Einstein sent to the Annalen der Physik, the leading
German physics journal, a paper with a new understanding of
the structure of light. He argued that light can act as though
it consists of discrete, independent particles of energy,
in some ways like the particles of a gas. A few years before,
Max Planck's work had contained the first suggestion of a
discreteness in energy, but Einstein went far beyond this.
His revolutionary proposal seemed to contradict the universally
accepted theory that light consists of smoothly oscillating
electromagnetic waves. But Einstein showed that light quanta,
as he called the particles of energy, could help to explain
phenomena being studied by experimental physicists. For example,
he made clear how light ejects electrons from metals.
The Annalen der Physik received another paper from
Einstein. The well-known kinetic energy theory explained heat
as an effect of the ceaseless agitated motion of atoms; Einstein
proposed a way to put the theory to a new and crucial experimental
test. If tiny but visible particles were suspended in a liquid,
he said, the irregular bombardment by the liquid's invisible
atoms should cause the suspended particles to carry out a
random jittering dance. Just such a random dance of microscopic
particles had long since been observed by biologists (It was
called "Brownian motion," an unsolved mystery).
Now Einstein had explained the motion in detail. He had reinforced
the kinetic theory, and he had created a powerful new tool
for studying the movement of atoms.
Einstein in the patent office.
Einstein discovered light quanta by pondering experiments
on particles discovered only a few years earlier. See our
Web exhibit, The Discovery of the Electron.