the vast majority of physicists believe in collapse theory, and no one has even suggested macroscopic decoherence over the last thirty years!”

Nohr raises his head, and begins to laugh.

“What’s so funny?” Huve says suspiciously.

Nohr just laughs harder. “Oh, my! Oh, my! You really think, Huve, that there’s a world out there where they’ve known about quantum physics for thirty years, and nobody has even thought there might be more than one world?”

“Yes,” Huve says, “that’s exactly what I think.”

“Oh my! So you’re saying, Huve, that physicists detect superposition in microscopic systems, and work out quantitative equations that govern superposition in every single instance they can test. And for thirty years, not one person says, ‘Hey, I wonder if these laws happen to be universal.’ ”

“Why should they?” says Huve. “Physical models sometimes turn out to be wrong when you examine new regimes.”

“But to not even think of it?” Nohr says incredulously. “You see apples falling, work out the law of gravity for all the planets in the solar system except Jupiter, and it doesn’t even occur to you to apply it to Jupiter because Jupiter is too large? That’s like, like some kind of comedy routine where the guy opens a box, and it contains a spring-loaded pie, so the guy opens another box, and it contains another spring-loaded pie, and the guy just keeps doing this without even thinking of the possibility that the next box contains a pie too. You think John von Neumann, who may have been the highest-g human in history, wouldn’t think of it?”

“That’s right,” Huve says, “He wouldn’t. Ponder that.”

“This is the world where my good friend Ernest formulates his Schrödinger’s Cat thought experiment, and in this world, the thought experiment goes: ‘Hey, suppose we have a radioactive particle that enters a superposition of decaying and not decaying. Then the particle interacts with a sensor, and the sensor goes into a superposition of going off and not going off. The sensor interacts with an explosive, that goes into a superposition of exploding and not exploding; which interacts with the cat, so the cat goes into a superposition of being alive and dead. Then a human looks at the cat,’ and at this point Schrödinger stops, and goes, ‘gee, I just can’t imagine what could happen next.’ So Schrödinger shows this to everyone else, and they’re also like ‘Wow, I got no idea what could happen at this point, what an amazing paradox.’ Until finally you hear about it, and you’re like, ‘hey, maybe at thatpoint half of the superposition just vanishes, at random, faster than light,’ and everyone else is like, ‘Wow, what a great idea!’ ”

“That’s right,” Huve says again. “It’s got to have happened somewhere.”

“Huve, this is a world where every single physicist, and probably the whole damn human species, is too dumb to sign up for cryonics! We’re talking about the Earth where George W. Bush is President.”

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I don't see how decoherence is an automatic win for MWI. Decoherence has been used in several different interpretations of quantum mechanics, notably in consistent histories and in certain hidden variable interpretations. Why should we choose MWI before those, particularly since it seems less parsimonious than consistent histories? For that matter, the language of Rovelli and Smolin's relational quantum mechanics very nearly turns decoherence into its own interpretation (if you compare papers on decoherence which shirk the metaphysical interpretation to the interpretation put forward by Rovelli, they're almost identical). Relational quantum mechanics requires much less in the way of grand assertions than MWI and is a natural framework for decoherence, so why pick MWI over relational quantum mechanics?

Relational quantum mechanics requires much less in the way of grand assertions than MWI and is a natural framework for decoherence, so why pick MWI over relational quantum mechanics?

Hear, hear.

That sure is far beyond my current educational horizon but I would love to see Eliezer answer that comment. Until now I haven't even heard of Relational Quantum Mechanics. I searched LW and that comment by Dustin2 seems to be one of two comments that mention it.

If you take decoherence realistically, you get something like MWI. CH is different because, like, CI, it is less realistic.

There's no form of decoherence which is equivalent to ontological collapse, to actually snipping off branches. (Penrose has a nice discussion of this somewhere). So decoherence as an interpretation can't be saying anything different to what MWI says as an interpetation.. Decoeherence just gives an criterion --albeit a fuzzy and subjective one -- for world-formation.

As far as I can tell, the only possible coherent state of affairs corresponding to RQM - the only reality in which you can embed these systems relating to each other - is MWI. To this is added some bad amateur incoherent epistemology trying to dance around the issue without addressing it.

You can quote me on the following:

  • RQM is MWI in denial.
  • Any time you might uncharitably get the impression that RQM is merely playing semantic word-games with the notion of reality, RQM is, in fact, merely playing semantic word-games with the notion of reality.
  • RQM's epistemology is drunk and needs to go home and sleep it off.

Some people consider it a good form to back up your accusations with examples, facts and proofs, even when discussing topics dear to their hearts. Give it a try some time.

Okay. Name a state of affairs that could correspond to RQM without being MWI.

PS: Whenever you say that something is 'true relative to' B, please replace it with a state of affairs and a description of B's truth-predicate over possible states of affairs.

Okay. Name a state of affairs that could correspond to RQM without being MWI.

First, the onus is on you to show that the above is both relevant to your claim of "bad amateur incoherent epistemology" and that there is no such state of affairs, since it's your claim that RQM is just a word game.

But, to indulge you, here is one example:

different observers may give different accounts of the same series of events: for example, to one observer at a given point in time, a system may be in a single, "collapsed" eigenstate, while to another observer at the same time, it may appear to be in a superposition of two or more states.

Whereas in MWI, unless I misunderstand it, each interaction (after the decoherence has ran its course) irrevocably splits the world into "eigenworlds" of the interaction, and there is no observer for which the world is as yet unsplit:

n DeWitt's formulation, the state of S after a sequence of measurements is given by a quantum superposition of states, each one corresponding to an alternative measurement history of S.

P.S. Just to make it clear, I'm not an adherent of RQM, not until and unless it gives new testable predictions not available without it. Same applies to all other interpretations. I'm simply pointing out that MWI is not the only game in town.

I'm not an adherent of RQM, not until and unless it gives new testable predictions not available without it.

By definition, interpretations don't give testable predictions. Theories give testable predictions.

EDIT: having said that, rQM ontology, where information is in relations, not in relata, predicts a feature of the formalism--that when you combine Hilbert spaces, what you have is a product not a sum. That is important for understanding the advantages of quantum computation.

By definition, interpretations don't give testable predictions. Theories give testable predictions.

Definitions can be wrong.

I understand that well-meaning physics professor may have once told you that. However the various quantum mechanics interpretations do in fact pre-suppose different underlying mechanisms, and therefore result in different predictions in obscure corner cases. For example, reversible measurement of quantum phenomenon results in different probabilities on the return path in many-worlds vs the Copenhagen interpretation. (Unfortunately we lack the capability at this time to make fully reversible experimental aparatus at this scale.)

A real testable difference between QM interpretations is a Nobel-worthy Big Deal. I doubt it will be coming.

Actually, Nobel does not begin to cover it, whether it would be awarded or not (even J.S. Bell didn't get one, though he was nominated the year he died). Showing experimentally that, say, there is an objective collapse mechanism of some sort would probably be the biggest deal since the invention of QM.

And even just formally applying all the complexity stuff that is alluded to in the sequences, to the question of QM interpretation, would be a rather notable deed.

There are real testable differences:

http://www.hedweb.com/manworld.htm#unique

That page lists three ways in which MWI differs from the Copenhagen interpretation.

One has to two with further constraints that MWI puts on the grand unified theory: namely that gravity must be quantized. If it turns out that gravity is not quantized, that would be strong evidence against the basic MWI explanation.

The second has to do with testable predictions which could be made if it turns out that linearity is violated. Linearity is highly verified, but perhaps it does break down at high energies, in which case it could be used to communicate between or simply observe other Everett branches.

Finally, there's an actual testable prediction: make a reversible device to measure electron spin. Measure one axis to prepare the electron. Measure an orthogonal axis, then reverse that measurement. Finally measure again on the first axis. You've lost your recording of the 2nd measurement, but in Copenhagen the 1st and 3rd should agree 50% of the time by random chance, because there was an intermediate collapse, whereas in MWI they agree 100% of the time, because the physical process was fully reversed, bringing the branches back into coherence.

We just lack the capability to make such a device, unfortunately. But feel free to do so and win that Nobel prize.

Finally, there's an actual testable prediction: make a reversible device to measure electron spin. Measure one axis to prepare the electron. Measure an orthogonal axis, then reverse that measurement. Finally measure again on the first axis. You've lost your recording of the 2nd measurement, but in Copenhagen the 1st and 3rd should agree 50% of the time by random chance, because there was an intermediate collapse, whereas in MWI they agree 100% of the time, because the physical process was fully reversed, bringing the branches back into coherence.

But such device is not physically realizable, as it would involve reversing the thermodynamic arrow of time.

? What aspect of measuring an electron's spin is not reversible? Physics at this scale is entirely reversible.

You can reversibly entangle an electron's spin to the state of some other small quantum system, that's not questioned by any interpretation of QM, but unless this entanglement propagates to the point of producing a macroscopic effect, it is not considered a measurement.

It's even worse than that. Zurek's einselection relies on decoherence to get rid of non-eigenstates, and reversibility is necessarily lost in this (MWI-compatible) model of measurement. There is no size restriction, but the measurement apparatus (including the observer looking at it) must necessarily leak information to the environment to work as a detector. Thus a reversible computation would not be classically detectable.

Which is why the experiment as described in the link I provided requires an artificial intelligence running on a reversible computing substrate to perform the experiment in order to provide the macroscopic effect.

That is, it would require inverting the thermodynamic arrow of time.

If you define a measurement as an the creation of a (FAPP) irreversible record....then, no.

Indeed. Truly reversing the measurement would involve also forgetting what the result of the measurement was, and Copenhagenists would claim this forgotten intermediate result does not count as a "measurement" in the sense of something that (supposedly) collapses the wave function.

So in MWI, this presumably arises when e.g. you've got 3 possible states of X, and version A of you decoheres with state 1 while version B is entangled with the superposition of 2+3. In RQM this is presumably described sagely as X being definitely-1 relative to A while X is 2+3 relative to B. Then if you ask them whether or not this statement itself is a true, objective state of affairs (where a 'yes' answer immediately yields MWI) there's a bunch of hemming and hawing.

Ignoring your unhelpful sarcastic derision... You should know better, really.

Take an EPR experiment with spatially separated observers A and B. If A measures a state of a singlet and the world is split into Aup and Adown, when does B split in this world, according to MWI?

In RQM, it does not until it measures its own half of the singlet, which can be before of after A in a given frame. Its model of A is a superposition until A and B meet up and compare results (another interaction). The outcome depends on whether A actually measured anything and if so, in which basis. None of this is known until A and B interact.

I confess I'm not quite clear on your question. Local processes proceed locally with invariant states of distant entanglement. Just suppose that the global wavefunction is an objective fact which entails all of RQM's statements via the obvious truth-condition, and there you go.

I confess I'm not quite clear on your question.

I confess I'm not quite clear on your answer.

Local processes proceed locally with invariant states of distant entanglement.

Not sure what this means, at least not past "local processes proceed locally", which is certainly uncontroversial, if you mean to say that interaction is limited to light speed.

Just suppose that the global wavefunction is an objective fact

"an objective fact"? As in a map from something to C? If so, what is that something? Some branching multiverse? Or what do you mean by an objective fact?

which entails all of RQM's statements via the obvious truth-condition

You lost me here, sorry.

Just suppose that the global wavefunction is an objective fact

Tell me what the basis is, and where it comes from, and I will...

I know I'm late to the party, but I couldn't help but notice that this interesting question hadn't been answered (here, at least). So here it is: as far as I know, B 'splits' immediately, but this in an unphysical question.

In MWI we would have observers A and B, who could observe Aup or Adown and Bup or Bdown (and start in |Aunknown> and |Bunknown> before measuring) respectively. If we write |PAup> and |PAdown> for the wavefunctions corresponding to the particle near observer A being in the up resp. down states, and introduce similar notation for the particle near observer B, then the initial configuration is:

|Aunkown> |Bunknown> (|PAup> |PBdown> - |PAdown> |PBup>) / \sqrt(2)

Now if we let person A measure the particle the complete wavefunction changes to:

|Bunknown> (|Aup> |PAup> |PBdown> - |Adown> |PAdown> * |PBup>) / \sqrt(2)

Important is that this is a local change to the wavefunction, what happened here is merely that A measured the particle near A. Since observer A is a macroscopic object we would expect the two branches of the wavefunction above (separated by the minus sign) to be quite far apart in configuration space, so the worlds have definitely split here. But B still isn't correlated to any particular branch: from the point of A, person B is now in a superposition. In particular observer B doesn't notice anything from this splitting - as we would expect (splitting being a local process and observers A and B being far apart). This is also why I called the question as to when B splits 'unphysical' above, since it is a property known only locally at A, and in fact the answer to this question wouldn't change any of B's anticipations.

This might seem a lot like RQM, and that is because RQM happens to get the answer to this question right. The problem with RQM (at least, the problem I ran into while reading the paper) was that the author claims that measurements are ontologically fundamental, and wavefunctions are only a mathematical tool. This seems to confuse the map with the territory: if wavefunctions are only part of our maps, what are they maps of? Also if wavefunctions aren't part of the territory an explanation is needed for the observation that different observers can get the same results when measuring a system, i.e. an explanation is needed for the fact that all observations are consistent. It seems unnecessarily complicated to demand that wavefunctions aren't real, and then separately explain why all observations are consistent as they would have been if the wavefunction were real.

I think this is what Eliezer might have meant with

As far as I can tell, the only possible coherent state of affairs corresponding to RQM - the only reality in which you can embed these systems relating to each other - is MWI

RQM seems to assert precisely what MWI asserts, except that it denies the existence of objective reality, and then needs a completely new and different explanation for the consistency between measurements by different observers. I found the insults hurled at RQM by Eliezer disrespectful but, on close inspection, well-deserved. Denying reality doesn't seem like a good property for a theory of physics to have.

It seems unnecessarily complicated to demand that wavefunctions aren't real, and then separately explain why all observations are consistent as they would have been if the wavefunction were real.

Denying reality, and denying the reality of the .WF aren't the same thing.

Suppose RQM is only doing the latter. Then, you have observers who are observing a consistent objective reality, and mapping it accurately with WFs, then their maps will agree. But that doesn't mean the terrain had all the features of the map. Accuracy is a weaker condition than identity.

Consider an analogy with relativity. There is a an objective terrain of objects with locations and momenta, but to represent it an observer must supply a coordinate system which is not part of the territory.

I am starting to get confused by RQM, I really did not get the impression that this is what was claimed. But suppose it is.

To stick with the analogy of relativity, great efforts have been made there to ensure that all important physical formulas are Lorentz-invariant, i.e. do not depend on these artificial coordinate system. In an important sense the system does not depend on your coordinates, although for actual calculations (on a computer or something) such coordinates are needed. So while (General) Relativity indeed satisfies the last line you gave, it also explains exactly how (un)necessary such coordinate systems are, and explains exactly what can be expected to be shown without choosing a coordinate system.

Back to RQM. Here this important explanation of which observables are still independent of the observer(/initial frame) and which formulas are universal are painfully absent. It seems that RQM as stated above is more of an anti-prediction - we accept that each observer can accurately describe his experimental outcomes using QM, and different observers agree with eachother because they are looking at the same territory, hence they should get matching maps, and finally we reject the idea that these observer-dependent representations can be combined to one global representation.

Again I stuggle to combine this method of thought with the fact that humans themselves are made of atoms. If we assume that wavefunctions are only very useful tools for predicting the outcomes of experiments, but the actual territory is not made of something that would be accurately represented by a wavefunction, I run into two immediate problems:

1) In order to make this belief pay rent I would like to know what sort of thing an accurate description of the universe would look like, according to RQM. In other words, where should we begin searching for maps of a territory containing observers that make accurate maps with QM that cannot be combined to a global map?

2) What experiment could we do to distinguish between RQM and for example MWI? If indeed multiple observers automatically get agreeing QM maps by virtue of looking at the same territory, then what experiment will distinguish between a set of knitted-together QM maps and an RQM map as proposed by my first question? Mind you, such experiments might well exist (QM has trumped non-mathy philosophy without much trouble in the past), I just have a hard time thinking of one. And if there is no observable difference, then why would e favour RQM over the stiched-together map (which is claiming that QM is universal, which should make it simpler than having local partial QM with some other way of extending this beyond our observations)?

My apologies for creating such long replies, summarizing the above is hard. For what it's worth I'd like to remark that your comment has made me update in favour of RQM by quite a bit (although I still find it unlikely) - before your comment I thought that RQM was some stubborn refusal to admid that QM might be universal, thereby violating Occam's Razor, but when seen as an anti-prediction it seems sorta-plausible (although useless?).

By the way, your complaint here...

To stick with the analogy of relativity, great efforts have been made there to ensure that all important physical formulas are Lorentz-invariant, i.e. do not depend on these artificial coordinate system. In an important sense the system does not depend on your coordinates, although for actual calculations (on a computer or something) such coordinates are needed. So while (General) Relativity indeed satisfies the last line you gave, it also explains exactly how (un)necessary such coordinate systems are, and explains exactly what can be expected to be shown without choosing a coordinate system.

Back to RQM. Here this important explanation of which observables are still independent of the observer(/initial frame) and which formulas are universal are painfully absent

..is echoed by no less than Jaynes:-

The title is taken from a passage of Jaynes [2], presenting the current quantum mechanical formalism as not purely epistemological; it is a peculiar mixture describing in part realities of Nature, in part incomplete human information about Nature – all scrambled up by Heisenberg and Bohr into an omelette that nobody has seen how to unscramble

http://arxiv.org/abs/1206.6024

RQM may not end in an I, but it is still an interptetation.

What the I in MWI means is that it is an interpretation, not a theory, and therefore neither offers new mathematical apparatus, nor testable predictions.

and finally we reject the idea that these observer-dependent representations can be combined to one global representation.

Not exactly, RQM objects to observer independent state. You can have global state, providing it is from the perspective of a Test Observer, and you can presumably stitch multiple maps into such a picture.

Or perhaps you mean that if you could write state in a manifestly basis-free way, you would no longer need to insist on an observer? I'm not sure. A lot of people are concerned about the apparent disappearance of the world in RQM. There seems to be a realistic and a non realistic version of RQM. Rovellis version was not realistic, but some have added an ontology of relations.

In other words, where should we begin searching for maps of a territory containing observers that make accurate maps with QM that cannot be combined to a global map?

its more of a should not than a cannot.

2) What experiment could we do to distinguish between RQM and for example MWI?

Well, we can't distinguish between MWI and CI, either.

Just because something is called an 'interpretation' does not mean it doesn't have testable predictions. For example, macroscopic superposition discerns between CI and MWI (although CI keeps changing its definition of 'macroscopic').

I notice that I am getting confused again. Is RQM trying to say that reality via some unknown process the universe produces results to measurements, and we use wavefunctions as something like an interpolation tool to account for those observations, but different observations lead to different inferences and hence to different wavefunctions?

There is nothing in Copenhagen that forbids macroscopic superposition. The experimental results of macroscopic superposition in SQUIDs are usually calculated in terms of copenhagen (as are almost all experimental results).

That's mainly because Copenhagen never specified macrsoscopic ...but the idea of an unequivocal "cut" was at the back of a lot of copenhagenists minds, and it has been eaten away by various things over the years.

So there are obviously a lot of different things you could mean by "Copenhagen" or "in the back of a lot of copenhagenist minds" but the way it's usually used by physicists nowadays is to mean "the Von Neumann axioms" because that is what is in 90+% of the textbooks.

The von Neumann axioms aren't self interpreting .

Physicists are trained to understand things in terms of mathematical formalisms and experimental results, but that falls over when dealing with interpretation. Interpretations canot be settled empirically, by definition,, and formulae are not self interpreting.

My point was only that nothing in the axioms prevents macroscopic superposition.

For some values of "wavefunction", you are going to have different observers writing different wavefunctions just because they are using different bases...that's a practical issue that's still true if you believe in, but cannot access, theOne True Basis, like a many worlder.

How are you defining territory here? If the territory is 'reality' the only place where quantum mechanics connects to reality is when it tells us the outcome of measurements. We don't observe the wavefunction directly, we measure observables.

I think the challenge of MWI is to make the probabilities a natural result of the theory, and there has been a fair amount of active research trying and failing to do this. RQM side steps this by saying "the observables are the thing, the wavefunction is just a map, not territory."

See my reply to TheAncientGeek, I think it covers most of my thoughts on this matter. I don't think that your second paragraph captures the difference between RQM and MWI - the probabilities seem to be just as arbitrary in RQM as they are in any other interpretation. RQM gets some points by saying "Of course it's partially arbitrary, they're just maps people made that overfit to reality!", but it then fails to explain exactly which parts are overfitting, or where/if we would expect this process to go wrong.

natural result of the theory

To my very limited understanding, most of QM in general is completely unnatural as a theory from a purely mathematical point of view. If that is actually so, what precisely do you mean by "natural result of the theory"?

Actually most of it is quite natural, QM is the most obvious extension that you get when you try to extend the concept of 'probability' to complex numbers, and there are some suggestions why you would want to do this (I think the most famous/commonly found explanation is that we want 'smooth' operators, for example if turning around is an operator there should also be an operator describing 'half of turning around', and another for '1/3 of turning around' etc., which for mathematical reasons immediately gives you complex numbers (try flipping a sign in two identical steps, this is the same as multiplying by i)).

To my best knowledge the question of why we use wavefunctions is a chicken-and-the-egg type question - we want square integrable wavefunctions because those are the solution of Schrodingers equation, we want Schrodingers equation because it is (almost) the most general Hermitian time-evolution operator, time-evolution operators should be Hermitian because that is the only way to preserve unitarity and unitarity should be preserved because then the two-norm of the wavefunction can be interpreted as a probability. We've made a full circle.

As for your second question: I think a 'natural part of the theory' is something that Occam doesn't frown upon - i.e. if the theory with the extra part takes a far shorter description than the description of the initial theory plus the description of the extra part. Informally, something is 'a natural result of the theory' if somehow the description for the added result is somehow already partly specified by the theory.

Again my apologies for writing such long answers to short questions.

Thank you, that was certainly insightful. I see now that it is some kind of natural extension of relevant concepts.

I have been told however that from a formal point of view a lot of QM (maybe they were talking only about QED) makes no sense whatsoever and the only reason why the theory works is because many of the objects coming up have been redefined so as to make the theory work. I don't really know to what extent this is true, but if so I would still consider it a somewhat unnatural theory.

I've since decided to not argue about what is and isn't in the territory, given how I no longer believe in the territory.

What's B? A many-worlds counterpart of A? Another observer enitrely?

In rQM, if one observer measures X to be in state 1, no other observer can disagree (How may times do I have to point that out?). But they can be uiniformed as to what state it is -- ie it is superposed for them.

Okay. Name a state of affairs that could correspond to RQM without being MWI.

Easy: no observer-independent state. No contradictory observations. No basis problem.

(Of course that isn't an empirical expectation-predicting difference, and of course there is no reason it should be, since interpretations are not theories).

"Quantum state is in the territory" versus "state is just model"

"Universal quantum state is a coherent notion" versus "universal quantum state cannot be correctly defined"

"We need to get a universal basis from somewhere" versus "we don't"

Etc, etc.

That is not a state of affairs, it is a list of questions you aren't trying to answer. I am asking for a concrete description of how the universe could possibly be that would correspond to RQM being true and MWI being false.

It isn't a list of questions, it is a list of assertions about state of the state of the universe made by rQM paired with differing ones made by MWI. If you can spot the MWI ones, you can figout the rQM ones. If you can't, Ill pull out the rQM ones:

There is no universal state.

There is universal basis.

State is a observer's map,

"Collapse" is receipt of information by an observer, not an objective process.

There is an ontology of relations.

Observers cannot disagree about information, but can have different levels of information.

"There is no universal state." is barely an assertion about the state of the universe. Okay, there's no "universal state". What is there instead? I can't write a simulation of a universe with "no universal state" without further information.

Some people consider it a good form to back up your accusations with examples, facts and proofs, even when discussing topics dear to their hearts. Give it a try some time.

I am disappointed that this move was validated with compliance.

To be fair, I should have pointed out what I meant, and I didn't:

bad amateur incoherent epistemology

That's three adjectives in a row with a negative connotation. In a reasonably rational discourse one would expect some comparative discussion of epistemology in both interpretations and pointing relative strength and weaknesses of each.

RQM is MWI in denial

This requires showing that RQM is a subset of MWI, so it's a repetition of the original statement, only with some extra derision.

RQM is merely playing semantic word-games with the notion of reality

How would you phrase it in a neutral way?

RQM's epistemology is drunk and needs to go home and sleep it off

That's just insults, surely not the best way to get your point across.

To be fair, my reply had some of the same faults:

Give it a try some time.

This was quite unfair of me. Most of your writings do have a good number of "examples, facts and proofs", as well as eloquence and lucidity. The problem arises when you get annoyed or frustrated, which is only human.

No, I understood what you meant. Otherwise I wouldn't have taken a shot at complying. Really RQM deserves its own post carefully dissecting it, but I may not have time to write it.

A very quick but sufficient refutation is that the same math taken as a description of an objectively existing causal process gives us MWI, hence there is no reason to complicate our epistemology beyond this to try to represent RQM, even if RQM could somehow be made coherent within a more complicated ontology that ascribed primitive descriptiveness to ideas like 'true relative to'. MWI works, and RQM doesn't add anything over MWI (not even Born probabilities).

A very quick but sufficient refutation is that the same math taken as a description of an objectively existing causal process gives us MWI, hence there is no reason to complicate our epistemology beyond this

Or MWI could be said to be complicating the ontology unnecessarily. To be sure, rQM answers epistemologically some questions that MWI answers ontologically, but that isn't obviously a Bad Thing. A realistitc interpretation of the WF is a postive metaphyscial assumption, not some neutral default. A realistic quantum state of the universe is a further assumption that buys problems other interpretations don't have.

RQM doesn't add anything over MWI

rQM subtracts objective state and therefore does not have MWI's basis problem.

I tend to agree with you. As I said before, to me RQM to MWI is what "shut up and calculate" is to Copenhagen. Unfortunately, I have a feeling that I am missing some important point Eliezer is making (he tends to make important points, in my experience). For example, in the statement

a description of an objectively existing causal process gives us MWI, hence there is no reason to complicate our epistemology beyond this to try to represent RQM

I do not understand where, in his opinion, RQM adds a complication to (what?) epistemology.

Instead of having causal processes which are real, we now need causal processes which are 'real relative to' other causal processes. To prevent the other worlds from being real enough to have people inside them, we need to insist very loudly that this whole diagram of what is 'real relative to' other things, is not itself real. I am not clear on how this loud insistence can be accomplished. Also, since only individual points in configuration space allow one particle to say that another particle is in an exact position and have this be 'real', if you take a blob of amplitude large enough to contain a person's causal process, you will find that elements of a person disagree about what is real relative to them...

...and all these complications are just pointless, there's no need for our ontology to have a notion like 'real relative to' instead of just talking about causes and effects. RQM doesn't even get any closer to explaining the Born probabilities, so why bother? It's exactly like a version of Special Relativity that insists on talking about 'real lengths relative to' instead of observer-invariant Minkowskian spacetime.

My best guess at the lack of agreement here is the difference in yours and mine ontology at a rather basic level. Specifically, your ontology seems to be

Since my expectations sometimes conflict with my subsequent experiences, I need different names for the thingies that determine my experimental predictions and the thingy that determines my experimental results. I call the former thingies 'beliefs', and the latter thingy 'reality'.

whereas mine does not have "the thingy that determines my experimental results" and treats these results as primitive instead. As a consequence, everything is a model ("belief"), and good models predict experimental results better. So there is no need to use the term "real" except maybe as a shorthand for the territory in the map-territory model (which is an oft useful model, but only a model).

You can probably appreciate that this ontological difference makes statements like

since only individual points in configuration space allow one particle to say that another particle is in an exact position and have this be 'real', if you take a blob of amplitude large enough to contain a person's causal process, you will find that elements of a person disagree about what is real relative to them...

where the term "real" is repeated multiple times, lose meaning if one only cares about making accurate models.

Now, I cannot rule out that your ontology is better than my ontology in some sense of the term "better" acceptable to me, but that would be a discussion to be had first, before going into the interpretational problems of Quantum Mechanics. I can certainly see how adopting your ontology of objective reality may lead one to dislike RQM, which evades pinning down what reality is in the RQM view. On the other hand, you can probably agree that removing objective reality from one's ontology would make MWI an unnecessary addition to a perfectly good model called relational quantum mechanics.

This sounds like 'shut up and calculate' to me. After applying "shut up and calculate" to RQM the results are identical to the results of applying "shut up and calculate" to MWI, so there's no reason to claim that you're shutting up about RQM instead of shutting up about MWI or rather just shutting up about quantum mechanics in general, unless you're not really shutting up. To put it another way, there is no such thing as shutting up about RQM or MWI, only shutting up about QM without any attempt to say what underlying state of affairs you are shutting up about.

If that's not what you mean by denying that you intend to talk about a thingy that generates your experimental results and treating the results as primitive, please explain what that was supposed to say.

First, I think that we agree that 'shut up and calculate' reflects the current unfortunate state of affairs, where no other approach is more accurate despite nearly a century of trying. It postulates the Born rule (measurement results in projection onto an eigenstate), something each interpretation also postulates in one form or another, where the term "measurement" is generally understood as an interaction of a simple transparent ( = quantum) system with a complex opaque ( = classical) one. The term decoherence describes how this simple system becomes a part of the complex one it interacts with (and separates from it once the two stop interacting).

Now, I agree that

applying "shut up and calculate" to RQM the results are identical to the results of applying "shut up and calculate" to MWI, so there's no reason to claim that you're shutting up about RQM instead of shutting up about MWI or rather just shutting up about quantum mechanics in general, unless you're not really shutting up.

And indeed I'm not shutting up, because the quantum-classical transition is a mystery to be solved, in a sense that one can hopefully construct a more accurate model (one that predicts new experimental results, not available in "shut up and calculate").

The question is, which are the more promising avenues to build such a model on. RQM suggests a minimal step one has to take, while MWI boldly goes much further, postulating an uncountable (unless limited by the Planck scale) number of invisible new worlds appearing all the time everywhere, without explaining the mysterious splitting process in its own ontology (how does world splitting propagate? how do two spacelike-separated splits interact?).

Now, I am willing to concede that some day some extension of MWI may give a useful new testable prediction and thus will stop being an 'I'. My point is that, unless you postulate reality as ontologically fundamental, MWI is not the smallest increment in modeling the observed phenomenon of the quantum-classical transition.

No approach is ever more accurate than 'shut up and calculate'. The 'Shut up and calculate' version of Special Relativity, wherein we claim that Minkowski's equations give us classical lengths but refuse to speculate about how this mysterious transition from Minkowski intervals to classical lengths is achieved, is just as accurate as Special Relativity. It's just, well, frankly in denial about how the undermining of your intuition of a classical length is not a good reason to stick your fingers in your ears and go "Nah nah nah I'm not listening" with respect to Minkowski's equations representing physical reality, the way they actually do. You believe this with respect to Special Relativity, and General Relativity, and every other "shut up and calculate" version of every physical theory from chemistry to nuclear engineering - that there's no reason to shut up with respect to these other disciplines. I just believe it with respect to quantum mechanics too.

there's no reason to shut up with respect to these other disciplines. I just believe it with respect to quantum mechanics too.

So do I, and have stated as much. Not sure where the misunderstanding is coming from.

You ought to, however, agree that QM is special: no other physical model has several dozens of interpretations, seriously discussed by physicists and philosophers alike. This is an undisputed experimental fact (about humans, not about QM).

What is so special about QM that inspires interpretations? Many other scientific models are just as counter-intuitive, yet there is little arguing about the underlying meaning of equations in General Relativity (not anymore, anyway) or in any other model. To use your own meta-trick, what is it so special about the Quantum theory (not about the quantum reality, if you believe in such) that inspires people to search for interpretations? Maybe if we answer this reasonably easy cognitive science question first, we can then proceed to productively discuss the merits of various interpretations.

You ought to, however, agree that QM is special: no other physical model has several dozens of interpretations, seriously discussed by physicists and philosophers alike. This is an undisputed experimental fact (about humans, not about QM).

Perhaps you mean the sheer quantity is so great. But there have been, an are, disputes about classical pysyics and relativity. Some of them have been resolved by just beiieving the theory and abandoning contrary intuitions. At one time, atoms were dismissed as a "mere calculational device". Sound familiar?

Sure, every new theory is like that initially. But it only takes a short time for the experts to integrate the new weird ideas, like relative spacetime, or event horizons, or what have you. There is no agreement among the experts about the ontology of QM (beyond the undisputed assertion that head-in-the-sand "shut up and calculate" works just fine), and it's been an unusually long time. Most agree that the wave function is, in some sense, "real", but that's as far as it goes. So the difference is qualitative, not just quantitative. Simply "trusting the SE" gives you nothing useful, as far as the measurement is concerned.

"shut up and calculate" works just fine

It doesn't work "fine", or at all, as an interpretation. It's silent as to what it means.

There is no agreement among the experts about the ontology of QM (b

There are slowly emerging themes, such as the uselessness of trying to recover classical physics at the fundamental level, and the importance of decoherence.

Simply "trusting the SE" gives you nothing useful, as far as the measurement is concerned.

I don't see what you mean by that. An interpretation that says "trust the SE" (I suppose you mean "reify the evolution of the WF according to the SE") won't give you anything results-wise, because its an interpretation

It doesn't work "fine", or at all, as an interpretation.

Uh, no. It's not an interpretation (i.e. "explanation"), it's an explicit refusal to interpret the laws.

Anyway, time to disengage, we are not converging.

Most agree that the wave function is, in some sense, "real", but that's as far as it goes

Yeah. Note also that if you are observing a probability distribution, that doesn't imply that something computed the probability density function. E.g. if you observe random dots positions of which follow Gaussian distribution, that could be count of heads in a long string of coin tosses rather than Universe Machine really squaring some real number, negating result, and calculating an exponent.

There's certainly one obvious explanation which occurs to me. There being a copy of you in another universe seems more counterintuitive than needing to give up on measuring distances, so it's getting more like the backlash and excuses that natural selection got, or that was wielded to preserve vitalism, as opposed to the case of Special Relativity. Also the simple answer seems to have been very hard to think of due to some wrong turns taken at the beginning, which would require a more complex account of human cognitive difficulty. But either way it doesn't seem at all unnatural compared to backlash against the old Earth, natural selection, or other things that somebody thought was counterintuitive.

You need to realize that the "simple answer" isn't so simple- no one has been able to use the axioms for many worlds to make an actual calculation of anything. By kicking away the Born amplitudes, they've kicked away the entire predictive structure of the theory. You are advocating that physicists give up the ability to make predictions!

Its even worse when you go to quantum field theories and try to make many worlds work- the bulk of the amplitude will be centered on "world's" with undefined particle number.

the simple answer seems to have been very hard to think of

You mean that "simple answer" that still can't make predictions?

What is so special about QM that inspires interpretations?

Cat neither dead nor alive until you open the box?

Yeah, that's pretty special, but why?

On a related note, in MWI there is an uncountable number of worlds with the cat is in various stages of decay once the box is open. Is that weird or what.

Yeah, that's pretty special, but why?

You're asking exactly what it is about a theory which speaks of unobserved cats as dwelling in existential limbo, that would inspire people to seek alternatives?

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