Longtime readers know that I’ve made a bit of an effort to help people understand, and perhaps even grow to respect, the Everett or Many-Worlds Interpretation of Quantum Mechanics (MWI) . I’ve even written papers about it. It’s a controversial idea and far from firmly established, but it’s a serious one, and deserves serious discussion.
Which is why I become sad when people continue to misunderstand it. And even sadder when they misunderstand it for what are — let’s face it — obviously wrong reasons. The particular objection I’m thinking of is:
MWI is not a good theory because it’s not testable.
It has appeared recently in this article by Philip Ball — an essay whose snidely aggressive tone is matched only by the consistency with which it is off-base. Worst of all, the piece actually quotes me, explaining why the objection is wrong. So clearly I am either being too obscure, or too polite.
I suspect that almost everyone who makes this objection doesn’t understand MWI at all. This is me trying to be generous, because that’s the only reason I can think of why one would make it. In particular, if you were under the impression that MWI postulated a huge number of unobservable worlds, then you would be perfectly in your rights to make that objection. So I have to think that the objectors actually are under that impression.
An impression that is completely incorrect. The MWI does not postulate a huge number of unobservable worlds, misleading name notwithstanding. (One reason many of us like to call it “Everettian Quantum Mechanics” instead of “Many-Worlds.”)
Now, MWI certainly does predict the existence of a huge number of unobservable worlds. But it doesn’t postulate them. It derives them, from what it does postulate. And the actual postulates of the theory are quite simple indeed:
- The world is described by a quantum state, which is an element of a kind of vector space known as Hilbert space.
- The quantum state evolves through time in accordance with the Schrödinger equation, with some particular Hamiltonian.
That is, as they say, it. Notice you don’t see anything about worlds in there. The worlds are there whether you like it or not, sitting in Hilbert space, waiting to see whether they become actualized in the course of the evolution. Notice, also, that these postulates are eminently testable — indeed, even falsifiable! And once you make them (and you accept an appropriate “past hypothesis,” just as in statistical mechanics, and are considering a sufficiently richly-interacting system), the worlds happen automatically.
Given that, you can see why the objection is dispiritingly wrong-headed. You don’t hold it against a theory if it makes some predictions that can’t be tested. Every theory does that. You don’t object to general relativity because you can’t be absolutely sure that Einstein’s equation was holding true at some particular event a billion light years away. This distinction between what is postulated (which should be testable) and everything that is derived (which clearly need not be) seems pretty straightforward to me, but is a favorite thing for people to get confused about.
Ah, but the MWI-naysayers say (as Ball actually does say), but every version of quantum mechanics has those two postulates or something like them, so testing them doesn’t really test MWI. So what? If you have a different version of QM (perhaps what Ted Bunn has called a “disappearing-world” interpretation), it must somehow differ from MWI, presumably by either changing the above postulates or adding to them. And in that case, if your theory is well-posed, we can very readily test those proposed changes. In a dynamical-collapse theory, for example, the wave function does not simply evolve according to the Schrödinger equation; it occasionally collapses (duh) in a nonlinear and possibly stochastic fashion. And we can absolutely look for experimental signatures of that deviation, thereby testing the relative adequacy of MWI vs. your collapse theory. Likewise in hidden-variable theories, one could actually experimentally determine the existence of the new variables. Now, it’s true, any such competitor to MWI probably has a limit in which the deviations are very hard to discern — it had better, because so far every experiment is completely compatible with the above two axioms. But that’s hardly the MWI’s fault; just the opposite.
The people who object to MWI because of all those unobservable worlds aren’t really objecting to MWI at all; they just don’t like and/or understand quantum mechanics. Hilbert space is big, regardless of one’s personal feelings on the matter.
Which saddens me, as an MWI proponent, because I am very quick to admit that there are potentially quite good objections to MWI, and I would much rather spend my time discussing those, rather than the silly ones. Despite my efforts and those of others, it’s certainly possible that we don’t have the right understanding of probability in the theory, or why it’s a theory of probability at all. Similarly, despite the efforts of Zurek and others, we don’t have an absolutely airtight understanding of why we see apparent collapses into certain states and not others. Heck, you might be unconvinced that the above postulates really do lead to the existence of distinct worlds, despite the standard decoherence analysis; that would be great, I’d love to see the argument, it might lead to a productive scientific conversation. Should we be worried that decoherence is only an approximate process? How do we pick out quasi-classical realms and histories? Do we, in fact, need a bit more structure than the bare-bones axioms listed above, perhaps something that picks out a preferred set of observables?
All good questions to talk about! Maybe someday the public discourse about MWI will catch up with the discussion that experts have among themselves, evolve past self-congratulatory sneering about all those unobservable worlds, and share in the real pleasure of talking about the issues that matter.
@Jacob Eliosoff, But “Under Copenhagen collapse happens; under MWI it doesn’t” are arguably both false. The former is false if “collapse” is required to be something other than a convenient boundary condition that could be perfectly well explained by including the apparatus and its interaction with the system as part of a larger system, and the latter by virtue of the theorems of Everett (and others) regarding what the measurement process would “look like” to the observed observer.
So far as I can see, Everett’s “relative state” results are just an elaboration of von Neumann’s – with perhaps more generality and a more complete accounting of how requiring the consistency of nested observations leads to the usual probabilistic interpretation of the experience of each observer. So I don’t think anyone would disagree with his claim that “The results of the present ‘relative state’ formalism agree with those of the conventional ‘external observation’ formalism in all those cases where that familiar machinery is applicable…When interaction occurs, the result of the evolution in time is a superposition of states, each element of which assigns a different state to the memory of the observer. Judged by the state of the memory in almost all of the observer states, the probabilistic conclusion of the usual ‘external observation’ formulation of quantum theory are valid. ”
Two points where I quarrel with Everett are in his objection to the “indefinite behaviour” of von Neumann’s account on the grounds that “physical objects always appear to us to have definite positions” (which is surely *not* the case if we want to have any idea of their momenta!), and in his apparent insistence that a superposition can in some meaningful sense be interpreted as multiple realities “existing” at once.
The only real controversy about relative state analysis of quantum systems is with regard to its characterization as a “Many Worlds Interpretation”. And that characterization is, I think, legitimately criticized as not having any testable consequences.
I find your post sort of disingenuous Carroll. Not too long ago you borderline mocked people for not accepting MWI, since then you have come to realize that there exist real technical problems for MWI and has toned it down a little. Which is good, but you still refuse to actually discuss these flaws, instead you want to spend energy on the people mocking MWI.
When you have a proper answer to the preferred basis problem, emergent ontology problem and Born Rule problem you should make a blog post about it.
Pingback: Many Quantum Interpretations | Ajit Jadhav's Weblog
Pingback: The many-worlds interpretation of quantum mechanics
It really takes a special kind of person to believe in MWI. I get what Sean is saying. People don’t want to believe in the MWI, because it sounds to extraordinary. Then isn’t Sean just believing in the MWI, because the Copenhagen Interpretation sounds too extraordinary as well? It’s just a logical trap that a lot of physicist fall into that learn about how non-classical the Copenhagen Interpretation really is. They would just rather believe in the MWI than admit that quantum mechanics is truly non-classical!
I don’t have a problem accepting the Copenhagen Interpretation, so I don’t feel a need to believe in the MWI. That mostly just comes from a firm belief in The Special Theory of Relativity. The theory breaks down when objects travel the speed of light. It just happens that quantum uncertainty is more prevalent when it comes to particles traveling the speed of light, and it is negligible for objects with mass that don’t. Then if it is impossible to describe a particle accurately when it is traveling the speed of light using The Special Theory of Relativity and the theory itself breaks down, then if it was a truly accurate theory it would mean that classical physics as we know it would break down. Therefore, quantum mechanics would then have to be truly non-classical!
Example; say an object is traveling the speed of light at a constant speed. It is traveling at a constant speed so it assumes that it has a velocity of zero. Then from that frame everything else is traveling the speed of light. Everything contracts to zero. Then it would appear to the object that it is everywhere at once. Then it would seem like it had infinite speed, but from the other frame of reference it is only traveling at about 300,000 km/s! The theory breaks down. The two observers do not agree on a single state of reality. Then it just so happens that particles traveling the speed of light can be measured to travel at 300,000 km/s, and those particles can also have an action at a distance instantaneously through things like entanglement at the same time! Then the solution is simple. Both states of realities are actually happening at the same time, and as a direct consequence we observe this type of non-classical behavior! Particles that travel the speed of light can also have an action at a distance that is instantaneous at the same time.
This “theory” does not need the Occam resor,it can be cut by the axe.
Rereading the article, the whole attack can be reduced to this one point:
“‘It’s trivial to falsify [MWI],’ boasts the Caltech cosmologist Sean Carroll, another supporter: ‘just do an experiment that violates the Schrödinger equation or the principle of superposition, which are the only things the theory assumes.’ But most other interpretations of quantum theory assume them (at least) too – so such an experiment would rule them all out, and say nothing about the special status of the MWI.”
Most quantum textbooks use the same propositions or an equivalent (sometimes weaker) set as the ones you claim. I think it would disarm your opponents if you provided an analytic argument for your claim. Analytically deriving the consequences unique to MWI from the minimal set of propositions necessary will make your claim immune from semantic attacks like this one. Without any formal argument/deduction, there isn’t really anything to argue rationally here.
I don’t get the Schrodinger’s Cat experiment. Until I look in the box, the cat is both dead and alive. What if, after 50 years, no one has looked inside the box? Can’t we be pretty sure, without looking, that the cat has died of old age?
Pingback: Multiverse in action – Quantum Bot
The way that I am going to understand this for the time being is that the universe is quantumly ringing like a bell. The only reason why our world seems finitely defined is because of our observational time and space frame. Every element within the universe is also “ringing”, so at any one instant in time the variation of one distant element to another could be quite large, and any one snapshot of the universe will be uniquely different to another from an entangled perspective yet not so much from an external on.
In this way I understand the Everetian notion of many worlds. As the universe oscillates or rings what we believe the universe to be is the most commonly observed universe of the many possible universes from our observational perspective.
Many wave functions are infinite in size; probability drops effectively to zero but never truly to zero. Copenhagen implies that my cars keys have a non-zero probability of tunneling to Mars (or anywhere in the universe), but it does not imply that this will ever happen in the lifetime of the universe. MWI implies that it does actually happen in a minority of the “almost” infinite universes being spawned at every instant. In my view, MWI does not stand up to thought experiments.
Before you destroy the world weigh all the “pros” and “cons”. But why waste time and not immediately get down to business?
“In the nature of man laid a natural desire to kill a woman. But some go further by doing more severely-marrying them” Verhaymer
In this tweet, I am pleased to list all the reasons that compel me every day to write a tweet. And So:
P091 tale of a princess who does not keep up with the times and, in desperation, to join with him in an intimate relationship
Exupery that night nightmares, as if he was not serious fetish for girls’ minds. “Do not come to this,” he thought waking up
http://rds-group.com.ua/banki/dolg-bankov-pered-cb-po-operaciiam-priamogo-repo-sostavil-2-trln-823-mlrd-7791-mln-ryb.html
http://rds-group.com.ua/biznes-i-finansu/finansyi/investbanki-staviat-na-ponijenie.html
http://rds-group.com.ua/banki/hmb-otkrytie-prokommentiroval-informaciu-o-priostanovke-vydachi-kreditov.html
http://rds-group.com.ua/banki/ekspert-ra-otozval-reiting-kreditosposobnosti-promsberbanka.html
http://rds-group.com.ua/biznes-i-finansu/it-tehnologii/predstavlen-mini-pk-zotac-zbox-pi320-pico-na-platforme-atom-z3735f.html
Sean writes:
“The quantum state isn’t just a probability distribution.”
Wouldn’t this statement be more accurate if you left out the word “just”? The quantum state isn’t a probability distribution. Period. One can obtain a probability distribution from it, but it is not itself a probability distribution. Perhaps emphasizing this fact would help eliminate some of the confusion about quantum mechanics.
Not even in one of the infinite universes are you going to get that $5 million check from the Nigerian prince to whom you wired $5K.
If MWI is correct, then Schrodinger killed a cat.