Sean Carroll is a cosmologist and physicist specializing in dark energy and general relativity. He is a research professor in the Department of Physics at the California Institute of Technology. A post on his blog recently caught my attention: “Are Many Worlds and the Multiverse the Same Idea?”
When talking about “parallel worlds” Carroll distinguishes between: a)the “multiverse” of inflationary cosmology; b) the “many worlds” or “branches of the wave function” of quantum mechanics; and c) “parallel branes” of string theory.” While branes represent a distinct idea, Carroll thinks that the multiverse and many worlds ideas might capture the same basic idea. Here’s how he explains the differences between those two ideas:
When cosmologists talk about “the multiverse,” it’s a slightly poetic term. 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 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. So there is a good reason to think about them as separate universes, even if they’re all part of the same underlying spacetime.
The situation in quantum mechanics is superficially entirely different. Think of Schrödinger’s Cat. 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 cat is neither alive nor dead; it is in a superposition of alive + dead. At least, until we observe it. In the simplistic Copenhagen interpretation, at the moment of observation the wave function “collapses” onto one actual possibility. We see either an alive cat or a dead cat; the other possibility has simply ceased to exist. 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. There are now two of us, both equally real, never to come back into contact.
Now clearly these ideas differ. Most notably, in the multiverse, the other universes are far away whereas, in quantum mechanics, they’re right here in different possibility spaces. (technically different parts of Hilbert space.) Still, some physicists have been wondering about the connection between the two ideas. And, after reading the recent literature, Carroll has “gone from a confused skeptic to a tentative believer.”
Carroll has changed his mind because of two ideas that fit together to make this crazy-sounding proposal plausible—quantum vacuum decay and horizon complementarity. Roughly quantum vacuum decay implies that “at any point in space you are in a quantum superposition of different vacuum states.” But horizon complementarity means that “you can talk about what’s inside your cosmological horizon, but not what’s outside.” Carroll concludes:
The result is: multiverse-in-a-box. Or at least, multiverse-in-an-horizon. On the one hand, complementarity says that we shouldn’t think about what’s outside our observable universe; every question that it is sensible to ask can be answered in terms of what’s happening inside a single horizon. On the other, quantum mechanics says that a complete description of what’s actually inside our observable universe includes an amplitude for being in various possible states. So we’ve replaced the cosmological multiverse, where different states are located in widely separated regions of spacetime, with a localized multiverse, where the different states are all right here, just in different branches of the wave function.
Carroll admits not knowing if any of this is true, although he is “inclined to think that it has a good chance of actually being true.” As for the implications for physics and for us, I’m in the dark. I’m simply don’t know what to do with the idea of a multiverse and parallel universes. Try as I may, our mysterious reality confounds me.