demagogue on 13/5/2022 at 10:55

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I thought about posting this in the UFO thread, since I guess it often gets lumped into the "pseudoscience" bucket (unfairly!), but it really deserves its own thread.

Anyway, I want to talk about parallel worlds. Do we live in a multiverse? Like I said, it just sounds like cheap pseudoscience when you hear it, but then you realize that a lot of the biggest names in physics not only take it seriously but seem to speak as if they just assume it's true.

Long story short, I've just read David Wallace's The Emergent Multiverse, which is great BTW. I recommend it. And this is coming off of going through an online QM course.

I now feel like I understand the basics of quantum physics for the first time in my life -- as in I get the basic math of the operators, simple bound states, etc. I mean like first semester undergrad QM. But it's a big deal for me.

And the Many Worlds Interpretation has quickly become my favorite interpretation. Now I'm also realizing how ubiquitous it is among physicists. Maybe there's a perception that it's a fringe idea. But I think it's not just mainstream, but the default interpretation that isn't the Copenhagen Interpretation (which amounts to just throwing your hands up and stop thinking about it).

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And for good reason. The only way it works is if you actually remove the problematic "measurement problem" and just apply the simple rules we already know are true. Every other interpretation you have to bend over backwards with hacks whose only job, it looks like, is to prevent parallel worlds, because they don't actually fix anything else, and they have to break a lot of other stuff to do it, like insisting on absurd new rules for which there is evidently no other basis than people's discomfort with what vanilla QM is telling us.

But it's something I think the general public still thinks is some scifi fever dream and couldn't possibly be real.

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That said, one shouldn't underestimate just how ludicrously wacko it is compared to our intuitions. It would say that every particle in our body (10^27) gets branched off into their own decohered worlds 10^21 times a second. Edit: Sorry, it's even worse than that because each "particle" (each point in a growing wave front) by itself is branching into a vast number of separate worlds, then you multiply that by the number of particles. The wave is continuous, so it's a matter of interpretation how you "count" its "points", and how different two separated entanglements need to be before you say they're "different worlds". But if you wanted to at least cap it, you could count it by Planck areas on the cosmic horizon, so it shouldn't be bigger than 10^90 branches (for just one particle at one instant!). If you do napkin math with numbers like that, you get an idea of how many parallel worlds there should be since the Big Bang, and how many new ones are being made every second.

It's so rapid that you shouldn't even say that we live in "a branch"; we live through a vast folio of branches whizzing by, those of us -- me writing this and you reading this -- that are traveling through this folio together. Note that you share this particular folio with me (as in you're reading the words I've written in this folio, not different words "I" wrote in another one) because the electron and photon waves from my fingers crash into and get entangled with the waves making up the keys on my keyboard, then with the parts of my computer, then the cables of the internet, your computer, your screen, your eyes reading these words, your consciousness, all entangling you and me into a shared reality. That's nuts. But that's (a toy explanation of) how it works.

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Well I'm on a high right now coming off of that book. It does such a good job explaining how and why it all works. I wanted to talk about it. It's not really fit for a forum discussion though. But if you've ever had a question about parallel worlds or the Many Worlds Interpretation of quantum mechanics, you can ask me and I'll try to give an answer within like two or three sentences. That would be fun for me to try anyway. YMMV. You can also give your own opinions or comments about the idea. It's just something I think is cool to think and talk about.

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Edit: I don't want to overplay the hype. I haven't even gotten to the part where there are other "mes" and "yous" sharing exactly this same space with us (our waves are overlapping), a truly vast number, each in their own reality separate from this one. Some of them have probably committed unspeakable crimes or have become super celebrities or done great good for the world, and it's unclear if we could recognize ourselves in them, or if we would want to even if we could.

And each is also in their own society that grows increasingly alien from this one as you pick branch-points further back in time, different languages, nations, histories, evolutions... Who or whatever you've loved in this universe, there's a universe where that thing never existed or (maybe worse) existed but was just never recorded and ended in obscurity, maybe even for the better; and they have their own beloved and sacred figures and events that we know nothing of. And there's just so damn many of them...

It can be kind of disheartening if you really think about it, if you really try to come to terms with the fact that so much of what we know is saying this is real. On the other hand, you could say it makes our little slice of reality and our history very special, because whatever else is out there, this one is ours and ours alone. You and me.

Anarchic Fox on 14/5/2022 at 02:18

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This physicist doesn't believe in it, and hasn't met any physicist who does. (Though admittedly I haven't been polling people.) The many worlds interpretation is a way of denying the non-determinacy of quantum mechanics. Or you could just... accept that quantum mechanics is not deterministic, and stop trying to force its strangeness to conform to human intuition.



Most of chemistry is quantum-mechanical in origin, so it isn't limited to small scales.

demagogue on 14/5/2022 at 04:13

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You're right I don't push it too strongly or be accusitorial, sorry. It's definitely challenged by a lot of people. I could just watch like 10 QM videos in a row on a given day ... Susskind, Witten, Carroll, the MIT course (Zweibach & Adams), and it felt like all of them took it for granted, and they've done really great work in other areas, so I respect their take.

But the point I actually wanted to make (I may not have always worded it this way) wasn't that it wasn't credibly challenged, but that it's a lot more serious proposal than people in the public tend to think.



Okay, that's a valid take. I respect it. As I understood it, there still is a non-deterministic aspect to it, which is that while the wave evolves deterministically on-wards somewhere, it's non-deterministic which branch you find yourself in, why you are in this one and not that one, well, depending on how one defines deterministic in that case.

People don't have a problem why when a zygote splits into twins, why a person would find themselves as twin A and not twin B. If it's just like that, then I suppose there isn't really a non-deterministic aspect, or it's only an apparent one.

I should also say I like thinking about all of the different interpretations, because even ones where the creators themselves say it's just a mental trick (like the transactional interpretation), you get different images in your head and it brings the math to life, I think anyway.

Pyrian on 14/5/2022 at 19:33

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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.)

Okay, okay, I got another, related one.

Premise 1: The development of local space seems to have a strong random element; as far as we can tell, this is from early quantum jiggliness that doesn't quite get smoothed out because inflation or whatever. The important point is that the specifics of our circumstances are random in origin.

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.

Conclusion: There are an infinite number of worlds that are almost exactly like this one, perhaps different in some tiny, barely noticeable way. We will never observe any of them, but they must be out there if premise 1&2 are correct.

rachel on 15/5/2022 at 11:55

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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.

With infinite possibilities at the quantum level we literally cannot imagine an infinite number of universes parallel to each other except at a conceptual level, but I suppose it’s either that or just the one, I’m not sure there can be anything in between.