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.
Diogenes,
While I’m fully on board deflating criticisms against MWI based on personal incredulity, I’m not sure I’d agree with the title “Disappearing Worlds” for the alternatives. You are making some assumptions about how QM must be interpreted.
When describing alternative interpretations as “Disappearing Worlds” interpretations, you are tacitly assuming a density operator must be a direct ontic representation of reality, as opposed to a device for preparing probabilities associated with observable properties. Similarly, you are assuming these interpretations frame the acquisition of knowledge as some reduction of reality on an ontological level.
Alternative interpretations would reject these assumptions, and would not be subject to your accusation of disappearing worlds.
@Luke Somers
Well, well. well! ” Minimalist!” So little bit of metaphysics is ok with you in physics!
Very interesting!
@diogenes
Imagine you’ve actually never seen your twins, but you assume everyone has twins because it best explains why some people have two heads. But others prefer a different explanation.
G.K. Chesterton: “When people stop believing in God, they don’t believe in nothing they believe in anything.”
Okay, he did not say it, but someone said it in a book about him. He could not have known how literally the saying is true. Believing in MWI is believing in anything.
You’re not “too polite.” But among the scientists that write popular books, tweet, and otherwise do your best to communicate real science to those of us in other fields, you’re one of the few (the only?) that does it without being a jerk, and while having a pretty sophisticated philosophical sense. And I appreciate that tremendously. If I had to rely on somebody else (whose name may or may not rhyme with Torrance Mouse) as my doorway to physics, then I would never get past the snideness and the derision for philosophy, and probably just stop paying attention. So please keep being polite.
@Diogenes “the scientific method by definition should prefer the simpler hypothesis among those that are equally good at predicting and explaining the observations.”
Simpler hypothesis does not mean one should buy anything. I will give you the simplest hypothesis: ” God designed all phenomena using his infinite wisdom!” Then every science book will have only this sentence and everyone will get Ph. D. in one minute!
More seriously, as I complained in my first comment, MWI proponents do not define what is meant by world. If it is just Hilbert space, that does not make any sense. When was the last time you walked in the Hilbert space? Everyone would agree that it is just a mathematical technique used by quantum mechanics. If the other worlds are anything like the world we are familiar with,then surely you are talking metaphysics!
Nice article.
With the MWI it seems that in assuming that the quantum state never collapses – and dropping the postulate that allows us to calculate the probabilities of possible outcomes – the theory has then lost some predictive power which it once had. Is this correct?
How does the question of interpretation go over to QFT?
Interesting thought:
“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”
Maybe focusing in on a question like are the law of physics computable (instead of whether they are deterministic)? could help to move things in new directions.
I’m grateful to Quentin at 10.29 on the 19th – yes, of course that was indeed the main point of my article, and I’m disappointed that Sean ignored it, because I’d genuinely like to know the answer. As others have remarked here, it really seems to be a question about what we mean by “worlds” here, and I’m heartened to see some suggestions emerging that the whole “worlds” terminology should be ditched altogether, as it is impossible to make that a precisely framed question. In other words, the MWI needs a caveat that we shouldn’t and can’t conclude from it anything about real observers. That’s probably a little wiser than Justin’s confident assertion that we can put a human in a quantum superposition and then ask them afterwards how they felt about it.
Hi Sean (or anyone else who understands MWI), could you please explain something to me? Suppose I have a state |psi> = a |x> + sqrt (1-a^2)|y>, where a =1/pi or a similar irrational number, then upon experiment how many new worlds of type x and y will be created? In the copenhagen interpretation, it’s like a Monte Carlo sampling, over time the ratios will approach the sq of coefficients. Thanks,
Shmi Nux argues – convincingly to me – that Carroll’s “‘rightness’ is rather hollow because [we] still have no definitive experiment that would convince [an] opponent [of MWI].” This whole discussion is just a rehash of the decades-old debate as to the proper interpretation of QM, with approximately zero hope of resolving it by empirical evidence.
Carroll laments the public’s “uncomfortable” lack of sophistication to chime in at the level of “experts” who have been rigorously trained to engage in “issues that matter.”
A possibility being overlooked here is that the unsophisticated public’s view is nevertheless the most sensible one. The prediction of “all those unobservable worlds” strikes the amateur scientist as so many angels dancing on pinheads. Perhaps in this case, being trained to discuss such things is more of a handicap than an advantage. It could be a handicap because it may be distracting physicists from more important unresolved empirical matters.
For example, a perfectly accessible regime in gravitational physics has yet to be explored. The common myth is that General Relativity (GR) has been well-tested on scales from mm to Astronomical Units. True as this may be for the Schwarzschild exterior solution, it is not at all true for the interior solution. The interior solution predicts that the rates of clocks in the field of a uniformly dense sphere decrease from the surface to a minimum at the center. How do we know that it’s not the other way around (i.e., maximum rate at the center)?
Human beings have never tested this prediction. As though distracted by seemingly loftier issues, we have not yet empirically established the validity of one of GR’s most basic predictions inside matter – the most ponderable half of the gravitational Universe.
The clock rate prediction could be at least indirectly tested because it is possible to test the corresponding consequence in Newtonian gravity. GR’s clock rate prediction corresponds to the Newtonian prediction that a test mass dropped into a hole through the center of a larger body oscillates between the extremities of the hole.
Carroll has extolled the importance of “Testing Your Theories” because “people are not especially logical creatures, and we’re just not smart enough to gain true knowledge about the world by the power of reason alone.” (Quotes from an earlier blog post: Testing Your Theories is not a Matter of Envy. )
Maybe in one of Carroll’s “other worlds” he takes this advice to heart by promoting interest in performing the experiment described above. Galileo first proposed the idea in 1632. As suggested in the paper linked here:
http://www.gravitationlab.com/Grav%20Lab%20Links/Galileo's-Belated-Experiment.pdf
the needed apparatus may be called a Small Low-Energy Non-Collider. Why continue belaboring the debatable, practically inconsequential implications of quantum theory when some real world empirical science beckons to be done? What would Galileo do?
I have always thought of (my own experience of) the universe as corresponding to (a very small part of) one particular configuration of a stochastic system, and that having a theoretical model for that system allows me to predict conditional probabilities of certain features (measurements) given others (state preparations). I suppose other configurations could be regarded as alternate worlds which *could* in some sense exist. But why is it necessary (and in fact, what would it mean) to suggest that they *do* exist?
Oh dear! Now I feel a little Feynmanesque “poem” coming on:
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Sean,
When a measurement is taken place and the particle can be observed to be in one of two states, is it true to state that, after the measurement, the universe splits into two: one for state |1> and another for state |2> ?? If not, then why is it called “Many Worlds”? And what is the definition of “world” in this case? If the universe does branch off, and if the original wavefunction had probabilities of 70% and 30%, then since both states get realized, what do those probabilities even mean? If I’m going to branch off into two versions of myself and observe both states after the measurement, then why aren’t the probabilities 50% and 50% ?
I also have a question about the relationship between MWI and your thoughts that you wrote down in a blog post from August 29th, 2014. Here they are:
“If God exists but has no effect on the world whatsoever ? the actual world we experience could be precisely the same even without God ? then there is no reason to believe in it, and indeed one can draw no conclusions whatsoever (about right and wrong, the meaning of life, etc.) from positing it.”
Sean, if these other universes exist but they have no effect on any particular world that the observed outcome of an experiment exist on – the actual world we experience could be precisely the same even without these other worlds – then does that also mean there is no reason to believe in them?
I mean, if there is a definite prediction that the MWI makes that we can test, and it is observed, then that would favor the theory over others — until a new theory comes along that accounts for the MWI’s prediction AND doesn’t posit the existence of these universes. Then Occam’s Razor would compel us to favor the simpler theory.
Do you have any thoughts about this?
Shmi Nux, you said, “g. If one knew the whole history of the universe (multiverse, actually) from beginning to end (let?s assume it has an end, for the moment), then all you see is the unitary evolution (a sort of spinning) of the state describing the whole universe, with different small but macroscopic parts of it behaving as though a bunch of inaccessible but ?real? worlds is constantly being created, as far as observers in those parts are concerned.”
Sorry, that makes absolutely no sense to me. Are there or aren’t there separate universes made after each measurement?
Shmi Nux, you also said, “And it does not mesh well with GR at all, but neither does any other formulation of QM. This is an open problem whose resolution requires the elusive Quantum Gravity, the current Holy Grail of High-Energy Physics.”
Then why do we even bother thinking about these things when, presumably, knowing the fundamental laws at the Planck Scale will probably clear things up about quantum mechanics?
Thanks!
I’m an applied mathematician specializing in statistical modeling not a physicist, but bear with me (when walking my two miles from high school in the late 50’s one of my theories I worked on involved a multiple universe I called the onion universe, layers expanding from the center, so I believe I have some legitimate intuition to work with here). Anyway, I’m thinking of the multiple worlds as “samples” in a population. I suspect that many of them are not noticeably different from each other on a macro level. What I’m wondering is that the world we are perhaps actually experiencing has “maximum likelihood”.
Dan,
The difference between God and MWI is that God does not follow from the laws of physics (specifically, Quantum Mechanics), whereas Many Worlds do (at least according to Deutsch, Carroll, Aaronson, and many others). They may be currently unobservable, but just knowing that they are likely to be out there provides an impetus for further research.
Dan, also,
Re QM and GR: studying the ontology of Quantum Mechanics is one of the directions of research toward the laws of Quantum Gravity, which is just as [un]promising as, say, the String Theory or Loop Quantum Gravity. The problem is much deeper than anyone expected some 50 years ago, and there is no breakthrough in sight. So every little bit helps, you never know where and when something truly exciting shows up.
DyutimanDas,
A couple of points. First, there are no irrational numbers in physics, as nothing can be measured with infinite precision. There are, of course, plenty of irrational numbers in our models of the physical worlds. The worlds, in the limit of the Planck scale being infinitely small, are not even countable, but continuous.
Are there or aren’t there separate universes made after each measurement?
It’s probably not a question worth answering if it can never be answered. It’s also probably wrong to extrapolate what we know about wave functions of simple systems to the entire universe.
Then why do we even bother thinking about these things when, presumably, knowing the fundamental laws at the Planck Scale will probably clear things up about quantum mechanics?
Thinking about these things highlights existing problems in the theory.
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The link in my previous post should be:
http://www.gravitationlab.com/Grav%20Lab%20Links/Mr-Natural-Says-LR.pdf
It is pretty much universally acknowledged that the biggest bugaboo in the ancient quest to “unite the forces,” solve the “cosmological constant problem,” and transcend the various other fundamental problems that plague physics, is a deeper understanding of gravity. This is exemplified by Elias Okun’s recent comment:
“It is the opinion of at least a sector of the fundamental theoretical physics community that such field is going through a period of profound confusion. The claim is that we are living in an era characterized by disagreement about the meaning and nature of basic concepts like time, space, matter and causality, resulting in the absence of a general coherent picture of the physical world.”
Shmi Nux has commented that:
The problem is much deeper than anyone expected some 50 years ago, and there is no breakthrough in sight. So every little bit helps, you never know where and when something truly exciting shows up.
In a similar vein, Eric Adelberger stated: ” It seems very likely that we are missing something huge in physics.”
Ironically, the hope that progress will be made by contemplating the Planck scale or by inspecting the debris from collisions between particles, is like looking for “something huge” in the tiniest places. If something huge is really missing, then most likely it will be found only when we look in those huge places where we have not yet looked.
The biggest such place is arguably the inside of ordinary bodies of matter. A gravity experiment proposed by Galileo in 1632 would suffice to look in that place.
Evermore abstract, loopy, stringy, inflatonic, holographic, multiply-universed (etc.) theories abound. Yet we still don’t know what happens when we drop a test mass into a hole through an ordinary body of matter. (Something huge.) Rather than perpetuate the morass of outlandish abstractions, why not at last fulfill Galileo’s dream by building and operating humanity’s very first Small Low-Energy Non-Collider?
http://www.gravitationlab.com/Grav%20Lab%20Links/Mr-Natural-Says-LR.pdf
Thanks Sean. The idea of many unobservable worlds never has sat well with me, however this post gives me plenty to consider.
Shmi Nux,
Are there or aren’t there separate universes made after each measurement?
If so, I don’t see how the laws of physics directly imply such a conclusion. If they did, I’m sure many many more people would be on board with this idea. If there aren’t separate universes made after each measurement, then why is it called the many “worlds” interpretation?
Shmi Nux, you said, “They may be currently unobservable, but just knowing that they are likely to be out there provides an impetus for further research.”
What does “likely out there” even mean? That has as much meaning as saying that God is “likely out there” or that heaven and hell are “out there”? Where? You can’t answer these questions for God, heaven, and hell, and I you can’t answer these questions about the MWI. It just seems to me like this is some kind of quantum mechanical religion disguised as science.
Shmi Nux,
Consider this example:
A rectangular garden has an area of 100 square meters. The length of the garden is 5 less than 3 times its width. Find the length of the garden. Now, when you set up the quadratic equation that solves this problem, you get (assuming I set up the problem correctly) two solutions: a positive solution and a negative one. Now, the laws of math imply the existence of a negative solution for length. Does that mean negative length is “out there”? No.
Do the laws of general relativity imply the existence of a point of infinite density and spacetime curvature at the center of a black hole? No.