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The Many Worlds Interpretation Is NOT Deterministic

It seems that quite a few philosophers have gotten an erroneous idea about the Many Worlds Interpretation (MWI) of quantum mechanics, namely that it is a deterministic interpretation. Even the Internet Encyclopedia of Philosophy makes this incorrect claim. So, I feel I ought to set the record straight.

The classic definition of determinism is that, given the way the world is at some time, plus the laws of nature, the future is completely fixed. Now, in one sense, this definition is clearly satisfied by the MWI. If, by "the way the world is at some time," we mean the quantum state of the universe at that time, and by "the future" we mean the quantum state of the universe at some later time, then the MWI is indeed deterministic. For the MWI takes the quantum state to be all that there is, and the unitary evolution of the quantum state to be the complete dynamics of the state. And that evolution is deterministic: there is one and only one state at a future time, and it is completely determined by the quantum state at the earlier time. (Here I am omitting the very serious difficulties of this approach, including the dubious nature of such an entity as "the quantum state of the universe" and the even more dubious assertion that that state evolves in a unitary fashion.)


But this sense of "determinism" is completely useless for normal philosophical discussion.

The problem is very easy to see in the famous example of Schroedinger's cat. Let's assume that some quantum process yields a 50-50 chance of outcomes A and B, and we rig up some equipment so that if outcome A happens then a vial of poison is smashed and the cat dies, but if outcome B happens, then the cat lives. And all of this takes place inside a perfectly impenetrable box, so that we have no way of determining the outcome until we open the box. Then, according to the rules of quantum mechanics, the cat will be in a superposition of alive and dead states until  the box is opened.

In the MWI, an experiment like this has a determinate outcome: that the quantum state evolves into two branches, one with a live cat and one with a dead cat. But in the real world we don't see such superposition states. We see a live cat or a dead one. The MWI solves this by invoking decoherence, yada, yada, yada, so that the two branches are effectively split into different worlds that never interfere thereafter.

But now you see the problem: the future, in the normal sense of the word, involves the cat being either alive or dead, not both. And the MWI does not determine which future we will see. Indeed, from the point of view of the MWI, there is not even any sense in asking which outcome actually occurred, because both actually occurred, in different "worlds."

So in the case of the cat, the future (in the normal everyday sense) is undetermined. But it's much worse than this: anything that occurs because of quantum effects has an indeterminate outcome. But in the MWI, everything that occurs at all occurs because of quantum effects. So (almost) everything is indeterminate.

(The sole exceptions would be those events that occur with 100% probability: outcomes that are represented by operators for which the quantum state of the universe is an eigenvector. But, given that the wave function splits every time a quantum event occurs, we can be sure that none of the events of ordinary interest will have the state of the universe as an eigenvector.)

The thing is, even though the MWI has restored determinism of a sort to the description of the universe, it completely fails to predict what we will see in our particular future. John Earman says that the MWI exhibits ontological determinism, but at the price of "radical epistemic indeterminism."

Clearly, though, it is the indeterministic aspect that is relevant for most philosophical concerns. For example, in discussions of free will the question is often asked, "Could I have done otherwise?" And the MWI gives the emphatic answer, "Yes!" Because not only could I have done otherwise, but in some other "world" I did do otherwise. Austin missed his putt, but in some other world where the quantum events in his brain fired the neurons in a different way he made the putt.

Maybe it is possible to continue to do philosophy while keeping in mind the ontological viewpoint of the MWI - though I have to say I doubt it. But it would at least be a very different philosophy than what we have now, and all of the classic discussions would have to be revisited in light of that ontology.

Beliefs Don't Matter (as much as you think)

We atheists spend a lot of time and energy addressing the beliefs of religious folks. But many people are religious, or at least go to a church or synagogue or mosque or something, not because of beliefs, but for reasons that we could call "social."

Jerry Coyne of Why Evolution Is True discusses an article by Philip Kitcher, a Columbia philosophy professor:

Kitcher distinguishes what he calls the “belief model”—the form of faith that is initially built on truth claims about God, Jesus, Mohamed and the like—from three other forms of what he calls the “orientation” model: the forms of faith that begin with a person identifying secular goals and beliefs that he shares with others, and then choosing a faith that properly frames these goals.

I think this is an important point that is often missed: as someone said, you can't rationally argue someone out a belief they weren't rationally argued into. (Or something like that.) And for most people, this must be the case. Most people follow the religion they were brought up in - it is a cultural issue, not a rational one.

It's an easy mistake to make, I think, because we are so used to thinking of religion in terms of beliefs. If you learn that someone is a two-seed-in-the-spirit predestinarian Baptist, you immediately ask, "What do they believe?"

But if we broaden our view and look beyond Western Christianity, we find there are many religions that are more interested in what you do than in what you believe. Strangely, in talking about non-Christian religions we often implicitly acknowledge the fact. We have no problem with the idea of "non-religious Jews," or people who are "culturally Jewish" without being "observant." When was the last time you heard of a "non-religious Christian"? Or heard someone say, "I'm culturally Christian, but I'm not observant"?

So, while New Atheist-type books and blogs may be very valuable in reaching a certain segment of the population, they may be missing an even larger segment. We need more secular versions of what the churches offer: communities with shared goals and values. Some existing groups - Quakers, Unitarian Universalists, and Ethical Societies - are based more on shared values than on shared beliefs, though of course the first two still involve some elements of religious belief. How do we build secular versions of these communities?

It's an interesting and important discussion, and I encourage you to read about Kitcher's article at the link above and at Russell Blackford's blog. (I linked the first post in a series: you can find the rest of the series in his sidebar.)

The Relativity of Wrong

In a previous post I argued that scientists choose their theories, not on the basis of which one is more likely true, but on the basis of which one gives a better approximation to reality.

Via a comment on Cosmic Variance, I came across an essay by Isaac Asimov that makes a similar point.

The young specialist in English Lit, having quoted me, went on to lecture me severely on the fact that in every century people have thought they understood the universe at last, and in every century they were proved to be wrong. It follows that the one thing we can say about our modern "knowledge" is that it is wrong. The young man then quoted with approval what Socrates had said on learning that the Delphic oracle had proclaimed him the wisest man in Greece. "If I am the wisest man," said Socrates, "it is because I alone know that I know nothing." the implication was that I was very foolish because I was under the impression I knew a great deal.

My answer to him was, "John, when people thought the earth was flat, they were wrong. When people thought the earth was spherical, they were wrong. But if you think that thinking the earth is spherical is just as wrong as thinking the earth is flat, then your view is wronger than both of them put together."

The basic trouble, you see, is that people think that "right" and "wrong" are absolute; that everything that isn't perfectly and completely right is totally and equally wrong.

However, I don't think that's so. It seems to me that right and wrong are fuzzy concepts, and I will devote this essay to an explanation of why I think so.

A No-Brainer?

This post is part of my series on physicalism.

When I started reading about physicalism I thought, "This is going to be a no-brainer - a piece of cake - a  clear shot at an open goal - a badly mixed metaphor." After reading these two books, I am impressed with the difficulty of the task. I think I understand a little better, too, how people like Trent can be so skeptical of the whole operation.

The issue with defining "physics", so that we can know what to reduce everything down to, seems almost a red herring. After all, if we could show that mental processes reduce down to events at the neural level, I don't think it would matter all that much (to philosophy) which definition of "physical" was taken to underlie the neural level. The biggest problem here is one that is glossed over briefly by Poland and sidestepped entirely by Melnyk: namely, the problematic nature of the ontology of quantum mechanics. If the wave function cannot be consistently interpreted as an actual, physical object (as I would argue), then it can't be the foundation for ontology for everything else. Poland waves such worries aside and says these things are for the physicists to worry about: whatever they come up with as the basic ontological entities are fine with him. But what if those basic entities can only be defined in term of large-scale, macroscopic quantities and measurements, as quantum mechanics seems to do? Then physicalism might justifiably be accused of circularity.

The more important point, it seems to me, is the issue of "realization." These two authors agree that supervenience is too weak, and identity theses are too strong. If realization gives a coherent account of how higher-level things are related to (reducible to) lower-level things, then something of real importance has been achieved. And this achievement is independent of the issue of what sort of physics is at the base of it all.

Finally, is realization physicalism (or something like it) is actually true? It is not so clear what, exactly would count as evidence in favor of, or against, physicalism. Melnyk spends two chapters considering the question and looking at the evidence, such as it is. He points out that some phenomena that might have turned out to be clear evidence against physicalism - psychokinesis, for example - have not panned out so well. On the other hand, more positively, every sort of mental activity seems to be associated with some sort of brain activity (e.g. as revealed in functional MRI investigations). He notes, too, that as recently as 1925 a prominent philosopher, C. D. Broad, could doubt that chemical phenomena were reducible to underlying physical laws, whereas now it is clear that the rules of quantum mechanics explain much of chemistry.

In short, physicalism has suffered no decisive defeat and has scored many major victories. This alone doesn't settle the matter, of course., and dualists and theists will continue to assail the physicalist position.

I still think that physicalism is clearly the most reasonable view - simply because of the complete lack of evidence for anything that is not physically realized. In this it is just like atheism: show me the evidence for your god, and perhaps I'll believe in him. (Or her.) (Or it.)

More About Physicalism

This post is part of my series on physicalism.

Some important points about Poland's and Melnyk's versions of physicalism:

The scope of physicalism. According to Poland (p.227),

The theses of physicalism apply to all natural phenomena and all claims to truth and knowledge concerning the natural order.

This leaves one wondering what, exactly, constitutes the "natural order." Poland lists

...physical, chemical, and biological phenomena, ... the psychological, the social, the moral, the aesthetic, and the commonsense world of our everyday experience.

However, he excepts purely abstract realms such as mathematics.

Melnyk is (again) more specific, saying that physicalism applies to everything that is either contingent or causal. The existence of a god who is not contingent (because he is a necessary being) but who causally affects, or is affected by, the world, would refute Melnyk's physicalism. (It seems that such a god would not refute Poland's version, as god is presumably outside of the natural order.) Mathematics, being composed of necessary (i.e. not contingent) truths that have no causal efficacy, is exempt here as well.


Realization physicalism entails supervenience. That is, the physical facts about the world determine all objective facts about the world.

But supervenience by itself is not enough to guarantee the kind of physicalism that we want. And supervenience doesn't entail realization physicalism. Thus, realization physicalism is a stronger claim than supervenience.


Physicalism is reductionistic. I was surprised to learn that "reductionism" is a bad word among philosophers. I thought that reduction was the whole point of science: to find explanations of things in terms of simpler, more fundamental things. Melnyk spends a whole chapter (cutely titled "Physicalism and R*d*ct**n*sm") discussing in what sense realization physicalism is, and in what sense it is not, reductionistic. He argues that it is reductionistic in the "core sense." The core sense, briefly, is that all higher-level facts have an explanation in terms of physical facts and necessary truths (such as logical and mathematical truths).

However, realization physicalism does not satisfy what Melnyk calls the "received sense" of reductionism, namely that each higher-level type is identical with some physical type. And Melnyk's version of physicalism is stated in terms of functional, not physical, types. In particular, a functional type can be one that satisfies a purely logical condition. (An example would be the computer program considered earlier.) While realization physicalism requires that all actual instances of such a higher-level type are realized physically, it does not identify the higher-level type with those instances, or the collection of all possible such instances, or anything like that.

Other Errors To Avoid: Poland writes (p.41)

[T]here is good reason to avoid such views at that everything is completely physical, physics is the one true theory, physics describes and explains everything, the only ... legitimate methods of inquiry are those of physics, and everything is best understood from the perspective of theoretical physics.

I have to agree with him here - except for the last one, which is self-evidently true.