All those Everett worlds

After republishing the old essay “Shadows and the concept of self,” which Richard L. Miller and I first wrote in 2005, I have been doing more reading and thinking about Hugh Everett‘s Relative State formulation of quantum mechanics, its popular interpretation as “Many Worlds,”, and alternative interpretations.

I recommend reading Everett’s biography “The Many Worlds of Hugh Everett III: Multiple Universes, Mutual Assured Destruction, and the Meltdown of a Nuclear Family,” by Peter Byrne, who is not a physicist himself but took the time to talk to top physicists and understand Everett’s theory, which he explains well. I found also interesting Byrne’s juicy account of Everett’s personal life — great scientists put their pants on one leg at a time like the rest of us, screw like us, and screw up like us.

See also John K Clark’s review of Byrne’s book.

Everett’s (or more precisely DeWitt’s, see below) Many Worlds are like “Many Sheets” of paper. Each sheet is a universe. When something is observed to happen, the sheet splits in parallel sheets. You (your memory, life history and all that) live in a sheet, but there are countless other versions of you in other sheets.

Everett’s theory, formulated in the fifties and first published in 1957, was hidden in total obscurity until Bryce DeWitt published a semi-popular outline in Physics Today titled “ Quantum Mechanics and Reality: Could the solution to the dilemma of indeterminism be a universe in which all possible outcomes of an experiment actually occur?” (1970). To bring Everett’s ideas to public attention, DeWitt used deliberately strong language, such as the “Many Worlds” label (not used by Everett) and passages like:

“[Every] quantum transition taking place on every star, in every galaxy, in every remote corner of the universe is splitting our local world on earth into myriads of copies of itself.”

Everett’s original papers, including previously unpublished papers, are collected in “The Many Worlds Interpretation of Quantum Mechanics,” edited by DeWitt and Neill Graham and published in 1973, after DeWitt’s Physics Today article. Since then, Everett’s ideas have often been referred to as “Many World Interpretation” (MWI) of quantum mechanics. Another collection titled “The Everett Interpretation of Quantum Mechanics: Collected Works 1955–1980 with Commentary,” edited by Byrne and Jeffrey A. Barrett and published in 2012, includes many of the same works but also new material.

Quantum mechanics doesn’t predict the outcome of a physical process, but only the probabilities of different possible outcomes. In the MWI, different possible macroscopic outcomes of a quantum process are realized in different parallel worlds, which together form the “multiverse” — the set of all parallel worlds, also called “branches of the multiverse.” The number of worlds where one specific outcome is realized — or in other words, the “size” of the region of the multiverse where one specific outcome is realized — is proportional to (and/or can be taken as a definition of) the probability of the outcome, which can be computed with the mathematical equations of quantum physics.

What did Everett mean?

Reading Everett’s original writings I have the impression that he didn’t think in terms of the universe splitting into parallel universes.

In fact, Everett just said that quantum mechanics is true. I suspect the multiverse that Everett really had in mind is One Big World (as opposite to the many “small” worlds of the MWI), too complex for us to perceive and understand. All possibilities are still realized out there in the multiverse, but the multiverse doesn’t have a simple visual interpretation as a set of parallel worlds.

Instead of a stack of sheets of paper, I prefer to think of a 3D object casting different shadows on different planes. For example a 3D cylinder can cast a rectangular shadow on a plane, and a circular shadow on another plane. The cylinder itself, it is neither a rectangle nor a circle, but a 3D object that can’t be captured on a plane. Generalize to a huge number of dimensions, and you get the idea.

On the other hand, Byrne and many of the scientists he consulted defend the MWI picture and claim it’s what Everett had in mind.

Decoherence

Quantum systems are delicate: unavoidable interactions with the environment force weird macroscopic quantum states (e.g. a stone in two places at the same time, a cat that is both dead AND alive) to become ensembles of possible unweird states: the stone becomes classically localized either here OR there, and Schrödinger’s cat is either dead OR alive.

This “decoherence” process, which is considered as a rigorous consequence of the equations of quantum physics and can be shown to take place in very short times for most physical systems, explains why we don’t see quantum weirdness unless we look for it in lab experiments.

However, while decoherence shows that a weird quantum superposition of a stone here AND there will evaporate rapidly and leave either a stone here OR a stone there, it doesn’t really say where is the stone: it can be here or there (though not here and there). Therefore, according to some physicists, decoherence seems to suggest the MWI: a version the stone is here in this world, and another version of the stone is there in another, equally real parallel world.

Like it or not?

Everett’s used to be my favorite interpretation of quantum mechanics. I am less sure now, because Everett assumes the universal validity of quantum mechanics, and perhaps it’s too early to be sure that a theory developed in the early 20th century is the ultimate scientific model of reality. I guess current science will seem very naive from the vantage point of future science. Of course, the core concept of multiple parallel realities could still be part of future scientific models.

Another objection to Everett — an emotional objection — is that I don’t like to worry about all those versions of me in all those words where I am much less happy than here. But there are also interesting ideas of “quantum resurrection” in MWI worlds where your life becomes better and better as you move life after life.

See “Racing Worlds: A visual model for downward causation in physics” for a mental toy model, based on Everett’s ideas, which I have made up to visualize a physical mechanism for downward causation.

All those Stapledon worlds

It’s worth noting that Olaf Stapledon anticipated Everett and DeWitt in his masterpiece “Star Maker” (1937). One of the creations of the Star Maker is a multiverse:

“In one inconceivably complex cosmos, whenever a creature was faced with several possible courses of action, it took them all, thereby creating many distinct temporal dimensions and distinct histories of the cosmos. Since in every evolutionary sequence of the cosmos there were very many creatures, and each was constantly faced with many possible courses, and the combinations of all their courses were innumerable, an infinity of distinct universes exfoliated from every moment of every temporal sequence in this cosmos.”

Everett was an avid science fiction reader, so perhaps he was influenced by Stapledon’s fiction. Or, alternatively, perhaps Stapledon was influenced by preliminary versions of multiverse concepts. Fred Hoyle noted that, much before Everett, he and his fellow students used to lightly speculate about “alter egos” in parallel universes, and suspects that the concept may have been speculated about from the earliest days of quantum mechanics.

Shadows image from Wikimedia Commons, Stacked paper sheets image from Public Domain Pictures.