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 essay, 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 ” and not “All electrons have spin before January 1, 2020” or “All electrons have spin 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 1030th 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 consequenceof 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, including 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 not have a split-remerge problem. Given unitary quantum physics, Born’s rule is the unique rule that prevents “observers” from having psychic powers—which doesn’t explain Born’s rule, but is certainly an interesting 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 singleneuron… 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.

    As Jess Riedel puts it:

    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 oneworld?

    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 moreobvious 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 that 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 twenty-first 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.

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

    False for three reasons.

    First: The Born probabilities. That is where all the 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 an... (read more)

    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, no... (read more)

    .> 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... (read more)

    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?


    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 ... (read more)

    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.
    6Rob Bensinger
    Although Tegmark incidentally endorses MWI, Tegmark's MUH does not entail MWI. Yes, if there's a model of MWI, then some world follows MWI; but our world 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.
    8Rob Bensinger
    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't live 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 20° angle to your own basis. From their perspective, too, the probability of getting either immediate result is ... (read more)

    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 nothing happens 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 argum... (read more)

    "it will have conscious observers in it if it performs computations" So your argument against Bohm depends on information functionalism?

    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.

    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 ... (read more)

    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.

    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 waveform the 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 t... (read more)

    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 st... (read more)

    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 know that at that point they 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 normal that many worlds is overwhelmingly likely. It just seems almost too good to be true that I now 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 ... (read more)

    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 there— 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.

    Günther, 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, o... (read more)

    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, 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.

    You seem to be talking about the wavefunction, which is a complex function defined over the configuration space (a set of configurations each with a complex number attached). But in that case you seem to be confusing a world with a configuration. A configuration defines only position. (Assuming we're t... (read more)

    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 c... (read more)

    (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.


    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:

    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?

    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 nonetheless determined by their causal ancestors. This is what makes the notion of "local causality" or Markov neig... (read more)

    constant - well, then, it is shaping up as follows: We need some concept 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 is a form of many-worlds that gives you the correct probabilities back. It's called consistent histories or decoherent histories. But it has two defi... (read more)

    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. A... (read more)

    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)

    So, decoherence, which implies Many Worlds, is the superior scientific theory because it makes the same predictions with strictly fewer postulates
    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 o... (read more)

    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 relational approach claims that a number of confusing puzzles raised by Quantum Mechanics (QM) result from the unjustified use of the notion of objective, absolute, ‘state’ of a physical system, or from the notion of absolute, real, ‘event’. The way out from the confusion suggested by RQM consists in acknowledging that different observers can give different accounts of the
    ... (read more)

    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 all the explanatory burd... (read more)

    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.


    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.


    Cheers, Günther

    Günther, 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... (read more)

    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.


    Many worlds is far from obviously true. The only logical stand point is single universe, there's no evidence against it or even suggesting ANYTHING else.

    Bohm is probably the correct one, and has been since 1926, before even Copenhagen was made up.

    If your such a MWI believer, realize it's self refuting faith. In MWI all the atoms making up your brain would be in many universes made to believe it was right while it was wrong.

    "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.


    Interesting quote from Stephen Hawking, apparently he's on board with MWI as the obvious best guess (and with Bayesian reasoning):

    HAWKING: I regard [the many worlds interpretation] as self-evidently correct.

    T.F.: Yet some don't find it evident to themselves.

    HAWKING: Yeah, well, there are some people who spend an awful lot of time talking about the interpretation of quantum mechanics. My attitude — I would paraphrase Göring — is that when I hear of Schrödinger's cat, I reach for my gun.

    T.F.: That would spoil the experiment. The cat would have been shot, a

    ... (read more)
    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.
    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.

    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'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... (read more)

    It's time to test the Grue Hypothesis! Anyone have some Emeralds handy?

    Just coming in to say that emeralds have now been confirmed as green, not "grue".