Can Many-Worlds Theory Rescue Us From Boltzmann Brains? | Hacker News Reader

Can Many-Worlds Theory Rescue Us From Boltzmann Brains?

nautil.us

60 pointsdnetesn56 comments

Can one unprovable and arguably imaginary idea save us from another unproveable and arguably imaginary idea?

Yeah it's sort of like religion all over again just in a different context. It seems that imagination for humans is more or less inescapable and in the end even if we get really nerdy about it, we end up in the same place we started. Undetermined and all about what you want to believe to be true.

Saying "Yeah it's sort of like religion all over again just in a different context" is sort of like a religion, but in a different context. It seems that making grand dismissals is more or less inescapable, and in the end we get really nerdy about it, and use atheistic language while we make our psuedoreligious grand dismissals.

In another words, it's all good.

Swap out prophets for celebrity "scientists"/eccentric billionaires, and swap out scripture with speculative pseudo-scifi publications, and you have a new religion!

Scientology is decades old. At this point so are the supposed real-life Jedi, too. And aura healers. And... So you're not just on to something, there are actual examples to back you up.

I agree but I take an even stronger view: religion provides answers to The Ultimate Questions: who am I? why am I here? is there a God (or are there gods)? what happens when I die? etc. So while the various naturalistic metaphysical theories are not themselves religious endeavors, there is a great deal of overlap with religion in terms of their applications to the mind.

Only artificial recursively self-improving super intelligences know the answer

Speaking of arguably imaginary ideas... :-)

They're unprovable but they're also straightforward applications of basic physical theories to scenarios far afield from what we've previously encountered. Induction isn't a logical inevitability but it is the basis of science and as a matter of practice we assume that if E=mc^2 has been true up to now it will continue to be true. And likewise that it continues to hold true for very large and very small values of m even well beyond values we've experimentally verified. You can posit that E=mc^2 wouldn't apply to masses larger than a galaxy, or that Schrodinger's equation doesn't apply to things with momentums that make their wavefunction undetectably small. Or that, in the steady state, some regions of a microcanonical ensemble won't be swept. But induction says that the equations that apply at one scale apply at another.

As a philosopher, I'm hesitant to just let the physicists off the hook with decoherence. Maybe decoherence as we know it is merely a gap in knowledge and the current dogma that the interference pattern is unobservable may be in fact untrue. Maybe it WILL be observable but we haven't gotten there yet and we're just guessing in the meantime using complicated statistics.

The point of my snarky comment wasn't that these ideas should be rejected outright.

My point is that since neither idea can be rejected or accepted based on current scientific understanding, it seems silly to ask whether one can "save us" from the other.

EDIT: I'm also somewhat disappointed that no one on HN has dinged me for spelling "provable" two different ways in one sentence (which I just noticed and will leave for the humor value).

Macroscopic decoherence (and therefore many worlds) is a straightforward prediction of the Schrödinger equation. It's not something you can just dismiss as imaginary.

Apart from that minor "Let's create an infinite number of universes out of nothing" problem.

Mathematical solutions are never guaranteed to be physical, or even potentially realisable.

What is your preferred interpretation? Mine is pilot wave, treating the wave function as a purely mathematical and nonphysical concept.

Pilot wave theory still requires the wavefunction to exist, with all the same detail as many-worlds, all the same computations. It just then picks a particular timeline and declares that this one is real, these are real particles, and everything else is shadows on the wall. It's thus incompatible with computationalism.

I consider this scenario improbable.

My limited understanding is that the pilot wave is more of a concept regarding embodied in the exact state of the universe. In other words, the pilot wave is the seed in the pseudorandom number generator of the universe.

I'll admit it does get a little philosophical, but:

No. The "pilot wave", in pilot wave theory, is a mathematical object identical to the universal wavefunction of the many-worlds interpretation. As such, if you believe that computation is all is required for consciousness to exist (and there's no separate "is real" tag on the math), then it's an incoherent concept.

I fall on that side myself, but I can't convincingly argue that it's the right philosophical stance to take. I've yet to see anyone give a proof, it just seems most likely.

Isn't this equivalent to a universe with non-determinism?

No, non-determinism on its own can't reproduce all the predictions of quantum mechanics. That'd be a different theory, and one I believe has been disproven experimentally.

The cardinality of a many-worlds multiverse isn't clear. It might be finite if all physical systems are in a superposition of a finite number of eigenstates in the collapse/split basis. You're also not creating anything out of nothing. The correct version of your complaint is "Let's create x > n things out of n things" (where "things" are quantum states), which happens all the time when you take the tensor product of two quantum systems, evolve the new overall system, and end up with a state that's not factorizable into a tensor product of two states.

Or that every probable action guarantees the creation of a new universe where the action didn't happen.

Keep in mind that the infinite universes still obey conservation laws collectively.

I tend to think of it like the number line. There are infinite numbers between 0 and 1. If we decide to define infinite unique ranges between 0 and 1, the space each one occupies becomes infinitely small and the total range is conserved.

But we don't know whether the multiverse is infinite or not, so that corresponds to the question of whether the range is from 0 to 1, or 0 to infinity.

Either way, the original point stands, and unitarity of the wavefunction (the most notable conservation law) is maintained in Many-worlds.

Where people get confused seems to be a mix-and-match of incompatible parts of the Copenhagen and Many-worlds interpretations, where you have both universe-splitting and the non-unitary wave function collapse in the same flawed interpretation.

What makes it seem imaginary to me is this: in what dimension(s) do all of these infinite "universes" reside? Why do they not interact in any way? What causes this creation of matter and energy?

If we didn't have telescopes, you could make the same argument about the existence of other galaxies.

You could, and that's a fair point, but you're more talking about philosophy of science and epistemology. My point is that Many Worlds seems to die to Occam's Razor, because of all the additional requirements to accept Many Worlds over something like wave function collapse.

Back when I read about this stuff I had the exact opposite reaction - Occam's Razor seemed to favour Many-Worlds.

Given direct experience of one universe, I think it's more ontologically conservative to introduce multiple universes than it is to introduce a whole new unprecedented category of cats that are simultaneously dead and alive.

Very loose analogy: if I see a mouse in front of me and simultaneously hear a squeak behind me, I'd probably assume two mice rather than one mouse which has learned ventriloquism.

Interesting. Occam's Razor says "don't multiply entities beyond need", not "don't multiply types of entities beyond need". Of course, you could argue that types are entities too. So many worlds minimizes entity types at the expense of total entity count.

Right.

Which makes it mildly ironic that John Wheeler, a leading early proponent of many-worlds, also proposed the one-electron universe...

[1] https://en.wikipedia.org/wiki/One-electron_universe

"Number of entities" is probably best understood through Kolmogorov complexity. So two regular mice, or million regular mice aren't much more complex than one regular mouse. But having one regular mouse and one "special" mouse is more complex all of that.

Something like Many Worlds is necessary to explain what it means for something like a wavefunction to collapse. Although it may multiply entities, that doesn't mean that it does so unecessarily. The trade-off is that new physical laws aren't introduced when doing so, and with other theories, you can say that introducing new laws is unnecessary, with respect to Many Worlds.

To quote from a quote in David Wallace's The Emergent Multiverse "Davies: so the parallel universes are cheap on assumptions but expensive on universes. Deutsch: Exactly right. In physics we always try to make things cheap on assumptions. (Interview between Paul Davies and David Deutsch)"

But is it even possible to view or otherwise observe these alternative universes? That's my biggest problem with the Many Worlds hypothesis: it's impossible to verify.

> Why do they not interact in any way?

I'm not sure if this theory is used by many-worlders, but eigenstates of the Hamiltonian don't interact with each other at all. It could be that a similar selection rule explains why many-worlds eigenstates don't seem to interact.

> What causes this creation of matter and energy?

You don't need to create energy to put a quantum system in a superposition of more states. You divide the energy of each state by its amplitude squared to get the total energy, and the amplitudes squared sum to one. So in Many Worlds, as with regular QM, the amplitudes of each state get smaller as you add more states. Total energy doesn't need to change.

I appreciate your points about the mathematical side, but what about the physical side? What is the embodiment of the energy that is being infinitely divided? We know of matter, dark matter, and dark energy. Is there a fourth form of energy?

What is the physical explanation as to why they don't interact? I.e. does gravity not travel along that space-time dimension? Is there a second time dimension? Are the universes created infinitely far apart?

> but what about the physical side?

That is the physical side. Your physical intuition may be getting in the way.

> What is the embodiment of the energy that is being infinitely divided?

What is the embodiment of the energy eigenstates of a quantum harmonic oscillator in a superposition? I'm not sure what kind of answer you want, but it's the same as the answer to that.

> What is the physical explanation as to why they don't interact?

Because they're (in my hypothetical scenario) eigenstates of the Hamiltonian. To get an intuition as to why they don't interact (again, in my scenario that might not correspond to leading many-worlds theories), look at the simplest possible case; one dimensional quantum harmonic oscillators. There's no short answer to your question.

> I.e. does gravity not travel along that space-time dimension? Is there a second time dimension? Are the universes created infinitely far apart?

Unfortunately, none of these questions are in the right ballpark. They are too complicated. If a selection rule in the universal Hamiltonian (or something) is the answer, they just won't interact at all. Bringing notions of other dimensions, or great distances, or whatever into it is unnecessary.

Unfortunately, I don't think any suggestion I can give here will alleviate your confusion without having a background in QM. You may want to read Griffiths QM textbook if you're really interested.

Calling them universes is problematic because everyone thinks of bubbles next to each other. That's not what many worlds says at all. Think of a Stern Gerlach experiment with the particle spin up or spin down. The wave function after traversing the magnet has two distinct humps. With classical observers, the universe goes one way or the other. Many worlds theory says that there is no classical observer, so the wave function of the whole universe ends up with two humps. If you squint and look at it funny you can call each hump its own world. Thus the name.

> Macroscopic decoherence (and therefore many worlds) is a straightforward prediction of the Schrödinger equation.

I was not aware of that development. in what way is the Copenhagen interpretation inconsistent with the Schrödinger equation?

The Many Worlds interpretation is equivalent to the Copenhagen interpretation. I prefer the Copenhagen interpretation. But since both are equivalent, both are equally good or bad. So one of the ideas is not "unprovable and arguably imaginary", it is only weird, extremely weird, as weird and all quantum mechanics.

(I'm not sure if they are using some extension of the Many Worlds interpretation. If it is not 100% equivalent then there must be an experiment to decide between the interpretations, and get a Nobel prize.)

I agree with you about the other idea. Moreover, I'm annoyed by

> A Boltzmann brain existing is more probable than a universe existing.

What are the odds? 10 to 1? 1000 to 1? How did they calculate it? Are the odds based in some experiments or only unsupported handwaving?

Can a Boltzmann brain exist in the absence of a universe?

I think the logical incoherence of the Boltzmann Brain saves us from it.

1) What we know about the world leads me to the belief that it's far more likely that a collection of particles in a thermal equilibrium have randomly and temporarily formed into a brain for an instant, than that the entire universe exists as I believe it to.

2) Since I am extremely likely to be a boltzmann brain, everything I know about the world is illusory, and nothing outside of my own mind exists, and my memories don't represent reality.

3) Therefore, everything I thought I knew about the world which led me to believe that I am a boltzmann brain is unlikely to be true.

4) I am unlikely to be a boltzmann brain.

But you also have to consider the probability of the brain containing precisely the self-consistent information that would allow you to develop a false theory. To me, the probability argument is more compelling than attacking the circular nature of the idea.

Step (3) is not valid. You can't use "I would reason badly in case A" as a proof that A can't happen. At best it's an argument that, when deciding what to do, case A isn't worth worrying about. But that's different from asserting case A can't have happened.

Step 3 doesn't quite work. If not being a Boltzmann brain implies that you are a Boltzmann brain, then it is irrelevant that this logic no longer works under the assumption that you are a Boltzmann brain. The first is enough to show that not being a Boltzmann brain is impossible, the second is not enough to cause a paradox.

Intuitively, I feel that it is extremely more probable for a Big Bang, giving rise to life that knows science, to randomly occur than for a Boltzmann Brain, self-consistently simulating the evolution of such a universe for billions of years, to randomly occur. However, it is hard for me to explain that feeling more precisely.

Maybe it's like this: it is much more likely to randomly simulate a game of chess where a pawn gets promoted to a queen than it is to randomly generate a move history from random characters, which resulted in a valid chess game of a pawn getting promoted to a queen.

We don't need it to simulate the universe for a billions of years. We just need it to simulate your belief that it happened for a couple of seconds.

Yes, a belief which relies on logical validity and self-consistency of memory and thought, which would in turn need to be the result of a "simulation". And by "need", I really mean "much more likely than the alternative" of producing such a state otherwise.

But are you really scanning all of your beliefs for consistency at every moment? Couldn't you just be presented with a simulated feeling that your beliefs are consistent? Wouldn't that be enough? It's certainly plausible that a Boltzmann brain could have that.

And in the end, does it really matter? The person I recognise as myself isn't the Boltzmann brain, but rather the person the Boltzmann brain thinks it is. Even if the Boltzmann brain swiftly disintegrates I will still live.

Given the internal and logical consistency apparent in this life, it stands to reason the Boltzmann brain would have to be simulating the entire thing -- the thought processes of other people, countries, wildlife, the movement of the stars, and so on.

Absent some calculation cost, which Boltzmann brains don't seem to provide, there's no reason everything wouldn't be fully rendered inside the Boltzmann brain.

At some point you just have a God, and are right back at old-timey religion, not that there's anything wrong with that.

Is a Boltzmann brain by definition unstable? Over what timescales? While it exists, does it function within its presumably more stable context, or is it somehow magically disengaged from its context in a way that necessitates instability?

What I'm getting at is that the idea of a Boltzmann brain, as juxtaposed with a "real brain", seems a little too " classical binary logic" for a universe that evolves. Perhaps some of our most resilient features have evolved with the help of spontaneous Boltzmann-brain-like intuitions, whims, or setups, but have been since then integrated in a stable way.

I imagine a Boltzmann brain to be something like a spark, but not immune from outside interaction or influence. Am I missing something?

What happens if a Boltzmann brain "catches on"? Is it still a Boltzmann brain, now that it has contextualized itself in a stable way? Was it ever a Boltzmann brain?

This also comes down to the meaning of "randomness", and I don't think the mathematical jury is quite out on that subject.

I'll point out that Botlzmann brains are only something you would ever think about in a universe going through a graceful heat death rather than the equally likely big crunch or big rip scenarios.

Dark energy suggests that a big crunch is unlikely.

A big rip, on the other hand...

Last I read, there is too much uncertainty in measurement to tell them apart. It will have to be resolved theoretically, since a heat death will only be the final result when omega exactly equals -1.

https://en.wikipedia.org/wiki/Betteridge's_law_of_headlines

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