1831

English physicist Michael Faraday, regarded as the father of the electrical age, proves the reverse of Oersted's discovery: A magnet moving through a coiled wire generates an electric current in the wire. Faraday uses this discovery to build the first dynamo, or electric generator.

1842

Austrian physicist Christian Doppler observes that waves of sound or light from a moving object are compressed if the object moves toward an observer and stretched if the object moves away. This effect shifts either the frequency of the sound waves or the wavelength of the light in a precise way. Astronomers use the Doppler shift today to track the motions of stars and galaxies in the universe.

1843

English physicist James Joule establishes the equivalence of work and heat in a series of experiments covering more than three decades. His inventions include a famous apparatus in which the gravitational potential energy of a falling weight converts to thermal energy within a jar of water via a set of rotating paddles. The principle of the conservation of energy stems largely from Joule's work.

1858

American astronomer Henry Draper pioneers astronomical photography by mounting photographic plates on the back of a reflecting telescope. In 1872, Draper takes the first successful photograph of the spectrum of a star, in which a prism spreads the star's light into its component colors.

1873

Scottish physicist James Clerk Maxwell publishes his unified theory of electromagnetism, in which light waves arise from electric fields and magnetic fields that stream through space. The fields oscillate together like water waves and travel at a fixed speed. Maxwell's work forms the basis for Albert Einstein's special theory of relativity.

1887

American physicists Albert Michelson and Edward Morley find that light moves at a constant speed, unaffected by Earth's motion through space. This finding, which disproves the notion of an invisible "ether" through which electromagnetic waves travel, also plays a key role in Einstein's theories.

1900

German physicist Max Planck proposes that energy radiates not in a smooth and continuous spectrum but in tiny packets called quanta. This insight sets into motion a true revolution in physics, called quantum mechanics. Many physicists develop this nonintuitive theory of the behavior of matter and light during the next three decades.

1901

Italian physicist Guglielmo Marconi sends the first wireless transmission across the Atlantic Ocean. His simple Morse Code signal presages the age of radio communication that dominates our technological society.

1905

German physicist Albert Einstein derives one of his landmark advances in physics, the special theory of relativity. It asserts that light always moves at a fixed speed regardless of the motions of an observer, a tenet with strange consequences for measurements of mass, length, and time. His monumental general theory of relativity, published in 1916, describes how gravity, matter, and space interact throughout the cosmos.

1909

Physicist Sir Ernest Rutherford, a New Zealand native working in England, determines that most of an atom's mass resides in a tiny positively charged nugget at its center, surrounded by a flitting cloud of electrons. The startling implication of Rutherford's work is that matter consists mostly of empty space.

1912

American astronomer Henrietta Swan Leavitt unveils a relationship between the brightness and the flickering rate of certain pulsating stars called Cepheid variables. The advance allows future astronomers, most notably Edwin Hubble, to gauge the distances to faraway galaxies by measuring these cosmic lightbulbs.

1913

Danish physicist Niels Bohr proposes a new model for the hydrogen atom. The atom's electron can occupy only a set of specific orbits, or energy levels, around the nucleus, Bohr claims; all other orbits are forbidden. Although the details of this model change over the years, it explains the distinctive patterns of light emitted by excited atoms.

1916

German astrophysicist Karl Schwarzschild uses Einstein's general theory of relativity to calculate how small an object would have to shrink for its gravitational field to become so intense that light could not escape. This Schwarzschild radius is about 5 miles for a star with three times the mass of our Sun. Schwarzschild's work forms the theoretical basis for our understanding of black holes.

1919

English astrophysicist Sir Arthur Eddington organizes expeditions to Africa and South America to look for small shifts in the positions of stars near the Sun during a total eclipse. He sees displacements exactly as predicted by the general theory of relativity. Eddington also is the first to propose that hydrogen atoms fusing into helium in the Sun's core might provide the source of its energy.

1925

Austrian physicist Wolfgang Pauli derives a quantum-mechanical rule now known as the Pauli exclusion principle. It states that no two electrons can inhabit the same location with the same motions at the same time. Pauli's principle explains the previously mysterious order of the periodic table of the elements.

1927

German physicist Werner Heisenberg publishes his famous uncertainty principle. One cannot simultaneously know the exact position and the exact momentum (velocity times mass) of a particle at any given moment, the principle maintains. This strange notion became a cornerstone of quantum mechanics.

1929

American astronomer Edwin Hubble discovers that the universe is expanding in all directions. His analysis is based on the distances to 25 galaxies which he and American astronomer Vesto Slipher determined from Cepheid variables and on their rates of motion away from Earth, determined from their Doppler shifts. This discovery remains one of the best pieces of evidence in support of the Big Bang theory.

1936

Swiss-American astronomer Fritz Zwicky observes that galaxies in the Coma cluster appear to move much more quickly than could be explained by the gravitational attraction of the galaxies alone. The cluster contains about 10 times as much invisible matter as visible matter, Zwicky concludes. This "missing mass" problem lurks in the background for 40 years but has now emerged as one of the central mysteries of astronomy.

1948

American physicist Ralph Alpher and Russian-born physicist George Gamow publish the first model that demonstrates how the elements arose in the early universe. They envision a hot and dense fireball that forges hydrogen, helium, and a smattering of other elements. Gamow also calculates that this inferno of creation left behind a faint glow that suffuses the universe today, a prediction verified by Arno Penzias and Robert Wilson in 1964.

1950

American astronomer Fred Whipple creates his dirty snowball model for the nucleus of a comet. He envisions a chunk of ice mixed with grains and dust left over from the birth of our solar system. Whipple's model accounts well for the overall appearance of comets, including their ethereal tails of gas and dust released under the heat of the Sun.