One question, however, still puzzles: how does the quantum world relate to the more familiar human-scale one? For a century, the Copenhagen interpretation, chiefly developed by Bohr and Heisenberg in that city, has been the standard answer taught in physics courses. It posits that the quantum scale is indeterminate; that is, operates according to the laws of probability. This world is utterly different from the deterministic and predictable “classical” human scale, yet the Copenhagen interpretation doesn’t clearly explain how reality changes between the two worlds.

Indeterminism also appears in the Schrödinger wave equation at the heart of the Copenhagen view. Einstein had shown that light waves can act like swarms of particles, later called photons; in 1924, Louis de Broglie assumed the inverse, that tiny particles are also wave-like. In 1926, Schrödinger published his equation for these “matter waves.” Its solution, the “wave function” denoted by the Greek letter Ψ (psi), contains all possible information about a quantum entity such as an electron in an atom. But the information is indeterminate: Ψ is only a list of probable values for all the different physical properties, such as position or momentum, that the electron could have in its particular surroundings. The electron is said be in a superposition, simultaneously present in all its potential states of actual being.

This superposition exists until an observer measures the properties of the electron, which makes its wave function “collapse”; the cloud of possible outcomes yields just one result, a definite value emerging into the classical world. It is as if, asked to pick a card out of a deck, the instant you select the three of hearts, the other fifty-one cards fade away. In this case, we know that the rejected cards still physically exist with definite properties, but in the Copenhagen view, subatomic particles aren’t real until they’re observed. Another problem is that the notion of a sudden wave function collapse seems an arbitrary addition to the Copenhagen interpretation; it contradicts the smooth evolution in time built into the Schrödinger equation.

These troubling features, called “the measurement problem,” were hotly debated in the 1920s. But overwhelming any objections was the fact that the Copenhagen interpretation works! Its results agree precisely with experiments, the final test of any theory, and inspire real devices. Even so, David Joseph Bohm (1917–1992) and Hugh Everett III (1930–1982) sought equally valid theories without any incongruities. In the 1950s, these two American physicists dared to challenge the conventional Copenhagen interpretation with their “pilot wave” and “many-worlds” theories, respectively. Though from different backgrounds, Bohm and Everett shared characteristics that helped them seek answers: mathematical aptitude, necessary to manipulate quantum theory; and unconventional career paths, which separated them from the orthodoxy of academic physics.

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