Why the Many-Worlds Interpretation May Not Have Significant Ethical Implications

At first glance, it seems like the many worlds interpretation of quantum mechanics (MWI) might have significant ethical implications. After all, MWI implies that there are many more sentient beings in the world than one would think given a naive classical view, indeed a much greater number of them. And so it seems quite plausible, at least on the face of it, that ethical considerations pertaining to MWI should dominate everything else in expectation, even if we place only a small credence on this interpretation being true. In this post, I shall outline some reasons why this may in fact not be the case, at least with respect to two commonly supposed implications: 1) extreme caution, and 2) exponentially greater value over time. However, questions concerning the ethical implications of our best physical theories and their interpretations remain open and worth exploring.

Would Branching Worlds Imply Extreme Caution?

“I still recall vividly the shock I experienced on first encountering this multiworld concept [MWI]. The idea of 10100 slightly imperfect copies of oneself all constantly splitting into further copies, which ultimately become unrecognizable, is not easy to reconcile with common sense.”

Bryce DeWitt

This is a common way to introduce the implications of MWI, and it seems plausible that this radically different conception of reality, if true, should lead us to change our actions in significant ways. In particular, it may seem intuitive that it should lead us to act more cautiously, as David Pearce argues:

So one should always act “unnaturally” responsibly, driving one’s car not just slowly and cautiously, for instance, but ultracautiously. This is because one should aim to minimise the number of branches in which one injures anyone, even if leaving a trail of mayhem is, strictly speaking, unavoidable. If a motorist doesn’t leave a (low-density) trail of mayhem, then quantum mechanics is false. This systematic re-evaluation of ethically acceptable risk needs to be adopted world-wide.

Yet, while intuitive, I would argue that this actually does not follow. For although it may be true that we should generally act much more cautiously than we do, this conclusion is not influenced by MWI, for various reasons.

First, if one is trying to reduce suffering, one should not “aim to minimise the number of branches in which one injures anyone”, but rather seek to reduce as much suffering as possible (in expectation) in the world. At an intuitive level, these may seem equivalent, yet they are not. The former is in fact impossible, as we are bound to injure others, even assuming the existence of just one world, whereas the latter — reducing the greatest amount of suffering possible throughout all branches — is possible by definition.

In particular, this argument for being highly cautious ignores the fact that such caution also carries risks — e.g. extreme caution might increase the probability that we will bring about more suffering by omission, by rendering our efforts to reduce suffering less effective. And these other risks may well be much larger, and thus result in the realization of a larger amount of suffering in a larger measure of branches. In other words, since it is far from clear that being ultracautious is the best way to reduce suffering in expectation throughout all branches, it is far from clear that we should practice such ultracaution in light of MWI.

Second, and quite relatedly, I would argue that, whether we live in many worlds or one, we should seek to minimize expected suffering regardless. For if we happened to exist in one world, a small probability of a very bad outcome would be equally worth avoiding, in expectation, as it would be if we happened to live in a quantum multiverse. Whether we do just one or an arbitrarily large number of “trials”, we should still pursue the same action: that which reduces the most suffering in expectation. 

Third, any argument of the kind made above concerning how all slightly probable outcomes will be realized can also be made by assuming that the multiverse of inflation exists. Thus, if one already believes that we live in a spatially infinite, or indeed “merely” extremely large universe, then the radical conclusions supposed to follow from MWI would already be implied by that belief alone (as we shall see below, many prominent proponents of MWI actually consider MWI not only equivalent but identical with the multiverse of inflation). And if one does not think a spatially very large universe should change how we act, then why think that a large, in many ways equivalent, quantum universe should? As argued above, it seems that no radical conclusions should follow either way. One world or many, we should still do what seems best in expectation.

Another way to arrive at the same conclusion is by embracing Stuart Armstrong’s Anthropic Decision Theory, according to which we, as altruists aiming to reduce suffering, should act the same way regardless of how many similar copies of us there may be in the world.

Would Branching Worlds Imply More Value Later?

Following Bryce DeWitt’s quote about rapidly splitting copies, one can reasonably wonder whether MWI implies that the net amount of value in the world, and hence the value of our actions’ impact on the world, is increasing exponentially over time. Indeed, if we naively interpret DeWitt’s claim to mean that the number of sentient beings that exists is multiplied by 10100 just about every second, this would imply that the value of the very last second of the existence of sentient life should massively dominate every thing else. If this interpretation of MWI is correct, it would have extremely significant ethical implications. Yet is it? It would seem not. Here is Max Tegmark:

Does the number of universes exponentially increase over time? The surprising answer is no. From the bird perspective, there is of course only one quantum universe. From the frog perspective, what matters is the number of universes that are distinguishable at a given instant—that is, the number of noticeably different Hubble volumes. Imagine moving planets to random new locations, imagine having married someone else, and so on. At the quantum level, there are 10 to the 10118 universes with temperatures below 108 kelvins. That is a vast number, but a finite one.

From the frog perspective, the evolution of the wave function corresponds to a never-ending sliding from one of these 10 to the 10118 states to another. Now you are in universe A, the one in which you are reading this sentence. Now you are in universe B, the one in which you are reading this other sentence. Put differently, universe B has an observer identical to one in universe A, except with an extra instant of memories.

Thus, it seems one should think about MWI in terms of an intertwining rope rather than a branching tree. A good way to gain intuition about it may be to think in terms of the multiverse of inflation instead. Indeed, according to prominent proponents of MWI, the many-worlds of quantum mechanics and the multiverse of inflation are not only closely related notions but indeed the same thing, cf. (Aguirre & Tegmark, 2010Nomura, 2011Bousso & Susskind, 2011). In that case, not only is thinking about copies of ourselves in worlds spatially far away from us a great way to gain intuition about MWI; it is the correct way to think about it.

And when we think about it in these terms, it suddenly all becomes quite straightforward and intuitive, at least relatively speaking. For on the inflationary model, there are copies of us in the universe located far away with whom we share our entire history from the big bang up until now. Yet as time progresses, and more different outcomes become possible, the distance to the copies of us that share our exact history becomes ever greater, at a rapid pace, cf. (Garriga & Vilenkin, 2001). Thus, there is indeed a rapid branching in a very real sense, only, this branching consists in departing from “nearby” copies of us who had been just like us up until this point. No new worlds are really added. The “other worlds” were always there, and then merely went their separate ways.

Hence, given the assumptions made here, the number of sentient beings in our world does not in fact increase exponentially in the way naively supposed above, unless one keeps on aggregating over an exponentially larger fraction of the space that already existed. (There is, however, an exponential increase in the number of new universes created by inflating regions of the universe, assuming inflationary theory is correct. Yet this process does not create an exponentially greater number of sentient beings from our point in space and time, i.e. Earth, 13.8 billion years after the big bang. Rather, these new worlds are all created “from scratch”.) In short, MWI does not appear to imply greater value later.


In sum, I have argued that we seem to have good reason to maintain something akin to one-world common sense in most of our decisions (decisions that might influence the creation of new universes would be an exception). This conclusion may, however, be strongly biased given that it comes from a brain that very much wants to preserve common sense.

Ontological Possibilities and the Meaningfulness of Ethics

Are there different possible outcomes of the future? Or phrased more broadly: do ontological possibilities exist? I think this is a profound question, and one may think much depends on our answer to it. For instance, if ‘ought’ truly implies ‘can’, and the only real ‘can’ there is is whatever in fact happens, it would seem to follow that the only thing that could ought happen is what in fact happens. That whatever happens is what ought to happen, if anything.

Ontological possibilities here stand in opposition to what we may call hypothetical, or ex ante possibilities. That is, we are clearly able to think in terms of different outcomes being possible, and to then plan and take action based on such thinking, but that does not imply that those outcomes were ever genuine possibilities in a deep ontological sense.

Worth noting is it that there seems to be a contradiction between two widely shared views that pertain to the existence of ontological possibilities. For on the one hand we have what appears a widely accepted distinction between necessary and contingent truths, necessary truths being ones that must be true because negating them would imply a contradiction with reality (commonly cited examples are 2+2=4 and syllogisms), while a contingent truth is one that could have been false, as its negation (supposedly) does not imply a contradiction with reality (e.g. “Life evolved on Earth”, “Hillary Clinton lost the 2016 election”).

Yet this does not seem consistent with another prevailing belief, namely that the entire world unfolds according to deterministic mathematical equations. If the latter is true, then any truth about the world would appear a necessary one, as its negation indeed would imply a contradiction with the fundamental equation(s) just as much as does 2+2=5.

So what are we to make of this? What could possibly be true about ontological possibilities?

Why Ask This Question?

My reason for examining this question here has to do with ethics – more specifically, it has to do with an objection one might be tempted to level against the sensibility and meaningfulness of ethics. For, in light of the ought-implies-can note above, one might claim that ontological possibilities are necessary in order for ethics proper to get off the ground, and indeed for engagement in ethical reasoning, decision-making, and action of any kind to even make sense.A combustible and controversial claim for sure, yet I shall entertain no discussion of it here.

Instead, my aim here is to argue that if one thinks ontological possibilities are required for “the meaningfulness of ethics” – i.e. required in order for engagement in ethical reasoning, decision-making, and action to make sense – then one cannot reasonably reject such “meaningfulness” with the claim that ontological possibilities are not real. The reason being, in short, that we simply do not know whether such possibilities are real or not, and, as far as I can tell, we all but surely never will.

The Nature of the World: Does Quantum Mechanics Preclude – or Describe – Ontological Possibilities?

In order to say whether ontological possibilities exist, it seems apt to look toward what is arguably our most fundamental and well-tested theory of the world, namely quantum mechanics, and see what it has to say about it.

The answer is that it depends on which interpretation of quantum mechanics is correct, a matter concerning which there is much disagreement among experts. And in light of such disagreement, it only seems reasonable to maintain a substantial degree of uncertainty concerning which interpretation is correct.

As for how one should break this uncertainty down more specifically, there seems plenty of room for reasonable disagreement. For instance, assuming the formalism of quantum mechanics indeed does describe the world in the first place, it seems defensible to place 50 percent probability on the claim that none of the well-known interpretations are correct, and to distribute the remaining 50 percent of one’s credence among the (more or less) mainstream interpretations. This could then be done based on how many subscribers these respective interpretations have among experts, in which case it seems one should assign roughly equal credence to the so-called Many Worlds Interpretation and the Copenhagen Interpretation – perhaps 20 percent to each – while distributing the remaining 10 percent to the remaining ~10 interpretations, resulting in about one percent credence in each of them.

That would be one way one could do it. I don’t necessarily agree with the exact numbers in this distribution; it is just an example that seems within the bounds of defensibility. What does not seem defensible, however, is to maintain complete certainty in the veracity of any one particular interpretation. And this is true for various reasons. For not only are there many competing interpretations that all seem to have at least some strengths and weaknesses, and not only can the possibility that the correct interpretation is a yet unformulated interpretation not be ruled out; the possibility that no interpretation of the formalism is true also seems a very open one.

After all, we have seen it happen before that a prominent physicist considered the, admittedly impressive, canon of physics of his time complete but for a couple of anomalies, only for those anomalies to then revolutionize physics completely within a few years. How can we maintain near-complete confidence that the same could not happen with the physics of our time? Indeed, who is to say that quantum mechanics isn’t still just one of the outer layers of the onion, no less a parochial approximation of the underlying dynamics of reality than classical mechanics ultimately?

Where does all this leave us with respect to ontological possibilities? It leaves us with substantial uncertainty. In particular, it leaves us uncertain about whether the world unfolds according to deterministic mathematical equations or not. Out of the 14 (at the time of writing) established interpretations of quantum mechanics listed on this Wikipedia page, only four are deterministic, seven are indeterministic, while three are agnostic. Consequently, although the deterministic interpretations include the relatively popular Many Worlds Interpretation in which all the mathematically possible outcomes of the formalism are realized, it seems overconfident to have anything near complete certainty that the right interpretation, to the extent there is one, is deterministic.

Indeed, among the 14 interpretation cited above, one of them, the transactional interpretation, is, at least as its proponent Ruth Kastner defends it, explicitly realist about ontological possibilities – a “Many Possibilities Interpretation”, if you will.2 Thus, according to this interpretation, the mathematical formalism of quantum mechanics is in fact the mathematics of ontological possibilities (apropos, perhaps also see possibility theory).

Now, my purpose here is not to settle which interpretation that is most plausible, much less which one is correct, but simply to argue that near-total confidence in the truth of deterministic interpretations is not defensible. On the rough sketch of a credence breakdown above, for instance, the transactional interpretation was given about one percent probability, and I don’t think one can defend giving it several orders of magnitude less than that. And even if one could, there are still other indeterministic interpretations that also deserve at least some weight, as well as a big space of “yet unknown interpretations that could be true” – what was assigned a 50 percent probability above – of which a substantial chunk should be expected to consist of indeterministic interpretations.

Combining these considerations, it seems overconfident to maintain a 99 percent credence in the proposition that the right interpretation of quantum mechanics is deterministic, and the same can reasonably be said, I believe, of any such credence above 95 percent as well.3
One could, of course, argue for uncertainty about this question through other routes as well, such as by invoking the idea that we always should maintain at least some degree of doubt, however minute, about any claim (an idea I have defended here), or the related claim that our entire conception of reality – not merely our physics – might be wrong, or perhaps not even that. Yet, as the argument above should make clear, one need not resort to these claims in order to reach the conclusion that it is reasonable to have a non-trivial degree of uncertainty regarding the matter of determinism and ontological possibilities.

[Another case for the existence of ontological possibilities, a case not made at the quantum level, is Helen Steward’s argument for the reality of “agential settling”, which she has presented in the book A Metaphysics for FreedomThus, one can be forced to increase one’s credence in the existence of ontological possibilities further by assigning a small credence to this position as well.]

Implications for (Objections Against) the Meaningfulness of Ethics

The conclusion above means that, if ontological possibilities are required for “the meaningfulness of ethics” – the conditional assumption that was our point of departure – the uncertainty we should maintain concerning the reality of such possibilities means that a rejection of ethics on the basis that ontological possibilities do not exist is unwarranted.

To be sure, one may reasonably argue that, if ontological possibilities are required for “the meaningfulness of ethics”, then it seems likely that no such meaningfulness exists. Yet that is a far cry from a refutation of such meaningfulness. For consider by analogy the claim that risks of terrible future outcomes are low, and hence that such outcomes most likely will not be realized. Even if such a claim were true, it by no means follows that such risks can reasonably be dismissed.

When the stakes are sufficiently high, it is not reasonable to dismiss low probabilities. And when we are discussing the meaningfulness of ethics, it seems that the stakes could not possibly be greater, as the subject in question comes down to whether there indeed are any stakes at all. In light of such stakes, even extremely low probabilities should be taken seriously; and yet the level of uncertainty we found reasonable to maintain concerning ontological possibilities was not extremely low, much less Pascallian by any stretch.4 Thus, if ontological possibilities are indeed required for the meaningfulness of ethics, the epistemic possibility of the reality of such ontological possibilities should be taken very seriously indeed.

Moreover, when considering the outcomes of the options before us, an emerging asymmetry appears to make the choice clear. For if ontological possibilities are real, and ethical action indeed amounts to trying to realize the best of these possibilities – to create the best ontologically possible world, if you will, or at the very least avoid the worst ones – it would seem that we have good reason to try to act accordingly, and no compelling reason not to. If, on the other hand, ontological possibilities are not real, trying to realize the best possible world appears to have no cost. We thus seem to have a strong reason in favor of trying to create a better world, and no reason against it.

Lastly, if we entertain the negation of the assumption that served as our starting point, namely that ontological possibilities are not required for the meaningfulness of ethics – again without saying whether this claim is true or not – we appear to arrive at the same conclusion: we have no reason to consider ethics meaningless or to not try our best. In conclusion, no matter our starting point, the meaningfulness of ethics seems on firm ground.

1. Alternatively, I could also say “the meaningfulness of trying to improve the world” or of “trying to improve one’s own situation.” These things ultimately all mean the same in my view, cf. You Are Them. Yet even if one considers these statements different, the argument I make here still applies equally to them all. That is, one can readily swap, say, “the meaningfulness of ethics” with “the meaningfulness of trying to improve one’s own situation”, and the argument would run just the same.

2. Although it should be noted that other indeterministic interpretations also hold ontological possibilities to be real, at least implicitly.

3. I have not discussed eternalism here, which is a deterministic view, but suffice it to say that, at the very least, one ought to maintain substantial doubt on this matter as well, one reason being that the mere fact that the physical equations do not appear to require an ontologically real present does not, contrary to what seems widely believed, imply that there is no ontologically real present. Much confusion about this issue seems to emerge from the belief that “ontological present” must mean “all clocks show the same”; presentism in that sense is surely as dead as can be, but presentism in general is not – there is no contradiction whatsoever about an ontological present in which there are (initially synchronized) clocks that show different times.

Beyond that, to turn the tables a bit, one might also ask why someone who holds an eternalist view would act to influence the future rather than the past, given that past and future both already exist on this view. It seems to me that eternatlists are aligned with common sense rather than their own view of time in this respect. Also, to what extent does it make sense to say that all moments exist “always”? After all, doesn’t “always” refer to something occurring over time? The meaning of claims of the sort that “every moment exists always” is, I believe, less obvious than proponents of eternalism appear to think, and seems in need of unpacking.

Yet, once again, the main point I wish to drive home here is not that people should consider presentism most plausible, but merely that we should maintain substantial uncertainty concerning this question as well.

4. What might perhaps be considered Pascallian, or at least more so, is the proposition that we are living in the multiverse described by the Many Worlds Interpretation and that ontological possibilities exist within this multiverse. Yet in this case the stakes appear to become more than great enough to justify even Pascallian probabilities. Hence, this possibility appears worth taking seriously as well.


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