The Deutsch-Yudkowsky argument for the Many Worlds Interpretation states that you can take the core of Quantum Mechanics—the Schrödinger wave equation, and the projection postulate—remove the projection postulate (also known as collapse and reduction ), and end with a simpler theory that is still adequate to explain observation. The idea is that entanglement can replace collapse: a scientist observing a superposed state becomes becomes entangled with it, an effectively splits into two, each having made a definite observation.
Moreover Yudkowsky, following David Deutsch, holds the many worlds interpretation to be obviously correct, in contrast to the majority of philosophers and physicists, who regard the problem of interpreting QM as difficult and unsolved.
This has some problems.
(Which are to do with the specific argument, and the level of certainty ascribed to it. To say that you cannot be certain about a claim is not to say it is false. To point out that one argument for a claim does not work is likewise not to say that the claim itself is false. There could be better arguments for these versions of many worlds, or better many worlds theories, for that matter).
The Problems.
The first thing to note is that there is more than one quantum mechanical many worlds theory. What splittng is...how complete and irrevocable it is … varies between particular theories. So does the rate of splitting, so does the mechanism of splitting.
The second thing to note is that many worlders are pointing at something implied the physical formalism and saying “that’s a world”....but whether it qualifies as a world is a separate question from whether it’s in the formalism , and a separate kind of question, from whether it is really there in the formalism. One would expect a world, or universe, to be large, stable, non-interacting, and so on . It’s possible to have a theory that has collapse , without having worlds. A successful MWI needs to jump three hurdles: empirical correctness, mathematical correctness and conceptual correctness—actually having worlds
The third problem to note is that all outstanding issues with MWI are connected in some way with quantum mechanical basis....a subject about which Deutsch and Yudkowsky have little to say.
Coherence versus Decoherence
There is an approach to MWI based on coherent superpositions, and a version based on decoherence. These are (for all practical purposes) incompatible opposites, but are treated as interchangeable in Yudkowsky’s writings.
Quantum superposition is a fundamental principle of quantum mechanics that states that linear combinations of solutions to the Schrödinger equation are also solutions of the Schrödinger equation. This follows from the fact that the Schrödinger equation is a linear differential equation in time and position.
(WP)
Coherent superpositions are straightforwardly implied by the core mathematics of Quantum mechanics. They are small scale in two senses: they can go down to the single particle level, and it is difficult to.maintain large coherent superpositions even if you want to. They are also possibly observer dependent, reversible, and continue to interact (strictly speaking , interfere) after “splitting”. The last point is particularly problematical. because if large scale coherent superposition exist , that would create naked eye, macrocsopic evidence:, e.g. ghostly traces of a world where the Nazis won. All in all, a coherent superposition isn’t a world you could live in.
I said complex coherent superpositions are difficult to maintain. What destroys them? Environmental induced decoherence!
Interference phenomena are a well-known and crucial aspect of quantum mechanics, famously exemplified by the two-slit experiment. There are many situations, however, in which interference effects are artificially or spontaneously suppressed. The theory of decoherence is precisely the study of such situations. (SEP)
Decoherence tries to explain why we don’t notice “quantum weirdness” in everyday life—why the world of our experience is a more-or-less classical world. From the standpoint of decoherence, sure there might not be any objective fact about which slit an electron went through, but there is an objective fact about what you ate for breakfast this morning: the two situations are not the same!
The basic idea is that, as soon as the information encoded in a quantum state “leaks out” into the external world, that state will look locally like a classical state. In other words, as far as a local observer is concerned, there’s no difference between a classical bit and a qubit that’s become hopelessly entangled with the rest of the universe.
Decoherence is the study of interactions between a quantum system (generally a very small number of microscopic particles like electrons, photons, atoms, molecules, etc. - often just a single particle) and the larger macroscopic environment, which is normally treated “classically,” that is, by ignoring quantum effects, but which decoherence theorists study quantum mechanically. Decoherence theorists attribute the absence of macroscopic quantum effects like interference (which is a coherent process) to interactions between a quantum system and the larger macroscopic environment.(www.informationphilosopher.com)
Decoherent branches are necessarily large, since decoherence is a high level phenomenon. They are also stable, non interacting and irreversible...everything that would be intuitively expected of a “world”. But there is no empirical evidence for them (in the plural) , nor are they obviously supported by the core mathematics of quantum mechanics, the Schrödinger equation.
We have evidence of small scale coherent superposition, since a number of observed quantum effects depend on it, and we have evidence of decoherence, since complex superposition are difficult to maintain. What we don’t have evidence of is decoherence into multiple branches. From the theoretical perspective, decoherence is a complex , entropy like process which occurs when a complex system interacts with its environment. But without decoherence, MW doesn’t match observation. So there is no theory of MW that is both simple and empirically adequate, contra Yudkowsky and Deutsch.
The original, Everettian, approach is based on coherence. (Yudkowsky says “Macroscopic decoherence, a.k.a. many-worlds, was first proposed in a 1957 paper by Hugh Everett III” … but the paper doesn’t mention decoherence[1]) As such, it fails to predict classical observations—at all—it fails to predict the appearance of a broadly classical universe. If everything is coherently superposed, so are observers...but the naturally expected experience an observer in coherent superposition with themselves, is that they function as a single observer making ambiguous, superposed observations … not two observers each making an unambiguous , classical observation, and each unaware of the other. Such observers would only ever see superpositions of dead and living cats, etc.
(A popular but mistaken idea is that full splitting happens microscopically, at every elementary interaction But that would make complex superpositions non-existent, whereas a number of instruments and technologies depend on them—so it’s empirically false).
Later, post 1970s, many world theorists started to include decoherence to make the theory more empirically adequate, but inasmuch as it is additional structure, it places the simplicity of MWI in doubt. In the worst case, the complexity is SWE+decoherence+preferred basis, whereas in the best case, it’s SWE alone, because decoherence is implicit in SWE, and preferred basis is implicit in decoherence. Decoherentists hope to show that the theory can be reduced to core QM, such as the Schrödinger equation, but it currently uses more complex math, the “reduced density matrix”. The fact that this research is ongoing is strong evidence that the whole problem was not resolved by Everetts’s 1957 paper. In any case, without a single definitive mechanism of decoherence, there is no definitive answer to “how complex is MWI”.
And single-universe decoherence is quite feasible. Decoherence adds something to many worlds, but many worlds doesn’t add anything to decoherence.
So, coherent superpositions exist, but their components aren’t worlds in any intuitive sense; and decoherent branches would be worlds in the intuitive sense, but decoherence isn’t simple. Also, theoretically and observationally, decoherence could be a single world phenomenon. Those facts—the fact that it doesn’t necessarily involve multi way branching, and the fact that it is hard to evaluate its complexity because there is not a single satisfactory theory for it—means it is not a “slam dunk” in Yudkowsky’s sense.
The Yudkowsky-Deutsch claim is that there is a single MW theory, which explains everything that needed explaining, and is obviously simpler than its rivals. But coherence doesn’t save appearances , and decoherence, while more workable, is not known to be simple. So neither theory has both virtues
Which makes the term *Everett branch” rather confusing. The writer possibly means a decohered branch, under the mistaken assumption that Everett was talking about them. Everett’s dissertation can be found here
The Deutsch-Yudkowsky argument for the Many Worlds Interpretation states that you can take the core of Quantum Mechanics—the Schrödinger wave equation, and the projection postulate—remove the projection postulate (also known as collapse and reduction ), and end with a simpler theory that is still adequate to explain observation. The idea is that entanglement can replace collapse: a scientist observing a superposed state becomes becomes entangled with it, an effectively splits into two, each having made a definite observation.
Moreover Yudkowsky, following David Deutsch, holds the many worlds interpretation to be obviously correct, in contrast to the majority of philosophers and physicists, who regard the problem of interpreting QM as difficult and unsolved.
This has some problems.
(Which are to do with the specific argument, and the level of certainty ascribed to it. To say that you cannot be certain about a claim is not to say it is false. To point out that one argument for a claim does not work is likewise not to say that the claim itself is false. There could be better arguments for these versions of many worlds, or better many worlds theories, for that matter).
The Problems.
The first thing to note is that there is more than one quantum mechanical many worlds theory. What splittng is...how complete and irrevocable it is … varies between particular theories. So does the rate of splitting, so does the mechanism of splitting.
The second thing to note is that many worlders are pointing at something implied the physical formalism and saying “that’s a world”....but whether it qualifies as a world is a separate question from whether it’s in the formalism , and a separate kind of question, from whether it is really there in the formalism. One would expect a world, or universe, to be large, stable, non-interacting, and so on . It’s possible to have a theory that has collapse , without having worlds. A successful MWI needs to jump three hurdles: empirical correctness, mathematical correctness and conceptual correctness—actually having worlds
The third problem to note is that all outstanding issues with MWI are connected in some way with quantum mechanical basis....a subject about which Deutsch and Yudkowsky have little to say.
Coherence versus Decoherence
There is an approach to MWI based on coherent superpositions, and a version based on decoherence. These are (for all practical purposes) incompatible opposites, but are treated as interchangeable in Yudkowsky’s writings.
Coherent superpositions are straightforwardly implied by the core mathematics of Quantum mechanics. They are small scale in two senses: they can go down to the single particle level, and it is difficult to.maintain large coherent superpositions even if you want to. They are also possibly observer dependent, reversible, and continue to interact (strictly speaking , interfere) after “splitting”. The last point is particularly problematical. because if large scale coherent superposition exist , that would create naked eye, macrocsopic evidence:, e.g. ghostly traces of a world where the Nazis won. All in all, a coherent superposition isn’t a world you could live in.
I said complex coherent superpositions are difficult to maintain. What destroys them? Environmental induced decoherence!
(http://scottaaronson.com/democritus)
Decoherent branches are necessarily large, since decoherence is a high level phenomenon. They are also stable, non interacting and irreversible...everything that would be intuitively expected of a “world”. But there is no empirical evidence for them (in the plural) , nor are they obviously supported by the core mathematics of quantum mechanics, the Schrödinger equation.
We have evidence of small scale coherent superposition, since a number of observed quantum effects depend on it, and we have evidence of decoherence, since complex superposition are difficult to maintain. What we don’t have evidence of is decoherence into multiple branches. From the theoretical perspective, decoherence is a complex , entropy like process which occurs when a complex system interacts with its environment. But without decoherence, MW doesn’t match observation. So there is no theory of MW that is both simple and empirically adequate, contra Yudkowsky and Deutsch.
The original, Everettian, approach is based on coherence. (Yudkowsky says “Macroscopic decoherence, a.k.a. many-worlds, was first proposed in a 1957 paper by Hugh Everett III” … but the paper doesn’t mention decoherence[1]) As such, it fails to predict classical observations—at all—it fails to predict the appearance of a broadly classical universe. If everything is coherently superposed, so are observers...but the naturally expected experience an observer in coherent superposition with themselves, is that they function as a single observer making ambiguous, superposed observations … not two observers each making an unambiguous , classical observation, and each unaware of the other. Such observers would only ever see superpositions of dead and living cats, etc.
(A popular but mistaken idea is that full splitting happens microscopically, at every elementary interaction But that would make complex superpositions non-existent, whereas a number of instruments and technologies depend on them—so it’s empirically false).
Later, post 1970s, many world theorists started to include decoherence to make the theory more empirically adequate, but inasmuch as it is additional structure, it places the simplicity of MWI in doubt. In the worst case, the complexity is SWE+decoherence+preferred basis, whereas in the best case, it’s SWE alone, because decoherence is implicit in SWE, and preferred basis is implicit in decoherence. Decoherentists hope to show that the theory can be reduced to core QM, such as the Schrödinger equation, but it currently uses more complex math, the “reduced density matrix”. The fact that this research is ongoing is strong evidence that the whole problem was not resolved by Everetts’s 1957 paper. In any case, without a single definitive mechanism of decoherence, there is no definitive answer to “how complex is MWI”.
And single-universe decoherence is quite feasible. Decoherence adds something to many worlds, but many worlds doesn’t add anything to decoherence.
So, coherent superpositions exist, but their components aren’t worlds in any intuitive sense; and decoherent branches would be worlds in the intuitive sense, but decoherence isn’t simple. Also, theoretically and observationally, decoherence could be a single world phenomenon. Those facts—the fact that it doesn’t necessarily involve multi way branching, and the fact that it is hard to evaluate its complexity because there is not a single satisfactory theory for it—means it is not a “slam dunk” in Yudkowsky’s sense.
The Yudkowsky-Deutsch claim is that there is a single MW theory, which explains everything that needed explaining, and is obviously simpler than its rivals. But coherence doesn’t save appearances , and decoherence, while more workable, is not known to be simple. So neither theory has both virtues
Which makes the term *Everett branch” rather confusing. The writer possibly means a decohered branch, under the mistaken assumption that Everett was talking about them. Everett’s dissertation can be found here