Parallel Worlds - TTLG Forums

Nicker on 15/5/2022 at 16:19

I started this reply.

It split into two replies.
It branched into two replies.

Those contradicted each other four times over...
Those deviated from each other four times over...
Those varied from each other four times over...
Those mirrored each other four times over...

As to determinism. Is there/what-is the difference between determinism as used in quantum mechanics/physics and as it is used/abused in philosophy/new-age woo woo?

Pyrian on 15/5/2022 at 18:09

Well, "determinism" philosophically contrasted with "free will" (for IMO no good reason) whereas in physics "determinism" contrasts with, well, "indeterminacy" i.e. randomness (a much more direct conflict).

Anarchic Fox on 15/5/2022 at 18:24

Quote Posted by Pyrian

That's what I'd lead with. What's the observable difference between these scenarios? Nothing at all. (Unless, of course, the many worlds aren't perfectly sequestered from each other.)

Well, some proponents claim that the MW interpretation is falsifiable, and there's probably enough variation among versions of the interpretation that they can't all be dismissed together. Anyway, when faced with an unfalsifiable or near-unfalsifiable belief like this I try to address the psychological needs that underlie it. In this case, this seems to be a reaction against non-determinism and the oddities that result from it.

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Premise 2: The universe is spatially infinite. This isn't really falsifiable, it could just be very large and we'd never know the difference, but as far as we can measure the universe is large-scale flat across the entirety of observable space, which implies that it goes on in all directions forever.

There are a couple problems here. First, a flat space can still be bounded. A simple example is an infinitely long cylinder, which is bounded in one direction, unbounded in another, and has no intrinsic curvature. (It does have extrinsic curvature.) Another is the classic RPG map, where you take a square and identify its edges. Second, due to the unexplained (https://en.wikipedia.org/wiki/Accelerating_expansion_of_the_universe) cosmic acceleration, we can't claim to know for sure what the average curvature is. The simplest explanation for the acceleration, which uses the cosmological constant, is very much not flat.

Quote Posted by Nicker

As to determinism. Is there/what-is the difference between determinism as used in quantum mechanics/physics and as it is used/abused in philosophy/new-age woo woo?

In physics, a deterministic system is one where identical initial conditions will always produce identical measurements. In a non-deterministic setting like quantum mechanics, they don't, because observation alters the system. As for how it's used in philosophy, I dunno; I suspect the meaning varies widely there.

mopgoblin on 16/5/2022 at 01:22

My thinking on this stuff is that you could represent a multiverse as the set of all possible states the universe can have, and then the progress of time becomes a wander though that state space, which you could (in principle) represent with a Markov chain of ungodly complexity - thanks to non-deterministic quantum malarkey, you can't know for sure where you're going or where you came from. It would also suggest that time travel, if you could do it, would just be a weird way to transition into another state rather than anything fundamentally different than the regular passage of time. It's not so much thinking that there's another universe over there, where the Soviet Union won the cold war or Oreos have the cream on the outside and the biscuit in the middle, but more like seeing the universe as a sort of weird thing smeared across an unfathomably immense mathematical object, where the question of whether time even exists is as much a philosophical question as an empirical one.

I don't know if/how that ties in with any formal theories, because I don't really know 'em, but it makes sense to me as a way of thinking about it. Also, if you think too hard about it then the concept of Boltzmann brains inevitably comes up, which is usually a good sign that it's time to stop thinking about it and go outside for a bit. Assuming there is an outside, or an inside for that matter.

Pyrian on 16/5/2022 at 04:43

Quote Posted by Anarchic Fox

Anyway, when faced with an unfalsifiable or near-unfalsifiable belief like this I try to address the psychological needs that underlie it. In this case, this seems to be a reaction against non-determinism and the oddities that result from it.

I really don't buy that "psychological need" thing at all; the MV solution is way harder for most people to swallow than "god rolling dice". Could just as easily be the vice-versa. Or, maybe it's just an elegant explanation that doesn't need ad hominems to explain its existence.

Quote Posted by Anarchic Fox

First, a flat space can still be bounded.

Yeah, but that's way outside the ol' Razor, and this is just speculation anyway.

Quote Posted by Anarchic Fox

The simplest explanation for the acceleration, which uses the cosmological constant, is very much not flat.

I'm no expert on these matters, but that is very much in opposition to what other sources say on the subject, and they're definitely taking the cosmological constant into account:

(https://en.wikipedia.org/wiki/Shape_of_the_universe#Curvature_of_the_universe)

Anarchic Fox on 16/5/2022 at 06:06

I may have phrased it in a condescending way, but I consider it a good thing to try to understand why people believe what they believe.

In an expanding universe the curvature due to mass varies over time, unlike the constant cosmological term. The early, matter-dominated universe would have positive curvature, and the late, lambda-dominated universe negative curvature. If the acceleration is explained by the cosmological constant, then the near-flat present era is an anomaly in the history of the universe.

Nicker on 16/5/2022 at 13:02

Quote Posted by Pyrian

That sounds largely like my take in different words, minus any specific understanding of the physics. By that I mean, in philosophical discussions of "free will vs determinism", both sides offer the physics as a slam dunk argument for their choice. At the very least, one of them is wrong. I suspect both are wrong to call either Newton or the Gottingen Gang to their moral cause.

Cipheron on 18/5/2022 at 18:57

Quote Posted by rachel

One thing that I've been thinking is that with quantum uncertainty the possibilities are infinite but like with the Schrödinger's cat thought experiment, once we observe it the infinite possibilities collapse into one observable reality.

I also think the word "observer" was a poor choice originally, because too many people think that quantum theory requires a *conscious* observer, which tends to divert people off into definite "woo" territory. "interact" would probably be more useful than "observe".

If I observe you in the macro-world, i only do so because of stuff that's bouncing off you: light. This light is interacting with you as an object, and it's definitely affecting you in the process. The difference is that light is very, very small compared to macro-sized objects so it just has less effect on you.

Imagine that instead of particles of light hitting you, there was no light and i tried to work out your position by hurling basketballs at you, and noting where they were deflected to. Would it seem "mysterious" that you were also affected by this process?

Obviously the 'rules' are different for quantum scale objects, but the fact is that the only way we even 'observe' them is by making them interact with other things, things which are actually bigger and heavier than they are. There's some exchange of energy going on. Sure the rules are mysterious, but the bit about things being perturbed by hitting them with energy is entirely understandable.

demagogue on 20/5/2022 at 12:51

Everything I try to say in this thread just doesn't want to stay short and properly responsive. But I feel compelled to say something. The best I could think of is maybe bullet points for now?

o Motivation for belief is incredibly important. I studied cognitive science, so it's the maybe most interesting part of science or any field to me. Determinism isn't my motivation anyway. Cf. Fred Hoyle rejected the Big Bang model because he thought its proponents' only motivation was bringing theological Creationism into science through a back door. On the other hand, Einstein never let go of locality based on over optimism about what "nature owes us", and didn't live to see Bell inequalities trash that idea. So motivation can cut both ways. (Einstein was wrong based on his motivation, but the Big Bang folks were right even with one of the architects being a monk & it looking admittedly theological.) I take it seriously though.

o My biggest motivation is probably the fact that MWI uses the actual existing rules of QM to answer the measurement problem, the rules that were already experimentally confirmed and in place by 1927 (well, plus the relativistic stuff that came a bit later), and have only been reconfirmed for the next ~100 years without any hint of any "new rules" that other interpretations seem to require. It just strikes me as a very clean answer that the existing rules of QM also appear to answer the measurement problem (it's a long story to explain why; I'm just saying it here, but to do it properly, one would need to walk through it) and that answer looks like the vanilla QM computations that physicists do literally every day. That feels cleaner to me than insisting that QM needs to have these other esoteric rules that no one has ever found or needed for their day to day work over the last century.

o Or another way to put it, it just strikes me as odd that the answer to the central problem of QM wouldn't look like QM but like something else. That may still be true, and there might be other rules and answers still out there (some we may never even be able to find, or even comprehend what "finding" even means), but IMO it makes the answer that already works (if it works; some people say it does) using only what we've figured out a good candidate. Or that's the kind of grounds on which I'd bet on it right now, if I had to bet on something. Of course if experiment or logic goes on to disprove it, I'd be fine trashing it. I don't have any special commitment to it based on my feelings. Actually I don't even like it; I'd much rather another interpretation make our universe the one unique one, if I had my choice on the matter. It's just what I'd bet on right now for the reasons I gave above.

o Did it make a prediction? Not really, although one could argue it predicted decoherence as the mechanism that makes it work. It's not really a prediction because it was always built into the original equations, but it (arguably) wasn't really well understood when it was developed in the 1950s, but by the 1970s it was realized that it would be the obvious candidate for doing the job they needed it to do. It depends on what one means by "prediction". If one insists on making a consequence manifest, and making a parallel world manifest is arguably prohibited because it's literally perfectly encrypted to this branch ... well, some people argue about recovering a superposition & its interference from a mixed state if they perfectly reverse every step, although that's not unique evidence for the MWI (but it'd support it)... Anyway, the nature of the problem may make falsifiability tricky. I understand and respect that problem. But people are clever and sometimes they can tease out experimental consequences if they think about it long enough. So never say never.

o The emergence of classical physics in MWI is a long story. But just as a bullet point I'd want to point out that the rules of QM under the MWI don't care about it. Nothing is special about any basis over any other as far as the rules of QM go. What's special about MWI isn't really about QM per se, it's why would classical physics emerge in a non-mixed state but not in mixed states, which is an emergence theory. One wants to point that out because sometimes people criticize the MWI because they think, if superpositions just layer on top of each other forever, something has to be special about simple non-mixed states, like I think was (https://www.youtube.com/watch?v=kF6USB2I1iU) Sabine's criticism. Sure you can show the interference terms disappear, but you still have all these basis states and what happens to them? But the whole point is there's nothing special about them. Once you have decoherence (once you have the interference terms disappear), your job as far as the rules of QM are concerned is done. Emergence is something you have to explain on top of that. It's a long story I can't fit here though. I might add bits and pieces of it later.

o Everett's son is E, the founder of (https://www.youtube.com/watch?v=V2yy141q8HQ&list=PLenuEuo1U9k11D7iWES5ll0BPixasWF9f) the Eels, one of my favorite bands. He made a great doc about his father ("Parallel Worlds, Parallel Lives"). I was a little frustrated by it because he clearly didn't understand QM, much less his dad's contribution. I mean that's understandable of course. But in the doc, when they were trying to explain it to him, they were throwing Dirac notation at him. But I was thinking, the guy is a professional musician, and I'm sure he's used music editing software. I bet he would recognize or understand a ton of quantum mechanics concepts if it had been presented to him in that context. Anyway, as a kind of poetic justice thing, I wrote out a summary of QM in terms of music editing, and the more I got into it, the more connections I found. It's actually really awesome how insightful it is to look at it through that lens. Pure tones (sine waves), timbre (superpositions), pitch (frequency), loudness (amplitude), timeline (position), Fourier transform (waveforms <=> spectrogram/overtones), Heisenberg uncertainty (thinner wavepacket = less pitch), etc., etc. It's too long to put here, but I may make a blog post on it and put the link up here.

o Okay, there's more I wanted to say, but this is good for now. This is actually the "short" version as best as I could get it.

Anarchic Fox on 20/5/2022 at 17:38

Quote Posted by Cipheron

Yeah, consciousness doesn't have anything to do with it. A quantum-mechanical observation is, roughly, a macroscopic system interacting with a microscopic one, in such a way that classical physics would give a definite outcome. Thought experiments involving cats are rather aggravating, since the cat is entirely able to observe its own state. That being said, observation can't be given an exact definition, because it's an empirical rather than a mathematical concept. QM is needed in order to explain observations; they aren't a mathematical concept introduced in the course of using QM.

Quote Posted by demagogue

o My biggest motivation is probably the fact that MWI uses the actual existing rules of QM to answer the measurement problem, the rules that were already experimentally confirmed and in place by 1927 (well, plus the relativistic stuff that came a bit later), and have only been reconfirmed for the next ~100 years without any hint of any "new rules" that other interpretations seem to require. It just strikes me as a very clean answer that the existing rules of QM also appear to answer the measurement problem (it's a long story to explain why; I'm just saying it here, but to do it properly, one would need to walk through it) and that answer looks like the vanilla QM computations that physicists do literally every day...

o Or another way to put it, it just strikes me as odd that the answer to the central problem of QM wouldn't look like QM but like something else.

It takes some argumentation to establish that the so-called measurement problem is a problem in the first place. It's so far from being "the central problem of QM" that I never once heard it mentioned in my QM classes (one undergraduate, three graduate), which did discuss non-locality and the Bell inequalities. I only seem to come across it in popular-science contexts. Admittedly, I work in mostly classical fields (plasma physics and relativity), where the topic wouldn't arise as often. Edit: After posting I remembered that one of my QM textbooks (Griffiths) does discuss wavefunction collapse as a problem, although my favorite one (Shankar) does not. I have some other QM textbooks, but they're packed away right now, so I can't poll them.

Rather than being a problem, I think wavefunction collapse is a central feature of QM. It's a fascinating physical fact that leads to interesting phenomena, like the (https://en.wikipedia.org/wiki/Quantum_Zeno_effect) Quantum Zeno effect. While future physics might shed light on it, this light won't be the MWI answer of "it doesn't actually happen." Which, as I pointed out earlier, replaces "the measurement problem" with an even worse multiverse problem.

Anyway, what is your argument that the measurement problem is a problem in the first place, as opposed to a central feature or phenomenon of QM? And, as before, how is the measurement problem worse than the multiverse problem introduced while trying to fix it?

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That feels cleaner to me than insisting that QM needs to have these other esoteric rules that no one has ever found or needed for their day to day work over the last century.

Those aren't your only two options! Rejecting the MWI doesn't put you in a position where you have to accept an even more objectionable interpretation.

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Anyway, the nature of the problem may make falsifiability tricky. I understand and respect that problem. But people are clever and sometimes they can tease out experimental consequences if they think about it long enough. So never say never.

Yeah, the "hidden variables" interpretation of quantum mechanics turned out to be falsifiable, and that (first its falsifiability, and then its falsity) was a big shock at the time. Now the study of Bell's inequalities has become an entire subfield of QM, with unanticipated consequences. I hold out some hope that the MWI will also turn out to be falsifiable, and that the physics involved in doing so will be similarly fruitful.