I’ve written about the bizarre nature of quantum physics many times, providing a lightning primer back in May on three major interpretations: Copenhagen, pilot-wave, and many worlds. The many worlds interpretation (MWI) is often summarily dismissed by people, often along with visceral shudders or high doses of outrage. I understand the discomfort. When I first read about the interpretation in detail, it seemed like an over the top extravagance, an attempt to solve the measurement problem by throwing the multiverse at it.
But as I noted in the primer, the original version of the interpretation by Hugh Everett is actually quite austere in what it postulates. It actually removes the postulate of the wave function collapse of Copenhagen, and then follows the wave dynamics described by the Schrödinger equation. The result is a deterministic and local explanation of experimental observations, but also all the additional extravagance many find objectionable.
Over time, most of my original objections to the MWI have been resolved. But one continues to be a nagging source of doubt: the question of where all the energy for this branching reality comes from. The most common explanation is that it’s simply the original energy of the universe constantly being diluted. But that assumes that energy can be infinitely divisible. Which could be true, but seems discordant with the original discovery of quantum physics, that energy is discrete.
One way to avoid this issue is to postulate that each interaction does not in fact result in a new world, but that all the worlds are already there. This appears to be the approach of David Deutsch.
Deutsch is a legendary advocate for the MWI. I know many of you find Sean Carroll’s aggressive selling of it objectionable. If so, I advise staying away from Deutsch. Deutsch leans heavily and unapologetically into multiverse terminology. His view is the MWI is the only explanation of quantum physics that makes any sense, and he is pretty scathing in his assessment of the alternatives, along with the philosophies behind them.
But it’s worth noting that Deutsch’s version has some striking differences from the original MWI. He sees the particle, not the wave, as the more fundamental reality. The wave like dynamics that we observe, even when only one particle is sent through the experiment at a time, results from interference with other versions of the particle in parallel universes, universes which are exactly like ours except for the position of this one particle.
In this version of the MWI, the “splitting” is not one reality becoming many, but the divergence of parallel realities away from each other. We don’t run out of other universes to have interference with because there are an infinite number of them, and so an infinite number of each variation.
(In some ways, this version of the MWI is similar to another interpretation developed some years ago: the many interacting worlds interpretation. The material for that interpretation doesn’t mention Deutsch, so the resemblance might be superficial.)
In this version, the energy concern disappears. The original energy isn’t being diluted because the other universes already exist. I have no idea if Deutsch gravitated to this version due to energy, it’s just a benefit I see of his approach.
Unfortunately it seems to raise a couple of new serious questions.
If there are an infinite number of parallel universes out there, what led to their existence? In the original MWI, the worlds can be seen as a consequence of taking quantum physics seriously. In this version, they become an additional postulate, somewhat undercutting the MWI’s claim to parsimony.
And what controls which universes interfere with each other? If there are an infinite number, it can’t be all of them that are still the same, since that number would itself be infinite. Infinite interference seems like it wouldn’t allow any dynamics at all, just a solid block of whatever the particle is, much less the wave interference patterns we observe. That means it must be some subset, implying there’s a higher dimensional topology of some sort between universes, which itself seems like a new thing to be explained.
So I’m not sure how much Deutsch’s modifications improve the situation, or how directly motivated the additional postulates are from the data. It seems like looser speculation than the tighter extrapolation of the original.
So for me, the energy concern remains. It’s worth noting that all of the interpretations have their own issues, so I don’t see it as invalidating the MWI, but it’s what currently prevents me from regarding it as the default.
But maybe I’m missing something?