# Many Worlds, One Best Guess

**Previously in series**: Collapse Postulates**Followup to**: Bell's Theorem, Spooky Action at a Distance, Quantum Non-Realism, Decoherence is Simple, Falsifiable and Testable

If you look at many microscopic physical phenomena—a photon, an electron, a hydrogen atom, a laser—and a million other known experimental setups—it is possible to come up with simple laws that seem to govern all small things (so long as you don't ask about gravity). These laws govern the evolution of a highly abstract and mathematical object that I've been calling the "amplitude distribution", but which is more widely referred to as the "wavefunction".

Now there are gruesome questions about the proper generalization that covers all these tiny cases. Call an object 'grue' if it appears green before January 1, 2020 and appears blue thereafter. If all emeralds examined so far have appeared green, is the proper generalization, "Emeralds are green" or "Emeralds are grue"?

The answer is that the proper generalization is "Emeralds are green". I'm not going to go into the arguments at the moment. It is not the subject of this post, and the obvious answer in this case happens to be correct. The true Way is not stupid: however clever you may be with your logic, it should finally arrive at the right answer rather than a wrong one.

In a similar sense, the *simplest* generalizations that would cover observed *microscopic* phenomena alone, take the form of "All electrons have spin 1/2" and not "All electrons have spin 1/2 before January 1, 2020" or "All electrons have spin 1/2 unless they are part of an entangled system that weighs more than 1 gram."

When we turn our attention to macroscopic phenomena, our sight is obscured. We cannot experiment on the wavefunction of a human in the way that we can experiment on the wavefunction of a hydrogen atom. In no case can you actually read off the wavefunction with a little quantum scanner. But in the case of, say, a human, the size of the entire organism defeats our ability to perform precise calculations or precise experiments—we cannot confirm that the quantum equations are being obeyed *in precise detail.*

We know that phenomena commonly thought of as "quantum" do not just disappear when many microscopic objects are aggregated. Lasers put out a flood of coherent photons, rather than, say, doing something completely different. Atoms have the chemical characteristics that quantum theory says they should, enabling them to aggregate into the stable molecules making up a human.

So in one sense, we have a great deal of evidence that quantum laws are aggregating to the macroscopic level without too much difference. Bulk chemistry still works.

But we cannot directly verify that the particles making up a human, have an aggregate wavefunction that behaves *exactly *the way the simplest quantum laws say. Oh, we know that molecules and atoms don't disintegrate, we know that macroscopic mirrors still reflect from the middle. We can get *many* high-level predictions from the assumption that the microscopic and the macroscopic are governed by the same laws, and every prediction tested has come true.

But if someone were to claim that the macroscopic quantum picture, differs from the microscopic one, in some as-yet-untestable detail—something that only shows up at the unmeasurable 20th decimal place of microscopic interactions, but aggregates into something bigger for macroscopic interactions—well, we can't *prove* they're wrong. It is Occam's Razor that says, "There are zillions of new fundamental laws you could postulate in the 20th decimal place; why are you even *thinking* about this one?"

If we calculate using the simplest laws which govern all known cases, we find that humans end up in states of quantum superposition, just like photons in a superposition of reflecting from and passing through a half-silvered mirror. In the Schrödinger's Cat setup, an unstable atom goes into a superposition of disintegrating, and not-disintegrating. A sensor, tuned to the atom, goes into a superposition of triggering and not-triggering. (Actually, the superposition is now a joint state of [atom-disintegrated * sensor-triggered] + [atom-stable * sensor-not-triggered].) A charge of explosives, hooked up to the sensor, goes into a superposition of exploding and not exploding; a cat in the box goes into a superposition of being dead and alive; and a human, looking inside the box, goes into a superposition of throwing up and being calm. The same law at all levels.

Human beings who interact with superposed systems will themselves evolve into superpositions. But the brain that sees the exploded cat, and the brain that sees the living cat, will have many neurons firing differently, and hence many *many* particles in different positions. They are very distant in the configuration space, and will communicate to an exponentially infinitesimal degree. Not the 30th decimal place, but the 10^{30}th decimal place. No particular mind, no particular cognitive causal process, sees a blurry superposition of cats.

The fact that "you" only seem to see the cat alive, *or* the cat dead, is exactly what the simplest quantum laws predict. So we have no reason to believe, from our experience so far, that the quantum laws are in any way different at the macroscopic level than the microscopic level.

And physicists have verified superposition at steadily larger levels. Apparently an effort is currently underway to test superposition in a 50-micron object, larger than most neurons.

The existence of other versions of ourselves, and indeed other Earths, is not supposed *additionally.* We are simply supposing that the same laws govern at all levels, having no reason to suppose differently, and all experimental tests having succeeded so far. The existence of other decoherent Earths is a *logical consequence* of the simplest generalization that fits all known facts*.* If you think that Occam's Razor says that the other worlds are "unnecessary entities" being multiplied, then you should check the probability-theoretic math; that is just not how Occam's Razor works.

Yet there is one particular puzzle that seems odd, in trying to extend microscopic laws universally incluing to superposed humans:

*If* we try to get probabilities by counting the number of distinct observers, then there is no *obvious* reason why the integrated squared modulus of the wavefunction should correlate with statistical experimental results. There is no known reason for the Born probabilities, and it even seems that, a priori, we would expect a 50/50 probability of any binary quantum experiment going both ways, if we just counted observers.

Robin Hanson suggests that if exponentially tinier-than-average decoherent blobs of amplitude ("worlds") are interfered with by exponentially tiny leakages from larger blobs, we will get the Born probabilities back out. I consider this an interesting possibility, because it is so normal.

(I myself have had recent thoughts along a different track: If I try to count observers the obvious way, I get strange-seeming results in general, not just in the case of quantum physics. If, for example, I split my brain into a trillion similar parts, conditional on winning the lottery while anesthetized; allow my selves to wake up and perhaps differ to small degrees from each other; and then merge them all into one self again; then counting observers the obvious way says I should be able to make myself win the lottery (if I can split my brain and merge it, as an uploaded mind might be able to do).

In this connection, I find it very interesting that the Born rule does

nothave a split-remerge problem. Given unitary quantum physics, Born's rule is theuniquerule that prevents "observers" from having psychic powers—which doesn'texplainBorn's rule, but is certainly aninteresting fact. Given Born's rule, even splitting and remerging worlds would still lead to consistent probabilities. Maybe physics uses better anthropics than I do!Perhaps I should take my cues from physics, instead of trying to reason it out a priori, and see where that leads me? But I have not been led anywhere

yet,so this is hardly an "answer".)

Wallace, Deutsch, and others try to derive Born's Rule from decision theory. I am rather suspicious of this, because it seems like there is a component of "What happens to me?" that I cannot alter by modifying my utility function. Even if I didn't *care* at all about worlds where I didn't win a quantum lottery, it still seems to me that there is a sense in which I would "mostly" wake up in a world where I didn't win the lottery. It is this that I think needs explaining.

The point is that many hypotheses about the Born probabilities have been proposed. Not as many as there should be, because the mystery was falsely marked "solved" for a long time. But still, there have been many proposals.

There is legitimate hope of a solution to the Born puzzle without new fundamental laws. Your world does not split into exactly two new subprocesses on the exact occasion when you see "ABSORBED" or "TRANSMITTED" on the LCD screen of a photon sensor. We are constantly being superposed and decohered, all the time, sometimes along continuous dimensions—though brains are digital and involve whole neurons firing, and fire/not-fire would be an extremely decoherent state even of a *single *neuron... There would seem to be room for *something* unexpected to account for the Born statistics—a better understanding of the anthropic weight of observers, or a better understanding of the brain's superpositions—without new fundamentals.

We cannot rule out, though, the possibility that a new fundamental law is involved in the Born statistics.

If there's one lesson we can take from the history of physics, it's that everytime new experimental "regimes" are probed (e.g. large velocities, small sizes, large mass densities, large energies), phenomena are observed which lead to new theories (special relativity, quantum mechanics, general relativity, and the standard model, respectively).

"Every time" is too strong. A nitpick, yes, but also an important point: you can't just *assume* that any particular law will fail in a new regime. But it's possible that a new fundamental law is involved in the Born statistics, and that this law manifests only in the 20th decimal place at microscopic levels (hence being undetectable so far) while aggregating to have substantial effects at macroscopic levels.

Could there be some law, as yet undiscovered, that causes there to be only *one* world?

This is a shocking notion; it implies that all our twins in the other worlds—all the different versions of ourselves that are constantly split off, not just by human researchers doing quantum measurements, but by ordinary entropic processes—are actually *gone*, leaving us alone! This version of Earth would be the *only* version that exists in local space! If the inflationary scenario in cosmology turns out to be wrong, and the topology of the universe is both finite and relatively small—so that Earth does not have the distant duplicates that would be implied by an exponentially vast universe—then this Earth could be the only Earth that exists *anywhere,* a rather unnerving thought!

But it is dangerous to focus too much on specific hypotheses that you have no specific reason to think about. This is the same root error of the Intelligent Design folk, who pick any random puzzle in modern genetics, and say, "See, God must have done it!" Why 'God', rather than a zillion other possible explanations?—which you would have thought of long before you postulated divine intervention, if not for the fact that you secretly started out already knowing the answer you wanted to find.

You shouldn't even *ask*, "Might there only be one world?" but instead just go ahead and do physics, and raise that *particular *issue only if new evidence demands it.

Could there be some as-yet-unknown fundamental law, that gives the universe a privileged center, which happens to coincide with Earth—thus proving that Copernicus was wrong all along, and the Bible right?

Asking *that particular* question—rather than a zillion other questions in which the center of the universe is Proxima Centauri, or the universe turns out to have a favorite pizza topping and it is pepperoni—betrays your hidden agenda. And though an unenlightened one might not realize it, giving the universe a privileged center *that follows Earth around through space* would be rather difficult to do with any *mathematically simple* fundamental law.

So too with asking whether there might be only one world. It betrays a sentimental attachment to human intuitions already proven wrong. The wheel of science turns, but it doesn't turn *backward.*

We have specific reasons to be highly suspicious of the notion of only one world. The notion of "one world" exists on a higher level of organization, like the location of Earth in space; on the quantum level there are no firm boundaries (though brains that differ by entire neurons firing are certainly decoherent). How would a *fundamental* physical law identify one *high-level* world?

*Much worse*, any physical scenario in which there was a *single *surviving world, so that any measurement had only a *single* outcome, would violate Special Relativity.

If the same laws are true at all levels—i.e., if many-worlds is correct—then when you measure one of a pair of entangled polarized photons, you end up in a world in which the photon is polarized, say, up-down, and alternate versions of you end up in worlds where the photon is polarized left-right. From your perspective before doing the measurement, the probabilities are 50/50. Light-years away, someone measures the other photon at a 20° angle to your own basis. From their perspective, too, the probability of getting either immediate result is 50/50—they maintain an invariant state of generalized entanglement with your faraway location, no matter what you do. But when the two of you meet, years later, your probability of meeting a friend who got the *same* result is 11.6%, rather than 50%.

If there is only one global world, then there is only a single outcome of any quantum measurement. Either you measure the photon polarized up-down, or left-right, but not both. Light-years away, someone else's probability of measuring the photon polarized similarly in a 20° rotated basis, actually *changes* from 50/50 to 11.6%.

You cannot possibly interpret this as a case of merely revealing properties that were already there; this is ruled out by Bell's Theorem. There does not seem to be any possible consistent view of the universe in which both quantum measurements have a single outcome, and yet both measurements are predetermined, neither influencing the other. Something has to actually *change,* faster than light.

And this would appear to be a fully general objection, not just to collapse theories, but to any possible theory that gives us one global world! There is no consistent view in which measurements have single outcomes, but are locally determined (even locally randomly determined). Some mysterious influence has to cross a spacelike gap.

This is not a trivial matter. You cannot save yourself by waving your hands and saying, "the influence travels backward in time to the entangled photons' creation, then forward in time to the other photon, so it never actually crosses a spacelike gap". (This view has been seriously put forth, which gives you some idea of the magnitude of the paradox implied by one global world!) One measurement has to change the other, so which measurement happens *first?* Is there a global space of simultaneity? You can't have both measurements happen "first" because under Bell's Theorem, there's no way local information could account for observed results, etc.

Incidentally, this experiment has already been performed, and if there is a mysterious influence it would have to travel six million times as fast as light in the reference frame of the Swiss Alps. Also, the mysterious influence has been experimentally shown not to care if the two photons are measured in reference frames which would cause each measurement to occur "before the other".

Special Relativity seems counterintuitive to us humans—like an arbitrary speed limit, which you could get around by going backward in time, and then forward again. A law you could escape prosecution for violating, if you managed to hide your crime from the authorities.

But what Special Relativity really says is that human intuitions about space and time are simply wrong. There *is *no global "now", there *is* no "before" or "after" across spacelike gaps. The ability to *visualize* a single global world, *even in principle,* comes from not getting Special Relativity on a gut level. Otherwise it would be obvious that physics proceeds locally with invariant states of distant entanglement, and the requisite information is simply *not* *locally present* to support a *globally single world*.

It might be that this seemingly impeccable logic is flawed—that my application of Bell's Theorem and relativity to rule out any single global world, contains some hidden assumption of which I am unaware -

- but consider the burden that a single-world theory must now shoulder! There is absolutely no reason *in the first place* to suspect a global single world; this is just *not what current physics says!* The global single world is an ancient human intuition that was *disproved*, like the idea of a universal absolute time. The superposition principle is visible even in half-silvered mirrors; experiments are verifying the disproof at steadily larger levels of superposition—but above all there is *no longer any reason* to *privilege the hypothesis *of a global single world. The ladder has been yanked out from underneath that human intuition.

There is no experimental evidence that the macroscopic world is single (we already know the microscopic world is superposed). And the prospect necessarily either violates Special Relativity, or takes an even more miraculous-seeming leap and violates seemingly impeccable logic. The latter, of course, being much more plausible in practice. But it isn't really *that* plausible in an absolute sense. *Without experimental evidence,* it is generally a *bad sign* to have to postulate arbitrary logical miracles.

As for quantum non-realism, it appears to me to be nothing more than a Get-Out-Of-Jail-Free card. "It's okay to violate Special Relativity because none of this is real anyway!" The equations cannot reasonably be hypothesized to deliver such excellent predictions *for literally no reason. *Bell's Theorem rules out the obvious possibility that quantum theory represents imperfect knowledge of something locally deterministic.

Furthermore, macroscopic decoherence gives us a perfectly *realistic* understanding of what is going on, in which the equations deliver such good predictions because they mirror reality. And so the idea that the quantum equations are just "meaningless", and therefore, it is okay to violate Special Relativity, so we can have one global world after all, is not *necessary.* To me, quantum non-realism appears to be a huge bluff built around semantic stopsigns like "Meaningless!"* *

It is not quite safe to say that the existence of multiple Earths is as well-established as any other truth of science. The existence of quantum other worlds is not so well-established as the existence of trees, which most of us can personally observe.

Maybe there is something in that 20th decimal place, which aggregates to something bigger in macroscopic events. Maybe there's a loophole in the seemingly iron logic which says that any single global world must violate Special Relativity, because the information to support a single global world is not locally available. And maybe the Flying Spaghetti Monster is just messing with us, and the world we know is a lie.

So all we can say about the existence of multiple Earths, is that it is as rationally probable as e.g. the statement that spinning black holes do not violate conservation of angular momentum. We have extremely fundamental reasons, having to do with the rotational symmetry of space, to suspect that conservation of angular momentum is built into the underlying nature of physics. And we have no specific reason to suspect this *particular* violation of our old generalizations in a higher-energy regime.

But we haven't actually checked conservation of angular momentum for rotating black holes—so far as I know. (And as I am talking here about rational guesses in states of partial knowledge, the point is exactly the same if the observation has been made and I do not know it yet.) And black holes are a more massive regime. So the obedience of black holes is not *quite* as assured as that my toilet conserves angular momentum while flushing, which come to think, I haven't checked either...

Yet if you make the *mistake* of thinking too hard about this one particular possibility, instead of zillions of other possibilities—and especially if you don't understand the fundamental reason *why *angular momentum is conserved—then it may start seeming more and more plausible that "spinning black holes violate conservation of angular momentum", as you think of more and more vaguely plausible-sounding reasons it *could* be true.

But the rational probability is pretty damned small.

Likewise the rational probability that there is only one Earth.

I mention this to explain my habit of talking as if many-worlds is an obvious fact. Many-worlds *is* an obvious fact, if you have all your marbles lined up correctly (understand very basic quantum physics, know the formal probability theory of Occam's Razor, understand Special Relativity, etc.) It is in fact considerably *more *obvious to me than the proposition that spinning black holes should obey conservation of angular momentum.

The only reason why many-worlds is not universally acknowledged as a direct prediction of physics which requires magic to violate, is that a contingent accident of our Earth's scientific history gave an entrenched academic position to a phlogiston-like theory which had an unobservable faster-than-light magical "collapse" devouring all other worlds. And many academic physicists do not have a mathematical grasp of Occam's Razor, which is the usual method for ridding physics of invisible angels. So when they encounter many-worlds and it conflicts with their (undermined) intuition that only one world exists, they say, "Oh, that's multiplying entities"—which is just flatly wrong as probability theory—and go on about their daily lives.

I am not in academia. I am not constrained to bow and scrape to some senior physicist who hasn't grasped the obvious, but who will be reviewing my journal articles. I need have no fear that I will be rejected for tenure on account of scaring my students with "science-fiction tales of other Earths". If *I* can't speak plainly, who can?

So let me state then, very clearly, on behalf of any and all physicists out there who dare not say it themselves: Many-worlds *wins outright* given our current state of evidence. There is no more reason to postulate a single Earth, than there is to postulate that two colliding top quarks would decay in a way that violates conservation of energy. It takes more than an unknown fundamental law; it takes magic.

*The debate should already be over. It should have been over fifty years ago. The state of evidence is too lopsided to justify further argument. There is no balance in this issue. There is no rational controversy to teach. The laws of probability theory are laws, not suggestions; there is no flexibility in the best guess given this evidence. Our children will look back at the fact that we were STILL ARGUING about this in the early 21st-century, and correctly deduce that we were nuts.*

We have embarrassed our Earth long enough by failing to see the obvious. So for the honor of my Earth, I write as if the existence of many-worlds were an established fact, because it *is.* The only question now is how long it will take for the people of this world to update.

Part of *The Quantum Physics Sequence*

Next post: "Living in Many Worlds"

Previous post: "If Many-Worlds Had Come First"

## Comments (76)

OldCorrection: Eliezer, you wrote "Jess Reidel" but correct is "Jess RIEDEL".

False for three reasons.

First: The Born probabilities. That is where

allthe predictive power of quantum theory is located. If you don't have those, you just have a qualitative world-picture, one of many possibilities.Second: There is no continuity of identity in time of a world, as I suppose we shall see in the Julian Barbour instalment; nothing to relate the worlds extracted from the wavefunction in one moment to those extracted in the next, nothing to say 'this world is the continuation of that one'. The denial of continuity in time is a radical step and should be recognized as such.

Third: If you favor the position basis, then as things stand, you have to talk about instantaneous spacelike states of the whole universe, i.e. there is a conceptually (though not dynamically) special reference frame. You are free to say 'maybe we can do it differently, in a way that's more relativistic', but for now that's just a hope.

For all these reasons, many worlds is

notobviously the leading contender.I suppose the basic intuition here is, "Superposition is real for small things, we have no evidence that it breaks down for large things, and superposition means multiple instances of the thing superposed; therefore, many worlds, not just many electrons."

But is it clear that superposition means multiple instances of the thing superposed? Consider the temporal zigzag interpretations. There it is supposed that there is only one history between first and final observed event, and that the amplitudes are just the appropriate form of probabilities, not signs of multiple coexisting actualities. The temporal zigzag theorists cannot yet rigorously show that this is so; but the many worlds people cannot show that they get the right probabilities either. Therefore, even at the level of the individual quantum process, there is no evidence to favor the interpretation of superposition as denoting multiple actuality rather than multiple possibility.

.> But is it clear that superposition means multiple instances of the thing superposed? Consider the temporal zigzag interpretations.

Consider also that supersposition is observer-relative.

Eliezer asked (of zigzag theories): "One measurement has to change the other, so which measurement happens

first?"It doesn't have to be that way. Events can be determined through a combination of local causality and global consistency; see the work on attempts to create time travel paradoxes using wormholes. For example, you may set things up so that a sphere, sent into one end of a wormhole, should emerge from the other in such a way as to collide with itself on the way in, thereby preventing its entry. It sounds like a grandfather paradox: what's the answer? The answer is that only nonparadoxical histories are even possible; such as those in which the sphere emerges and perturbs its prior course, but not by so much as to prevent its entry into the wormhole.

The harmony of distant outcomes in an EPR experiment may similarly be due to a global consistency.

Ideally, in order to apply the description-length version of Occam's razor to competing and wildly dissimilar theories, such as we have in these attempts to explain quantum mechanics, one would first take the rival theories, embed them in a common superfamily of possible theories, deploy some prior across that superfamily, and then condition on experimental results. However, neither many worlds nor temporal zigzag is even capable of reproducing experimental results, so long as they cannot derive the Born probabilities. There are two types of realist theories which

areexperimentally adequate: stochastic objective collapse theories (e.g. Ghirardi-Rimini-Weber), and deterministic nonlocal hidden-variable theories (e.g. Bohm). In theory, if we're trying to figure out our best current guess, we have to choose between those two! In practice, it seems obvious that theoretical pluralism is still called for, and that much more work needs to be done by the advocates of interpretations which remain qualitative but could become quantitative.Have you considered nonlocal hidden variables (Bohm's version in particular)? The "pilot-wave" model does away with many worlds and the problems that you see many worlds addressing as far as I can tell.

Eliezer, continued compliments on your series. As a wise man once said, it's remarkable how clear explanations can become when an expert's trying to persuade you of something, instead of just explaining it. But are you sure you're giving appropriate attention to rationally stronger alternatives to MWI, rather than academically popular but daft ones?

Mitchell,

there is another argument speaking for many-worlds (indeed, even for all possible worlds - which raises new interesting questions of what is possible of course - certainly not everything that is imaginable): that to specify one universe with many random events requires lots of information, while if _everything_ exists the information content is zero - which fits nicely with ex nihilo nihil fit :-)

Structure and concreteness only emerges from the inside view, which gives the picture of a single world. Max Tegmark has paraphrased this idea nicely with the quip "many words or many worlds" (words standing for high information content).

Max's paper is quite illuminating: Tegmark, Max. 2007. The Mathematical Universe http://arxiv.org/abs/0704.0646

So we could say that there a good metaphysical reasons for preferring MWI to GRW or Bohm.

*0 points [-]Now THAT's an interesting argument for MWI. It's not a final nail in the coffin for de Broglie-Bohm, but the naturalness of this property is certainly compelling.

*2 points [-]Although Tegmark incidentally endorses MWI, Tegmark's MUH does not entail MWI. Yes, if there's a model of MWI, then

someworld follows MWI; butourworld can be a part of a MUH ensemble without being in an MWI-bound region of the ensemble. We may be in a Bohmian portion of the ensemble.Tegmark does seem to think MWI provides some evidence for MUH (which would mean that MUH predicts MWI over BM), but I think the evidence is negligible at best. The reasons to think MWI is true barely overlap at all with the reasons to think MUH is. In fact, the failure of Ockham to resolve BM v. MW could well provide evidence against MUH; if MWI (say) turned out to be substantially more complex (in a way that gives it fewer models) and yet true, that would give strong anthropic evidence against MUH. MUH is more plausible if we live in the kind of world that should predominate in the habitable zone of an ensemble.

But MWI is not the doctrine 'everything exists'. This is a change of topic. Yes, if we live in a Tegmark universe and MWI is the simplest theory, then it's likely we live in one of the MWI-following parts of the universe. But if we

don'tlive in a Tegmark universe and MWI is the simplest theory, then it's still likely we live in one of the MWI-following possible worlds. It seems to me that all the work is being done by Ockham, not by Tegmark.Sure, but why is the information content of the current state of the universe something that we would want to minimize? In both many-worlds and alternatives, the complexity of the ALGORITHM is roughly the same.

"If the same laws are true at all levels - i.e., if many-worlds is correct - then when you measure one of a pair of entangled polarized photons, you end up in a world in which the photon is polarized, say, up-down, and alternate versions of you end up in worlds where the photon is polarized left-right. From your perspective before doing the measurement, the probabilities are 50/50. Light-years away, someone measures the other photon at a 20째 angle to your own basis. From their perspective, too, the probability of getting either immediate result is 50/50 - they maintain an invariant state of generalized entanglement with your faraway location, no matter what you do. But when the two of you meet, years later, your probability of meeting a friend who got the same result is 11.6%, rather than 50%.

"If there is only one global world, then there is only a single outcome of any quantum measurement. Either you measure the photon polarized up-down, or left-right, but not both. Light-years away, someone else's probability of measuring the photon polarized similarly in a 20째 rotated basis, actually changes from 50/50 to 11.6%."

I don't see how you claim many-worlds gets you around the special relativity problem, the measurements can only be compared within one world - how would postulating other non-interacting (after the split) worlds help?

Also I have been having trouble following your posts. Your writing here has the same problem many weirdos (IDers, perpetual-motion-machine makers, etc) has. Any facts and arguments are getting lost in your wordiness. You might want to try to post brief explanations of what **specifically** your claims are in each post (maybe as occsasional summing-up posts).

Brains, as far as we currently understand them, are not digital. For a neuron fire / not fire is digital, but there is a lot of information involved in determining weather or not a neuron fires. A leaky integrator is a reasonable rough approximation to a neuron and is continuous.

Your writing here has the same problem many weirdos . . . has. Any facts and arguments are getting lost in your wordiness.Unfair. Eliezer has been trying to keep the series accessible to nonspecialists, and of course that means that the specialists are going to wade through more words than they would have preferred to wade through. Boo hoo.

Brains, as far as we currently understand them, are not digital. For a neuron fire / not fire is digital, but there is a lot of information involved in determining weather or not a neuron fires. A leaky integrator is a reasonable rough approximation to a neuron and is continuous.The point is that by the time two brains differ

by a whole neuron firing, they are decoherent - far too many particles in different positions. That's why you can't feel the subtle influence of someone trying to think a little differently from you - by the time a single neuron fires differently, the influence has diminished down to an exponentially tiny infinitesimal. Even a single neurotransmitter in a different place prevents two configurations from being identical.@Billswift: The point is that

nothinghappens differently as a result of distant events - no local evolution, no probabilistic chance, no experience, no "non-signaling influence",nothing changes- until the two parties meet, slower than light. You can (I think) split it up and view it in terms of strictly local events with invariant states of distant entanglement.@Recovering irrationalist: I haven't encountered any stronger arguments for the untestable SR-violating single-world theory. Sure, no one knows what science doesn't know. But given that I believe single-worlds is false, I should not expect to encounter unknown strong arguments for it. Do you have a particular stronger argument in mind?

@Jason: Bohm's particles are epiphenomena. The pilot-wave must be real to guide the particles; the particles themselves have no effect. If the pilot-wave is real, the amplitude distribution we know is real, and it will have conscious observers in it if it performs computations, etc. And there is simply no reason to suppose it.

@Mitchell: Of Born I have already extensively spoken (your 1), and postulating a single world doesn't help you at all; it is strictly simpler to say "The Born probabilities exist" than to say "The Born probabilities exist and control a magical FTL collapse" or "The Born probabilities exist and pilot epiphenomenal points [also FTL]." On your 2, it so happens that I

don'tdeny causal continuity, and plan to speak of this later. And regarding (3) quantum physics describes a covariant, local process so it seems like a good guess that there exists a covariant, local representation; but regardless the essence of Special Relativity is in the covariance and locality, whether we can find a representation that reveals it, or not.*2 points [-]"it will have conscious observers in it if it performs computations"

So your argument against Bohm depends on information functionalism?

Shouldn't it be possible for a tinier-than-average decoherent blobs of amplitude to deliberately become less vulnerable to interference from leakages from larger blobs, by evolving itself to an isolated location in configuration space (i.e., a point in configuration space with no larger blobs nearby)? For example, it seems that we should be able to test the mangled worlds idea by doing the following experiment:

1. Set up a biased quantum coin, so that there is a 1/4 Born probability of getting an outcome of 0, and 3/4 of getting 1. 2. After observing each outcome of the quantum coin toss, broadcast the outcome to a large number of secure storage facilities. Don't start the next toss until all of these facilities have confirmed that they've received and stored the previous outcome. 3. Repeat 100 times.

Now consider a "world" that has observed an almost equal number of 0s and 1s at the end, in violation of Born's rule. I don't see how it can get mangled. (What larger blob will be able to interfere with it?) So if mangled worlds is right, then we should expect a violation of Born's rule in this experiment. Since I doubt that will be the case, I don't think mangled worlds can be right.

So the Bohm interpretation takes the same amplitude distribution as many-worlds and builds something on top of that. So what? That amplitude distribution is just a mathematical object, but it having a physical existence certainly doesn't change the truth or falsehood of any mathematical statements, so I could just as easily say that the amplitude distribution itself is an "epiphenomenon" (and therefore can't exist).

Dynamically, "secure storage facilities" are not at all secure against world mangling. Perhaps quantum error correction could do better.

Robin, can you offer some intuitive explanation as to why defense against world mangling would be difficult? From what I understand, a larger blob of amplitude (world) can mangle a smaller blob of amplitude only if they are close together in configuration space. Is that incorrect? If those "secure storage facilities" simply write the quantum coin toss outcomes in big letters on some blackboards, which worlds will be close enough to be able to mangle the worlds that violate Born's rule?

Dynamically, I think the problem is that for everything you try that would render your world "distant" in the configuration space, it naturally tends to make your world smaller and more vulnerable, too. The worlds mangling yours aren't

close, it's just that, collectively, they're so much larger than yours, that even very tiny stray amplitude flows from them can mangle you.@Goplat: In Bohm's theory, the amplitude distribution has to be real because it affects the course of the particles. But the amplitude distribution itself is not affected by the particles. So any people encoded in the amplitude distribution - which can certainly compute things - would have no way of knowing the particles existed.

*3 points [-]Rather a late comment... but this response to Goplat reminds me of one of David Lewis's arguments for modal realism. Namely, he argues that "merely possible" people have exactly the same evidence that they are "real" as we do (it all looks real to them), and hence we ourselves have no evidence that we are "real" rather than merely possible.

An objection to this is "No! Merely possible people DON'T have evidence that they are real, because they don't exist. They don't have any evidence at all. They WOULD have the same evidence that we do if they DID exist, but then of course they WOULD be real."

A similar objection is that the wave function amplitudes can't do any real computation (as opposed to possible computation) unless they have real particles to compute with. So any people who find themselves existing can infer (correctly) that they are made out of real particles and not mere amplitudes.

It always amuses me that the particle motions in Bohm's theory are described as "hidden variables". Rather to the contrary, they are the ONLY things in the theory which are NOT hidden (whereas the wave function pushing the particles around is...)

"it will have conscious observers in it if it performs computations" I'm at a loss for what this means.

"In Bohm's theory, the amplitude distribution has to be real because it affects the course of the particles. But the amplitude distribution itself is not affected by the particles. So any people encoded in the amplitude distribution - which can certainly compute things - would have no way of knowing the particles existed." How is not being able to know where the particular particles are in a particular amplitude distribution an argument against it?

Oh, that's

subtle.Check me if I'm wrong: according to the MWI, the evolving waveform itself can include instantiations of human beings, just as an evolving Conway's Life grid can include gliders. Thus, if we're proposing that humans exist (a reasonable hypothesis), they exist in the waveform, and if the Bohmian particles do not influence the evolution of the waveform, they exist

in the waveformthe same way whether or not Bohm's particles are there. And, in fact, if they do not influence the amplitude distribution, they're epiphenomenal in the same sense that people like Chalmers claim consciousness is.If the particles

doinfluence the evolution of the amplitude distribution, everything changes (of course). But that remains to be shown.Eliezer, I think your (and Robin's) intuition is off here. Configuration space is so vast, it should be pretty easy for a small blob of amplitude to find a hiding place that is safe from random stray flows from larger blobs of amplitude.

Consider a small blob in my proposed experiment where the number of 0s and 1s are roughly equal. Writing the outcomes on blackboards does not reduce the integrated squared modulus of this blob, but does move it further into "virgin territory", away from any other existing blobs. In order for it to be mangled by stray flows from larger blobs, those stray flows would somehow have to reach the same neighborhood as the small blob. But how? Remember that in this neighborhood of configuration space, the blackboards have a roughly equal number of 0s and 1s. What is the mechanism that can allow a stray piece of a larger blob to reach this neighborhood and mangle the smaller blob? It can't be random quantum fluctuations, because the Born probability of the same sequence of 0s and 1s spontaneously appearing on multiple blackboards is much less than the integrated squared modulus of the small blob. To put it another way, by the time a stray flow from a larger blob reaches the small blob, its amplitude would be spread much too thin to mangle the small blob.

This seems like a devastating objection to the mangled worlds idea. Any counterarguments?

Question: how does MWI not violate SR/no-faster-than-light-travel itself?

That is, if a decoherence happens with a particle/amplitude, requiring at that point a split universe in order to process everything so both possibilities actually happen, how do all particles across the entire universe

knowthatat that pointthey must duplicate/superposition/whatever, in order to maintain the entegrity of two worlds where both posibilities happen?Eliezer:

But given that I believe single-worlds is false, I should not expect to encounter unknown strong arguments for it.Indeed. And in light of your QM explanation, which to me sounds perfectly logical, it seems

obvious and normalthat many worlds is overwhelmingly likely. It just seems almost too good to be true thatInow get what plenty of genius quantum physicists still can't.The mental models/neural categories we form strongly influence our beliefs. The ones that now dominate my thinking about QM are learned from one who believes overwhelmingly in MWI. The commenters who

already hadnon-MWI-supporting mental representations that made sense to them seem less convinced by your arguments.Sure I can explain all that away, and I still think you're right, I'm just suspicious of myself for believing the first believable explanation I met.

Well, now I think I understand why you chose to do the QM series on OB. As it stands, the series is a long explication of one of the most subtle anthropocentric biases out thereâ€” the bias in favor of a single world with a single past and future, based on our subjective perception of a single continuous conscious experience. It takes a great deal of effort before most of us are even willing to recognize that assumption as potentially problematic.

Oh, and one doesn't even have to assume the MWI is true to note this; the single-world bias is irrationally strong in us

even if it turns out to correspond to reality.GĂźnther, I am aware of that argument, but it has very little to do with favoring many worlds in the sense of Everett. See Tegmark's distinction between Level III and Level IV. The worlds of an Everett multiverse are supposed to be connected facets of a single entity, not disjoint Level IV entities.

This allows me to highlight another aspect of many worlds, which is the thorough confusion regarding causality. What are the basic cause-and-effect relationships, according to many worlds? What are the entities that enter into them? Do worlds have causal power, or are they purely epiphenomenal? Remember, that-which-exists at any moment does not just consist of a set of worlds, but a set of worlds

each with a complex number attached. And that-which-exists in the next moment is - the same set of worlds, but now with different complex numbers attached. The more I think about it, the less sense it makes, but people have been seduced by the simple-sounding rhetoric of worlds splitting and recombining.To say it all again: what are we being offered by this account? On the one hand, a qualitative picture: the reality we see is just one sheet in a sheaf of worlds, which split and merge as they become dissimilar and then become similar again. On the other hand, a quantitative promise: the picture isn't quite complete, but we hope to get the exact probabilities back somehow.

Now what is the

realityof quantum mechanics, applied to the whole universe? (If we adopt the configuration-centric approach.) There is a space of classical-looking "configurations" - each an arrangement of particles in space, or a frozen sea of waves in fundamental fields. Then, there is a complex number, a "probability amplitude", associated with each configuration. Finally, we have an equation, the SchrĂśdinger equation, which describes how the complex numbers change with time.That's it.If we just look at the configuration space, and ignore the complex numbers, there is no splitting and merging, nothing changes. We have a set of instantaneous world-states, just sitting there.

If we try to bring the complex numbers into the picture, there are two obvious options. One is to identify a world with a particular static configuration. Then nothing ever actually moves in any world, all that changes is the mysterious complex number globally associated with it. That's one way to break down a universal wavefunction into "many worlds", but it seems meaningless.

The other way is to break down the wavefunction at any moment in that fashion, but to deny any relationship between the worlds of one moment and the world of the next, as I described it up in my second paragraph. So once again, reality ends up consisting of a set of static spatial configurations, each with a complex number magically attached, but there is no continuity of existence.

There is actually a third option, however - an alternative way to assert continuity of existence, between the worlds of one moment and the world of the next. Basically, you go against the gradient in configuration space of the angles of the complex numbers, in order to decide which world-moments later continue the world-moments of now. That defines a family of nonintersecting trajectories, each of which resembles a perturbed version of a classical history. In fact, we've just reinvented a form of Bohmian mechanics.

But enough. I hope

someonegrasps that we have simply not been given a picture which can sustain the rhetoric of worlds splitting. Either the worlds sit there unchanging, or they only exist for a moment, or they are self-sufficient Bohmian worlds which neither split nor join. If I try to understand mangled worlds in this way, it seems to say, "ignore those configurations where the amplitude is very small". But they're either there or not; and if they are literally not, we're no longer using the SchrĂśdinger equation.Mitchell, you already know about Barbour, so why are you asking this?

Remember, that-which-exists at any moment does not just consist of a set of worlds, buta set of worlds each with a complex number attached. And that-which-exists in the next moment is - the same set of worlds, but now with different complex numbers attached.You seem to be talking about the wavefunction, which is a complex function defined over the configuration space (

a set of). But in that case you seem to be confusing a world with a configuration. A configuration defines only position. (Assuming we're talking about positional configuration space.)configurationseach with a complex number attachedIt seems I can save myself some trouble explaining by quoting Eliezer:

If Conway's Game of Life managed to support a multiverse, then a single universe in this multiverse would not correspond to a

cell. It would correspond to some section of the whole pattern quite a bit larger than a single cell - a section which was for the most part causally separated from the rest of the pattern. And this section might move around over Conway's gameboard (or whatever it's called), just as a glider can move across Conway's gameboard.So far, we're still implicitly in a framework where there's time evolution, so I have described ways of implementing the many worlds vision in that framework. I am a little hesitant to preempt your next step (after all, I don't know what idiosyncratic spin you may put on things), but nonetheless: Suppose we adopt the "timeless" perspective. The wavefunction of the universe is a standing wave in configuration space; it does not undergo time evolution. My first option means nothing, because now we just have a static association of amplitudes with configurations. The second option is Barbour's - disconnected "time capsules" - only now there isn't even a question of linking up the time capsules of one moment with the time capsules of the next, because there's only one timeless moment throughout configuration space. I don't know if the third option is still viable or not; you can still compute the phase gradients of the standing wave according to the Bohmian law of motion, but I don't know about the properties of the resulting trajectories.

There may be a problem for mangled worlds peculiar to Barbour's model; there are no dynamics, therefore no mangling in any

dynamicalsense. You will have to come up with a nondynamical notion of decoherence too.(Previous comment was in response to Eliezer's 02:38 AM.)

constant, part of my objective is to highlight the vagueness of the concept of "world" as used by many-worlds advocates, and the problems peculiar to the various ways of making it exact, having previously argued that leaving it vague is not an option. I have certainly seen many-worlds people talk as if worlds were "wave packets" or other extended substructures within the total wavefunction. But I await a precise statement of what that means.

mitchell,

I think Eliezer recognizes the the vagueness of "world" but sees it as a problem for

single-worlders. This is what he seems to be saying here:What flows is not time, but causality. As you guessed, I shall expand on that later. I think Barbour's time capsules reflect his lack of cog-sci-phil background - a static disk drive should never contain any observers; something has to be processed. You cannot identify observer-moments with individual configurations, which seems to be what Barbour is trying to do.

From the perspective outside time, nothing

changes, but things are nonethelessdeterminedby their causal ancestors. This is what makes the notion of "local causality" or Markov neighborhoods meaningful. This flow ofdeterminationis what supports computation, which is what supports the existence of observers. This means that no observer is ever embedded in a single configuration; only a determination of future configurations' amplitude by past configurations' amplitude, can support computation and consciousness.Which I consider as common sense. Timelessness, also, adds up to normality; there's still a future, there's still a past, and there's still a causal relation between throwing a rock and breaking a window. None of that goes away when you take a standpoint outside time.

constant - well, then, it is shaping up as follows: We need

someconcept of world. We can try to be exact about it, and run into various problems, as I have suggested above. Or we can be determinedly vague about it - e.g. saying that a world is a roughly decoherent blob of amplitude - and run into other problems. And then on top of this we can't even recuperate the quantitative side of quantum mechanics.There

isa form of many-worlds that gives you the correct probabilities back. It's called consistent histories or decoherent histories. But it has two defining features. First of all, the histories in question are "coarse-grained". For example, if your basic theory was a field theory, in one of these consistent histories, you don't specify a value for every field at every space-time point, just a scattering of them. Second, each consistent history has a global probability associated with it - not a probability amplitude, just an ordinary probability. Within this framework, if you want to calculate a transition probability - the odds of B given A - first you consider only those histories in which A occurs, and then you compute Pr(B|A) by using those apriori global probabilities.Those global probabilities don't come from nowhere. The basic mathematical entity in consistent histories is an object called the decoherence functional (which can be derived from a familiar-to-physicists postulate like an action or a Hamiltonian), which takes as its input

twoof these coarse-grained histories. The decoherence functional defines a consistency condition for the coarse-grained histories; a set of them is "consistent" if they are all pairwise decoherent according to the decoherence functional. You then get that apriori global probability for the individual history by using it for both inputs (in effect, calculating its self-decoherence, though I don't see what that could mean). The whole thing is reminiscent of a diagonalized density matrix, and if I understood it better I'm sure I could make more of that similarity.Anyway, technical details aside, the important point is that there is a form of many-worlds thinking in which we

doget the Born probabilities back, by conditioning on a universal prior computed from the decoherence functional. If we try this out as a picture of reality, we now have to make sense of the probabilities associated with the histories. Two possibilities suggest themselves to me (I will neglect subjectivist interpretations of those probabilities): (a) there's only one world, and a world-probability is the primordial probability thatthatwas to be the world which became actual; (b) all the worlds exist, in multiple copies, and the probabilities describe their relative multiplicities. They're both a little odd, but I think either is preferable to the whole "dare to be vague" line of argument.Eliezer: would you agree with the following, as a paraphrase of the physical ontology you propose?

Quantum theory is just field theory in the infinite-dimensional space formerly known as configuration space. What we thought were "locations in space" are actually directions in configuration space. If I see a thing at a place, it actually means there's a peak in the ψ-field in a certain region of configuration space, a region which somehow corresponds to my seeing of the thing just as much as it corresponds to the thing itself being in that state. And if the peak splits into two, there are now two of me.

I think I get it finally. Not that I believe it now. But expressed that way, I can put it into communication with the other interpretations, as part of the one spectrum of theoretical possibilities. I still strongly doubt that, after you employ a Kolmogorovian razor, theories with branching worlds will be favored over theories without. And I still advance the vagueness objection; but there are extra directions in which this idea might be taken. For example, though the boundaries of a wave are vague, the existence of a peak is not. So a quest for ontologically sharp entities, as the ostensible correlates of 'world' and 'mind', could focus on topological structures in the ψ-field, like inflection points, rather than geometric ones like blobs. Indeed, the whole description in terms of a smoothly varying ψ-field might be dual to a discrete combinatorial one; there are many such correspondences in algebraic geometry.

So, decoherence, which implies Many Worlds, is the superior scientific theory because it makes the same predictions with strictly fewer postulates, and academic physicists only believe otherwise because of deeply ingrained biases.

There, that didn't take 4,000 words, now, did it?

j/k,j/k, you're good, you're good ;-)

(Don't ban me)

No. Decoherence as an interpretation is not a scientific theory, it is an ontology. Decoherence as an interpretation does not imply Many Worlds unless the wavefunction is considered to be metaphysically real. That ascription of reality to the wavefunction is not a scientific postulate, it is a metaphysical one. Many worlds does not predict anything -- quantum theory makes the predictions, Many Worlds is an ontology, a reification of that theory.

In any case, my last question was ignored, and I don't suspect that further questions about considering things in a less realistic light will be taken seriously because of the glib dismissal and flippant mischaracterization Eli has given the very serious objections from instrumentalists. But I'm going to throw out another paper on the relational interpretation in the hopes that someone here will take seriously the idea that all of this confusion over which interpretation is the right one comes from an unreasonable committment to bad metaphysics.

Dustin said: "Decoherence as an interpretation does not imply Many Worlds unless the wavefunction is considered to be metaphysically real."

Dustin's referenced paper said:

The final step in the proof is left as an exercise to the reader.

A further implication of "quantum theory as field theory of configuration space": It means that "spatial configurations" are merely coordinates, labels; and labels are merely conventions. All that really exists in this interpretation are currents in a homogeneous infinite-dimensional space. When such a current passes through a point notionally associated with the existence of a particular brain state, there's no picture of a brain attached anywhere. This means that the currents and their intrinsic relations bear

allthe explanatory burden formerly borne by spatial configurations in classical physics.Dustin, what question are you talking about? Question to whom? The only comments I see from you are addressed to Caledonian, in the previous post in this series.

I am afraid that I find the relational interpretation to be gibberish. "The character of each quantum event is only relative to the system involved in the interaction." Can we apply this to Schrรถdinger's cat? "The cat is only dead relative to its being seen to be dead", perhaps? The cat is dead, alive, neither, or both. It is not "relative".

No, you can't fet inconsistent interpretations:-

"This relativisation of actuality is viable thanks to a remarkable property of the formalism of quantum mechanics. John von Neumann was the first to notice that the formalism of the theory treats the measured system (S ) and the measuring system (O) differently, but the theory is surprisingly flexible on the choice of where to put the boundary between the two. Different choices give different accounts of the state of the world (for instance, the collapse of the wave function happens at different times); but this does not affect the predictions on the final observations. Von Neumann only described a rather special situation, but this flexibility reflects a general structural property of quantum theory, which guarantees the consistency among all the distinct "accounts of the world" of the different observing systems. The manner in which this consistency is realized, however, is subtle."--SEP

It's good to know that somewhere I won the World Series of Poker last year; and the idiot that went all in over my 3x raise with 7-2 off suit and sucked out to beat my AA with is poor and broke somewhere today and that's good to know too. Not that I'm bitter or anything, of course. Not in those other worlds anyway.

Live in your own world.

Mitchell,

your concerns concerning vagueness of the world concept is addressed here:

Everett and Structure (David Wallace) http://arxiv.org/abs/quant-ph/0107144v2

Also, the ontology proposed here fits very nicely with the currently most promising streak of Scientific Realism (also referred to in the Wallace paper) -in it's ontic variant.

http://plato.stanford.edu/entries/structural-realism/

Cheers, G端nther

G端nther, I have previously argued that vagueness is not an option for "mind" and "world", even if it is for "baldness" or "heap of sand" or "table". The existence of some sort of a world, with you in it, and the existence of a mind aware of this, are epistemic fundamentals. Try to go vague on those and you are in effect saying there's some question as to whether anything at all exists, or that that is just a matter of definition. Your mind in your world is the medium of your awareness of everything. You are somewhat free to speculate as to the nature of mind and world, but you are not free to say that there's no fact of the matter at all.

This whole situation exists because of the particular natural-scientific models we have. But rather than treat the nonvagueness of mind and world as an extra datum to be used in theoretical construction, instead we get apologetics for the current models, explaining how we can do without exactness in this regard. It's all rationalization, if you ask me.

Live in your own world. Sure except when I need the MWI Spaghetti Monster to get the opposite of my result.

Collapse/MWI are the new wave/particle duality. The metaphysical cube fell over and rotated 90 degrees. Collapse/MWI only looks different because the cube looks unchanged.

A superposition doesn't imply that the simpler component waveforms exist. It can also mean you drove the speakers to eleven, reached the limit the fabric of spacetime could handle, and are receiving distortion.

"realize it's self refuting faith. [...] all the atoms making up your brain would be [...] made to believe it was right while it was wrong."That's not an argument against the MWI; that's an argument against

physics.Only if Many worlds is assumed true, yeah, cause then EVERY possibility is true. Like right now in this universe you read this post. In another you have intercourse with your neighbours dog. In another your hair just fell off. EVERY physical possibility being true = not science = cop out = end of science.

Anyway, MWI is inconsistent with all forms of realism so it's a incoherent hypothesis.

Please save your breathe, don't even try to say "NONO Many worlds is the REALIST" approach to QM. That's bohm, he came 3 years before Everett, he saved realism in QM. Actually no, de Broglie did in the early 1920's.

Read Travis Norsen's article in Foundation of Physics: "Against realism". It'll show you just HOW deluded MW proponents claim they are.

You can find it on arxivs I think

Note the ellipses, Dave.

*3 points [-]Interesting quote from Stephen Hawking, apparently he's on board with MWI as the

obviousbest guess (and with Bayesian reasoning):...though I am a bit confused by how he describes it in the last line — doesn't that sound more like non-realism (or at least "shut up and calculate") than MWI?

Do you have a link to the source? I would be interested it seeing more context.

It's from here, but no further context was given, unfortunately.

*0 points [-]Isn't the point of the "best" explanation (in the Bayesian sense) that it is the one most at peace with the "shut up and calculate" mentality? My reaction, which please feel free to disregard, is that nothing could be more "real" than saying something like, "Okay, here's the theory, it's self-evident given our observations. Great. Now shut up and multiply. Onto the next question."

It's saying that there is no mysticism inherent in MWI - you can be just as practical about it as you would otherwise.

We're still arguing whether or not the world is flat, whether the zodiac should be used to predict near-term fate and whether we should be building stockpiles of nuclear weapons. There's billions left to connect to the internet, and most extant human languages to this day have no written form. Basic literacy and mathematics is still something much of the world struggles with. This is going to go on for awhile: the future will not be surprised that the finer details of after the 20th decimal point were being debated when we can't even agree on whether intelligent design is the best approach to cell biology or not.