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The field of quantum mechanics dates to 1900, the year German scientist Max Planck (1858–1947) discovered that energy could come in discrete packages called quanta. It advanced in 1913, when Danish physicist Niels Bohr (1885–1962) used quantum principles to explain what had until then been inexplicable, the exact wavelengths of light emitted or absorbed by a gas of hydrogen atoms. And since the 1920s, when Werner Heisenberg (1901–1976) and Erwin Schrödinger (1887–1961) built new quantum theories, quantum mechanics has consistently proven its value as the fundamental theory of the nanoscale and as a source of technology, from computer chips and lasers to LED bulbs and solar panels.

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.

Heisenberg and Bohr developed the Copenhagen interpretation amidst the blossoming of new quantum theories in the first half of the twentieth century. In 1927, Heisenberg announced his important uncertainty principle: at the quantum level, certain pairs of quantities, such as momentum and position, cannot be simultaneously measured to any desired degree of precision. The more exactly you measure one, the less well you know the other. Thus, we can never fully know the quantum world, a key feature of the Copenhagen interpretation.

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