Practical Implications of the Quantum Multiverse
According the many-worlds interpretation (MWI) of quantum mechanics, the universe is constantly splitting into a staggeringly large number of decoherent branches containing galaxies, civilizations, and people exactly like you and me.1
You might think such a metaphysically radical theory should have pretty radical implications for how one should live. The quantum physicist John Bell once wrote “if such a theory were taken seriously, it would hardly be possible to take anything else seriously”.
But proponents of MWI have typically concluded the opposite. David Wallace, in his masterful, book-length treatment, claims we should not expect any practical implications: “Friendship is still friendship. Boredom is still boredom. Sex is still sex.”2
I’ll argue that it's important to distinguish between two kinds of practical implications we might expect MWI to have.
It’s true that there are no major implications for decision-making. We should look instead for the implications for virtue and wisdom.
Decision theory
The main reason for thinking MWI has no major practical implications comes from decision theory.
In standard decision theory you try to calculate the expected value for a particular course of action by multiplying the utility of each possible consequence by the probability of that consequence occurring.
The main difference in MWI is that each possible consequence can correspond to a world that actually ends up existing, rather than being just hypothetical.
But the probabilities you assign to each consequence should still be the same.
Quantum mechanics includes a standard rule, the Born rule, which states that the mod-squared amplitudes of a wave function should be treated as probabilities.
Normally, when we say an event has a 60% probability of occurring, we imply that it either will or won’t happen, and we’re not sure which.
According to MWI, by contrast, it can both happen and not happen - in different worlds. A 60% probability can mean there’s a 60% chance that you see the event in your branch of the multiverse, since it happens in 60% of the worlds that result from your action - and not in the other 40%.
But this makes no difference to the expected value you get in decision theory. Whether or not MWI is true, the expected value of your action is (60% x U1) + (40% x U2), where U1 and U2 are the utilities you assign to the event occurring and not occurring respectively.
So we basically end up making the same decisions in MWI as we would otherwise3.
Virtue
But this doesn’t mean that MWI has no practical implications at all. The focus on decision theory can lead us to overlook other kinds of practical implications MWI could have.
Discussions about how one should live have always been central to philosophy. And a significant part of the content of such discussions - whether historically or in contemporary work - is not about decision-making at all, but rather: what kind of person to be, what kinds of character traits are desirable and how one should think and feel about situations.
These questions are often addressed under the heading of ‘virtue ethics’ - understood as one of three main approaches to ethical theory.
Virtue ethicists seek to develop an understanding of both specific ‘virtues’ or character traits, such as being courageous, loving, generous or just, as well as a general understanding of what virtue consists in, and how such virtues must operate together in a well-lived life.
The other approaches in the standard triad of ethical theories are the deontological (Kantian) and consequentialist (deriving from Bentham and Mill’s Utilitarianism) approaches - both of which focus more on rules for decision-making.
But you don’t need to be in the ‘virtue ethics’ camp to think virtue is worth understanding.
Consequentialist and deontological approaches to virtue exist as well. For instance a consequentialist might propose that what makes a trait virtuous is its tending to lead to good consequences. And the well-known difficulties involved in actually quantifying the utilities of all possible consequences are among the reasons for consequentialists to show interest in virtue.
Virtue can also be understood from a religious or spiritual point of view. The Christian tradition explicitly adopted the concept, proposing specific ‘cardinal’ and ‘theological’ virtues. And the philosopher Pierre Hadot has proposed that the virtues described in ancient philosophy are best understood in a framework of inner development based on secular ‘spiritual exercises’.
In short, it’s possible for those from a variety of perspectives to agree that the focus on decision-making leaves out a great deal of ethics.
Equanimity
To make such implications seem not only technically possible but also plausible, I'll now sketch out some specific practical implications of MWI (I'll go into more detail on these in future posts).
Firstly, consider that there's a certain kind of anxiety and regret associated with having to choose between two mutually exclusive good options. It seems plausible that MWI could help us feel better about such choices, if it's true there's a world where you actually experience the other good option.
Secondly, consider the simple fact that if you find yourself in an extremely unlucky personal situation - a car crash, say, or getting cancer - then MWI implies that there are other worlds, with high quantum weights, in which you are not so unlucky.
Again this is plausibly a consoling thought, similar in kind to the consoling thoughts recommended by philosophical traditions like Stoicism.
Likewise, if you're in what you estimate as the bad end of the spectrum of physically possible 'timelines' of global history, it seems consoling to know that those other timelines are real.
From a virtue-theoretic perspective, we could say that it’s good to develop the disposition to take the wider cosmic perspective these thoughts assume, and so enhance one’s equanimity - a standard goal of classical virtue theory4 which is arguably linked to both well-being and ethical action.
Wisdom
Zooming out further, there is also the ability to take the widest possible cosmic perspective, and consider one’s place in the quantum multiverse as whole.
The idea that an appreciation of science is important for wisdom goes right back to the origins of virtue ethics in the Ancient world.
Plato and Aristotle based their understanding of virtue on theories of human nature and humanity’s place in the cosmos. Their ideas were developed in Stoicism and Epicureanism, and integrated with the idea that the cosmos is governed by natural laws governing the motions of atoms.
Modern physical theories have also been used to help us interpret our place in the universe.
The second law of thermodynamics, for instance, has been taken to show that the universe as a whole is essential hostile to human interests.
In 'A free man's worship'. Bertrand Russell proposed that this law is a kind of foundation for an overall view of existence:
all the labours of the ages, all the devotion, all the inspiration, all the noonday brightness of human genius, are destined to extinction in the vast death of the solar system, and that the whole temple of Man's achievement must inevitably be buried beneath the debris of a universe in ruins - all these things, if not quite beyond dispute, are yet so nearly certain, that no philosophy which rejects them can hope to stand.
In the same vein, it seems plausible that MWI can help us assess the goodness or badness of the cosmos as a whole: whether we should feel at home in nature, or set against a hostile universe.
Or to put it another way: an accurate appreciation - both intellectually and emotionally - of our place in the quantum multiverse could make up an essential component of wisdom.
Whether this is the best interpretation of quantum mechanics is of course controversial, and I’m not going to get into that debate here. I’ll just point to the best book-length defense of MWI, namely David Wallace's rigorously argued The Emergent Multiverse (2014). The anthology Many Worlds (2012) provides a useful entry point into surrounding debates.
See p.273 of The Emergent Multiverse. Another prominent proponent of MWI, Eliezer Yudkowski, quotes Egan's Law: “It all adds up to normality”. Egan’s law is named after science fiction author Greg Egan, whose multiverse-themed novel Quarantine ends with that line.
There’s a complication with this point of view that I just want to mention, though I won’t go into the details here.
Some people have thought that the number of worlds in the quantum multiverse is infinite or rapidly increasing, and that this causes problems for decision-theory.
But this problem is addressed in Wallace’s treatment, in which the number of worlds is actually undefined. What is precisely defined in this approach are the quantum weights associated with Born rule, which allows us to still assign probabilities in the usual way.
In the discussion of practical implications here and in future posts I’ll be careful to avoid talking about the number of alternate worlds, and instead focus on the probability or ‘proportion’ of worlds that involve a particular outcome.
Glad I could read this article. Nicely written! MWI certainly leads to interesting philosophical ideas and questions. Especially since there is a nonzero chance for pretty much any decision to be made. Plus the idea of scenario branching!