There are two ideas that fit together to make this crazy-sounding proposal into something sensible. So I’m going to explain what I think has a chance of being true I believe it’s pretty close to what is being proposed in these papers, but don’t hold the authors responsible for anything silly that I end up saying. However, for better or for worse, my interpretation of these papers is strongly colored by my own ideas. This happened for a very common reason: I realized that these ideas fit very well with other ideas I’ve been thinking about myself! So I’m going to try to explain a bit about what is going on. But these two new ones go explicitly for the “multiverse = many-worlds” theme.Īfter reading these papers I’ve gone from a confused skeptic to a tentative believer. Related ideas have been discussed recently under the rubric of “how to do quantum mechanics in an infinitely big universe” see papers by Don Page and another by Anthony Aguirre, David Layzer, and Max Tegmark. The Multiverse Interpretation of Quantum Mechanics, Raphael Bousso and Leonard Susskind Physical Theories, Eternal Inflation, and Quantum Universe, Yasunori Nomura But some physicists have been musing for a while that they might actually be the same, and now there are a couple of new papers by brave thinkers from the Bay Area that make this idea explicit. different parts of Hilbert space, if you want to get technical). In the cosmological multiverse, the other universes are simply far away in quantum mechanics, they’re right here, but in different possibility spaces (i.e. There are now two of us, both equally real, never to come back into contact. In the Many Worlds or Everett interpretation, both possibilities continue to exist, but “we” (the macroscopic observers) are split into two, one that observes a live cat and one that observes a dead one. We see either an alive cat or a dead cat the other possibility has simply ceased to exist. In the simplistic Copenhagen interpretation, at the moment of observation the wave function “collapses” onto one actual possibility. The cat is neither alive nor dead it is in a superposition of alive + dead. Quantum mechanics describes reality in terms of wave functions, which assign numbers (amplitudes) to all the various possibilities of what we can see when we make an observation. The situation in quantum mechanics is superficially entirely different. So there is a good reason to think about them as separate universes, even if they’re all part of the same underlying spacetime. When you combine this with string theory, the emergent local laws of physics in the different pocket universes can be very different they can have different particles, different forces, even different numbers of dimensions. We really just mean different regions of spacetime, far away so that we can’t observe them, but nevertheless still part of what one might reasonably want to call “the universe.” In inflationary cosmology, however, these different regions can be relatively self-contained - “pocket universes,” as Alan Guth calls them. When cosmologists talk about “the multiverse,” it’s a slightly poetic term. (I think the branes are still a truly distinct notion.)Īt first blush it seems crazy - or at least that was my own initial reaction. Increasingly, however, people are wondering whether the first two concepts might actually represent the same underlying idea.
There is the “multiverse” of inflationary cosmology, the “many worlds” or “branches of the wave function” of quantum mechanics, and “parallel branes” of string theory.
When physicists are asked about “parallel worlds” or ideas along those lines, they have to be careful to distinguish among different interpretations of that idea.
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