APS News

A Century of Physics

1900 - 1915 Foundations of the New Physics

By Hans Christian von Baeyer

The twentieth century began with a flurry of inventions such as the airplane, the mass-produced automobile, and transatlantic radio communication. These innovations transformed the world, but the changes sweeping over physics at the same time were far more radical. They brought about not just different lifestyles, but new ways of thinking.

Modern physics, which grew out of classical physics, rests on three pillars: The quantum theory, which governs atoms and their nuclei, Special Relativity, which deals with the relationship between space and time, and General Relativity, which explains gravity. The latter two were the sole creations of Albert Einstein and even the former received a crucial early contribution from him.

Einstein's miracle year came in 1905 when he was 26 years old and working as a patent examiner in Bern, Switzerland. In March he submitted a paper in which he proposed that light, which classical physics treats as a wave phenomenon, could also be thought of as consisting of discrete bits of energy he called quanta. The implied wave/particle duality of light became the cornerstone of the quantum theory. In May, Einstein explained the erratic motion of pollen floating in water as due to the jostling of innumerable invisible atoms. When this theory was verified in the laboratory, even the most skeptical of physicists were forced to accept atoms, which until then had been mere conjecture, as real, material objects.

In June of the same year Einstein submitted his historic paper on the Special Theory of Relativity, which demolished the rigid imaginary scaffolding of space and time that held up classical physics. In the preamble, he declared the troublesome ether hypothesis to be superfluous. In September, as an afterthought, he added the formula E=mc2 which would later be used to account for the unexplained enormous energies liberated by radioactivity. In seven frenetic months Einstein had torn down the foundations of physics and begun to build them up anew.

The success of the quantum theory of light, together with the lessons learned from radioactivity and the discoveries of the electron and the atomic nucleus, led to Niels Bohr's model of the hydrogen atom as a miniature planetary system. It explained the colors of light emitted by hydrogen gas, and the way X-rays originate in rearrangements of electrons deep within the atom.

Although the model was fundamentally flawed (for one thing, undisturbed hydrogen atoms are shaped like balls, not disks), and abandoned by its inventor within six years, it has survived to this day as a popular representation of the atom. But for scientific purposes, what could replace it? Who would find the key to the interior of the atom?

Editor's Note: To celebrate its centennial, the APS is producing A CENTURY OF PHYSICS, a dramatic illustrated timeline wallchart of over a hundred entries on eleven large posters intended for high schools and colleges. Each poster covers about a decade and is introduced by a thumbnail essay to provide a glimpse of the historical and scientific context of the time.

In May, APS News will feature the third introductory essay 1915-1924: Physics Extends its Reach.

APS encourages the redistribution of the materials included in this newspaper provided that attribution to the source is noted and the materials are not truncated or changed.

Editor: Barrett H. Ripin