A fictional multiverse

The collection of essays Science and Ultimate Reality: From Quantum to Chaos was put together to celebrate John Wheeler’s 90th birthday in 2001. Max Tegmark’s contribution, “Parallel Universes,” begins as follows:

Tegmark goes on to explain why some astronomers and physicists take the idea of parallel universes, or multiverses, seriously. For one thing, if the universe is infinite—as presumed by the most popular cosmological model, the concordance or ΛCDM model —then somewhere, no matter how unlikely, a parallel “you” exists. You and your twin are most likely too far apart to ever communicate. Yet, in what Tegmark calls a level-1 multiverse, the two of you experience the same physical laws.

Tegmark’s level-2 multiverse accounts, in principle, for the suspicious fine-tuning of physical constants and initial conditions that makes life, the universe, and everything possible. If the inflationary phase that’s presumed to follow the Big Bang begat a froth of baby universes—each one occupying a bubble where different constants, particles, and dimensionality exist—then the fine-tuning is no longer suspicious. Our universe happens to be one among a diverse multitude.

Levels 3 and 4 are more esoteric. Inspired by Hugh Everett’s many-worlds interpretation of quantum mechanics, the level-3 multiverse consists of universes that correspond to quantum states embodied in wavefunctions. Contrary to the so-called Copenhagen interpretation, the quantum states don’t cease to exist when you measure a system and its wavefunction collapses. Rather, each state gives birth to a new universe in which its associated outcome exists on an equal footing with the universe that your measurement seemingly selected.

Level 4 is inspired by Wheeler himself. He wondered whether the mathematical structures that humans presume to underlie our physical understanding of the universe, complete or not, are unique. Conceivably, a multitude of different structures could exist that give rise to a multitude of different universes.

Multiverses are weird

Tegmark concludes his essay with a discussion of the pros and cons of the multiverse concept. The pros are briskly summarized in a single paragraph. Without favoring one level over another, he notes that all four succeed more or less in explaining a perceived problem with the single universe we think we inhabit.

Five times as many words are devoted to dealing with the cons, which Tegmark identifies as coming in two flavors: wasteful and weird. The “wasteful” objection amounts to evoking William of Ockham ‘s famous razor. Tegmark counters that a single, infinite universe is hardly parsimonious when it comes to mass, space, or energy. What’s more, some ensembles—Tegmark cites natural integers and solutions to the Einstein field equations—can be described more simply than their components can.

As for the “weird” objection, Tegmark counters that a) weirdness is in the eye of the beholder—that is, it’s a matter of aesthetics, not science; and b) certain firmly established phenomena, such as time slowing down at high speeds and superfluid helium flowing uphill, appear weird principally because they exist under conditions that lie outside our classical, everyday world and not because of some more fundamental reason.

I don’t know whether Tegmark’s essay inspired or influenced Iain Banks, but Banks’ 2009 Transition is based on what seems like a level-1 multiverse, albeit one whose constituent universes are more accessible to one another. The plot’s principal characters belong to, or are estranged from, a morally murky organization called the Concern. Thanks to a drug called septus, members of the Concern can transition or “flit” between parallel universes, ostensibly to keep the universal peace. But as the plot unfolds, more sinister motives emerge.

Whatever your opinion of the multiverse concept, I recommend both Tegmark’s essay and Banks’s book. Both are enjoyable and thought-provoking—in our universe or any other.