by Peter Byrne
Collapsing the wave function was a way of making a separate peace with the measurement problem. As once you allowed a superposed quantum system to propagate according to the Schrödinger equation, there was no way to explain why the macroscopic world does not mirror microscopic superpositions.
According to Everett’s account, the large does mirror the small.
Bohr’s complementarity
Although Bohr did not dispute that the world is quantum mechanical (if unknowable as such), he called for “neglecting” the atomic constitution of measuring instruments and observers, as to do otherwise would allow for no external ground from which to make a measurement.8 And although Bohr did not employ von Neumann’s wave reduction terminology, his interpretation was analogous to collapsing the wave function. In essence, Bohr made the cut by privileging experimental context and making experimental results the sole criterion for knowledge.9
But unlike Wigner and von Neumann, Bohr did not overtly make “consciousness” into a force that extracts material results out of the ether of superposed ideas. Shying away from the heuristic abyss posed by the measurement problem, Bohr acknowledged that classical terms do not completely describe the quantum world, not even in his idealized model of complementarity. Nevertheless, he said, classical terms and concepts are the only guides we have at hand:
It is decisive to recognize that, however far the phenomena transcend the scope of classical physical explanation, the account of all evidence must be expressed in classical terms. The argument is simply that by the word ‘experiment’ we refer to a situation where we can tell others what we have done and what we have learned and that, therefore, the account of the experimental arrangement and of the results of the observations must be expressed in unambiguous language with suitable application of the terminology of classical physics.10
But as the scholar Max Jammer points out, Bohr’s interpretation tends to sequester physics from the world it purports to describe:
Another problematic aspect whose serious implications were only gradually understood was the fact that as long as a quantum mechanical one-body or many-body system does not interact with macroscopic objects, as long as its motion is described by the deterministic Schrödinger time-dependent equation, no events could be considered to take place in the system. Even such an elementary process as the scattering of a particle in a definite direction could not be assumed to occur since this would require a ‘reduction of the wave packet’ without an interaction with a macroscopic body. In other words, if the whole physical universe were composed only of microphysical entities, as it should be according to atomic theory, it would be a universe of evolving potentialities (time-dependent ψ-functions) but not of real events.11
For Bohr, the physical design of an experiment a priori divides the effable from the ineffable. Bohr chose not to ask what went on before or after he obtained a measurement result:
Our task is not to penetrate into the essence of things, the meaning of which we don’t know anyway, but rather to develop concepts which allow us to talk in a productive way about phenomena in nature.12
And there is logic to his method, as measuring instruments are classical and produce classical results for parsing by classical minds:
The experimental conditions can be varied in many ways, but the point is that in each case we must be able to communicate to others what we have done and what we have learned, and that therefore the functioning of the measuring instruments must be described within the framework of classical ideas.13
In brief: Bohr declared that although there may be a reality underlying quantum phenomena, we cannot know what the reality is. It is accessible to human understanding only through the mediation of experiment and classical concepts. Consequently, generations of physicists were taught that there is no quantum reality independent of experimental result. And that the Schrödinger equation, while incredibly useful as a predictive tool, should not be interpreted literally as a description of reality.
Everett took the opposite view.
Bohr and free will
Although Bohr contended that his interpretation did not posit human consciousness as a causal agent, it easily supported that conclusion, because nothing outside of a measurement registered by a human consciousness counted as real in his interpretation of quantum mechanics. It has been suggested, consequently, that Bohr was influenced by Immanuel Kant, and that Bohr regarded the quantum world as Kant’s forever-inaccessible “the thing itself.”14
In 1968, Bohr’s assistant (and Everett’s friend), Aage Petersen, claimed that quantum physics “reintroduced Kantian ideas.”15 He elaborated:
The most important feature of the ontological part of Kant’s philosophy is his distinction between the world of things as they are in themselves, and the world of phenomena, or things as they appear to us. Human reason is a lawgiver, but it can only legislate for phenomena, not for things in themselves…. Thus phenomena are conditioned by our way of knowing…. We are members of two worlds. In the world of phenomena, the will is subject to the law of causality. But as a thing in itself it is not subject to that law, and is therefore free … Just as Aristotle was convinced that he had given a complete account of the basic structure of being, Kant held that his critique of pure reason exhausted the subject.16
Bohr’s philosophy of complementarity can be viewed as an epistemological framework for holding mutually exclusive opposites: the quantum world is the inaccessible thing itself, the classical world reflects the quantum, bringing it into the realm of reason and knowledge as classically described phenomena. Bohr, in a Kantian echo, wrote that experimental evidence, “exhaust[s] all definable knowledge about the objects concerned.”17 Bohr’s resolution of the measurement problem was to declare, congruent with his philosophical basis, that classically defined measurements tell us everything we can know about a quantum system, period.
In part, Bohr’s stance was a reaction against the determinism of 19th century classical mechanics that was seen as undermining the agency of free will. Quantum mechanical indeterminism, therefore, had serious religious, philosophical, and sociological implications. No less a scientific light than A.S. Eddington claimed in 1928 that quantum indeterminism liberated free will.18 And Philipp Frank, an “instrumentalist” who profoundly influenced the development of American philosophical physics (and was admired by Everett), observed in 1954,
In twentieth century physics, we note clearly that a formulation of the general principles of subatomic physics (quantum theory) is accepted or rejected according to whether we believe that introduction of ‘indeterminism’ into physics gives comfort to desirable ethical postulates or not. Some educators and politicians have been firmly convinced that the belief in ‘free will’ is not compatible with Newtonian physics but is compatible with quantum physics.19
By the end of his life, Bohr was convinced that his philosophy of complementarity applied to the hard and soft sciences alike and that indeterminism, not determinism, was a basic rule of the universe. In a lecture at the University of Oklahoma on December 13, 1957, a few months after Everett’s thesis was published, Bohr pronounced that, “A rigorous deterministic approach leaves no room for the concept of free will.”20
In Everett’s determinist scheme, free will was consigned to the realm of illusion, but it also recognized that we act as if we have choices, as if rationality is an option.
A few years later, physicist Frederik J. Belinfante, who was in the process of “reinterpreting” Everett’s theory, pointed out, according to Jammer,
that quantum mechanical indeterminism, may be conceived as harmonizing with the belief in God, who continuously makes his own decisions about the happenings in this world, decisions unpredictable for us, while “if nature would be fully deterministic, one might reason that there would be no task for ‘God’ in this world.”21
The determinism inherent in Everett’s theory broke decisively with Bohr’s view of the universe; although it was not long before free will-loving
theists found in his many worlds model room for Divinity: for if everything that is possible occurs, they reasoned, then God must exist.22 Everett, naturally, viewed that argument as absurd. It is less absurd, however, to ask: What is the nature of probability if everything that is physically possible occurs somewhere in an “Everettian” universe?
12 The Philosophy of Quantum Mechanics
A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because it opponents eventually die, and a new generation grows up that is familiar with it.
Max Planck1
Copenhagen catch-all
American physicists in the mid 20th century were notoriously unphilosophical about quantum mechanics. Oppenheimer, Wiener, Linus Pauling, John van Vleck, E. U. Condon—leading American physicists—considered von Neumann’s “orthodox” postulate of wave function collapse to be a pragmatically acceptable, if mysterious, explanation of experimental results. To the extent that they thought about interpretation, American physicists usually favored the implicit “don’t ask, don’t tell” attitude of the Copenhagen interpretation because, among other attractions, it left free will intact.2
But not all physicists were satisfied with the status quo.
In 1974, Max Jammer, a German-born physicist based at Bar-llan University in Israel, published a comprehensive history, The Philosophy of Quantum Mechanics. Chronicling the articulation of interpretive issues in quantum mechanics, Jammer’s book accounts for the uneasy marriage between quantum physics and philosophy from 1926 through the early 1970s, concluding with a detailed explication of the many worlds interpretation.3
The task of interpretation, said Jammer, is to connect mathematical logic and physical systems. It is not enough for an explanation to be logically consistent; it must explain how the world works by making accurate predictions. And that is why physics concerns many philosophers, even if philosophy does not concern many physicists.
By the 1950s, the identification of Bohr’s complementarity with the Copenhagen interpretation had taken on a life of its own (Everett thought them one and the same). But the so-called Copenhagen interpretation was not completely identical with Bohr’s complementarity, nor with von Neumann’s postulate of wave function collapse, although many physicists treated it that way. Jammer noted,
The Copenhagen interpretation is not a single, clear-cut, unambiguously defined set of ideas but rather a common denominator for a variety of related viewpoints. Nor is it necessarily linked with a specific philosophical or ideological position. It can be, and has been, professed by adherents to most diverging philosophical views, ranging from strict subjectivism and pure idealism through neo-Kantianism, critical realism, to positivism, and dialectical materialism.4
However,
In the early 1950s the almost unchallenged monocracy of the Copenhagen school in the philosophy of quantum mechanics began to be disputed in the West. The previous lack of widespread criticism in this field was explained in some quarters as the result of a somewhat dictatorial imposition of what was called ‘The Copenhagen dogma’ or ‘orthodox view.’5
Challenging monocracy
In addition to Einstein’s well-known skepticism of Bohr’s complementarity, Schrödinger agitated against the idea of wave function collapse for decades. And despite the international prestige of Bohr and von Neumann during much of the 20th century, significant challenges to their views were mounted by Americans. Researching his thesis, Everett studied alternatives to the “official” interpretations, while constructing his own model reality.
In 1957, Henry Margenau, a professor of physics and natural philosophy at Yale University, read a preprint of Everett’s thesis with pleasure. Subsequently, he sent the young theorist a paper he had recently written. In it, Margenau laid out his own case against wave function collapse, calling it,
a mathematical fiction … used persuasively by von Neumann and later by others who were able to derive from this fiction the correct formalism of quantum mechanics, thus adding another example to the vast array of scientific instances in which correct conclusions were deduced from insupportable premises…. Current disbelief in the correctness of the present formulation of quantum mechanics has its source, at least in part, in the grotesque claim of the projection postulate.6
Nor did Margenau generally have kind words for Bohr:
Bohr does not ask science to make a choice—he asks science to resign itself to an eternal dilemma. He wants the scientist to learn to live while impaled on the horns of that dilemma, and that is not philosophically healthy advice. [Such dualism] relieves its advocates of the need to bridge a chasm in understanding by declaring that chasm to be unbridgeable and perennial; it legislates a difficulty into a norm.7
Margenau concluded, as had Everett, that causal relations hold in quantum mechanics and that, “the causal relation is Schrödinger’s equation.”8
Bohm’s hidden variables
Bohm broke with the Copenhagen model in the early 1950s, after his misgivings about it were amplified at a meeting with Einstein, who “talked me out of it,” encouraging him to focus on a determinist theory.9 Before he was driven out of Princeton by McCarthyites, Bohm unleashed his “hidden variables” interpretation, which eliminated the necessity of an external observer. Along the way, he shed the superposition principle and the measurement problem, (albeit at the expense of supplementing the quantum mechanical formalism with new variables). Declaring Bohr’s interpretation of quantum mechanics to be subjective and “totally inadequate,”10 Bohm said that his own “non-local” interpretation,
is based on the simple assumption that the world as a whole is objectively real and that, as far as we can know, it can be correctly regarded as having a precisely describable and analyzable structure of unlimited complexity.11
Bohm proposed that an undiscovered physical field permeates the universe. The hidden field guides particles on single trajectories according to the classical laws of motion. Starting from hidden initial conditions, hidden variables determine the visible paths of quantum systems.12 One problem with his interpretation is that its predictions do not differ from those of the conventional interpretation—and vice versa. It is not falsifiable, because any experiment will give the same results for the new theory as for the old theory. This is also a weakness in Everett’s theory—but that does not mean that the Copenhagen approach is better.
Bohr’s long time assistant at his institute, Leon Rosenfeld, instantly attacked Bohm’s “deterministic” interpretation, not on the grounds of its formal argument, but on the grounds that it did not conform to Bohr’s position that quantum mechanics is fundamentally indeterminist.
Everett was influenced by Bohm’s work, although he saw no need to add new terms to the Schrödinger equation. In his thesis, Everett opined that hidden variable theories, “are indeed possible.”13 Adumbrating an aspect of his own theory, he wrote,
Bohm succeeds in showing that in all actual cases of measurement the best predictions that can be made are those of the usual [collapse] theory, so that no experiments could ever rule out his interpretation in favor of the ordinary theory.14
Nor did Everett care to join in the ballyhoo about God playing or not playing dice:
The question of determinism or indeterminism in nature is obviously forever undecidable in physics, since for any current deterministic (probabilistic) theory one could always postulate that a refinement of the theory would disclose a probabilistic (deterministic) substructure, and that the current deterministic (probabilistic) theory is to be explained in terms of the refined theory on the basis of large numbers (ignorance of hidden variables).15
Everett believed that the best interpretation of quantum mechanics was embedded in the wave equation and that he could tease it out.
Einstein’s skepticism
Einstein did not endorse Bohr’s complementarity model; he reluctantly favored a “statistical,” classically oriented interpretation of quantum mechanics.
16 Although Einstein found von Neumann’s projection postulate empirically useful, he did not find it explanatory, and he therefore considered quantum mechanics to be unfinished as a descriptive method. In 1936, he wrote,
The ψ function does not in any way describe a condition which could be that of a single system: it relates rather to many systems, to ‘an ensemble of systems’ in the sense of statistical mechanics.17
For Einstein, it made no more sense to say that a single particle acts probabilistically than to say that single atom has a temperature. In other words, probability is not in and of itself explanatory, it is a clue that there is a more foundational reality.
In his thesis, Everett cited a recently published essay by Einstein in which he highlighted the implausibility of reducing superpositions governed by the Schrödinger equation into single states by acts of observation. Einstein commented that Schrödinger was “in principle” correct to call for treating the object measured and the measuring apparatus as a combined quantum mechanical system.18 Referring to the view that reality depends upon acts of observation, Einstein reflected,
Such an interpretation is certainly by no means absurd from a purely logical standpoint; yet there is hardly likely to be anyone who would be inclined to consider it seriously.19