# If Many-Worlds Had Come First

**Followup to**: Collapse Postulates, Decoherence is Simple, Falsifiable and Testable

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

Macroscopic decoherence—the idea that the known quantum laws that govern microscopic events, might simply govern at all levels without alteration—also known as "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 exist* until 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 *that* point 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."

Part of *The Quantum Physics Sequence*

Next post: "Many Worlds, One Best Guess"

Previous post: "Collapse Postulates"

## Comments (179)

Old> may now be the majority viewpoint (or not).

Or both at the same time?

I hope the following isn't completely off-topic:

What exactly does a hypothetical scenario where "person X was born Y years earlier" even look like? I could see a somewhat plausible interpretation of that description in periods of extremely slow scientific and technological progress, but the twentieth century doesn't qualify. In the 1920s: 1) The concept of a turing machine hadn't been formulated yet. 2) There were no electronic computers. 3) ARPANET wasn't even an idea yet, and wouldn't be for decades. 4) Television was a novelty, years away from being used by a significant number of people. 5) WW1 was recent history.

Two persons with the same DNA and, except for results of global changes, very similar local environments during their childhood, would most likely turn into completely different adult humans if one of them was born in the 1920s and the other at some point in the last 30 years (roughly chosen to guarantee exposure to the idea of the internet as a teenager), and they both grew up in industrialized countries. The scientific and technological level one is born into is critical for mind development. What does it mean to consider a hypothetical world where a specific person was born into an environment very different in those respects? Why is this worth thinking about?

What if it had seemed that there was no way to get the Born rule with just simple decoherence - what if that seemed to clearly imply a uniform probability rule. Would the random collapse view seem more plausible then?

What if it had seemed that there was no way to get the Born rule with just simple decoherence - what if that seemed to clearly imply a uniform probability rule. Would the random collapse view seem more plausible then?No. Eight strikes and it's out. There is no possible reason for adopting a theory that unphysical, or even spending more than thirty seconds thinking about it, without crushingly unmistakable experimental evidence that nails it down.

If you're postulating new fundamental physics, things that don't show up microscopically but do show up macroscopically, to explain the Born statistics, there would be a hundred better possibilities that

don'tviolate Special Relativity.One thing you're currently having trouble explaining is not an excuse to import magic out of nowhere and say, "Oh,

thatmust be the explanation." Doesn't work for intelligent design and it doesn't work for collapse either.If the Born rule comes from decoherence, and if decoherence comes from the SWE, the Born rule comes from what you would class as acceptable physics. In fast, since the Born rule is part if what makes QM work, any MWI type theory must justify it. You replied as tthough you read "the Born rule " as "mysterious nonlocal collapse process". The Born rule is just a piece of maths,

If MWI has no observable consequences, does it matter other than as a point of principle? Or are you going to get to ethical consequences, like the spaceship that doesn't disappear when it passes the horizon?

I'm surprised by the last sentence. Politics is the mind-killer, and all that.

Correct me if I am wrong, but MWI does have noticeable consequences, or at least implications: for example, interference at all length-scales and proper evaluation of the waveform equations implying the Born probabilities. Neither of these are implicit in the Copenhagen interpretation - in fact, the first is contradicted.

03:16 was me - curse you, Typepad!

If there really are consequences of one of the hypotheses that differ from the consequences of others, that is extremely important to know.

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?

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.

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:

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.

*6 points [-]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:

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:

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.

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.

*0 points [-]Tell me what the basis is, and where it comes from, and I will...

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

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.

"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?

You lost me here, sorry.

*1 point [-]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

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.

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.

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?).

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."

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"?

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.

*0 points [-]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.

*1 point [-]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.

Definitions can be wrong.

I understand that well-meaning physics professor may have once told you that. However the various quantum mechanics interpretations

doin 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.)*0 points [-]A real testable difference between QM interpretations is a Nobel-worthy Big Deal<tm>. I doubt it will be coming.

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.

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

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.

*1 point [-]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).

*0 points [-]"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.

Or here's another way of looking at it:

MWI = Minkowskian spacetime. Clear objective state of affairs, observer-invariant intervals separating events.

Single-world QM = Pre-Minkowski mysterious "Lorentz contractions" as a result of moving through the ether. The ether seems mysteriously unobservable and it's really odd that the Lorentz contractions just happen to be exactly right to make motion undetectable, when in principle the ether could be doing anything (just like it's mysterious that the worldeater eats off parts of the wavefunction according to the Born probabilities rather than something else, and only leaves one world behind). Also, since you don't know about the Lorentz transformation for time at this point in the history of physics, your equations will yield the wrong answers for extreme circumstances (just as a large enough quantum computer could contain observers who still wouldn't collapse).

"Shut up and calculate" = Use Minkowskian spacetime but refuse to admit that your equations might refer to something.

RQM = Relational Special Relativity = You repeatedly talk about how "motion" can only be defined relative to an observer, and it's impossible for the universe as a whole to move because it would have to be moving relative to something; you use this to insist that every observer has their private reality in which objects

really aremoving at a certain rate relative to them, and timereally isprogressing at a certain rate, and there's no conflict with other observers and their observed rates of motion because reality is not objective. If anyone shows you Minkowskian spacetime and asks why they should adopt your weird epistemology when there's all these perfectly natural invariants to use, or asks you what it would even mean for everyone to have a private reality, yell at them that the universe as a whole clearly can't have an objective state of motion because there's nothing else it could be moving relative to. Basically, Special Relativity only you'd rather give up the attempt to describe a coherent state of affairs than give up on talking separately about space and time the way you're accustomed to.(If that didn't make sense check SEP or Wikipedia on RQM.)

*-2 points [-]Reversing the direction of the analogy, what are the "invariants" of MWI? A natural, emergent multiversal basis? nah. A natural, emergent Born's law? Nah...

That's actually a perfectly reasonable argument.

rQM is coherent, observers can't make contradictory observations. It just isn't objective. It also isn't anything-goes philosophical subjectivism. It is an interpretation that agrees with all the results of the formalism, like any interpretation properly so called, so it does not break anything or make anything unscientific.

*0 points [-]Any interpretation could be

calledsemantic word game, since the whole point is tointerpreta mathematical formalism. To do that you have to usewords(shock!) and discuss what things might really mean (horror!).Why is there so much effort spent on philosophical interpretations of QM, when there probably will be more fundamental levels of description such as string theory?

Is it to be expected that the least complex interpretation of QM will also apply to the one-day victorious string theory model?

It would be unlikely for any more fundamental theory not to be subject to the same set of evasions as QM. Roughly, we have people claiming that atoms are just theoretical figments of the imagination which merely yield good predictions, discovering neutrons isn't going to change their arguments. String theory in particular doesn't help.

*4 points [-]I once asked a QM person (who shall remain nameless) why people argue about interpretations despite their untestability, and (s)he conjectured that what they are

reallyarguing about is ramifications of these interpretations for "hard problems" (e.g. consciousness) which was an answer that surprised me.It is written: a physicist does not live on instrumentalism alone.

The way that we currently build theories in physics is to write down a classical theory, and then 'quantize it' (which involves replacing classical numbers with operators and enforcing some non-commutation. Or it involves promoting the idea that the action is extremized with a path integral over the action). String theory is no exception, you typically start with a classical string-action.

Because of this, most of the underlying structure of quantum mechanics comes along for the ride. Unfortunately, this usually leads to formal problems (no one has yet developed a satisfying axiomatic quantum field theory, and the situation in string theory is even worse), but physicists ignore these issues, because such theories, while not formally developed, make the right predictions.

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.

*0 points [-]If you take decoherence realistically, you get something like MWI. CH is different because, like, CI, it is less realistic.

Bravo, Eliezer, bravo. Have you sold the screen rights yet?

Inspired by this post, I was reading some of the history today, and I learned something that surprised me: in all of his writings, Bohr apparently never once talked about the "collapse of the wavefunction," or the disappearance of all but one measurement outcome, or any similar formulation. Indeed, Huve Erett's theory would have struck the historical Bohr as complete nonsense, since Bohr didn't believe that wavefunctions were real in the first place -- there was nothing to collapse!

So it might be that MWI proponents (and Bohmians, for that matter) underestimate just how non-realist Bohr really was. They ask themselves: "what would the world have to

be likeif Copenhagenism were true?" -- and the answer they come up with involves wavefunction collapse, which strikes them as absurd, so then that's what they criticize. But the whole point of Bohr's philosophy was that you don't evenasksuch questions. (Needless to say, this is not a ringing endorsement of his philosophy.)Incidentally, I'm skeptical of the idea that MWI never even

occurredto Bohr, Heisenberg, Schrรถdinger, or von Neumann. I conjecture that something like itmusthave occurred to them, as an obviousreductio ad absurdum-- further underscoring (in their minds) why one shouldn't regard the wavefunction as "real". Does anyone have any historical evidence either way?I think you're being a bit hard on Schrรถdinger here. I thought the whole point of Schrรถdinger's cat was to point out that the "observers cause collapse" idea was kind of stupid.

The "One Christers" are a nice SF touch.

Nice one Eli, I haven't been able to read OB for about a month, and whith your breakneck pace it was tough to catch up, but this has been good. I enjoyed this post in particular!

Schrodinger killed his cat.

First, W Bush was just 11 in 1957. However, that does make me wonder over what fraction of the many-worlds he ended up being an idiotic asshole -- much less President now... And, wow, imagine the possible alternate world where he was a good President!

Second, though I generally liked your post, I feel it was a bit disingenuous to not mention the hidden variable hypothesis in regard to the Copenhagen interpretation. Early 20th century physicists weren't thinking collapse was an extraordinary violation of know physics -- they thought it was a temporarily opaque -- and deceptively random in appearance -- layer on an underlying deterministic physics.

It wasn't until 1964 that the traditional interpretation started to really fall apart. And the modern split is, I suspect, largely down a deterministic/stochastic universe preference. The CI survivors are waiting for a workable replacement to hidden variable. The growth in the MWI camp is because they haven't come up with anything in the last four decades.

What the hell are the Born statistics?

Jeeves: whaaaa?

Smedly: the Born rule... the whole probability of what you seem to experience observing is proportional to the squared magnitude thing. ie, if you had a two state system, say a qbit, in a superposition of, say, 2/3*|0> + sqrt(5)*i/3*|1>, then if you take a measurement of a bunch of qbits that are independantly in that state, then you'd expect about 4/9 of them to be 0, and 5/9 of them to be 1.

Given that QM is linear, you can see why the existance of such a rule may be a bit confusing. And given the many worlds perspective, the question of "probability of... what, exactly?" is a question too. Seems hard to even phrase the rule without invoking consciousness. Thus, we, or at least I (did everyone else solve it and simply keep me out of the loop? :)) am confused on this matter.

Not really; anticipation seems easy enough to define without consciousness.

Nick: anticipation of... what? Don't misunderstand, I'm not saying "oooh, Born probabilities transcend understanding" sort of thing, I just mean that I'm unsure how, in the context of many worlds, to state it. Robin's Mangled Worlds idea, if it pans out, would certainly help, but until then, I'm stumped about how to really say it in any way other than "anticipation of experience"

Anticipation of input, which at least doesn't seem like it immediately implies conscious experience - does a minimalist Bayesian decision system feel anything?

Nick: But... what do you mean? ie, if you have some sort of decoherence event so that one can meaningfully distinguish between world with input A occuring and world with input B occuring...

What are we anticipating? ie, both input A occurs _and_ input B occurs.

If they have different amplitudes, so we square the magnitudes to figure out the anticipation... anticipation of... what? ie, _both_ outcomes occur.

Yet in some sense they well be "weighted" differently. What do we mean by that other than "how much do we anticipate experiencing one or the other?"

Again, presumably there's some way to clear this all up, but right now "anticipation of input" doesn't really seem to be reducing my confusion on the subject.

"Anticipation of input" is the same as "anticipation of experience", but without any reference to consciousness - a non-conscious Bayesian decision system should also derive the squared-modulus law, and "anticipate" (in an qualia-free way) future "observations" (again qualia-free) to follow it. (Shouldn't it?) IMO, this is actually more confusing.

Nick: presumably in same way it would... but I don't really see how. Remember, this is indexical uncertainty. It doesn't correspond to uncertainty about what actually happened so much as uncertainty about which branch of reality this version of you is in.

So... There's a version of you in A, and a version of you in B.

In A, all the computations that happen are more or less analogous to those in B, except that B uses slightly larger numbers to represent the computations...

So exactly why would that change any anticipation of anything? I'd be unsure what a nonconscious Bayesian decision system would be computing/anticipating, unless the Born rule was already hard coded into it.

Yes, presumably there's _some_ actual physical reason which gives rise to the Born statistics, and once we know that, that may even help us talk about it better. But right now, I don't really even see any obvious way to state the rule without invoking anticipation of experience.

Since both branch A and B are real... what exactly are we weighing other than something along the lines of "where is more of our consciousnes experience flowing?"

And it's really annoying to have to phrase it like that. I know I'm confused on this. But right now, I don't see any obvious way to state the rule without saying something to that effect.

Meanwhile, imagine yet another alternate Earth, where the very first physicists to notice nonlocality, said, "Holy brachiating orangutans, there's a non-local force in Nature!"

In the years since, the theory has been successfully extended to encompass every observed phenomenon. The biggest mystery in physics is the relationship between nonlocality and relativity. The basic equations have a preferred reference frame, but it's undetectable. Everyone thinks that there must be a relativistic way to write the equations, but no-one knows how to do it.

One day, Bavid Dohm walks into the office of Huve Erett...

Bavid gestures to the paper he'd brought to Huve Erett. It is a short paper. The title reads, "The Solution to the Relativity Problem". The body of the paper reads:

"There is no classical trajectory. The pilot wave already contains the world that we see, along with infinitely many others."

"Let me make absolutely sure," Erett says carefully, "that I understand you. You're saying that there is no space-time, as we know it, separate from Hilbert space. There's just the pilot wave, evolving according to the Schrodinger equation. But the pilot wave actually contains space-time - infinitely many space-times."

"Right!" says Bavid.

"Where?" says Erett.

"Everywhere throughout configuration space!" says Bavid. "The configurations

arethe worlds.""But if every possible configuration exists, how do you predict anything?" asks Erett.

"Er, well, it's not the configurations which are the worlds, then", says Bavid. "It's the blobs of amplitude hovering

overthe configurations.""I still don't see how you make predictions. Or eliminate a universal time coordinate", says Erett.

"Decoherence!" says Bavid. "If you don't count the blobs where the amplitude really thins out, then the numbers come out correctly."

"But the blobs are still there?" asks Erett.

"Yes... they're just... thinner", says Bavid.

"Why shouldn't I count them, then?" asks Erett.

"Because the numbers won't come out right otherwise!" says Bavid.

"I

see", says Erett. "And relativity? You did say this is a relativistic theory.""Yes, well, my idea is to get rid of time entirely", says Bavid.

"Ah yes, the old 'H=0' approach. The pilot wave is a

standingwave. But how is that relativistic? Relativity mingles space and time. H=0 just abolishes time and leaves space", says Erett."Er..." says Bavid.

At which point Erett politely but firmly shows Mr Dohm out of his office.

I'm a chemist; we actually have to use quantum physics on a routine basis. The main reason many-worlds never got traction is that it doesn't make a testable prediction. Most physicists realize that making a model of reality that predicts experiment (as far as possible) is, well, science; BSing about what the implications are is more of a late night and beer thing.

In other words, if the model implies that there may be other worlds, but they can't conceivably be detected, then who cares?

One last thing: there's some pretty good evidence of nonlocal physics these days. It's inconsistent with general relativity, but no biggie. We already knew that general relativity and quantum physics were incompatible. The current situation in physics (for the last 30 years or so) is considerable confusion at the level of fundamental theory, but extremely robust models for every actual physical situation that we can probe. The robustness of the models is exactly what has halted progress.

If resources (including mental ones) are being spent fighting for a less plausible theory, isn't that enough?

What are you referring to? The kind of non-locality exhibited in the EPR paradox is consistent with special relativity - or at least there's an elegant way of looking at this in which it is consistent. So are you talking about something totally different? Something incompatible with GR but

notSR? Or both?I am not sure that it is possible to interpret this sentence without admitting to what amounts to Eliezer's position. In other words, for this to be either right

orwrong, Eliezer has to be right.This sentence is most plausibly unpacked as assuming that the Copenhagen Interpretation and MWI are consistent with all findings, and that pride of place is naturally given to the first interpretation that makes predictions no other interpretation has. Science may not be wrong to, in general and as a heuristic, only accept new theories that make

betterpredictions than the old. After all, even creationism or magic faerieism can be molded to beconsistentwith all known observations, whatever they are.Eliezer simply asserts that MWI is simpler. He appeals to the Occam's razor heuristic, not the "new testable predictions" one, as reason for the reader to accept MWI. (If you caught it, MWI is making a prediction - that no quantum superposition will be too small to cause a result interpreted as a collapse under CI - but that's relatively small potatoes, since MWI is succeeding where CI is agnostic. However, that testable position isn't the point here, the non-socially scientific criteria of theoretical simplicity is.)

Eliezer says: MWI is better that CI under Occam's razor. You say: scientists care about subsequent theories having

superiortestable positions, not their being simpler under Occam's razor.Eliezer may reply: OK, there is good reason for science to work like that, since theoretically more complicated theories can always be just as predictive as previously discovered simpler ones by cheating and stealing their results, plus adding complexity, while never being more predictive. However, there is good reason to believe in the theoretically superior theory. (Perhaps he might add: also if you look closely CI is doing the cheating by looking at MWI to see when to declare a superposition.)

Ultimately, you have failed to dispute that MWI is simpler or that it is superior, and your offering a sociological explanation (CI's coming before MWI) for why CI may be more broadly accepted despite theoretical inferiority does not engage Eliezer's points in opposition, it shows the strength of one particular argument

that assumes his point: CI is accepted only because it came before MWI.*0 points [-]haha, somebody who has connections should send these posts to David Deutsch or other proponents of MWI.

It sounds like you are mocking the post, not expressing genuine amusement.

I imagine that wasn't your actual intent, though; judging tone over the internet is notoriously difficult. Your comment follows the same format as other mocking posts, so I'd avoid it - i.e. starting off with "haha", "scream with laughter", it comes off as sarcasm.

In addition to that it's badly written and not really insightful;)

Hilarious and 100% true! Thank you! The only thing I might add to this is that in Huve's theory, information is created out of nowhere.

Eliezer, don't you have a whole post about why you shouldn't use examples from politics if you can possibly avoid it?