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Phd student Simon Gröblacher sits behind the floating table where he spends evenings doing Schrödinger’s cat-style experiments. *Photograph by Mark Mahaney.*

**In the summer of 1925,** Werner Heisenberg was stricken with hay fever and having trouble with math. He asked his advisor for two weeks off and left for a barren island in the North Sea. He spent his mornings swimming and hiking, but every evening Heisenberg tried to describe atoms in a theory that included only what could be measured. One night, feverish with insight, he calculated until dawn. After Heisenberg put down his pencil as the sun began to rise, he walked to the tip of the island, confident he had discovered quantum mechanics.

By this time a quarter century had passed since Max Planck first described energy as whole-number multiples of a basic unit, which he called the quantum. When two of the quantum’s other leading progenitors, Niels Bohr and Albert Einstein, heard about Heisenberg’s completion of the work they began, their reactions were almost immediate; Bohr was impressed, Einstein was not. Heisenberg’s theory emphasized the discrete, particle-like nature of matter, and Einstein, who tended to think in images, could not picture it in his head.

In Switzerland, Erwin Schrödinger had also been “repelled” by Heisenberg’s theory. In the fall of 1925, Schrödinger was 38 years old and rife with self-doubt, but when Einstein sent him an article describing a possible duality between particles and waves, Schrödinger had an idea. Over a period of six months, he published five papers outlining a wave theory of the atom. Though it proved difficult to physically interpret what his wave was, the theory felt familiar to Schrödinger. Heisenberg, who had moved to Copenhagen to become Bohr’s assistant, thought the theory “disgusting.”

Schrödinger and Heisenberg independently uncovered dual descriptions of particles and atoms. Later, the theories proved equivalent. Then in 1926 Heisenberg’s previous advisor, Max Born, discovered why no one had found a physical interpretation for Schrödinger’s wave function. They are not physical waves at all; rather the wave function includes all the possible states of a system. Before a measurement those states exist in *superposition,* wherein every possible outcome is described at the same time. Superposition is one of the defining qualities of quantum mechanics and implies that individual events cannot be predicted; only the probability of an experimental outcome can be derived.

The following year, in 1927, Heisenberg discovered the uncertainty principle, which placed a fundamental limit on certain measurements. Pairs of specific quantities are incompatible observables; momentum and position, energy and time, and other measurable pairs cannot be known together with absolute accuracy. Measuring one restricts knowledge of the other. With this quantum mechanics had become a full theory. But what physicists ended up with was a world divided. There was an inherent distinction between atoms unseen and their collective motion we witness with our eyes—the quantum versus the classical. While the distinction appeared physical, many, like Bohr, thought it philosophical; the theory lacked a proper interpretation.

According to Bohr every measuring device affects what it is used to observe. The quantum world is discrete and so there can never be absolute precision during a measurement. To know about quantum mechanics, we rely on classical devices. To Bohr this implied that the hierarchy between observer and observed had no meaning; they were nonseparable. Concepts once thought to be mutually exclusive, such as waves and particles, were also complements. The difference was only language.

By contrast Einstein was a realist who believed in a world independent of the way it is measured. During a set of conferences at the Hotel Metropole in Brussels, he and Bohr argued famously over the validity of quantum mechanics and Einstein presented a number of thought experiments intended to show the theory incorrect. But when Bohr used Einstein’s own theory of relativity to evade one of these thought experiments, Einstein was so stung he never tried to disprove quantum mechanics again, though he continued to criticize it.

In 1935, from an idyllic corner of New Jersey, Einstein and two young collaborators began a different assault on quantum mechanics. Einstein, Podolsky, and Rosen (EPR) did not question the theory’s correctness, but rather its completeness. More than the notion that god might play dice, what most bothered Einstein were quantum mechanics’ implications for reality. As Einstein prosaically inquired once of a walking companion, “Do you really believe that the moon exists only when you look at it?”

The EPR paper begins by asserting that there’s a real world outside theories. “Any serious consideration of a physical theory must take into account the distinction between the objective reality, which is independent of any theory, and the physical concepts with which the theory operates.” If quantum mechanics is complete, then “every element of physical reality must have a counterpart in the physical theory.” EPR argued that objects must have preexisting values for measurable quantities and that this implied that certain elements of reality could not be determined by quantum mechanics.

Einstein and his colleagues imagined two electrons that collide and fly apart. After the collision the electrons exist in a state of superposition of the possible values for their momenta. Mathematically and physically, it makes no sense to say that either electron has a definite momentum independent of the other before measurement; they are “entangled.” But when one electron’s momentum is measured, the value of the other’s is instantly known and the superpositions collapse. Once the momentum is known for a particle, we cannot measure its position. This element of reality is denied us by the uncertainty principle. Even stranger is that this occurs even when the electrons fly vast distances apart before measurement. Quantum mechanics still describes the electrons as a single system across space. Einstein could never stomach that an experiment at one electron would instantaneously affect the other.

In Copenhagen Bohr began an immediate response. It didn’t matter if particles might affect one another over vast distances, or that particles had no observable properties before they are observed. As Bohr later said, “There is no quantum world. There is only an abstract quantum physical description.” Physicists’ discourse on reality began just as the world slid inexorably toward war. During WWII physicists once interested in philosophy worried about other issues. David Bohm, however, did worry. After the war Bohm was a professor at Princeton, where he wrote a famous textbook on quantum mechanics. Einstein thought it was the best presentation of quantum mechanics he had read, and when Bohm began to challenge the theory, Einstein said, “If anyone can do it, then it will be Bohm.”

In 1952, during the Red Scare, Bohm moved to Brazil. There he discovered a theory in which a particle’s position was determined by a “hidden variable” even when its momentum was absolutely known. To Bohm reality was important, and so to preserve it, he was willing to abandon locality and accept that entangled particles influenced one another over vast distances. However, Bohm’s hidden variables theory made the same predictions as quantum mechanics, which already worked.

In America Bohm’s theory was ignored. But when the Irishman John Bell read Bohm’s idea, he said, “I saw the impossible done.” Bell thought hidden variables might show quantum mechanics incomplete. Starting from Bohm’s work, Bell derived another kind of hidden variables theory that could make predictions different from those of quantum mechanics. The theories could be tested against one another in an EPR-type experiment. But Bell made two assumptions that quantum mechanics does not; the world is local (no distant influences) and real (preexisting properties). If quantum mechanics were correct, one or both of these assumptions were false, though Bell’s theorem could not determine which.

Bell’s work on local hidden variables theory stirred little interest until the 1970s, when groups lead by John Clauser, Abner Shimony, and others devised experimental schemes in which the idea could be tested with light’s polarizations instead of electrons’ momentum. Then in 1982 a young Frenchman named Alain Aspect performed a rigorous test of Bell’s theory on which most physicists finally agreed. Quantum mechanics was correct, and either locality or realism was fundamentally wrong.

During the 1980s and 1990s, the foundations of quantum mechanics slowly returned to vogue. The theory had been shown, with high certainty, to be true, though loopholes in experiments still left some small hope for disbelievers. However, even to believers, nagging questions remained: Was the problem with quantum mechanics locality, realism, or both? Could the two be tested?

**in may of 2004** Markus Aspelmeyer met Anthony Leggett during a conference at the Outing Lodge in Minnesota. Leggett, who had won the Nobel Prize the year before, approached Aspelmeyer, who had recently become a research assistant to Zeilinger, about testing an idea he first had almost 30 years before.

In 1976 Leggett left Sussex on teaching exchange to the University of Science and Technology in Kumasi, the second largest city in Ghana. For the first time in many years, he had free time to really think, but the university’s library was woefully out of date. Leggett decided to work on an idea that didn’t require literature because few had thought about it since David Bohm: nonlocal hidden variables theories. He found a result, filed the paper in a drawer, and didn’t think about it again until the early 2000s.

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