No Born Rule in Many Worlds Theory

There are dozens of papers on trying to reconcile the Born rule with the many-worlds interpretation of quantum mechanics. At first this seem bizarre. The Born rule is the main empirical prediction of QM, and any interpretation would have to obey it. How else could it be an interpretation?

The position I have repeatedly taken on this blog is that many-worlds is not an interpretation, and is wholly incompatible with the concept of probability, whether expressed by the Born rule or anything else.

Now a new paper agrees with me, and says it well:

Against Many Worlds
Emily Adlam, Jacob A. Barandes

Any viable interpretation of quantum theory needs to account for the Born rule, from which the theory gets its probabilistic empirical predictions. In this paper, we give an overview of possible approaches to this problem in the context of the Many Worlds interpretation. We argue that, for structural reasons, none of them can possibly succeed. More precisely, we argue that the Many Worlds interpretation must obtain the Born rule by proceeding either axiomatically, deductively, or inductively, and that all three of these approaches run into general, fundamental obstructions.

I previously posted a video explanation by Barandes.

You cannot just postulate probabilities for the many-worlds branches of the wave function. It is essential to many-worlds that the branches emerge as a result of decoherence. The branches cannot be assumed to have probabilities, and in fact no one has been able to make sense out of some branches being more likely than others.

The issue goes right to the core of what we mean by probability. For this, the paper recites a textbook probability problem for comparison.

A more fundamental obstruction is that merely having multiple things is not enough, by itself, to imply a conception of probability. To see why, consider, for example, a jar containing 43 red marbles among a total of 100 marbles. On the one hand, it would be entirely reasonable to say that the fraction of red marbles was 43%. On the other hand, it would not yet be reasonable to call this fraction a probability, because, at this point in the thought experiment, the probability would not be a probability of anything. It would not make sense to say that there was a 43% probability simpliciter.

Of course, if we were to stipulate that an agent were picking a marble ‘at random’ from the jar, under some suitable notion of ‘at random’ and with the further assumption that the marbles were ‘well-mixed,’ then we would be in a reasonable position to assert that there was a probability of 43% of the agent picking a red marble. But absent such a picking process, or the introduction of some other selection process to serve a similar functional role in the argument, there would be no probability, because, again, there would simply be nothing for the probability to be a probability of.

It is worth dwelling for a moment more on what premises are needed for a ‘picking process’ to legitimate the move from 43% qua fraction to 43% qua probability. Assigning a picking probability of 43% amounts to adopting a ‘principle of indifference’ (Eva 2019) assuming that all the marbles are ‘equally likely’ to be picked. And the philosophical literature on indifference principles makes clear that they are justified and successful only when they reflect relevant features of the process by which an outcome is selected, such as the symmetries of the picking process (van Fraassen 1989, Shackel 2007).

In particular, assigning a probability of 43% makes sense if the agent responsible for picking a marble is doing so by some process that is blind to color, meaning that there is a symmetry with respect to color – hence our invocation of terms like ‘at random’ and ‘well-mixed.’ Obviously if the agent is picking marbles in some way that is biased toward red marbles, then it would not make sense to assign a probability of 43%.

Just as you cannot abstractly assign probabilities to a jar of marbles, you cannot assign them to a wave function either. You have to have some notion of a measurement, and that is what the many-worlds folks refuse to do.

I doubt that any of these arguments are new. Many worlds theory is wholly incompatible with a scientific outlook, and is a crackpot belief.

The only way to understand many-worlds is as a complete rejection making scientific predictions, and a complete rejection of probability. That is, to accept it you have to say that no predictions can be made, and that probability theory does not make sense.

In Sean M. Carroll's recent podcast, he rambles in support of many-worlds theory:

I have heard you describe more unlikely worlds, thinner branches as not mattering as much. I don't know what your ontology of mattering is here, but it brought is it broadly speaking the fact that there are just more of the likely versions and therefore we should care about those more? If so, would you consider this a fundamentally utilitarian approach to mattering?

Well, uh it's not that there are more likely versions. ...

That's that's the entire trickiness of many worlds is that you can't get the counting that you need to get the Born rule right just from counting the numbers of worlds. The worlds don't count equally. They count as much as the amplitude squared. So 99% for this one, 1% for that one.

So I use the word mattering just to be indicative of whatever matters to you. So it might be the probability that you find yourself in such a world. It might be how much energy such a world has. It might be the utilitarian uh utility function that you have for such a world. Whatever you think is assigned to the world mattering goes along with the amplitude squared in many worlds. I think that's a good motto to keep in mind to get you through figuring out how to think about this scenario.

In quantum mechanics, the amplitude squared gives the probability. In many worlds, you cannot count the worlds or get more likely worlds. What you get is that some worlds subjectively matter more than others. You can think of the mattering as being based on probability or energy or utility function.

Really? This is all nonsense. There is nothing scientific about some worlds mattering more than others.