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 may influence the creation of new universes seem an exception). This conclusion may, however, be strongly biased given that it comes from a brain that very much wants to preserve common sense. The upshot of all of this, I think, is that we should 1) primarily seek to minimize suffering in expectation in the one, concrete world we appear to inhabit, while 2) still further explore ways in which this first, rather convenient conclusion may be confounded.

A Brief Note on Eternalism and Impacting the Future

Something I find puzzling is that many people in intellectual circles seem to embrace the so-called eternalist view of time, which holds that the past, present, and future all equally exist already, yet at the same time, in terms of practical ethics, these same people focus exclusively on impacting the future. These two positions do not seem compatible, and it is interesting that no one seems to take note of this, and that no attempt seems to be made at reconciling them, or otherwise examining this issue. 

For why, given an eternalist view of time, should one focus on impacting the future rather than the past? After all, the eternalist view of time amounts precisely to the rejection of the common sense view that the past is fixed while the future is not, which is the common sense view of time that seems to underpin our common sense focus on trying to impact the future rather than the past. So how can one reject the common sense view of time that seems to underlie our common sense practical focus, yet then still maintain this focus? If the past and the future equally exist already, why focus more on trying to impact one rather than the other?

The only attempted reply I have heard to this question so far, which came from Brian Tomasik, is that if, hypothetically, the present were different, then the future would be different, and hence it makes sense to focus on such changes that would render the future different. The problem, however, is that the same argument applies to the past: if, hypothetically, the present were different, then, for the equations of physics to be consistent, the past would also have to be different. Tomasik seemed to agree with this point. So I fail to see how this is an argument for focusing on impacting the future rather than the past given an eternalist view of time.

Possible Responses

There are various ways to respond to this conundrum. One can, for instance, try to argue that there is no conflict between eternalism and focusing only on impacting the future (which seems the prevailing assumption, but I have yet to see it defended). Another path one could take is to argue that we in fact should focus on impacting the past just as much as the future (something I find highly dubious). Alternatively, one could argue that it is just as senseless to try to change the future as it is to change the past (something few would be willing to accept in practice). Lastly, one could take the tension between these two widely esteemed views to imply that there may be something wrong with the eternalist view of time, and at the very least that we should lower our credence in eternalism, given its ostensible incompatibility with other, seemingly reasonable beliefs.

My Preferred Path: Questioning Eternalism

I would be curious to see attempts along any of the four paths mentioned above. I myself happen to lean toward the last one. I think many people display overconfidence with respect to the truth of eternalism. The fact that the equations of the theory of relativity, as they stand, do not demand an ontologically existing “now does not imply that no such thing exists (where this now, it must be noted, is not defined by “clocks all show the same”, as such a now clearly is impossible; yet there is no contradiction whatsoever in the existence of a unique, ontologically real “present” in which initially synchronized clocks show different times). In other words, although the equations of relativity do not demand the existence of such a now, they do not rule it out either. Yet it seems a widely entertained fallacy that they do, and people thus seem to accept that eternalist view as though it were a matter of logical certainty, when it is not. I think this is bad philosophy. And I think it is important to point this out, since false certainties can be dangerous in unexpected ways (for example, if the above-mentioned fallacy led us to falsely conclude that trying to impact the future is senseless).

Beyond that, as I have noted elsewhere, one can also question to what extent it makes sense to say — as eternalists often do, and as the name eternalism itself implies — 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.

A General Note on Our Worldview

I think the tension explored here speaks to a more general point about our worldview, namely that we often do not derive the more practical views we hold (such as the view that we can influence the future but not the past), from our fundamental ontological theories of how the world works. Instead, such views are often derived mostly from tacit common sense notions and intuitions (which is not to say that they should necessarily be rejected, and certainly not on this ground alone). This means that sometimes — quite often, in fact — the views we hold on various subjects, such as the philosophy of time and practical ethics, are scarcely compatible. The project of bringing the various beliefs we hold across these different areas in concert is, I believe, an important and potentially fruitful one, for our theoretical views in themselves, as well as for our pratical efforts to act reasonably in the world.

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