Oh, boy, parallel universe #57339! That's where I'm a Viking.

May 24, 2008 5:39 PM

How would the many-worlds interpretation work on the human level, if at all?

(I'm no quantum physicist, so please forgive me if the following is woefully simplistic, ridiculously naive, and/or hopelessly wrong)

The way I've heard it explained, the many-worlds interpretation of quantum mechanics holds that for every situation in which multiple outcomes are possible, each one of those outcomes does happen -- albeit in its own universe. That the universe we perceive is just part of an inconceivably large multiverse of infinitely branching possibilities, and that every interaction between every atom everywhere in the universe creates another one, or multiple ones, all the time.

I've also heard that because every possible outcome occurs, even the most bizarrely improbable event has happened in at least one universe. This makes sense if the ideas in the above paragraph are true.

For instance, it is incredibly unlikely that a fair coin could come up heads twenty times in a row -- the odds are about 1,048,576 to 1. But if each coin toss branches into a universe where it lands heads and a universe where it lands tails, then at the end of the line one of the million+ universes would see it land heads all twenty times. Of course, most of the rest of the million branches would see a mixed outcome, so from the point of view of a single universe the odds are still very unlikely. But the many-worlds theory says it does happen somewhere.

But when you think about it, it wouldn't be that simple. For starters, each coin toss would make more than two universes. A lot more. For example, a single coin toss could have two universes where the coin lands tails, but one universe sees it land one centimeter further to the right than the other. And there would be a small set of worlds where the coin landed perfectly on its side. And an even smaller minority where all the molecules of the coin spontaneously evaporated at the same time. A colossally improbable event, but possible.

And hey, if this holds true for molecules and small objects like coins, could you not also extend it to the rest of the world (which is just a collection of 10^huge molecules)? Must there be a universe out there where, say, every person who bought a Florida State lottery ticket happened to pick the same number, which was the winning one? Or where every building on Earth suffered simultaneous structural failure? Or where everyone spontaneously decided to break into Broadway-style song and dance? And a trillion variations on these and other scenarios, each slightly different from the other? And that the only reason we (most of our selves?) don't experience these things is because the infinity of "normal" universes where probable things happen outnumbers the infinity of universes where "impossible" things happen?

It feels absurd, like I'm talking about the Infinite Improbability Drive instead of a theory of physics, but I'm not seeing why it shouldn't be true. I've tried finding answers, but most of the literature out there (with the occasional exception) deals with quantum physics in a dry, academic context that limits the discussion to the atomic level. And of the material that imagines crazy outlier universes like the ones I described, I don't have a good way of telling if the physics involved is real or just taking artistic liberties for the sake of interesting fiction.

Must I mourn for the Earth somewhere out there that suffered Total Existence Failure? (And the one that suffered it five minutes later, and the other one that suffered it sixteen years later, and the one that suffered it partially, losing the western hemisphere, and the one that lost the eastern hemisphere, and the one that suddenly split into two planets, and the one where Australia turned into gelatin, and etc.)

Oh, and I know that we still aren't sure about which interpretation of quantum mechanics is correct, and that even if we knew the many-worlds theory were right, we'd have no way to observe other universes. I just want to know if the things I described are allowable in the context of the theory as it's understood today.

posted by Rhaomi to science & nature (36 comments total)

On a human-level, you end up with Larry Niven's "All the Myriad Ways": why make any choices at all, when you, or some alt-universe version of you, will make every possible choice?
posted by SPrintF at 6:00 PM on May 24

I think you're bleeding together the sci-fi application of multiple universes with what it would actually mean scientifically if it's true. Quantum effects aren't really happening on the macrolevel in this way -- quantum uncertainty doesn't kick in when you flip a coin, for example -- but sci-fi writers have generally run with the concept as if they did.
posted by gerryblog at 6:04 PM on May 24 [1 favorite]

Larry Niven's "All the Myriad Ways."
posted by gerryblog at 6:05 PM on May 24

This one of those situation where your speculation is as valuable as anyone else's, so long as your speculation is constrained by logic and accepted math.

You're asking if wildly improbable events happen in potential other universes, and the answer, within the context of "many worlds theory as basis for alternate universes" is yes. Wildly improbable events happen in this universe - quite a few of them are responsible for life on earth, for example. So if there are other universes, wildly improbable things happen in them as well.

The improbable events you're choosing are pretty wildly improbable however. Every single building on earth losing integrity is way, way, way more improbable than a self-replicating DNA molecule developing naturally. The broadway thing is probably even more improbable than that. But remember that nothing happens "spontaneously". Every happening this or any universe is the result of causality. If 6 billion people were to burst into synchronized song, it would be because something lead to that happening. What that may be is hard to imagine, and therefore hard to quantify.

But it's all math, so if you acknowledge that something is possible, you pretty much have to acknowledge that it will happen, given a long enough timeframe. And if you accept that their are infinite universes, you have to accept that they represent infinite timeframes in which things can happen. As an example, 20 consecutive "heads" is unlikely if you flip a coin 20 times, but it isn't unlikely if you flip the coin 11 billion times. If there really are infinite universes, then for every cross-universe occurance of the Florida State Lottery, it becomes likely that at least one iteration will feature every entrant choosing the same numbers.
posted by chudmonkey at 6:11 PM on May 24 [1 favorite]

that limits the discussion to the atomic level

I just reread your question and saw this, which is basically explained by what I said above. Quantum mechanics is only a description of reality when it's sliced very thinly—it's not really relevant to events on the level we experience day-to-day.

The many worlds hypothesis doesn't really hold that there's a world where Hitler won the war, or where crazy thing X or Y happened -- just that when we observe the electron going through Slit A rather than both slits, that there's in some meaningful sense also a universe where it went through Slit B. (This is also the idea that will make quantum computers work, if they can work -- that we can make the quantum computer run multiple calculations at once, each in its own universe.)

Most of the thought experiments obscure this fact by pushing quantum effects to the macro-level for comprehensibility, like Schrödinger's cat does. If some perverse individual actually wanted to run the cat in the box experiment, the cat wouldn't really be alive and dead at the same time, just like flipping a coin doesn't really fork the universe every time you do it.

I'm not at all an expert on this, though -- this is just how it's been explained to me when I've spoken to people who are.
posted by gerryblog at 6:18 PM on May 24 [1 favorite]

I'm no quantum physicist either, but Michael Clive Price, who wrote the Everett Interpretation FAQ, is. (The Everett interpretation is what's colloquially known as "many-worlds".) Basically, a "splitting" occurs whenever there's a thermodynamically irreversible event -- one that causes an expenditure of energy and an increase in entropy.

Essentially, many-worlds is simply an alternative to the idea of "collapsing the wavefunction" when you measure some aspect of a quantum system. In the Copenhagen interpretation, a measurement of a system causes the wavefunction to somehow instantaneously collapse from a continuous probability distribution to a single outcome. In many-worlds, the same effect is explained by the fact that the observer's quantum state becomes entangled with the system. We only experience one world because the entire state of the universe as we know it is just one branch of the wavefunction.
posted by teraflop at 6:19 PM on May 24

Oh, and for a neat science-fiction exploration of many-worlds, check out "Quarantine" by Greg Egan, and the short story "Ten Sigmas" by Paul Melko.
posted by teraflop at 6:37 PM on May 24

Every happening this or any universe is the result of causality

No, quantum events are random (though following a probability curve).


Quantum effects aren't really happening on the macrolevel in this way

Usually, no, because whatever particle behaves weirdly is counterbalanced by trillions of particles that behave normally. But in the mindbogglingly unlikely (but AFAIK not theoretically impossible) universes described above, the surrounding particles also choose improbable states, and thus we see effects on the macro level.

As far as I have been able to tell, the MWI has a pretty good chance of being correct (it's certainly more theoretically parsimonious than the Copenhagen interpretation). And if it is, there will be a near-infinte number of universes where I turn into a smurf who will be tortured by Oompa-Loompas for billions of years, even after the sun turns into a red giant. I hope I'm not in one of them.

I'm serious. This is science.
posted by martinrebas at 6:39 PM on May 24 [2 favorites]

Just imagine the tenth dimension.
posted by self at 6:41 PM on May 24

Just because it's bugging me: twenty heads in a row is precisely as unlikely as every other outcome. It's just a more memorable outcome than one that looks "random" to us.
posted by mendel at 6:46 PM on May 24 [4 favorites]

chudmonkey's comment captures well the idea I was getting at -- essentially that infinite universes + variation between universes = anything can and will happen.

But if I'm interpreting gerryblog's comment right, the reality (as the theory describes it) is that if there are multiple physical universes, they are all identical at the macroscopic scale, and the only difference is what happens at the quantum level.

If that is the case, then how do things like Schroedinger's Cat or quantum suicide work? The outcomes of these experiments are dependent on what happens at the quantum level. If a gun is wired to go off if quantum event X happens (with a 50-50 chance of it happening), then there will be one universe where it happens and one universe where it doesn't. Wouldn't that logically mean that in one universe the cat would die while in the other it would live? Isn't that the expansion of quantum effects into the "real" world?

Furthermore, would such a specialized experiment be the only situation where quantum effects would have an impact on our level? Or are there other ways that quantum interactions affect our world? It just seems counterintuitive to have another universe different from ours at the quantum level, but identical beyond that. I mean, quantum particles are the building blocks of the universe -- are the effects of their actions and reactions completely divorced from the things that are made from them?

On preview: martinrebas echoes what I was thinking about quantum effects influencing the material world when probability is out of the picture. Consider the Total Existence Failure scenario. Quantum particles have an odd habit of popping in and out of existence randomly. But they're small enough, numerous enough, and disappear rarely enough that it has no visible effect on us. Presumably their patterns of movement and disappearance vary randomly from one universe to another. Well, given that there are an infinite number of universes, wouldn't there be one where all the trillions of trillions of trillions (etc.) of quantum particles that compose Earth would all vanish at the same time? This is extraordinarily improbable, given the number of particles and the timing necessary, but like chudmonkey said, given infinite universes, even the most improbable events will eventually happen somewhere.

And the existence failure thing is just an example. I'm sure there are plenty of ways that large masses of quantum particles appearing and disappearing in an (improbably) coordinated way could change things in the macroscopic world.

Oh, and I've seen that 10dimensions site before. But the Mefi discussion about it knocked it down to pseudoscience pretty convincingly.
posted by Rhaomi at 6:52 PM on May 24

Quantum effects aren't really happening on the macrolevel in this way

this is something i've been thinking about as well. i think one component of this question is how quantum events do/do not effect things like human consciousness. since no one understands consciousness in any meaningful way, this is all conjecture, right?

i mean, it's feasible that quantum events spark things that could be called "decisions" but it's also possible that they don't. so if they don't, there might be a universe where your finger twitched while deciding to pull a trigger (a physical action), but not one where you "decided" not to, to reference the Niven story. [in case it's not obvious, i'm not a physicist or a quantum mechanic.]

and like chudmonkey points out, things have to have an explicable way of arriving at the event you're picturing. why would everyone break into broadway song? well... maybe there was a contest involving, uh, everyone and Cats. but what sort of "universe" would spawn that? it wouldn't be almost indistinguishable from ours; so you can't really expect that there's a universe just like ours except everyone breaks into song spontaneously.
posted by xz at 7:11 PM on May 24

why would everyone break into broadway song?

The molecules in your body are moving in response to quantum events, which generally cancel each other out. In an unlikely universe, coordinated quantum events might produce a muscle spasm, which makes you utter the beginning syllable of a Brodway song. In an even more unlikely universe, another collection of unlikely quantum events creates another spasm, making you follow the first syllable with another syllable. In an extremely, extremely unlikely universe, a series of quantum-event based muscle spasms will make you sing an entire song. In an exponentially more unlikely universe, everyone experiences similar muscle spasms simultaneously.

If I'm interpreting MWI correctly, this HAS to happen in billions of universes.
posted by martinrebas at 7:36 PM on May 24 [1 favorite]

^ Rhaomi: "Oh, and I've seen that 10dimensions site before. But the Mefi discussion about it knocked it down to pseudoscience pretty convincingly."

Yes, it is indeed speculation- as is pretty much everything in your post.

The thing that bothers me about parallel universes is if quantum particles are so easily subjected to change, then one change would simply set off a domino effect, essentially changing everything about the universe- including the various laws of physics, existence of certain atoms, and if there's even consciousness to perceive that change. I mean, what stops these changes once set in motion?
posted by self at 7:50 PM on May 24

I'm a nuclear physicist and I think the many-worlds interpretation of quantum mechanics is silly. This NYT article, which I linked to a month ago in another many-worlds question, basically elaborates on your observation that "it wouldn't be that simple."
posted by fantabulous timewaster at 7:59 PM on May 24 [1 favorite]

I think you're bleeding together the sci-fi application of multiple universes with what it would actually mean scientifically if it's true. Quantum effects aren't really happening on the macrolevel in this way -- quantum uncertainty doesn't kick in when you flip a coin, for example -- but sci-fi writers have generally run with the concept as if they did.

They have to "bleed together" - they're part of the same universe. Some people try to use the atomic level theory as if it wouldn't affect "middle sized objects" but there's no scientific reason given to separate the domains. Einstein said that if the copenhagen interpretation were right it would mean the moon was splayed across the sky (that's why he couldn't accept it). Many-worlds starts because of very real activity we see at the quantum level. But its repercussions go beyond just what we already see.

There are other ideas, and people are still thinking things through... Basically, the double slit experiment (among other things) produces some really weird results, and scientists are trying to come up with ways to explain it, but so far every theory has down sides.
posted by mdn at 8:12 PM on May 24

This NYT article, which I linked to a month ago in another many-worlds question, basically elaborates on your observation that "it wouldn't be that simple."

I read it and was disappointed. It doesn't mention the MWI at all. It mentions Boltzmann brains and that "cosmologists are desperate to eliminate these freaks from their theories, but so far they can’t even agree on how or even on whether they are making any progress". Apparently, some rates of dark energy decay will make Boltzmann brains impossible, but that's not relevant to MWI weirdness, which could be happening "right now" in another universe. And someone named Raphael Bousso thinks that because "People have their own favorite measures of probability in the multiverse [...] anytime your measure predicts that something we see has extremely small probability, you can throw it out", but doesn't offer any justification in the article.

I don't see anything there that argues against the MWI or against the idea that the MWI would involve really bizarre parallel universes.
posted by martinrebas at 8:40 PM on May 24

It's my own personal belief that the many-worlds interpretation wouldn't change anything on the human level at all, because all the contradictory worlds just cancel each other out and what we experience is the tiny amount left over. In other words, every possible thing is happening all the time, but only the self-consistent stuff persists. In fact, the more self-consistent it is, the longer it persists. My vague suspicion is that this is what's actually described by the Heisenberg relationship between energy and time.

I've never liked many-worlds, taken at face value, simply because it's about as multiplicative of needless entities as a model could possibly be.
posted by flabdablet at 8:48 PM on May 24 [1 favorite]

What if there are no odds? What if we live in a universe where all events are equally probable, but in order to function we must ignore that? What if, in order to make sense of the world, our brains have evolved to see generalized patterns in unique events, ignore their uniqueness, and relegate them to a background of "sameness" against which we navigate those other unique events that affect our welfare?

Years ago, on a hot August day, when picking apricots from my trees as I do every summer, I stopped about noon to make lunch, which I set on a small card table under one of the trees. The plate was completely full. I started by eating half an avocado, and when I had done that I reached for a sip of beer. At exactly that moment an apricot fell from the tree above and landed on the only vacant spot on the plate--the spot where the avocado had been. The apricot was neither hard enough to bounce, nor soft enough to splatter, but exactly the right consistency to flatten slightly on the bottom and remain on the plate as if I had placed it there. I went in the house for my camera, and took a picture--which is unremarkable except for the story. But is the story remarkable?

My mind raced. What were the odds? If anything had been different--the placement of the table, the moment I had decided to eat, my choice of food, my choice of beer, the number of steps I had taken, the clothes I decided to wear, the temperature, the temperature the previous day, the way I had pruned that branch, the life history of the bee that pollinated that particular apricot flower at that particular moment on that particular day, the life history of that bee's ancestors--this remarkable event would not have happened.

But the more I thought about it, the more I realized that although this event was unique, I was comparing it to a background of apparently "non unique" apricots falling. But this is an illusion. There are no "non-unique" apricots falling. The fall of every apricot is unique. The odds of each apricot falling exactly where and when and how it does are exactly the same as the apricot that fell on my plate.

Every event in the universe is unique. Every coin toss is unique. No two coins ever fall exactly the same way, but we humans have chosen to look at only one aspect of the coin toss, force it into "heads or tails", and infer "odds" which appear to contradict the uniqueness of each toss. We do this because we could not function if we saw each event in the universe as unique. There is too much information to process.

It is true that the odds against everyone breaking into song simultaneously are astronomical. But the mistake we make is to treat each "not breaking into song simultaneously" moment as being the same. They are not--they are also unique. The odds against any of those moments happening exactly the same way again are equally astronomical. But we humans couldn't have evolved to hunt and eat and reproduce if we acknowledged that.
posted by weapons-grade pandemonium at 9:34 PM on May 24 [2 favorites]

For explorations of this, I'll ditto the recommendation of Egan's Quarantine and suggest also Robert Charles Wilson's "Divided by Infinity" (in Starlight 2 and The Perseids and Other Stories.)
posted by Zed_Lopez at 11:05 PM on May 24

Well, I kind of agree with you, wgp, except from where I sit the probability of that apricot having fallen where it did (or the probability of any other unique event, of which I fully agree the universe is entirely composed) are not (inversely) astronomical, but exactly 1.

Just because we don't and often can't know in advance what is going to happen doesn't mean that it won't.
posted by flabdablet at 11:22 PM on May 24

Over time, flabdablet, yes. My central point is that the difference in the probabilities of various events is an illusion.
posted by weapons-grade pandemonium at 12:01 AM on May 25

The odds are "astronomical" at any given moment.
posted by weapons-grade pandemonium at 12:15 AM on May 25

The Many-Worlds theory is awesome and all, but a more practical (and infinitely more entertating) creation of Everett's is his progeny.

I'm pretty sure he was lit when he created both, but was definitely so when he created the many-worlds hypothesis (at least, according to pretty much everyone in Parallel Worlds, Parallel Lives.
posted by vertigo25 at 9:20 AM on May 25

martinrebas, I guess I tend to conflate the many-worlds interpretation and the idea of a “multiverse,” even though they have some subtle differences. Let me see whether I can explain less flippantly.

The observable universe has some properties that depend on some pretty unlikely-seeming coincidences. Most people know about the “Goldilocks problem,” that the earth has a stable orbit at the magic distance from the sun where water can remain liquid for geologically long timescales. What are the odds of that? Most stars have massive partners that disrupt such orbits; there are other considerations. Drake’s estimate of the average number of intelligent civilizations in a galaxy the size of the Milky Way was much less than one.

But there are other Goldilocks problems, too. The one that comes to mind first is the existence of nuclei heavier than beryllium (four protons, five neutrons). It turns out that the pathway to the rest of the periodic table is for three helium nuclei to effectively meet at once, to make carbon. But of course, “at once” is an approximation that doesn’t hold in the real world. It turns out there is a state in beryllium-8 --- that is, two helium nuclei stuck together --- that lives for a longish time, maybe a microsecond. And this state happens to have an energy near an excitation in carbon that doesn’t fall apart. If the strong nuclear force were just a little stronger or weaker, relative to the electric force, this pathway would be orders of magnitude slower and the periodic table would end with beryllium.

For that matter, there was a time in the first hour after the big bang when the universe contained roughly equal numbers of free protons and neutrons. At the end of this time all the neutrons were gone. Some were bound up in helium-4; some very few in lithium and beryllium and the rarer hydrogen and helium isotopes; some decayed into protons. If the weak nuclear force were just a little stronger, all the neutrons would have decayed and the universe would be entirely hydrogen. If the weak force were just a little weaker, all the protons would have been gobbled up and the universe would be entirely helium.

So, here’s a fair question to ask: did it have to go this way? There are several delicate balances between apparently unrelated things here: between the strengths of the strong and weak and electromagnetic forces, which are pretty different creatures, and in the collective behavior of the eight- and twelve-nucleon systems. This is on top of the coincidence of gravitational stability of the universe that cosmologists talk about. Maybe the big bang was some phase change where our little piece of the multiverse cooled off and chose our values for the fundamental constants from some distribution. Maybe that happens all the time, but most of the other universes don’t last long enough for anyone inside them to develop the smarts to notice. If so, then a theory that predicts the fundamental constants of physics doesn’t have to prefer ours. If a long-lived universe like ours is allowed at all, that’s where slow-to-develop critters like ourselves will be.

In Everett’s many-worlds interpretation of quantum mechanics, another universe splits off from ours every time some value gets chosen from a quantum-mechanical distribution. In a multiverse, you have a very large number of independent universes, some of which will follow indistinguishable trajectories for a long but not infinite time. I think that any argument about one can be applied to the other.

So, that brings us to this business about universes containing only disembodied brains. Which requires less energy and entropy: a fluctuation of the multiverse vacuum where the laws of physics act stably for tens of billions of years, and (at least) tens of billions of creatures evolve consciousness complex enough to notice? Or a universe which contains a solipsist with some convincing imagined memories for a fleeting instant, and then vanishes? If I understand the Times article correctly (and I admit to not having read the technical literature on the subject), it’s the second thing.

So I’ll repeat what I said before: if there are an infinite number of parallel universes, the number that contain you is small enough to be uninteresting.
posted by fantabulous timewaster at 1:23 PM on May 25

that the earth has a stable orbit at the magic distance from the sun where water can remain liquid for geologically long timescales. What are the odds of that?

1. It's a done deed. It's a given. It's already happened. Any calculation of odds leading to a result other than 1 are therefore clearly flawed.

By asking how many other otherwise comparable stars have a planet in a stable orbit at the right distance yadda yadda, you might come up with some figure for the rarity of this condition. But that figure will depend most strongly on what you consider "otherwise comparable" to mean. The rarity measure is a feature of your model, not of the reality you're modelling.

did it have to go this way?

Yes. We may not yet understand all the underlying connections, but yes it did. Because it did.
posted by flabdablet at 6:31 PM on May 25

It's a great fictional device, but in "reality" it's a problem of scale. "Many world" splits occur every time a particle (or something like it) might change in two or more ways. So, there is a universe where atom #10,101,786,931 of 100,000,000,000 did X instead of Y. And another one where atom #10,101,786,932 of 100,000,000,000 did X instead of Y. Only, because the atoms deviate simultaneously, we have four universes: XX, YY, XY, YX. Now expand that out to the number of particles in the universe (10⁸⁷?), all simultaneously deviating.
posted by aeschenkarnos at 7:22 PM on May 25

Now we can of course simplify that a bit by "chunking" them (this set of 100,000,000,000 atoms is "me", this set is "you") and only examining those deviations "of interest" to "us" ... but that's looking through static for patterns. That we find the patterns does not imply those patterns have any reality of their own, apart from that we impose upon them.

Time can be seen as nothing more than a journey through these patterns. Our random walk has brought us together, here, and we think of each other, and our particles interact in subtle ways, and then move on.

Whether this is a correct view, is as yet uncertain, which is why it's called the many worlds hypothesis. But it's pretty.
posted by aeschenkarnos at 7:29 PM on May 25

a related question: do we (or any hive-minders with specific knowledge) know/theorize that quantum events have any connection with "decisions?" i know that consciousness isn't understood, but...

asked another way; do we think that there would be a world-split when decisions are made or not? if decisions are the result of quantum events, then with MWI it would follow yes. if they aren't, then no. physical events that affect decision-making might seem to connect decisions with world-splits, but that wouldn't actually be the case.
posted by xz at 8:00 PM on May 25

xz, a decision isn't more privileged than any other thermodynamic process.

In the Bell-inequality experiments credited with showing that quantum mechanics is fundamentally nondeterministic, you get consistent results whether the "decisions" at either end of the apparatus are made by a person, or by a pseudorandom number generator, or by a "true" random number generator seeded by a radioactive source.
posted by fantabulous timewaster at 8:48 PM on May 25

a decision isn't more privileged than any other thermodynamic process.

right, fantabulous timewaster, but what you're in essence saying is that a "decision" -- a creature choosing one course over another -- is part of, or the result of, a quantum process. can a neurologist watch a decision happen? is there a physical element to a "decision?" i know that we can see the effects of emotion/thought in the brain, but i'm not sure we can see or isolate the physical component of a decision, making me wonder if one exists at all?

does that make my question clearer? i'm not sure anyone knows the answer at this point, i'm just curious to hear what people think.

i guess it's also part of a larger question i have, which is is there a physical component to consciousness? if we asked you a question, then instantly made a particle-for-particle clone of you, complete with electrical impulses, etc, would you and the clone of you answer the question in exactly the same way? or would there be something missing?

yes. i think of weird things. it's a living...
posted by xz at 10:58 PM on May 25

Consciousness certainly has a physical component, xz, but not necessarily a simple one. There is a close relationship between information theory and thermodynamics. Regardless of the mechanism, storing a new fact in a brain reduces its entropy: the overall state of (some part of) the brain has less freedom than before, which requires the entropy of the outside world to increase, which requires a flow of heat. The magic of thermodynamics is that you know these things regardless of the microscopic mechanism. We don't have to identify this mechanism to have confidence it exists.
posted by fantabulous timewaster at 3:29 AM on May 26

thanks. that's helpful. if i wanted to learn more about this, could you recommend any sources a novice might understand?
posted by xz at 5:19 AM on May 26

xz, I first learned about the connection between thermodynamics and information theory from a little book "Warmth Disperses and Time Passes" by Hans Christian von Baeyer.

The professor that recommended that book made an interesting observation: even though most practicing physicists are experimentalists, most popular physics books are written by theorists. His suggested reasoning was that experimentalists are busy and theorists spend more time casting about for something to do. A corollary to this observation seems to be that people who write about the speculative fringe of physics --- string theory, the many-worlds interpretation, etc. --- are really starved for work. I can't think of a book to recommend about interpretations of quantum mechanics; most of them just upset me. I did enjoy the perspective offered by Robert Laughlin in his recent "A Different Universe," but I'm not sure it's quite what you have in mind.
posted by fantabulous timewaster at 6:00 AM on May 26

The things that Boltzmann Brains and Many Worlds have in common is that they both deal with infinity applied on a human scale and they both freak the frack out of some people enough to have them spend a lot of time discounting them as silly or trying to disprove them. Shrug. Thing is, you can't ever think large enough about infinity, but thinking small is just plain out.

Science owes its existence to philosophy. Don't forget that. This story does go back to the Allegory of the Cave in a lot of ways, because if there are strange worlds where extremely improbable things happen, then there are infinite numbers and variations of these strange worlds just as surely as there are infinite variations on our own. As pointed out by weapons-grade pandemonium and others, all probabilities are equally likely, equally valid and equally strange. Therefore, a universe where all quantum particles act randomly according to the physics we experience, but coincidentally, by sure improbability, are observable by an entirely different and consistent set of physics is possible. Who is in the Cave? The strange Universe or our own?

IIRC, the NYT article's main focus wasn't to disprove Boltzmann Brains, per se, but just to show that a Boltzmann Brain is less probable than the situation as currently understood, because it's kind of comforting to think that, even if you can't prove with utmost certainty that you're not some hallucinating solipsist brain floating around in the ether, at least it's less probable that you are. (To be sure, I would be the Boltzmann Brain and you, dear reader, would be my hallucination... But I digress.)

Another way that problem is phrased is, "if computers can become powerful enough to simulate a universe, how do we know we're not a simulation running on someone's cell phone." And that can't even be known because maybe the (n-1) recursive universe is more computationally capable than our own.

Scientists tend to dislike this kind of philosophy, because it steps all over Occam's Razor, turns induction into a tool for chaos and, of course, it really fails the whole empirical provability deal. Meh.

But if there are many worlds, why do we observe just one? Why do we observe our Universe through time in the direction of entropy? And what about Zeno's Paradoxes? When does the arrow fly when it is motionless in infinitesimal time? Infinity is the deepest rabbit hole.

You might be able to deny these strangenesses if you believe in quanta, limits to time and space, determinism and/or other limits to probability. What if there is an absolute zero to probability? Or maybe all probabilities are not created equal. Maybe some dead-end. Maybe there's enough impossible happenings and dead-ends that the number of possible universes is a really, really big number, too big to hold in your mind, but actually finite.

Sincerely asked: Where does Planck's constant fit in to this discussion? What the heck is going on in the border between the macro-world and the quantum world that make so many scientists seem so confident about these sci-fi questions' silliness? I get that the term "observer" has a scientific meaning that has nothing to do with consciousness, but quantum computers are freaking weird in their own right. What's this deal about quantum particles not collapsing because of observation? And its relevance to consciousness and the mind-body dichotomy, is that quackery?
posted by Skwirl at 7:03 AM on May 26 [1 favorite]

Skwirl, nice points. To selectively answer your questions:

I have an awful lot of experience with the macro world, and I have in my head a pretty good idea about how it works. This gives me some intuition about how the micro world ought to work, too. Most of that intuition, unfortunately, turns out to be wrong. I've done enough physics that I have a pretty good idea how microscopic things will turn out in a given situation. I have some feeling about when the rules are the same, when they're different, and why. But it would take enormous (and unjustified) confidence in my skill as a logician to interpret my erroneous assumptions about the micro world as evidence that the macro world is fundamentally different than it seems. Quantum mechanics isn't surprising just to be quirky --- quantum mechanics is surprising because it describes situations where most people don't have enough relevant evidence to make smart guesses. If someone wants to argue that QM makes strange predictions about the macro world, too, I want more evidence than "you could never tell otherwise."

As to your last question: quantum interference effects tend to be pretty delicate, smearing out into classical distributions if you couple too tightly to them. Consciousness is pretty robust.

For your other questions, I recommend you locate a patient physicist to drink beer with.
posted by fantabulous timewaster at 12:09 PM on May 26