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Comment by bmh100 | Hacker News Reader

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?

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> 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.

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.

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