The Search for a Unified Field Theory

The Goal

General relativity had explained gravity as the geometry of spacetime. In the 1920s the only other fundamental force known was electromagnetism. Einstein became convinced that the two must be aspects of a single field, and that a deep enough theory would produce particles, and perhaps even quantum behavior, as consequences of that field's equations.

He was not alone at first. Hermann Weyl, Theodor Kaluza, and others proposed ways of enlarging the geometry of spacetime to make room for electromagnetism. Kaluza's idea, adding a fifth dimension, particularly impressed Einstein.

Thirty Years of Attempts

Einstein tried one mathematical framework after another: extra dimensions, a geometry of "distant parallelism," and finally a generalized version of his own theory with a non-symmetric field. Several times he believed he had succeeded. A 1929 attempt made front pages in New York and caused a sensation in London, and was abandoned by 1931.

Each theory failed, usually because it could not reproduce the known behavior of electric charges and particles. He kept going. By his biographer's account, twelve pages of unified-field calculations lay at his bedside in Princeton Hospital in April 1955.

Why It Did Not Work

Two things counted against the program, and both are clearer in hindsight.

• The missing forces. The strong and weak nuclear forces were being uncovered during these same decades. Einstein's attempts ignored them. Any unification of only gravity and electromagnetism was aiming at an incomplete picture.
• The missing quantum. He hoped to derive quantum phenomena from a classical field theory, where the rest of physics built quantum theory in from the start. That path led to the triumphs of twentieth-century particle physics, and it was not his.

As a result he grew isolated from the mainstream of physics. Younger colleagues regarded his later work with respect for the man and little interest in the mathematics.

What People Get Wrong

These were not the wasted years of someone who had lost his way. The work was mathematically serious, he knew the odds, and he reportedly felt that someone with a secure position and an established name had a duty to take on problems that younger scientists, needing results, could not afford to.

Where It Stands

The aim outlived the attempts. Electromagnetism and the weak force were unified in the 1960s. Theories that seek to include gravity, such as string theory, revive ideas from Einstein's era, including extra dimensions. The unification he wanted has still not been achieved.