Eliezer recently posted an essay on "the fallacy of privileging the hypothesis". What it's really about is the fallacy of privileging an *arbitrary* hypothesis. In the fictional example, a detective proposes that the investigation of an unsolved murder should begin by investigating whether a particular, randomly chosen citizen was in fact the murderer. Towards the end, this is likened to the presumption that one particular religion, rather than any of the other existing or even merely possible religions, is especially worth investigating.

However, in between the fictional and the supernatural illustrations of the fallacy, we have something more empirical: quantum mechanics. Eliezer writes, as he has previously, that the many-worlds interpretation is *the one* - the rationally favored interpretation, the picture of reality which rationally should be adopted given the empirical success of quantum theory. Eliezer has said this before, and I have argued against it before, back when this site was just part of a blog. This site is about rationality, not physics; and the quantum case is not essential to the exposition of this fallacy. But given the regularity with which many-worlds metaphysics shows up in discussion here, perhaps it is worth presenting a case for the opposition.

We can do this the easy way, or the hard way. The easy way is to argue that many-worlds is merely *not favored*, because we are nowhere near being able to locate our hypotheses in a way which permits a clean-cut judgment about their relative merits. The available hypotheses about the reality beneath quantum appearances are one and all unfinished muddles, and we should let their advocates get on with turning them into exact hypotheses without picking favorites first. (That is, if their advocates can be bothered turning them into exact hypotheses.)

The hard way is to argue that many-worlds is actually *disfavored* - that we can already say it is unlikely to be true. But let's take the easy path first, and see how things stand at the end.

The two examples of favoring an arbitrary hypothesis with which we have been provided - the murder investigation, the rivalry of religions - both present a situation in which the obvious hypotheses are homogeneous. They all have the form "Citizen X did it" or "Deity Y did it". It is easy to see that for particular values of X and Y, one is making an arbitrary selection from a large set of possibilities. This is not the case in quantum foundations. The well-known interpretations are extremely heterogeneous. There has not been much of an effort made to express them in a common framework - something necessary if we want to apply Occam's razor in the form of theoretical complexity - nor has there been much of an attempt to discern the full "space" of possible theories from which they have been drawn - something necessary if we really do wish to avoid privileging the hypotheses we happen to have. Part of the reason is, again, that many of the known options are somewhat underdeveloped as exact theories. They subsist partly on rhetoric and handwaving; they are mathematical vaporware. And it's hard to benchmark vaporware.

In his latest article, Eliezer presents the following argument:

"... there [is] no concrete evidence whatsoever that favors a collapse postulate or single-world quantum mechanics. But, said Scott, we might encounter *future* evidence in favor of single-world quantum mechanics, and many-worlds still has the open question of the Born probabilities... There must be a trillion better ways to answer the Born question without adding a collapse postulate..."

The basic wrong assumption being made is that quantum superposition *by default* equals multiplicity - that because the wavefunction in the double-slit experiment has two branches, one for each slit, there must be two of something there - and that a single-world interpretation has to add an extra postulate to this picture, such as a collapse process which removes one branch. But superposition-as-multiplicity really is just another hypothesis. When you use ordinary probabilities, you are not rationally obligated to believe that every outcome exists somewhere; and an electron wavefunction really may be describing a single object in a single state, rather than a multiplicity of them.

A quantum amplitude, being a complex number, is not an ordinary probability; it is, instead, a mysterious quantity from which usable probabilities are derived. Many-worlds says, "Let's view these amplitudes as realities, and try to derive the probabilities from them." But you can go the other way, and say, "Let's view these amplitudes as derived from the probabilities of a more fundamental theory." Mathematical results like Bell's theorem show that this will require a little imagination - you won't be able to derive quantum mechanics as an approximation to a 19th-century type of physics. But we have the imagination; we just need to use it in a disciplined way.

So that's the kernel of the argument that many worlds is *not favored*: the hypotheses under consideration are still too much of a mess to even be commensurable, and the informal argument for many worlds, quoted above, simply presupposes a multiplicity interpretation of quantum superposition. How about the argument that many worlds is actually *disfavored*? That would become a genuinely technical discussion, and when pressed, I would ultimately not insist upon it. We don't know enough about the theory-space yet. Single-world thinking looks more fruitful to me, when it comes to sub-quantum theory-building, but there are versions of many-worlds which I do occasionally like to think about. So the verdict for now has to be: not proven; and meanwhile, let a hundred schools of thought contend.

The remaining uncertainty in QM is about which

slower-than-light, differentiable, configuration-space-local, CPT-symmetric, deterministic, linear, unitaryphysics will explain the Born probabilities, possibly in combination with some yet-unrealized anthropic truths - and combine with general relativity, and perhaps explain other experimental results not yet encountered.The uncertainty

withinthis space does notslop overinto uncertainty over whether single-world QM - that is, FTL, discontinuous, nonlocal, CPT-asymmetric, acausal, nonlinear, nonunitary QM - is correct. Just because this was a historical mistake is no reason to privilege the hypothesis in our thought processes. It's dead and should never have been alive, and uncertaintywithinthe unmagical versions of QM won't bring the magic back. You don't get to say "It's not resolved, so probability slops around whatever possibilities I happen to be thinking about, and I happen to be thinking about a single world." This reallyisthe classic theistic tactic for keeping God alive.In similar wise, any difficulties with natural selection are to be resolved

withinthe space of naturalistic and genetically endogenous f... (read more)The theories actually used in particle physics can generally be obtained by starting with some classical field theory and then "quantizing" it. You go from something described by straightforward differential equations (the classical theory) to a quantum theory on the configuration space of the classical theory, with uncertainty principle, probability amplitudes, and so forth. There is a formal procedure in which you take the classical differential equations and reinterpret them as "operator equations", that describe relationships between the elements of the Schrodinger equation of the resulting quantum field theory.

Many-worlds, being a theory which says that the universal wavefunction is the fundamental reality, starts with a quantum perspective and then tries to find the observable quasi-classical reality somewhere within it. However, given the fact that the quantum theories we actually use have not just a historical but a logical relationship to corresponding classical theories, you can start at the other end and try to understand quantum theory in basically classical terms, only with something extra added. This is what Hadley is doing. His hypothesis is that... (read more)

One of these elegances could be wrong. But

allof them? In exactly the right way to restore a single world?It's not worth thinking about, at this stage.If you're a reductionist about things like objects and minds -- if you believe it's enough that there are patterns -- then you can find such patterns in quantum superpositions without further assumptions. You may not be such a reductionist, but most of us are.

More Wallace linkage: http://users.ox.ac.uk/~mert0130/papers/proc_dec.pdf

So you say many-worlds isn't rationally favored over other interpretations because those other interpretations haven't been stated clearly enough? I'm pretty sure I could argue against evolution or gravity in the same manner.

If mere clarity were the issue, then Bohmian mechanics would be #1, spontaneous collapse theories would be #2, and many-worlds and the "zigzag in time" approach would be tied for third place.

The reason for this ranking is that Bohmian mechanics and collapse theories actually have equations of motion which allow you to make the correct predictions. But the collapse theories come off as slightly inferior because there is no principle constraining the form of collapse dynamics.

Zigzag-in-time refers to John Cramer's transactional interpretation and Mark Hadley's QM-from-gravity approach (mentioned above). They're in third place with many-worlds because they cannot presently make predictions.

But the situation is way more complex than this summary suggests. You can have Bohmian mechanics without a pilot wave (the "nomological" version of Bohm), you can have a collapse theory without superpositions (you just quantum jump from one "collapse" to the next), you can have many-worlds without a universal wavefunction (just use the world-probabilities in a "consistent histories" ensemble). Like I said, the known options have been expressed in a babel of theoretical frameworks, and anything resembling objective comparison has hardly begun. The human race is still thinking this through.

This article makes frequent references Eliezer's arguments against the quantum collapse postulate, including those harking back to OvercomingBias days. Yet I find no reference anywhere in the article to 'faster than light' or, in fact, any of the critical elements to Eliezer's claim.

Michael's argument is founded on the principle behind the "the fallacy of privileging the hypothesis". This is only made relevant by truncating all references to the evidence which supports the hypothesis. Deny the evidence if you will, but even if you do, the problem with the argument would be 'the reasoning behind the premises is bogus'. Privileging the hypothesis doesn't even remotely apply.

Many worlds is favored. It is what you get if you just apply the same laws of physics which correctly describe the observed behaviors of microscopic systems on a large scale without postulating any additional laws of physics which are not suggested by the evidence. If you model a measuring device as a system of particles, then measuring a particle in superposition puts that device into superposition, and if you model a human observer as a system of particles then observing the results of that measurement on the device in superposition puts the human in sup... (read more)

You are arguing a strawman. Many-worlds is contended only

conceptuallycorrect, in the same way classical illusions of our billiard ball world are conceptually correct. Obviously, quantum mechanics is technically imprecise, and there is likely another conceptual picture that gives the more accurate layer of description of reality, in the same way as classical physics is technically imprecise, and quantum mechanics serves as a shift in perspective allowing to fix some of its imprecision (theories of relativity working on the same problem on the other end, a... (read more)My main argument is that most likely none of us really knows enough about mathematics of quantum mechanics to follow emergence of patterns of observable universe out of MWI. My quantum maths stops at quantum computing, which is MWI-interpretable, and Copenhagen-interpretable equally well.

The second argument is that our view of physics is incomplete - we don't know about quantum gravity, our cosmology is ridiculous, filled with inflation, dark matter, dark energy etc., we don't know if there are any tiny non-linearities in 200th decimal place with quantum s... (read more)

This would be significantly more useful if there were a bit of argument clarifying the point. EY has argued at length why MWH is the best explanation we have; merely saying, "Privileging the hypothesis!" is not adequate.

Upon reading Collapse Postulates, or If Many-Worlds Had Come First, I would say that Eliezer_Yudkowsky is not merely arguing this correct a la "billiard ball world". Quote from the latter article:

Also, we hang on to the b... (read more)

Go for it. I have extreme difficulty trying to work out how it might even make sense that all possible(*) realities don't exist....

To me, the killer arguments are:

How arbitrary both the arrangement of the universe, and the universe itself is,

How impossible it is to pin down what existence is, compared to an abstracted implementation#

How consciousness itself implies uncertainty and indescernibility between contexts.

(*) In a meaningful sense, of course.

Your title claims that MWI is incorrectly elevated to high probability without rational evidence. Eliezer spent ~30 posts explaining his understanding of Quantum Mechanics. You provide no evidence that many worlds is promoted wrongly. Did your article get posted early? It seems to just drift off, with no discussion of your disagreement with many worlds.