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

There is more than one many worlds interpretation. The version stated above is not known to be true.

There is an approach to MWI based on coherent superpositions, and a version based on decoherence. These are (for all practical purposes) incompatible. Coherent splitting gives you the very large numbers of "worlds"..except that they are not worlds, conceptually.

Many worlders are pointing at something in the physics and saying "that's a world"....but whether it qualifies as a world is a separate question , and a separate kind of question, from whether it is really there in the physics. One would expect a world, or universe, to be large, stable, non-interacting, objective and so on . A successful MWI needs to jump three hurdles: mathematical correctness, conceptual correctness, and empirical correctness.

Decoherent branches are expected to be large, stable, non interacting, objective and irreversible...everything that would be intuitively expected of a "world". But there is no empirical evidence for them , nor are they obviously supported by the core mathematics of quantum mechanics, the Schrödinger equation. Coherent superpositions are small scale , down to single particles, observer dependent, reversible, and continue to interact (strictly speaking , interfere) after "splitting".

(Note that Wallace has given up on the objectivity of decoherent branches. That's another indication that MWI is not a single theory).

There isn’t the slightest evidence that irrevocable splitting, splitting into decoherent branches occurs at every microscopic event -- that would be combining the frequency of coherent style splitting with the finality of decoherent splitting. We dont know much about decoherence , but we know it is a multi-particle process that takes time, so decoherent splitting, if there is such a thing, must be rarer than the frequency of single particle interactions. ( And so decoherence isn't simple ). As well as the conceptual incoherence, there is In fact plenty of evidence—eg. the existence of quantum computing—that it doesnt work that way

Also see

https://www.lesswrong.com/posts/wvGqjZEZoYnsS5xfn/any-evidence-or-reason-to-expect-a-multiverse-everett?commentId=o6RzrFRCiE5kr3xD4

I’m not going to argue for this view as that was done very well by Eliezer in his Quantum Physics.

Which view? Everetts view? DeWitts view? Deutsch's Zeh's view? Wallace's view? Saunders view?

@dr_s

I feel like branches being in fact an uncountable continuum is essentially a given

Decoherent branches being being countable, uncountable, or anything else is not given, since there is no established theory of of decoherence.

It's a given that some observables have continuous spectra..but what's that got to do with splitting? A observed state that isn't sharp (in some basis) can get entangled with an apparatus, which then goes into a non-sharp state, and so on. And the whole shebang never splits , or becomes classically sharp.

I mean that the amount of universes that is created will be created anyway, just as a consequence of time passing. So it doesn’t matter anyway. If your actions e.g. cause misery in 20% of those worlds, then the fraction is all that matters; the worlds will exist anyway, and the total amount is not something you’re affecting or controlling.

That's a special case of "no moral responsibility under determinism". which might be true , but it's very different from "utilitarianism works fine under MWI".

**Enough of the physics confusions -- onto the ethics confusions!""

As well as confusion over the correct version of many worlds, there is of course confusion about which theory of ethics is correct.

There's broadly three areas where MWI has ethical implications. One is concerned with determinism, freedom of choice, and moral responsibility. One is over the fact that MW means low probability events have to happen every time -- as opposed to single universe physics, where they usually don't. The other is over whether they are discounted in moral significance for being low in quantum mechanical measure or probability

MWI and Free Will

MWI allows probabilities of world states to change over time, but doesn't allow them to be changed, in a sense amounting to libertarian free will. Agents are just part of the universal wave function, not anything outside the system, or operating by different rules.MWI is, as it's proponents claim, a deterministic theory, and it only differs from single world determinism in that possible actions can't be refrained from, and possible futures can't be avoided. Alternative possibilities are realities, in other words.

MWI, Moral Responsibility, and Refraining.

A standard argument holds that causal determinism excludes libertarian free will by removing alternative possibilities. Without alternative possibilities, you could but have done other than you did, and , the argument goes, you cannot be held responsible for what you had no choice but to do.

Many world strongly implies that you make all possible decisions: according to David Deutsch's argument that means it allows alternative possibilities, and so removes the objection from moral responsibility despite being a basically deterministic theory.

However, deontology assumed that performing a required act involves restraining from alternatives.. and that it is possible to retain from forbidden acts. Neither is possible under many worlds. Many worlds creates the possibility, indeed the necessity, of doing otherwise, but removes the possibility of refraining from an act. Even though many worlds allows Alternative Possibilities, unfortunately for Deutschs argument, that other aspects create a similar objection on the basis of moral responsibility: why would you hold someone morally responsible for an act if they could not refrain from it?

MWI, Probability, and Utilitarian Ethics

Its tempting to think that you can apply a standard decision theory in terms of expected value to Many Worlds, since it is a matter of multiplying subjective value by probability. One wrinkle is that QM measure isn't probability -- the probability of something occurring or not -- because all possible branches occur in MWI. Another is that it is reasonable to assess the moral weight of someone else's experiences and existence from their point of view. That is the intuition behind the common rationalist/utilitarian/EA view that human lives don't decline in moral worth with distance. So why should they decline with lower quantum mechanical measure? There is quandary here: sticking to the usual "adds up to normality" principle,as an apriori axiom means discounting the ethical importance of low-measure worlds in order to keep your favourite decision theory operating in the usual single-universe way, even if you are in a multiverse. But sticking to the equally usual universalist axiom, that that you dont get to discount someone's moral worth on the basis of factors that aren't intrinsic to them, means you should not

Measure is not probability.

Mathematically, Quantum mechanical measure—amplitude—isn’t ordinary probability, which is why you need the Born rule.The point of the Born rule is to get a set of ordinary probabilities, which you can then test frequentistically, over a run of experiments. Ontologcally, it also not probability, because it does not represent the likelihood of one happening instead of another. And it has its own role, unlike that if ordinary probability, which is explaining how much contribution to a coherent superposition each component state makes (although what that means in the case of irrevocably decohered branches is unclear)

Whether you are supposed to care about them ethically is very unclear, since it is not clear how utilitarian style ethics would apply, even if you could make sense of the probabilities. But you are not supposed to care about them for the purposes of doing science, since they can no longer make any difference to your branch. MWI works like a collapse theory in practice.

The Ethical Weight or Low Measure Worlds

MWI creates the puzzle that low probability outcomes still happen, and have to be taken into account ethically. Many rationalists assume that they simply matter less, because that is the only way to restore anything like a normal view of ethical action -- but one should not assume something merely because it is convenient.

It can be argued that most decision theoretic calculations come out the same under different interpretations of QM...but altruistic ethics is different. In standard decision theory, you can tell directly how much utility you are getting; but in altruistic ethics , you are not measuring your suffering/happiness, you are assessing someone else's...and in the many worlds setting, that means solving the problem of how they are affected by their measure. It is not clear how low measure worlds should be considered in utilitarian ethics. It's tempting to ethically discount low measure worlds in some way, because that most closely approximates conventional single world utilitarianism. The alternative might force one to the conclusion that overall good outcomes are impossible to attain , so long as one cannot reduce the measures of worlds full of suffering zero. However, one should not jump to the conclusion that something is true just because it is convenient. And of course, MWI is a scientific theory so it doesn't comes with built in ethics

One part of the problem is that QM measure isn't probability, because all possible branches occur in MWI. Another stems from the fact that what other people experience is relevant to them, wheareas for a probability calculation, I only need to be able to statistically predict my own observations.. Using QM to predict my own observations, I can ignore the question of whether something has a ten percent chance of happening in the one and only world, or a certainty of happening in one tenth of possible worlds. However, these are not necessarily.equivalent ethically.

Suppose they low measure worlds are discounted ethically. If people in low measure worlds experience their suffering fully, then a 1%, of creating a hell-world would be equivalent in suffering to a 100% chance, and discount is unjustified. But if people in low measure worlds are like philosophical zombies, with little or no phenomenal consciousness, so that their sensations are faint or nonexistent, the moral hazard is much lower, and the discount is justified. A point against discounting is that our experiences seem fully real to us, although we are unlikely to be in a high measure world

A similar, but slightly less obvious argument applies to causing death. Causing the "death" of a complete zombie is presumably as morally culpable as causing the death of a character in a video game...which, by common consent, is not problem at all. So... causing the death of a 50% zombie would be only half as bad as killing a real person...maybe.

Classical Measure isn't Quantum Mechanical Measure

A large classical universe is analogous to Many Worlds in that the same structures -- the same people and planets -- repeat over long distances. It's even possible to define a measure, by counting repetitions up to a certain level of similarity. And one has the option if thinking about Quantum Mechanical measure that way,as a "head count"....but one is not forced to do so. On one hand, it features normality, on the other hand It is not "following the maths" ,because there's nothing in the formalism to suggest summing a number of identical low measure states is the only way to get a high measure one. So, again, it’s an extraneous assumption, and circular reasoning .

Ethical Calculus is not Decision Theory

Of course, MWI doesn't directly answer the question about consciousness and zombiehood .You can have objective information about observations, and if your probability calculus is wrong , you will get wrong results and know that you are getting wrong results. That is the negative feedback that allows physics to be less wrong. And you can have subjective information about your own mental states, and if your personal calculus is wrong , you will get wrong results and know that you are getting wrong results. That is the negative feedback that allows personal decision theory to be less wrong.

Altruistic ethics is different. You don't have either kind of direct evidence, because you are concerned with other people's subjective sensations , not objective evidence , or your own subjectivity. Questions about ethics are downstream of questions about qualia, and qualia are subjective, and because they are subjective, there is no reason to expect them to behave like third person observations.

"But it all adds up to normality!"

If "it all" means every conjecture you can come up with, no It doesn't. Most conjectures are wrong. The point of empirical testing is to pick out the right ones -- the ones that make correct predictions, save appearances, add up to normality That's a difficult process, not something you get for free.

So "it all adds up to normality" is not some universal truth And ethical theories relating to someone else's feelings are difficult to test, especially if someone else is in the far future, or an unobservable branch of the multiverse. Testability isn't an automatic given either.

There are no major ethical implications at all...Wallace makes a similar claim in his book: “But do [the many worlds in MWI] matter to ordinary, banal thought, action and language? Friendship is still friendship. Boredom is still boredom. Sex is still sex

That's very narrow circle ethics, if it's ethics at all --he just likes a bunch of things that impact him directly And it's rather obvious that small circle ethical theories have the least interaction with large universe physical theories. So it likely he hasn't even considered the question of altruistic ethics in many worlds, and is therefore coming to the conclusion that it all adds up to normality rather cheaply. It's his ethical outlook that is the structural element , not his take on MWI.