by Peter Byrne
14 Genesis of Many Worlds
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.
Bryce DeWitt, 1970.1
I believe that basing quantum mechanics upon classical physics was a necessary provisional step, but that the time has come … to treat [quantum mechanics] in its own right as a fundamental theory without any dependence on classical physics, and to derive classical physics from it.
Hugh Everett III, 1957.2
It was Everett who gave us permission to think about the universe as wholly quantum mechanical.
W. H. Zurek, 2006.3
Dice-playing mice
After Ivy Mike exploded, Wheeler returned to full-time teaching at Princeton. He taught a graduate course in general relativity. One spring day, Wheeler and eight of his relativity students took tea with Einstein at his Mercer Street home. They elicited the great man’s thoughts on “everything from the nature of electricity and the unified field theory to the expanding universe and his position on quantum theory.”4
A year later, on April 14, 1954, Einstein gave the last public lecture of his life at the Palmer Laboratory to Wheeler’s relativity class. Misner recalled that Everett attended the lecture. That day, Einstein said that quantum mechanics is true, as far as it goes, but he did not see it as fully describing the quantum world:
It is difficult to believe that this description is complete. It seems to make the world quite nebulous unless somebody, like a mouse, is looking at it.5
(On the cocktail party tape a quarter century later, Everett did not remember attending Einstein’s talk, but he had used the mouse metaphor to great effect in his thesis.)
A few months later, Wheeler convened a group of graduate students, including Misner, to explore using Feynman’s sum over histories technique as a model for quantizing gravity. This was an extremely ambitious project: it meant trying to figure out how to create wave functions for gravity particles, which were purely theoretical entities (and still are). Misner eventually wrote his doctoral dissertation on that topic.
Misner recalled, “Everyone talking to Wheeler at that time was likely to be encouraged to think about quantum gravity. The question of how to give meaning to the wave function of the universe appears to have played a role in Wheeler’s interest in Everett’s views of the quantum.”6 Everett did not sign up for Wheeler’s relativity class, but he kept a copy of Wheeler’s mimeographed “Notes on quantization of gravity and electromagnetism,” dated October 15, 1955. The notes are a summary of Wheeler’s conversations with students about quantizing gravity. He feared that the equations of general relativity and quantum mechanics were too dissimilar to ever mesh, but he was bound to try. And, as we shall learn, Wheeler considered Everett’s theory of a universal wave function to potentially be a key for unlocking the secret of quantum gravity.
Out of the basement
The great find for the history of science in Mark Everett’s funky basement was his father’s correspondence with prominent physicists and philosophers, and his half-century old handwritten thesis: the penciled first draft, complete with scratched out paragraphs, and pages and pages of private notes about his theory. It is the raw stuff of the many worlds interpretation—and it illuminates how he viewed his own work, particularly his belief on whether or not the branching universes exist.
Fifteen years after the thesis was published, Everett penned a letter (found in the basement) to Max Jammer, who was writing his book on the philosophy of quantum mechanics. Everett told Jammer that “the principle intellectual influences” on the development of his theory were Misner, Petersen, and Wheeler. The latter
encouraged me to pursue the matter further as a thesis. During the course of this pursuit I would say that perhaps the primary influences were von Neumann’s book and the later chapters of Bohm’s Introduction to Quantum Mechanics [sic].
I must answer in all candor the primary motive was, of course, to obtain a thesis. However, I must also admit to a strong secondary motive to resolve what appeared to me to be inherent inconsistencies in the conventional interpretation.
I was of course struck, as many before and also many since, by the apparent paradox raised by the unique role assumed by the measurement process in quantum mechanics as it was conventionally espoused. It seemed to me unnatural that there should be a ‘magic’ process in which something quite drastic occurred (collapse of the wave function), while in all other times systems were assumed to obey perfectly natural continuous laws.7
In 1954, Everett was not alone in his feeling that the collapse postulate was illogical, but he was one of the very few physicists who dared to publicly express deep dissatisfaction with it:
I thought at that time that perhaps the pursuit of this apparent difficulty would lead to a new and different theory which, while resolving the apparent paradoxes, would also lead to new predictions. Unfortunately, as it turned out, the theory which I constructed resolved all the paradoxes and at the same time showed the complete equivalence with respect to any possible experimental test of my theory and that of conventional quantum mechanics. The net result of my theory therefore is simply to give a complete and self-consistent picture (without any particular ‘magic’ associated with the measurement) that in all practical predictions will of course be identical to the predictions of the conventional formulation.8
In other words, Everett had hoped to reinvent quantum mechanics on its own terms and was disappointed that his revolutionary idea was experimentally unproveable, as the only “proof” of it was that quantum mechanics works—a fact which was already known. He saw his interpretation as valuable, nonetheless:
To me, therefore, the real usefulness of this picture or theory of quantum mechanics is simply as an alternative which could be acceptable to those who sense the paradoxes in the conventional formulation, and therefore save much time and effort by those who are also disturbed by the apparent inconsistencies of the conventional model. As you know there have been a large number of attempts to construct different forms of quantum mechanics to overcome these same apparent paradoxes. To me, these other attempts appear highly tortured and unnatural.
I believe that my theory is by far the simplest way out of the dilemma, since it results from what is inherently a simplification of the conventional picture, which arises from dropping one of the basic postulates—the postulate of the discontinuous probabilistic jump in state during the process of measurement—from the remaining very simple theory, only to recover again this very same picture as a deduction of what will appear to be the case to observers.
I therefore believe that my formulation is by far the simplest from an axiomatic point of view. The acceptability, however, clearly is a matter of personal taste.9
It is important to note that Everett believed he had deduced from the laws of quantum mechanics the “appearance” of probability to an observer in most of the branches of a branching universe in which everything physically possible happens.10 In other words, he did not necessarily believe that probability was an objective property of the universe of universes, but that it was a subjective measure of our ignorance about what goes on outside our single branch.
Bohm again
In thinking about the measurement paradox, Everett said he was influenced by Bohm’s 1951 textbook, Quantum Theory. This highly regarded work explains the basic equations of quantum mechanics in clear language accessible to non-physicists. Bohm employed Bohr’s method of treating opposites as complementary processes, but he ignored Bohr’s rule that one must not speak about what goes on inside the quantum world. And he spent
his last chapter climbing the edifice of the measurement problem:
At the quantum level of accuracy the entire universe (including, of course, all observers of it) must be regarded as forming a single indivisible unit with every object linked to its surroundings by indivisible and incompletely controllable quanta [i.e. events not controlled by experimental design]. If it were necessary to give all parts of the world a completely quantum-mechanical description, a person trying to apply quantum theory to the process of observation would be faced with an insoluble paradox. This would be so because he would then have to regard himself as something connected inseparably with the rest of the world. On the other hand, the very idea of making an observation implies that what is observed is totally distinct from the person observing it.11
Bohm described the contradiction that Everett sought to remedy:
If the quantum theory is to be able to provide a complete description of everything that can happen in the world, however, it should also be able to describe the process of observation itself in terms of wave functions of the observing apparatus and those of the system under observation [including] the human investigator as he looks at the observing apparatus and learns what the results of the experiment are, this time in terms of the wave functions of the various atoms that make up the investigator, as well as those of the observing apparatus and the system under observation. In other words, the quantum theory could not be regarded as a complete logical system unless it contained within it a prescription in principle for how all these problems were to be dealt with.12
Everett took this to mean that if the object is a superposition of properties, then the observer who is correlating to that object by looking at it will also be in a superposition. A superposition in which each state of the observer is linked to a particular state of the superposed object. In his textbook, Bohm avoided that conclusion by falling back on von Neumann’s collapse postulate as an explanation of why macroscopic objects are not superposed. He retreated to Bohr’s quantum-classical duality, asserting that, “quantum theory presupposes a classical level,” and that,
In order to obtain a means of interpreting the wave function, we must therefore at the outset postulate a classical level in terms of which the definite results of a measurement can be realized.13
This was necessary, he said, because,
the classically definite aspects of large-scale systems cannot be deduced from the quantum-mechanical relationships of assumed small-scale elements … instead … the nature of what can exist at the nuclear level depends to some extent on the macroscopic environment.14
Everett took a different tack. He thought that classicality emerged from the quantum womb, not the reverse. But the basic interpretive questions that Bohm grappled with in the textbook—how single results emerge from super-positions—and his attempt to break out of the Bohr-von Neumann model, must have encouraged Everett, showing him that his idea of a fundamentally quantum universe was not hallucinatory.
15 Alone in the Room
This problem of getting the interpretation proved to be rather more difficult than just working out the equation.
P. A. M. Dirac, 19771
In the fall of 1954, Everett sat at his desk in his room at the Graduate College. In front of him were sharpened pencils, a yellow legal pad, and a manual on dissertation writing given to him by Wheeler. The guide advised: “In your investigation, think of the subject. In your presentation, think of the reader…. The general tone of one’s statements should be cautious, and the strength of the report should come through the nature of the evidence and its logical presentation, rather than through strong personal assertions.” Although Wheeler was fond of using provocative language in his own work, he preferred that his students be more circumspect, a bridle at which Everett champed.
During the next year, Everett spent most of his time researching and writing the thesis. In the spring and summer of 1955, he took breaks to take Nancy Gore to parties and football games and ballroom dancing. But back at the desk, he focused on his theme:
Quantum mechanics is reformulated in a way which eliminates its present dependence on the special treatment of observation of a system by an external observer.2
Nothing too ambitious: just the reformulation of quantum mechanics.
Because Everett’s thesis evolved through multiple versions it had several different titles. To clarify: In his son’s basement are the original sheaves of yellow legal paper upon which Everett began writing in pencil during his third semester of graduate school. In this rough draft each chapter evolved through several versions. The equations changed as he refined his mathematical argument; and he repeatedly toyed with images of “splitting” amoebas, cannonballs, and observers. It appears that Wheeler read through some handwritten sections, making suggestions with a red pencil.
Gore, who was an accomplished typist, typed up three mini papers that Everett extracted from his evolving thesis. He gave them to Wheeler as progress reports during the fall term of 1955. They were titled “Probability in Wave Mechanics,” “Quantitative Measure of Correlation,” and “Objective vs. Subjective Probability,” and Wheeler read them carefully, making notations.
The arguments presented in the mini papers explain the main points of his theory. One, that the universe is governed in its entirety by the Schrödinger equation; there is, objectively, no such thing as wave function collapse. Two, that the macroscopic world of our experience emerges from the microscopic world through entanglement. Three, that information theory can generate a probability measure in quantum mechanics without having to postulate the Born rule. Everett originally called his work, “Correlation Interpretation of Quantum Mechanics.”
In January 1956, Everett submitted his typed, 137-page dissertation to Wheeler (the “long” thesis), now entitled, “Quantum Mechanics by the Method of the Universal Wave Function.”3 A few months later, after a minor revision, bound copies entitled “Wave Mechanics Without Probability” were distributed to select physicists, including Bohr.
In April, Wheeler wrote to Bohr:
I would be appreciative of comments by you and Aage Petersen about the work of Everett…. The title itself, “Wave Mechanics Without Probability”, like so many of the ideas in it, need further analysis and rephrasing, as I know Everett would be the first to say.
But I am more concerned with your reaction to the more fundamental question, whether there is any escape from a formalism like Everett’s when one wants to deal with a situation where several observers are at work, and wants to include the observers themselves in the system that is to receive mathematical analysis.4
In May, Wheeler visited Bohr in Copenhagen and presented the case for Everett’s theory. Bohr and his circle vehemently rejected it. Wheeler put Everett’s degree on hold pending a drastic revision of the thesis. In June, Everett took a job at the Pentagon doing operations research for the Weapons Systems Evaluation Group. Many months later, in February, 1957, Wheeler and Everett sat down and rewrote the dissertation,5 excising and condensing three-quarters of it. The final version (the “short” thesis) was retitled, “On the Foundations of Quantum Mechanics,” and that is the title of the doctoral thesis that was officially accepted by Princeton on April 15, 1957.
For publication in the July Reviews of Modern Physics, it was renamed, at Wheeler’s insistence, “‘Relative State’ Formulation of Quantum Mechanics.” Fifteen years later, the unedited version (the “long” thesis) was published for the first time, by Princeton University Press, in The Many Worlds Interpretation of Quantum Mechanics, edited by Bryces. DeWitt and Neill Graham. Before sending the original manuscript of “Wave Mechanics Without Probability” to DeWitt for publication, Everett made extensive handwritten notes on it, restructuring the section on probability and information theory.6 He retitled it “The Theory of the Universal Wave Function.” Seeking to generate controversy, DeWitt ginned up the phrase “many worlds interpretation” for the book title and it stuck to the theory itself.7
&
nbsp; Introducing many worlds
In the fall of 1955, Everett outlined for Wheeler the main argument of his interpretation. This unpublished, typed, nine-page work, “Probability in Wave Mechanics,” is essentially an abstract, light on mathematical notation, but heavy on metaphor, with numerous descriptions of the observer as “splitting” into multiple copies embarking on different histories in variously branching universes that cannot communicate with each other in any ordinary sense.8 It is a compressed version of his long thesis.
After defining the measurement problem as the contradiction between the Schrödinger equation and the wave collapse axiom, Everett pointed out that the observer of a quantum system, who is himself a quantum system, is necessarily “correlated” to the object observed. (By “correlated” he meant “entangled.”) What happens, asked Everett, when a quantum mechanical observer looks at the quantum mechanical needle (or “pointer”) of a meter that correlates with a superposed quantum object? Say, a particle the wave function of which describes a superposition of possible positions.
