The Quantum Revolution (1900-1928)

Max Planck presented a new derivation of the law of blackbody radiation to the German Physical Society, proposing that oscillating atoms emit and absorb energy only in discrete units he called quanta, with energy proportional to frequency through the constant h. Planck himself treated the idea as a mathematical device rather than a physical reality. The date of his lecture is conventionally regarded as the birth of quantum theory, and the constant h became the fundamental scale of the microscopic world.

Location: Berlin, Germany

Albert Einstein published a paper arguing that light itself travels in discrete packets, later called photons, explaining the photoelectric effect in ways wave theory could not. The proposal was widely rejected by leading physicists of the day, including Planck, precisely because it seemed to contradict a century of evidence for light's wave nature. Einstein received the 1921 Nobel Prize in Physics for this work rather than for relativity.

Location: Bern, Switzerland

The Belgian industrialist Ernest Solvay convened the first international physics conference devoted to radiation and quanta, gathering Einstein, Planck, Marie Curie, Hendrik Lorentz, and most of the era's leading physicists. Lorentz's chairing and Einstein's closing summary marked the younger generation's ascendancy. The meeting made quantum problems a central concern of European physics and inaugurated the Solvay Conferences, which became the century's most famous scientific gatherings.

Location: Brussels, Belgium

Niels Bohr published a trilogy of papers proposing that electrons occupy only certain allowed orbits around the nucleus, emitting or absorbing light when they jump between them. The model explained the spectrum of hydrogen with striking precision, the first quantum account of atomic structure. It won Bohr the 1922 Nobel Prize and made Copenhagen a world center of the new physics.

Location: Manchester, United Kingdom (work); Copenhagen, Denmark

Louis de Broglie presented his doctoral thesis proposing that just as light had shown particle properties, matter should show wave properties, with every particle having an associated wavelength. His examiners were skeptical until Einstein endorsed the idea. The proposal was confirmed experimentally by electron diffraction in 1927 and earned de Broglie the 1929 Nobel Prize in Physics, the first awarded for a doctoral thesis.

Location: Paris, France

Werner Heisenberg, working on the island of Helgoland to escape hay fever, developed a new mechanics built only from observable quantities, with no electron orbits at all. His formulation, put into mathematical form by Max Born and Pascual Jordan at Gottingen as matrix mechanics, gave the field the name it still carries: the word "Quantenmechanik" first appeared in a Born-Jordan paper of September 1925. Heisenberg received the 1932 Nobel Prize for the achievement.

Location: Helgoland, Germany

Erwin Schrodinger published the first of his papers on wave mechanics, presenting the equation that describes how a quantum system's wave evolves in space and time. His approach arrived as a rival to Heisenberg's matrices and initially seemed to restore a more familiar picture of continuous waves. Within months the two formulations were proven mathematically equivalent, giving physicists a single, if strange, new mechanics. Schrodinger shared the 1933 Nobel Prize with Dirac.

Location: Zurich, Switzerland

Max Born proposed that Schrodinger's wave does not describe a spread-out physical object, but gives only the probability of finding a particle at a given place and time. The interpretation made chance fundamental to physics for the first time, replacing the clockwork determinism of Newton. It was among the most controversial steps of the revolution and earned Born the Nobel Prize in 1954.

Location: Gottingen, Germany

Heisenberg showed that a particle's position and momentum cannot both be known with arbitrary precision at the same time; the more exactly one is fixed, the less exactly the other can be. Far more than a measurement limitation, the principle declared that such pairs of properties do not simultaneously exist with exact values. It became the public face of quantum mechanics' departure from classical physics.

Location: Copenhagen, Denmark

At the fifth Solvay Conference, the founders of quantum mechanics confronted the theory's meaning in person. Niels Bohr defended what became known as the Copenhagen interpretation, in which probability and complementarity are fundamental features of nature, while Einstein argued that the theory, though correct in predictions, must be incomplete. The Bohr-Einstein debate continued for decades and shaped the philosophy of science; Einstein never accepted the probabilistic picture, famously objecting that God does not play dice.

Location: Brussels, Belgium

Paul Dirac married quantum mechanics to special relativity in an equation for the electron that unexpectedly required states of negative energy. He interpreted these as an antiparticle, and the positron was discovered in cosmic rays in 1932, the first antimatter found. Dirac's work opened quantum field theory and completed the revolutionary decade; he shared the 1933 Nobel Prize with Schrodinger. Quantum mechanics, built in under thirty years, became the foundation of chemistry, electronics, and all later physics of matter.

Location: Cambridge, United Kingdom