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Comment by lalaithion | Hacker News Reader

> The many-worlds interpretation of quantum mechanics imagines our universe as one node in an infinitely branching tree of universes where every possible quantum outcome exists in its own universe. And each time a universe branches, it creates a child universe that is slightly different from the parent universe, e.g., universal constants such as gravity and the speed of light might differ.

Pretty sure this is flat out wrong; the Many Worlds hypothesis does not include universes in which universal constants differ.

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Author here. Sorry, I guess I misunderstood that part of the hypothesis. Do you have a source, so I can edit my post and correct it?

I'm not sure which Sean Carroll talk you saw, but I think he only mentions the differences between worlds in terms of different "choices"-- and these universal constants don't "choose" to be what they are, right?

This is the video that inspired this project, but I don't think that I'm knowledgeable enough to answer your question with 100% certainty.

https://www.youtube.com/watch?v=gpEvv349Pyk

if you read "Our Mathematical Universe: My Quest for the Ultimate Nature of Reality" by Max Tegmark, many worlds falls in the bucket of L3 multiverse. What you're describing is a L4 multiverse (see comments below. seems it's L2, not L4. will leave my mistake in, although i'm sure there is a parallel universe where i did not make this mistake)

Whoa, I'm so out of my depth. This is really cool. Thanks for sharing. I didn't know about the multiverse levels!

Level 2 is universes generally like ours but with different physical constants: https://en.wikipedia.org/wiki/Multiverse#Level_II:_Universes.... Level 4 includes all abstract mathematical structures, like a universe embedded in a Game of Life simulation.

i guess i got it wrong but it's still L2 != L3

This is great. I'm learning so much from this thread.

It seems you're mixing up MWI with a Max Tegmark Level II multiverse: https://en.wikipedia.org/wiki/Multiverse#Level_II:_Universes.... Tegmark considered MWI a separate idea from that and used the label "Level III" for it.

MWI is just one of multiple multiverse ideas. Most multiverse ideas (like Tegmark's Levels I, II, and IV) are basically what-if ideas without any direct evidence, but MWI specifically happens to be a more-grounded idea based on trying to make sense of what the (well-tested) Schrodinger equation says about reality.

The first part of your description of MWI ("The many-worlds interpretation of quantum mechanics imagines our universe as one node in an infinitely branching tree of universes where every possible quantum outcome exists in its own universe.") is pretty good, if a slight though common simplification (different branches aren't entirely separate, so envisioning it as a tree is only mostly correct; different branches can sum together or cancel each other out if their configurations are identical).

Ah, I think I'm beginning to understand. Perhaps I should just remove the sentence about constants changing.

This is a great comment. I updated my post based on your suggestion.

> trying to make sense of what the (well-tested) Schrodinger equation says about reality.

Nitpick: The Schrödinger equation predicts unitary time evolution (which is another way of saying that physical systems evolve in a deterministic manner). Interpretations of quantum mechanics exist to make sense of the part of quantum mechanics that doesn't follow unitary time evolution, namely the measurement process.

Well; another problem is that saying that branching happens at all is to miss the point of MWI; which is to avoid collapse. There aren't any discrete "branching" events.

That sounds unlikely because empirically Certain Things Happen and Other Things Don't, and you need to be able to explain why. With examples.

MWI honestly sounds like the philosophical equivalent of No True Scotsman with added Meatloaf - no one is really sure what it means in detail, but they're somehow sure it doesn't mean that.

MWI explains that "Certain Things Happen and Other Things Don't" is an illusion - everything that can happen, given the quantum probabilities, does happen. There is an infinity of branching outcomes. The only reason you perceive that Certain Things Happen is because, despite there being infinite paths from the root of the tree to a node, there is exactly one path from any node (where you are) to the root of the tree. So you look back and each moment appears to have followed from the previous, because it did. But the moments you experienced aren't the only ones that proceeded from the moments prior.

I think a typical criticism of that is that Many Worlds replaces the mystery of wave function collapse with the mystery of why we are on this branch rather than one of the many copies of us on other branches.

I don't think this is a particularly effective criticism. It's about as insightful as asking why I'm me and not you.

right. it's more like copy-on-write than a completely different replica. also, the probability of the outcomes is not the same (ie some worlds may be more equal/present than others).

it's questionable if it's one world or a split happens and we have multiple worlds. I would lean toward one world with phenomena that we don't really understand.

The main thing that's wrong is that we have no reason to believe the fundamental constants change.

Regular quantum mechanics is compatible with the many-worlds interpretation (of course it is), but talk of fundamental constants changing would require new theories that we don't have (not to mention evidence of those theories). That would be something incompatible with quantum mechanics as we know it, within which the fundamental constants are, well, constant.

just because those constants are incompatible with our current understanding does not mean it's not possible, even if only on a theoretical level.

The JS equivalent would be for this to use different babel, and node.js versions? Random compatible dependencies based on semver?

That's hilarious. I should have done that.

Within one of the Everett worlds you’ll likely have eternal inflation with many pocket universes that effectively have different constants.

There are even some papers that suggest the worlds of eternal inflation and MWI are the same.

I might be conflating this with parallel universes, but what would be an example of this?

If variations of ‘me’ exist in all worlds, am I the universal constant?

Speaking as the center of the universe as far I’m concerned over here :p

an example would be the speed of light is different. or the mass of certain particles is different. or the dark matter to normal matter ratios are changed.

versions of you could not exist in all worlds, but there are still an infinite number of universes where you exist

I don't understand how this could be true. The chances of you coming into existence require an astronomical number of things to happen in an exact time and order. Changing any number of those variables will cause a chain reaction that will make your existence not happen.

If we assume the universe is deterministic, then changing any of the starting variables of the universe will cause a different course of events playing out. If the speed of light were different, for example, a different course of events would have played out and you and I would have never come into existence.

If we assume the universe is not deterministic, then even if we had another universe with the exact same starting variables, the chances that things would play out exactly the same are also absurdly small.

Just because a set of things is infinite does not mean that it will contain everything. I could have an infinite set of odd numbers, and none of those numbers will be even. It strikes me that infinite number of universes would be the same.

Yeah usually the Many Worlds hypothesis has the same constants, certainly as Everett proposed it.

The multiverse theory is usually used for having many universes with different constants and the like. Of course word use may vary.

You're thinking of the Cosmic Landscape. Many Worlds [Everett] interpretation is branching of the quantum wave function as described by GP. It solves the probabilistic nature of quantum mechanical observables by saying that all possibilities occur, but we only see one of them because we exist in only one branch and branches can't interact. The copy of us in a different branch would see a different outcome.

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