Parallel Worlds, Parallel Lives

Read Original Documents by Peter Byrne

Both of the original documents presented here, which were written by Hugh Everett in the 1950s and are accessible at right, have never before been published.

The first is two pages from an early draft of Everett's doctoral dissertation at Princeton University. Using an amoeba as a metaphor, the excerpt describes the concept of splitting into multiple universes that lies at the heart of Everett's Many Worlds theory, as well as how such multiple universes never come into contact with one another. Though the amoeba metaphor never made it into either the short version of Everett's thesis published in 1957 or the expanded version published in 1973, one physicist recently told me he considers it one of the classic metaphors of all time.

The second document is a four-page letter of May 31, 1957 that was Everett's response to cosmologist Bryce DeWitt, who had written to Everett with concerns about Everett's theory—concerns that DeWitt later abandoned. In this letter, Hugh Everett's sense of humor, his self-assurance even at such a young age (he was just 27), and the depth of his thinking on the subject, shine through.

To better appreciate both documents and to better understand Everett's theory, the following provides brief context. For more in-depth background, see The Theory Today.

The "monstrosity"

As a graduate student at Princeton in the early 1950s, Hugh Everett enjoyed discussing a central paradox in quantum mechanics with his classmates. As these physics doctoral students understood, the Schrödinger Equation, devised by Austrian physicist Erwin Schrödinger, shows that quantum systems exist in continually evolving "superpositions" of all physically possible states of being. (Think of photographic super-impositions, in which more than one photograph is exposed on the same piece of film.)

Oddly, nobody had ever been able to prove mathematically—and nobody still has—that quantum superpositions "collapse" into a single state of being, as the so-called Copenhagen Interpretation of quantum mechanics holds. Despite the lack of proof, it certainly appears to us that a single state emerges—the world as we see it at any given moment.

According to Everett, the laws of quantum mechanics dictate that the splitting observers will lead different lives in separate universes.

For decades, Everett and his classmates knew, physicists had been perplexed by the absence of a mathematical derivation of a single quantum reality from the multiple realities afforded by the Schrödinger Equation. To cope with this problem, physicists "postulated" that not all elements in a superposition are real. That must be true, they felt, because it accords with our experience: The only real element is the single one, the single world we see before us. We don't see multiple worlds superimposed on top of one another.

In the letter that appears here, Hugh Everett deemed this paradoxical "collapse" postulate "a philosophic monstrosity." Casting it aside, he decided to take the Schrödinger Equation at face value and work out what happens if there is no physical collapse. Everett considered the collection of all objects in the universe, including large or macroscopic systems (what we can see with the naked eye) to be fundamentally composed of atoms and molecules (microscopic quantum systems that we can't see). He suggested that these objects existed in ever-evolving superpositions without ever collapsing into a single reality, even though our reality suggests otherwise.

The Geiger Counter metaphor

Everett started handwriting his doctoral dissertation in the fall of 1954. His wife-to-be, Nancy Gore, began typing it up soon after they began dating in the summer of 1955. That September, Everett showed his thesis advisor at Princeton, the physicist John Wheeler, several typed excerpts of the larger work-in-progress. One excerpt, called "Probability in Wave Mechanics," introduced the idea that when two quantum systems interact, or exchange energy, the larger of the two systems automatically correlates with each element in the "related" superposed system.

In this scheme, a Geiger counter correlates with every physically possible instance of atomic decay that can be emitted from a radioactive metal (as opposed to measuring only one emission per instant). This makes logical sense, Everett said, because in a superposition, no single state is "privileged"—all possibilities are equally real. Of course, to us it seems as if the Geiger counter is registering only one decay at a time, instead of all possible decays simultaneously. But Everett argued that what actually occurs is that the Geiger counter "splits" into multiple instruments, each one registering one decay at one moment in time.

A human observer, however, sees the needle of the Geiger counter in only one position at a time, not "smeared" into multiple positions at a single moment. We do not see the needle simultaneously mirroring all of the possible energies of a radioactive particle flying off the metal at a given instant. Using the new science of information theory, Everett made a case that the observer does not see a "smear" because she herself has split, or "branched" into multiple observers (just as the amoeba does in his unpublished draft).

Each copy of the human observer is correlated with a possible event in the quantum system: radioactive metal-particle-Geiger counter-human observer. According to Everett, the laws of quantum mechanics dictate that the splitting observers will lead different lives in separate universes that continually branch off from interactions at the speed of light. Each branching universe creates a distinct record of its own history in the quantum environment.

The footnote

Ultimately, John Wheeler insisted that Everett remove all mention of "splitting" observers from his final Ph.D. thesis, including any mention of intelligent amoebas. Shortly before the greatly truncated thesis went to print in the July 1957 issue of Reviews of Modern Physics, however, Everett, without Wheeler's approval, inserted a footnote that summarized his position, affirming the splitting process. The rebellious note (see below) was inspired by the aforementioned letter from Bryce DeWitt, who happened to be the acting editor of that July 1957 issue. (The original, much longer version was published by DeWitt in 1973 and is available for the first time online here).

Recalling Everett's response to him decades later, DeWitt wrote, "All I could say was touché!"

In his letter, DeWitt had claimed that he could not feel himself split, so, as mathematically attractive as Everett's theory was, he said, it could not be true. Everett replied in his letter to DeWitt that, hundreds of years ago, after Copernicus had made his radical assertion that the Earth revolved around the sun instead of the reverse, his critics had complained that they could not feel the Earth move, so how could it be true? Recalling Everett's response to him decades later, in which he pointed out how Newtonian physics revealed why we don't feel the Earth move, DeWitt wrote, "All I could say was touché!"

Everett's now-famous footnote in his Reviews of Modern Physics paper, "'Relative State' Formulation of Quantum Mechanics," summarizes the core of his theory that the Schrödinger Equation rules both the microscopic and macroscopic worlds:

In reply to a preprint of this article some correspondents have raised the question of the "transition from possible to actual," arguing that in "reality" there is—as our experience testifies—no such splitting of observer states, so that only one branch can ever actually exist. Since this point may occur to other readers the following is offered in explanation.

The whole issue of the transition from "possible" to "actual" is taken care of in the theory in a very simple way—there is no such transition, nor is any such transition necessary for the theory to be in accord with our experience. From the viewpoint of the theory all elements of a superposition (all "branches") are "actual," none any more "real" than the rest. It is unnecessary to suppose that all but one are somehow destroyed, since all the separate elements of a superposition individually obey the wave equation with complete indifference to the presence or absence ("actuality" or not) of any other elements. This total lack of effect of one branch on another also implies that no observer will ever be aware of any "splitting" process.

Arguments that the world picture presented by this theory is contradicted by experience, because we are unaware of any branching process, are like the criticism of the Copernican theory that the mobility of the earth as a real physical fact is incompatible with the common sense interpretation of nature because we feel no such motion. In both cases the argument fails when it is shown that the theory itself predicts that our experience will be what in fact it is. (In the Copernican case the addition of Newtonian physics was required to be able to show that the earth's inhabitants would be unaware of any motion of the earth.)

To learn how Everett's Many Worlds theory has fared in the five decades since he introduced it to the scientific world, see The Theory Today.

Read Original Documents

Click on the image above to read an original draft in which Hugh Everett describes his concept of splitting "observers" using an amoeba metaphor.

Read Original Documents

Click on the image above to read a never-before-published letter that Everett wrote to cosmologist Bryce DeWitt defending Everett's Many Worlds theory.

Peter Byrne

Peter Byrne, a journalist based in Petaluma, California, is writing a biography of Hugh Everett III. The book, The Devil's Pitchfork: Multiple Universes, Mutually Assured Destruction, and the Meltdown of a Nuclear Family, is due out from Oxford University Press in 2009.

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