As someone who spends a good deal of time writing, Im generally fond of language and literary devices as part of the science-communication toolkit. Tricks like analogies, similes, metaphors, and all the rest have considerable power when it comes to speaking to people, and not making use of them would be foolish.

That said, there are times when these tools sort of tip over into becoming counterproductive. That is, it can be helpful and vivid to use a metaphor in describing a physical theory, but taken too literally, this can actually create more confusion as people latch on to ancillary features of the metaphor and try to take them too literally. A classic example of this is the rubber sheet analogy for spacetime curvature in General Relativity, where the warping of space by mass is visualized as being like the stretching of an elastic sheet with a mass pulling it down. This is a vivid image and can be useful for getting the basic idea, but some people will take it too far and start thinking that the universe is literally stretched in some other direction, or asking about the elastic properties of the sheet, and so on. (As memorably spoofed in this excellent xkcd cartoon.)

Im increasingly convinced that the Many-Worlds Interpretation of quantum mechanics is one of these places. The very name of the theory is derived from a vivid metaphor for its approach, but I think that metaphor is too often taken too literally, in a way that practically begs for unhelpful diversions into arguing about what are really ancillary elements of the metaphor.

Cover of Sean Carroll's SOMETHING DEEPLY HIDDEN

The proximate cause of this is reading Sean Carrolls new book, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime (well, the first two-thirds of it, anyway, which is where the bulk of the MWI description is), and discussions of it on social media, but its not a problem particular to Carroll this is something thats been bugging me since I was writing about it in How to Teach [Quantum] Physics to Your Dog, a bit over ten years ago. I actually like the start of Carrolls presentation quite a bit, where he casts MWI as Austere Quantum Mechanics, with the only postulates being that the universe is described by a wavefunction, and that the wavefunction evolves according to the Schrdinger equation.

That austerity is the core of MWI, and central to its appeal. Its a theory that avoids the measurement problem of quantum mechanics by pointedly not introducing some new phenomenon that changes the wavefunction in a mysterious way at the instant of a measurement. Quantum wavefunctions evolve smoothly and predictably at all times, and theres an undeniable elegance to that.

The problem is that after that austere beginning, Carroll dives back into the somewhat baroque metaphor thats grown up around the simple initial idea, talking at great length about branches of the wavefunction that contain copies of everything in the universe that differ only in the results of particular measurements. This language is really an additional interpretive superstructure on top of the actual austerity of MWI, an extended metaphor for the experience of observers within the theory. Its also where everything goes wrong, from the standpoint of communication.

Talking about parallel worlds or even branches of the wavefunction as real separate things invites a whole bunch of questions that are really about the metaphor, not the theory, and thus ultimately unproductive. It brings in the fundamentally aesthetic objection that all these extra universes run afoul of Occams razor, and questions about why making copies of everything doesnt violate some other principle of physics, and what triggers the making of copies, etc. These arent dumb questions, given the language in which MWI is often presented, but theyre fundamentally questions about the language in which MWI is presented, not the austerely quantum central idea.

That is, of course, a strong claim for me to be making, and suggests that I have an approach I think would be better, and of course I do. I think that most popular treatments of MWI lean into the parallel universe language way too much, when in fact its just a bookkeeping trick.

null

That is, the right way to think about MWI or at least the approach to it that allowed me to make my peace with it is just the Austere Quantum Mechanics approach. You have a collection of quantum things maybe particles, maybe fields, whatever suits your fancy that are described by a wavefunction, and those things evolve according to the Schrdinger equation. As these quantum things evolve and interact, they necessarily end up in complicated superpositions of multiple states, superpositions that are entangled with each other. Its still all one giant wavefunction, though no branches, no copies, no extra universes its just not one that you would want to attempt to write down on a sheet of paper. But thats fine the universe is under no obligation to operate in a manner that allows humans to conveniently write down a description of it.

However, being lazy humans, we often want to write down descriptions of things, and so we use a bookkeeping trick: we choose to pull out pieces of that one giant wavefunction and treat them as if they exist in isolation. This, strictly speaking, isnt a complete and correct description, in the same way that your household budget, strictly speaking, isnt completely separable from the rest of the global financial system if youre making payments on a loan, youre directly or indirectly affected by the complicated assets and obligations of the bank you owe money to. But for the purposes of keeping your household books balanced, you can bracket all the bank stuff off as an external influence whose internal details dont matter, and work only with the tiny piece of the system about which you have direct knowledge.

The same trick works with the giant unwieldy wavefunction of the universe. Strictly speaking, the state of every quantum object is at least potentially bound up with every other one, in a way that defies compact description. Its even worse than accounting, because while banks are classical objects, quantum objects can affect each other in a non-local way. Happily, though, in the same way that you can get away with thinking about only the one set of financial accounts about which you have detailed knowledge, we can carve out a tiny piece of the universal wavefunction and treat it as an isolated system where we have detailed knowledge of the specific outcomes of measurements. We bracket everything else off as the environment which is a black box in the same way that Seventh National Bank is in finance.

Niels Bohr (L) and Werner Heisenberg on vacation (Photo by ullstein bild/ullstein bild via Getty Images)

How can we get away with this? Ironically, the key to understanding it comes from two guys who come in for a lot of abuse in most pop-quantum books: Niels Bohr and Werner Heisenberg. Bohr and Heisenberg get disparaged as anti-realists for running off into a weird does the Moon exist when nobodys looking? land of observer-created reality. While they arguably took it too far, though, their initial insight is a critical one: It makes no sense to talk about the properties of a thing unless you also talk about how you are going to measure those properties.

How does this help with MWI? The problematic aspect here is that the wavefunction of the universe has everything in complicated superposition states, but when we select out a tiny piece of it as our system of interest, we often see that system only in single states, not a superposition of multiple states. The question thats too often un-asked, though is: What measurement would you do to demonstrate that your system is really in a superposition?

The answer to this doesnt need to be a procedure specific enough to actually do the experiment; a general outline would be sufficient. And, in fact, we have a couple of centuries of experience at doing exactly this: When we want to show that something has been in two states at the same time, we do an interference experiment. We put our system of interest in a superposition of two states, arrange for those two states to evolve at slightly different rates for some time, and then bring them back together and measure the final state. If a superposition exists, there will be some oscillation in the probability of a given final state that depends on the differential evolution in the middle. This takes lots of forms if the two states of the superposition correspond to passing through spatially separated slits, itll show up as an interference fringe pattern in space; if theyre two states of a cesium atom in an atomic clock, itll show up as a varying probability of ending up in one of those states as you adjust the frequency of your microwave oscillator.

Quantum physics books with dice.

In every case, though, youre measuring a probability. And not even a Bayesian can accurately measure a probability from a single experiment. To get a good measurement of a probability of some outcome let alone the variation in probability that is the signature of a superposition state you need a large number of repeated measurements. And those measurements have to be made under the same conditions every time.

Thats the key feature that lets you carve out some parts of the giant wavefunction of the universe and choose to treat them as systems in definite states, while others need to be treated as full quantum superpositions. The vast majority of the universe that were bracketing off as the environment affects the measurement conditions, which changes the probabilities youre measuring. If the interaction with the environment is small, though, you can ensure that the conditions are close to identical for enough trials to unambiguously see the changing probabilities that show a superposition exists. That subpart of the universal wavefunction needs to be dealt with as a fully quantum system.

If the interaction with the environment is strong and poorly controlled, though, the conditions of your measurement change enough from one repetition to the next that youre not really doing the same measurement multiple times. If you could know the full state of the environment for a given trial, you would predict one probability, but knowing the full state of the environment for the next trial would lead you to predict a different probability. In the absence of that knowledge, adding together repeated results just gets you junk you wont see a clear dependence on the different evolution of the different states in the superposition, because its swamped by the unknown effect of the environment. If you cant see the interference effect, that system looks classical, and you can treat it as having a definite state.

That process of interaction with the changing state of an unknown environment gets the name decoherence, and its what enables the bookkeeping trick that lets us split off pieces of the wavefunction and consider them in isolation. If the piece youre interested in is big enough and interacts with the environment strongly enough, theres no hope of doing the interference measurement that would show its in a superposition state. If you cant do a measurement that would show the existence of the other piece(s) of the superposition, you can safely treat it as being in a single definite state.

It should be emphasized, though, that this is just bookkeeping, not a real separation between copies of the universe, or even copies of the system of interest. Theres only one universe, in an indescribably complex superposition, and were choosing to carve out a tiny piece of it, and describe it in a simplified way. Its not even true, strictly speaking, that the results of a given experiment for a particular object are unaffected by the presence of the other parts of the superposition for that specific object. If you could do the full probability calculation for the whole wavefunction, including all of the environment, the probability you would predict for that experiment would include a contribution from all the various states that are superposed. In the absence of that complete knowledge, though, you can get away with ignoring them, because youll never be able to repeat the measurements in the way you would need to see the influence.

(If you would like a version of this picture that includes a more detailed physical example, this is essentially the picture I give in How to Teach [Quantum] Physics to Your Dog That version is more humorous and less exasperated.)

A shopkeeper doing his monthly financial planning and bookkeeping

Thinking about MWI in this way as a bookkeeping trick to simplify an otherwise incomprehensibly vast wavefunction clears up most of the typical objections that arise from taking the separate worlds metaphor too literally. Theres no Occams Razor problem because theres only one wavefunction obeying one set of rules. Theres no issue with creating copies of everything, because there are no copies: theres one universe, with one set of components described by one wavefunction. Its not even a problem that the criteria for splitting are kind of nebulous, because its clear that its a fundamentally arbitrary process the choice of which pieces to isolate and discuss is purely a matter of bookkeeping convention for the convenience of puny human physicists.

So, thats my argument for why the way we talk about the Everettian interpretation of quantum mechanics sucks, and should be revisited. Please note that Im not saying that Sean Carroll or any of the other super-smart people who spend time and energy thinking about and working with MWI are Doing It Wrong in terms of the physics mathematically, thinking of the different pieces we can carve out of the giant wavefunction of the universe as separate branches works perfectly well. Thats how you keep the books. All Im arguing is that, on a conceptual level and in terms of the language used to communicate to non-experts, we should do a better job of making clear that it is just bookkeeping.

Thats also why, despite a general distaste for the (over)use of abbreviations and acronyms in physics, Ive been using MWI through most of the above. Id suggest that it continues to work perfectly well as a shorthand reference for this particular take on quantum theory, it just needs a slight tweak. Rather than Many-Worlds Interpretation, Id go with Metaphorical Worlds Interpretation, to reflect the fact that all the different ways of cutting up the wavefunction into sub-parts are fundamentally a matter of convenience, a choice to talk about pieces of the wavefunction as if they were separate, because the whole is too vast to comprehend.

See the rest here:

Many Worlds, But Too Much Metaphor - Forbes