"I soon learned to scent out
what was able to lead to fundamentals and to turn aside from everything else,
from the multitude of things that clutter up the mind."
March 1905
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.
Einstein discovered light quanta by pondering experiments on particles discovered
only a few years earlier. See our Web exhibit, The
Discovery of the Electron.
May 1905
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.
"When the Special Theory of Relativity began to germinate in me, I was visited
by all sorts of nervous conflicts... I used to go away for weeks in a state
of confusion."
June 1905
Einstein sent the Annalen der Physik a paper on electromagnetism and motion.
Since the time of Galileo and Newton, physicists had known that laboratory measurements
of mechanical processes could never show any difference between an apparatus
at rest and an apparatus moving at constant speed in a straight line. Objects
behave the same way on a uniformly moving ship as on a ship at the dock; this
is called the Principle of Relativity. But according to the electromagnetic
theory, developed by Maxwell and refined by Lorentz, light should not obey this
principle. Their electromagnetic theory predicted that measurements on the velocity
of light would show the effects of motion. Yet no such effect had been detected
in any of the ingenious and delicate experiments that physicists had devised:
the velocity of light did not vary.
Einstein had long been convinced that the Principle of Relativity must apply
to all phenomena, mechanical or not. Now he found a way to show that this principle
was compatible with electromagnetic theory after all. As Einstein later remarked,
reconciling these seemingly incompatible ideas required "only" a new and more
careful consideration of the concept of time. His new theory, later called the
special theory of relativity, was based on a novel analysis of space and time
-- an analysis so clear and revealing that it can be understood by beginning
science students.
September 1905
Einstein reported a remarkable consequence of his special theory of relativity:
if a body emits a certain amount of energy, then the mass of that body must
decrease by a proportionate amount. Meanwhile he wrote a friend, "The relativity
principle in connection with the Maxwell equations demands that the mass is
a direct measure for the energy contained in bodies; light transfers mass...
This thought is amusing and infectious, but I cannot possibly know whether the
good Lord does not laugh at it and has led me up the garden path." Einstein
and many others were soon convinced of its truth. The relationship is expressed
in an equation: E
equals m c squared.
Learn more in essays on Einstein's Time (Peter Galison) and Einstein's Relativity (John Stachel)