Bohr summarized the apparent paradox of particles and waves under the concept of complementarity. After a guest lecture he gave at Moscow University, he left the following aphorism on the blackboard where famous visitors were sup posed to leave comments: Contraria non contradictoria sed complementa sunt (Opposites do not contradict but rather complement each other).
But back to Heisenberg, Plato, and the ancient Greeks: As the American philosopher of science Thomas S. Kuhn realized, science in times of scientific revolutions is particularly vulnerable to nonscientific influences. When changes to the scientific paradigm cause a shift in the generally accepted problems and solutions and thus also in the general perception and scientific world view, rational reasons like conformity with facts, consistency, scope, simplicity, and usefulness are not sufficient to understand the evolution of a new theory.
During these times, personal factors such as cultural back ground can also play a decisive role. And Heisenberg’s background was almost as Greek as it was German: As the son of a professor of Greek language, he became accustomed to Greek philosophy and culture and their reception in early twentieth-century Germany long before he himself learned Latin and ancient Greek in school. His biographer Armin Hermann suggests that the encounter with Plato’s philosophy influenced Heisenberg more than anything else. And not long after Heisenberg studied, climbed, and calculated in Helgoland, Paul Dirac in Cambridge and Erwin Schrodinger in Vienna worked out different but mathematically equivalent versions of quantum physics.
Since the standard interpretation of these works was developed basically in the inner circle around Bohr and Heisenberg, Heisenberg’s background seems particularly relevant for its appreciation. Also, Schrodinger made statements such as “Almost our entire intellectual heritage is of Greek origin” and “science can be correctly characterized as reflecting on the Universe in a Greek way.”And Dirac left on the blackboard in Moscow, right next to Bohr’s principle of complementarity, only the laconic remark, “A physical law has to have mathematical beauty,” a statement that reminds us strongly of Goethe’s transfiguration of the classical worldview:
Nature and art, they seem each other to repel
Yet, they fly together before one is aware;
The antagonism has departed me as well,
And now both of these seem to me equally fair.
And sure enough, quantum physics seems to be a Greek theory after all. This becomes evident when reading the thoughts in the book Die Einheit der Natur (The Unity of Nature) by Heisenberg’s student and friend, Carl Friedrich von Weizsacker, the brother of the subsequent German president, on the centerpiece of quantum physics — the wave-particle duality — and how it can be traced back to the arguments in Plato’s dialogue Parmenides.
Parmenides of Elea (Fig. 3.3) was a Greek philosopher in the pre-Socratic era around the fifth century BCE. Of his writing only the fragment of a philosophical poem remains; it deals with the unity of all being. It describes how an unnamed goddess-often understood as Persephone- invites the poet to perceive the truthful being-again a likely reference to the mystical experience in the mystery cults of Eleusis.
The truthful being then is distinguished from mere appearances and described as the all-embracing One — uncreated and indestructible, alone, complete, immovable, and without an end — reminiscent of Aldous Huxley’s stage of egolessness. One is the All is correspondingly the central statement followed up by Weizsacker when he discusses the argument between Socrates and Parmenides chronicled by Plato, which, according to the Italian author Luciano De Crescenzo, was the “most boring and complicated discussion in the entire history of philosophy.”
In this battle of words, which supposedly took place on the occasion of a visit of Parmenides to Athens, Socrates tried to refute the identity of One and All. To this end Socrates argued that One is not Many and thus has no parts. On the other hand All refers to something which does not miss any of its parts. Consequently the One would consist of parts if it were All, and thus finally One cannot be the All.
At this point Weizsacker comes to Parmenides’s defense by stressing the connection with quantum mechanics. And it is really astounding how the quantum mechanical interpretation of the One suddenly bestows this incomprehensible debate with lucidity and meaning. After all, in quantum mechanics the All is the wave function and, in its fullest manifestation, the all-embracing wave function of the universe.
Moreover, in quantum mechanics the analysis of the individual parts of an object without destroying the object is impossible, since the measurement, as explained above, affects the object and thus distorts the unity of its parts. And of all possible states an object can assume, only an infinitesimally small fraction are states in which the parts of the object actually correspond to a clearly defined outcome of a measurement. Only in these states can one truthfully assign reality or existence to these parts.
For example, only two among the infinitely many possible states that Schrodinger’s cat can assume (such as 90 percent alive and 10 percent dead or 27.3 percent alive and 72.7 percent dead) — namely totally dead or totally alive-correspond to possible outcomes in a measurement. But quantum mechanically, a pair of two cats, half of them dead and the other half alive, is realizable not only with one living and one dead but also with two half-dead cats or one being 70 percent alive and one being 30 percent alive.
Consequently, in quantum physics the All is really more than its parts, the partial objects actually constituting through their association a new entity, or, just as postulated by Parmenides, a new unity, a new One.
Now Parmenides, according to Plato, required further that the One possesses no properties: It has ho beginning, no center and no end, no shape and no location; it is neither in itself nor in anything else; it is neither at rest nor is it moving. Weizsacker can argue that a quantum mechanical object fulfills these requirements perfectly.
After all, a determination of any of these properties relies on a measurement, which implies a collapse of the wave function and thus destroys the unity of the collective object. On the other hand, isolation of the object from the surrounding universe is impossible: The object would not exist in the universe if it were not connected to the universe via some kind of interaction.
Thus, strictly speaking, only the universe as a whole can constitute a real quantum mechanical object.
Then, however, nobody would remain who could observe it from outside. Next Weizsacker and Parmenides follow the discussion backward: how the One — meaning the all-embracing universe barring all properties — unfurls into the colorful and multifaceted appearances of our everyday life. The argument relies here on the quirky assumption that the One, in the instant where it “is” — in the sense of exists — is already two things. It is the One and it is the Is. This argument can be iterated. Again both the One and the Is are two things: the Is is and is the One, and the One is and is the One. By repetition of this consideration the One acquires an infinite multiplicity: The being One unfolds itself into the universe. And again Weizsacker clarifies the discourse by referring to the quantum mechanical object.
After all, the way an object can exist is via interaction with other objects, which again results in the collapse of the wave function and the loss of quantum mechanical unity: In order to establish that an object exists, the object has to be measured and thus is affected in a way that implies that it is no longer one object according to the meaning of Parmenides’s One. In summary, Weizsacker arrives at an amazing conclusion, that the notion of complementarity has its source in ancient Greece: “We find . . . the foundation of complementarity already foretold in Plato’s Parmenides.” We actually can recover the feel of what the ancient Greeks experienced in their mystery cults in modern twentieth-century physics!
But this is not the end of the story: The atomism of Democritus, the idea that, the world is not continuously divisible but made out of indivisible particles, makes sense only in the context of quantum mechanics, where matter consists of compound objects that correspond to standing waves and thus can absorb or emit energy only in indivisible portions the quanta.
Also, the idea of tracing the laws of nature back to fundamental symmetries, as proposed in Plato’s Timaeus is an integral part of contemporary particle physics. Finally, consider Einstein’s objection to the fundamental importance of probabilities. Because of that objection, he remained a lifelong opponent of quantum mechanics: God doesn’t play dice with the world. This statement appears as a direct response to the 2,500-year-old fragment of Heraclitus: “Eternity is a child moving counters in a game; the kingly power is a child’s.”
How can one really comprehend the lack of causality inherent in quantum physics and in particular the role of the puzzling quantum collapse, which are not described by the mathematical formalism and remain controversial today? The most modern and consistent interpretation of these puzzling phenomena seems to be at the same time the craziest one: Every thing that can happen does happen-albeit in different parallel universes.
This idea was formulated for the first time in 1957 by Hugh Everett III while he was working on his doctoral dissertation at Princeton University. With the bizarre concept of parallel universes he asked too much of his contemporary physicists, even though Everett — like Richard Feynman, a founder of quantum electrodynamics, and Kip Thorne, the father of the wormhole time machine — was a student of the eminent John Archibald Wheeler, who was himself a rather unorthodox and creative associate of Einstein and who, among many other achievements coined the term black hole for the timeless star corpses in the universe.
But even with this first class mentor, Everett’s colleagues didn’t take him seriously. Everett left the academic world shortly after finishing his dissertation. During a frustrating visit in Copenhagen, during which Everett tried to convince Niels Bohr to take some interest in his work, he (Everett) transformed a standard approach in classical mechanics into a method for optimization that he could apply to commercial and military problems and that helped him to become a multimillionaire — but didn’t make him happy. He became a chain-smoking alcoholic and died of a heart attack when he was only fifty-one years old.
According to his explicit wish, his ashes were disposed of in the garbage. Fourteen years after his death, his daughter Elizabeth, who suffered from schizophrenia, committed suicide. In her suicide note she wrote that she was going into a parallel universe, to meet her father. His son Mark Everett became the famous rock star E, lead singer of the Eels. He described his father as distant, depressed, and mentally absent, and his own childhood as strange .and lonely. Only his music saved him. But he also expressed sympathy for his father: “These guys, I don’t think they should be held to subscribe to normal rules. I think that about rock stars, too.” Hugh Everett’s ideas about quantum physics were finally popularized in the 1970s by his advisor Wheeler and Bryce DeWitt, who had also worked with Wheeler. It was DeWitt who added the “many-worlds” label, a term that Wheeler never liked.
The interpretation essentially states that every measurement results in a split of the universe. Every possible outcome of a measurement — or more generally of any physical process — is being realized, but in different parallel universes. If a guy chats up a girl in a dance club, there is always a universe where the two of them get happily married and remain in love until they die, but also another one where she tells him to back off, he has too much to drink, and he wakes up the next morning with a serious hangover. This very insight made me particularly nervous when I prepared to jump out of an airplane 4,000 meters above Oahu’s north shore. After all, even if I survived in this universe, there are always countless universes where the parachute did not open. So somewhere one loses, every time. But somewhere there is also a parallel universe where Everett still lives happily together with his daughter.
The major advantage of the many-worlds interpretation, compared with the classical Copenhagen interpretation, is that no collapse of the wave function — which, in any case, is not really part of the theory — has to be assumed. Even after the measurement has been performed, both possible outcomes like an electron at place A and an electron at place B — coexist, but they decouple, so that an observer who measures the electron at place A does not notice the alternative reality with the electron at place B.
In contrast to the collapse of the wave function, this process of decoupling can be described within the formalism of quantum mechanics. Perhaps this process so-called decoherence — is the only reason we witness so little quantum weirdness in our everyday lives. The drawback of the many-worlds interpretation, however, is that we have to give up the concept of a unique reality.
The interaction of different parallel universes is suppressed after a measurement, but not totally lost. So even in our daily lives we don’t reside in clearly defined conditions such as dead or alive. The parallel universes in which we and our fellow human beings experience totally different fates instead resonate as unobservable tiny admixtures of alternative realities into our universe.
Thus the many-worlds interpretation exhibits the Parmenidic-neo-Platonic nature of quantum mechanics most clearly. According to this point of view, the unity of the different realities is not completely lost. It is actually possible to recognize the multiverse — the collection of all of Everett’s parallel universes — directly as Parmenides’s primeval One: the unity of the world the ancient Greeks felt they had lost in the charted modern world, and for whose reunification with the individualized ego they looked in the ecstasy of their mystery cults, in their Dionysian arts, or in the flush induced by psychedelic drugs.
The bizarre properties of quantum physics naturally inspired the fantasies of both journalists and authors. The parallel existence of different realities in quantum physics, for example, became the subject of a Physics World cover in 1998, which depicts a couple on the phone arguing as follows: “Oh Alice . . . you’re the one for me”-”But Bob . . . in a quantum world . . . How can we be sure?”
An even more radical take on the many-worlds interpretation can be found in Douglas Adams’s The Hitchhikers Guide to the Galaxy. Whenever the extraterrestrial crackpot Zaphod Beeblebrox, double-headed and addicted to Pan Galactic Gargle Blasters, starts the Infinite Improbability Drive, his stolen spaceship gets located in all places in the universe simultaneously, and tiny probabilities are amplified. In the novel this allows the spaceship to travel faster than light, and also causes various strange incidents, such as when a threatening pair of rockets gets suddenly transformed into a dumbfounded whale and a flowerpot.
Finally, and now I am serious again, the many-worlds interpretation could protect time travelers from ludicrous paradoxes, and in this way make time travel a meaningful physics concept. But we’ll get to that later…