Essay: John Stachel
Einstein's annus mirabilis
One hundred years ago, Einstein published five papers that led to revolutionary changes in our understanding of the properties of space, time and the microscopic world.
As a student, Einstein said, he had no feeling for the important problems in mathematics, whereas in physics he always had a sense of what is really significant. He certainly proved this in 1905. In that miraculous year he: 1) made major contributions to the classical kinetic theory of atoms and molecules; 2) resolved the apparent conflict between classical mechanics and the optics and electrodynamics of moving bodies; and 3) laid the foundations for the quantum theory of light.
Kinetic Theory: To this day, Einsteins two papers on molecular behavior and Brownian motion (the random jostling of small particles, like dust in water) are cited more often than his other 1905 papers because of the many practical applications of his formulas for diffusion and viscosity. The paper on Brownian motion is also of great theoretical interest because it is the first successful mathematical treatment of a stochastic (random) process.
Relativity: People like Lorentz and Poincaré were trying to understand why experimental attempts to observe a background ether, assumed to pervade space, had failed. Einstein realized that, since the days of Fresnel and Fizeau, such experiments had been trying to tell us that the traditional way of understanding motion break down for speeds approaching that of light. What was needed was to replace the old mechanics, in which the time was absolute, with a new mechanics, in which the speed of light is absolute and space and time behave differently from the perspective of observers in different states of motion.
It took years before this new viewpoint prevailed. Indeed, a look at some current textbooks treatment of the special theory shows that it still has not prevailed everywhere.
Quantum theory: The only 1905 paper Einstein described as revolutionary is the one on quantum theory, which introduced the idea that there are smallest possible elements (quanta) of light. This work on the photoelectric effect won the Nobel prize in 1921. In contrast to attempts by Planck and others to patch up classical theories in order to incorporate quantum phenomena, Einstein proclaimed from the start that neither classical mechanics nor classical electrodynamics would survive the quantum revolution. His concept of light quanta was widely derided until the discovery of the Compton effect (involving the scattering of light by electrons) in 1923 made clear the need for a quantum theory of light.
On the other hand, Einsteins explanation in 1907 of certain puzzling thermal properties of crystalline solids treating the crystals molecules as quantum oscillators convinced most physicists of the need for a quantum theory of matter, making quantum theory a major research topic even before Bohrs 1913 theory of the hydrogen atom.
By 1907, Einstein was at work on an extension of relativity theory to include gravitation. He soon realized that gravity and inertia are two sides of the same coin and their equivalence is the key to understanding gravity. This insight implies that inertial (non-accelerating) frames of reference, which had maintained their privileged status in the special theory of relativity, must be dethroned. To include gravitation, his 1905 theory had to give way to a generalized theory of relativity, with no privileged frames of reference. Most physicists scoffed and continued to seek ways to incorporate gravitation with special relativityindeed, many still do!
However, Einstein persevered, and by 1915 had developed a theory of gravity in which all space-time structures are dynamical objects, in bold contrast to all other physical theories, including the quantum theories developed later. These all depend on some fixed background space-time structures, and hence are called background-dependent theories.
Attempts to create a quantum theory of gravity must somehow reconcile general relativity with quantum mechanics. They face this dilemma: Is it possible to formulate a background-independent quantum gravity theory, as general relativity suggests? Or must we give up background independence in order to quantize gravity as quantum theory suggests? Most physi- cistsincluding string theoristsbelieve in the latter alternative. But if the former proves possible and the resulting theory is physically fruitfulas loop quantum gravity advocates have shown there is reason to hopethen the formulation of the first background-independent physical theory will surely rank as Einsteins greatest achievement.
John Stachel is Professor Emeritus of Physics and Director of the Center for Einstein Studies, Boston University. He is the founding editor of The Collected Papers of Albert Einstein, author of Einstein from B to Z, and he edited Einsteins Miraculous Year, annotated translations of all the 1905 papers.
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