by Peter Byrne
As a cosmologist interested in plumbing the initial conditions of the universe—where quantum mechanics and gravity ought to have been united—DeWitt was pleased to have a method of thinking about the whole universe from inside that universe, even if it had certain weaknesses. One weakness, he thought, was,
As to why the good Lord chose to construct the world along these particular mathematical lines, Everett and Wheeler are silent.24
Another weakness:
Although it is a beautifully self-consistent philosophy it can never receive operational support in the laboratory.25
These papers were but the opening salvos in DeWitt’s life-long battle as Everett’s champion. As quantum mechanics became more and more important to the study of conditions in the early universe, interest in the theory of the relative state soared.
DeWitt coins “many worlds”
DeWitt recalled that in 1969, he “had a visit in Chapel Hill from Max Jammer, who was writing a book on the foundations of quantum mechanics, and he had never heard of Everett. And I thought this was scandalous, because Everett had a brand new idea, it was the first fresh idea in quantum theory in decades, and he was being completely ignored. So I decided to write an article for Physics Today, which really put Everett on the map, and Wheeler promptly disowned Everett. The reason, as far as I can see, was that it was too revolutionary an idea, this idea of many worlds. It was anti-Copenhagen; Bohr was one of Wheeler’s heroes, and he didn’t want to be associated with it. He has refused to have anything to do with it in all the years since.”26
DeWitt’s September 1970 article in Physics Today, “Quantum Mechanics and Reality,” was subtitled: “Could the solution to the dilemma of indeterminism be a universe in which all possible outcomes of an experiment occur?”
It began by criticizing Bohr:
Bohr convinced Heisenberg and most other physicists that quantum mechanics has no meaning in the absence of a classical realm capable of unambiguously recording the results of observations. The mixture of metaphysics with physics, which this notion entailed, led to the almost universal belief that the chief issues of interpretation are epistemological rather than ontological: The quantum realm must be viewed as a kind of ghostly world whose symbols, such as the wave function, represent potentiality rather than reality.27
Indeed, that statement sums up how many physicists and philosophers still feel about the quantum mysteries. DeWitt continued:
If a poll were conducted among physicists, the majority would profess membership in the conventionalist camp, just as most Americans would claim to believe in the Bill of Rights, whether they had read it or not. The great difficulty in dealing with activists in this camp is that they too change the rules of the game, but, unlike Wigner and Bohm, pretend that they don’t…. The Copenhagen view promotes the impression that the collapse of the state vector [wave function], and even the state vector itself, is all in the mind. If this impression is correct, then what becomes of reality? How can one treat so cavalierly the objective world that obviously exists all around us?28
After castigating Wigner’s “non-linear” solution to measurement, i.e. his theory that consciousness has a privileged role in the universe, DeWitt introduced the “Everett-Wheeler-Graham metatheorem.” DeWitt later said, “The Physics Today article was deliberately written in a sensational style. I introduced terminology (‘splitting,’ multiple ‘worlds,’ etc.) … to which a number of people objected because, if nothing else, it lacked precision.”29 And in the article DeWitt was eloquent, not shying away from the “split” word:
This universe is constantly splitting into a stupendous number of branches, all resulting from the measurement like interactions between its myriad of components. Moreover, 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…. Here is schizophrenia with a vengeance.30
He was concerned, however, about how to tie any type of probability measure to physical reality:
The alert reader may now object that [Graham’s probability] argument is circular, that in order to derive the physical probability interpretation of quantum mechanics, based on sequences of observations, we have introduced a non-physical probability concept … alien to experimental physics because it involves many elements of the superposition at once, and hence many simultaneous worlds, that are supposed to be unaware of one another.31
DeWitt’s confession summarized the probability problem in Everett: how can we prove a concrete, physical measure of the relative frequency of certain occurrences over sets of branching universes when we have no hope of physically accessing those universes to make tests? He as much as admitted that the Born Rule was an assumption in the Everett interpretation, (a sentiment with which Everett disagreed). This admission was not in accord with his extravagant claim that the many worlds interpretation emerged from the quantum formalism without tinkering:
Without drawing on any external metaphysics or mathematics other than the standard rules of logic, EWG are able … to prove the following metatheorem: The mathematical formalism of the quantum theory is capable of yielding its own interpretation.32
Shortly after the Physics Today article appeared, DeWitt made a more detailed, formalistic analysis of the Everett interpretation at a summer conference in Varenna, Italy. “The Many-Universes Interpretation of Quantum Mechanics” expanded on the problem of trying to derive the Born rule from the quantum formalism:
This reality is not the reality we customarily think of, but is a reality composed of many worlds. [What we know about the meaning of quantum mechanics] is unfortunately not yet sufficient to tell us how to apply the formalism to practical problems. The symbols that describe a given [quantum] system … describe not only the system as it is observed in one of the many worlds comprising reality, but also the system as it is seen in all the other worlds. We, who inhabit only one of these worlds, have no symbols to describe our world alone. Because we have no access to the other worlds it follows that we are unable to make rigorous predictions about reality as we observe it. Although reality as a whole is completely deterministic, our own little corner of it suffers from indeterminism.33
DeWitt was not suggesting that we cannot use quantum mechanics to make sufficiently accurate predictions on “our” branch, but only that we can never know the exact composition of the universal wave function and, therefore, we cannot know or predict what happens in the universe as a whole, the multiverse.
Evoking Einstein
DeWitt concluded the Physics Today article asserting that the “EWG” interpretation would “breathe new life” into the philosophy of science. He posited: “Yet it is a completely causal view, which even Einstein might have accepted.”
Letters to the editor streamed into Physics Today,34 most of them taking furious exception to the evocation of Einstein. The journal gave substantial space to six prominent physicists to pick apart Everett and DeWitt, zeroing in on problems with preferred basis and the derivation of probability.35 But they all treated Everett’s interpretation as worthy of serious consideration, even when they were reluctant to endorse it as physics, or were ontologically repelled by it. And there were supporters: one scientist saw in many worlds an opportunity to apply parapsychology to quantum theory; another commented, wistfully, that people on a crashing airplane could take comfort in the certainty that they would survive in some branches of their personal universes.
DeWitt answered his critics point by point. Regarding the crashing plane, he said he would worry, because, “It’s me I’m concerned about, not those other guys!”36
But he remained convinced that Einstein would have been
surprised and pleased at Everett’s conception … for it is the only conception that appears capable of unifying general relativity in a profound way with the quantum theory, without changing either theory or adding any new formal elements. It is the only conception that, within the framew
ork of the presently accepted formalism, permits quantum theory to play a role at the very foundation of cosmology.37
And then he backtracked:
I do confess to having somewhat overstated the case in my article in implying that the EWG metatheorem has been rigorously proven.38
He said that true rigor would require replacing all of the quantum mechanical symbols with new symbols stripped of formal rules for manipulating them. Rigor would require a proof
empty of an a priori meaning…. This remains a program for the future, to be carried out by some enterprising analytical philosopher.39
So, thanks to DeWitt and Graham, Everett became a minor celebrity in physics circles. Multiple universes were now a palatable topic for academic discussion. Philosophers started to pay attention. David Lewis, who was in the process of developing a theory of multiple worlds based purely upon philosophical concepts, signed Everett’s dissertation out of the Princeton library.40 Lewis’ well-known “modal” construct of a plurality of worlds based on “counterfactuals” is purely philosophical, and not quantum mechanical,41 but his work has influenced modern interpretations of how probability fits into the Everett worlds.
DeWitt and Graham proceeded to publish a resource letter in the American Journal of Physics analyzing 500 papers written on the interpretation of quantum mechanics; many were critical of Bohr and von Neumann. Clearly, they were not in the business of endearing themselves to colleagues:
No development of modern science has had a more profound impact on human thinking than the advent of quantum theory. Wrenched out of centuries-old thought patterns, physicists of a generation ago found themselves compelled to embrace a new metaphysics. The distress which this reorientation caused continues to the present day. Basically physicists have suffered a severe loss; their hold on reality.42
Saying that the impulse to interpret quantum mechanics is a “malaise,” the authors offered their bibliography so that successive generations of physicists and philosophers would not waste time serving up “half-forgotten ‘solutions.’”
As Everett’s controversial theory gained traction, Wheeler began distancing himself, asking colleagues to remove the “W” from “EWG.” But DeWitt, who had not communicated with Everett since their exchange of letters in 1957, did not want to be left out on a limb as the “sole spokesperson” for the many worlds interpretation, as Everett declined to publicly participate in the spirited debate. DeWitt decided to produce a collection of commentary on Everett:
I was also convinced that Everett’s ‘Relative State’ paper could not have constituted a complete statement of his views…. With Wheeler’s help, however, I was able to get Everett to send me a thick, faded, dog-eared manuscript entitled ‘The Theory of the Universal Wave Function.’43
This was the original thesis (retitled) that Everett gave Wheeler in January, 1956. DeWitt convinced Princeton University Press to publish it, along with reprints of Everett’s short thesis and Wheeler’s companion piece from Reviews of Modern Physics. Also included in the collection were DeWitt’s Physics Today article and his Varenna paper; Graham’s paper on probability; and the Cooper-van Vechten paper.
Everett was happy to have the long thesis published, although he drew the line at copy-editing or reading proofs—a task left to Graham. But he did invest some time rewriting the section of the manuscript where he deduced the Born rule, i.e. as a probability measure over sets of branching universes. After the intense criticism of his derivation from his detractors and supporters, he was determined to improve his explanation, without spending too much time on it.44
In October 1973, the collection was published as The Many Worlds Interpretation of Quantum Mechanics, edited by DeWitt and Graham. In its first five months, the combined hardcover and paperback editions sold 811 copies netting Everett a royalty payment of $500. He purchased a dozen copies of the paperback and kept them on a shelf, occasionally giving one away. Most of them ended up in his son’s basement.
Quantum corrections
Although Everett told one correspondent that he was pleased with the way DeWitt had presented his theory,45 there is evidence that he was not pleased with DeWitt’s and Graham’s analysis of his derivation of probability.
George Wesley, M.D., works for the inspector general at the Veterans Administration; collecting physics books is his hobby. In the mid 1990s, he was making the rounds of second-hand bookshops in Washington D.C. He discovered that Everett (of whom he had never heard) had scrawled his name on the inside cover of the first (and only) edition of The Many Worlds Interpretation of Quantum Mechanics. He had also corrected two typographical errors and penciled several acerbic comments in margins. It was Everett’s personal copy.46
Where De Witt wrote, in the Varenna paper:
Everett’s original derivation of this result invokes the formal equivalence of measure theory and probability theory, and is rather too brief to be entirely satisfying.
Everett noted: “!Only to you!”
Where Graham wrote:
In short, we criticize Everett’s interpretation on the grounds of insufficient motivation. Everett gives no connection between his measure and the actual operations involved in determining a relative frequency, no way in which the value of his measure can actually influence the reading of, say, a particle counter. Furthermore, it is extremely difficult to see what significance such a measure can have when its implications are completely contradicted by a simple count of the worlds involved, worlds that Everett’s own work assures us must all be on the same footing.
Everett commented: “bullshit.”
Everett also penciled remarks on a bound copy of DeWitt’s Varenna paper, which the author must have sent to him. Next to the exact same paragraph about probability (“rather too brief …”) he made a slashing down stroke with his pencil, scrawling: “Goddamit you don’t see it.”
But he was pleased on the next page when DeWitt wrote:
All the worlds are there, even those in which everything goes wrong and all the statistical laws break down. The situation is similar to that which we face in ordinary statistical mechanics. If the initial conditions were right the universe-aswe-see-it could be a place in which heat sometimes flows from cold bodies to hot. We can perhaps argue that in those branches in which the universe makes a habit of misbehaving in this way, life fails to evolve, so no intelligent automata are around to be amazed by it.
Everett slashed downwards again to bracket these sentences, scrawling a satisfied “yes.”
33 Records in Time
Most physicists end up as footnotes.
Susanne Misner1
Bell ringing
Although he never published another word of quantum mechanics after his dissertation was printed, Everett kept tabs on his theory as it matured and drew attention. He paid particular mind to a critique written by John Stewart Bell, a staff physicist at CERN, the particle accelerator complex in Geneva, Switzerland.
Bell was an experimentalist whose hobby was exploring quantum foundations. Attracted to Bohm’s hidden variables theory, he published a famous paper in 1964 showing that—contrary to Einstein’s EPR speculation in 1935—quantum mechanics acts non-locally, spookily.2 In a mere six pages, he proved that quantum objects correlate instantaneously over light-speed-separated distances, and that this correlation ruled out the locally deterministic hidden variables that Einstein had searched to define, but not non-local hidden variables. This means that two entangled particles separated by light years may be treated as a single quantum system.3
Motivated by DeWitt’s Physics Today and Varenna papers, Bell investigated the work of Everett and Louis DeBroglie. In 1927, DeBroglie had constructed a hidden variables theory. Decades later, Bohm, independently, had arrived at a similar formulation. The positions of particles in the deterministic quantum universe of DeBroglie and Bohm are governed by non-collapsing “pilot waves.” Bell saw a link between pilot waves and the determinism of Everett’s branching worlds.
; Amongst Everett’s effects, is a preprint of a paper written in 1971 by Bell, “On the Hypothesis that the Schrödinger Equation is Exact.” (It was later revised and renamed, “Quantum Mechanics for Cosmologists.”)4 Bell had subtitled a section on the many worlds interpretation: “Everett (?).” Upon reading this, Everett penciled: “?why?”
It is too bad they never met, because Everett had a lot to say to Bell, who was treating his controversial theory seriously, as worthy of thoughtful criticism. Where Bell wrote,
Now it seems to me that this multiplication of universes is extravagant, and serves no real purpose in the theory and can simply be dropped without repercussions. So I see no reason to insist on this particular difference between the Everett theory and the pilot-wave theory – where, although the wave is never reduced, only one set of values of the variables χ is realized at any instant.
Everett wrote: “not consistent.”
