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Eliezer_Yudkowsky comments on If Many-Worlds Had Come First - Less Wrong
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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?
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.
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.
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.
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.
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.
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 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<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.
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.
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 are moving at a certain rate relative to them, and time really is progressing 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.)
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.
Any interpretation could be called semantic word game, since the whole point is to interpret a mathematical formalism. To do that you have to use words (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.
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 really arguing 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.