# This Month in Physics History

Albert Einstein |

## Einstein's quest for a unified theory

After having become famous for several brilliant breakthroughs in physics, including Brownian motion, the photoelectric effect, and the special and general theories of relativity, Albert Einstein spent the last thirty years of his life on a fruitless quest for a way to combine gravity and electromagnetism into a single elegant theory.

Einstein was motivated by an intellectual need to unify the forces of nature. He felt very strongly that all of nature must be described by a single theory. “The intellect seeking after an integrated theory cannot rest content with the assumption that there exist two distinct fields totally independent of each other by their nature,” Einstein said in his Nobel lecture in 1923.

In addition, he believed there was a link between the need to resolve apparent paradoxes of quantum mechanics and the need to unify electromagnetism and gravity. Einstein always insisted that quantum mechanics could be derived from some more complete theory. For Einstein, who was never satisfied with the weirdness and randomness inherent in quantum theory, any acceptable unified field theory had to have quantum mechanics as a consequence.

In the 1920s, when Einstein began his work on a unified field theory, electromagnetism and gravity were the only known forces, and the electron and the proton were the only known subatomic particles. Most physicists at the time were excited about the newly discovered quantum theory, and spent their time absorbed with exploring its bizarre and interesting consequences. They didn’t see any great need for a theory uniting electromagnetism and gravity.

But Einstein, and several other scientists, did work on the problem of unification. In 1918, Hermann Weyl proposed a unification scheme based on a generalization of Riemannian geometry. Inspired by Weyl’s work, Theodor Kaluza showed that by extending space-time to five dimensions, one could produce the Einstein equations in four dimensions, plus an extra set of equations that is equivalent to Maxwell’s equations for electromagnetism. The fifth dimension would be compact, curled up so small that we can’t detect it. Oskar Klein later refined this idea.

Photo credit: Alan Richards Einstein's last blackboard, Institute for Advanced Study (1955). |

Einstein liked the five-dimensional approach. In 1919, he wrote to Kaluza, "The idea of achieving unification by means of a five-dimensional cylinder world would never have dawned on me...At first glance I like your idea enormously." Kaluza published his paper in 1921, and Einstein published his first paper on unified field theory, with Jacob Grommer, following a similar approach, in 1922.

Another approach Einstein tried involved extending general relativity to include the equations of electromagnetism by generalizing the metric tensor while keeping the 4-dimensional geometry.

Einstein worked on these two basic approaches persistently for the last thirty years of his life, but neither method ever produced the complete unified theory he was looking for.

He pursued and then soon rejected idea after idea. “Most of my intellectual offspring end up very young in the graveyard of disappointed hopes,” Einstein wrote in a letter in 1938.

But he never gave up on his quest for a unified theory. Even while lying on his deathbed, he continued his work. The day before he died, he asked to have his latest notes brought to him.

One reason for Einstein’s failure to discover a unified theory may be his rejection of quantum mechanics, which caused him to ignore new developments in physics and distance himself from the rest of the physics community. Einstein was aware of his position, and commented in 1954 that "I must seem like an ostrich who forever buries its head in the relativistic sand in order not to face the evil quanta." But the more he worked on unification, the farther away Einstein drifted from the rest of the physics community.

He also became more and more absorbed in formal mathematical arguments, rather than following the physical intuition that had guided him in his youth to his great discoveries.

Many people say that Einstein failed because he was simply ahead of his time. The knowledge and tools needed to complete a unified theory simply hadn’t been developed before Einstein died in 1955.

Today, many physicists are taking up his quest. The most promising approach appears to be string theory, which requires 10 or more dimensions and describes all elementary particles as vibrating strings, with different modes of vibration producing different particles.

String theory has not yet made any testable predictions, and some scientists worry that string theorists have, like Einstein in his later years, strayed too far from physical reality in their obsession with beautiful mathematics. But many others believe string theory does indeed hold the key to completing Einstein’s quest, and researchers are hoping to find ways to test some of the predictions of string theory.

Though his own work never produced a useful physical theory, Einstein established unification as an important goal of physics. Indeed a theory of everything is commonly called the “holy grail” of modern physics. Einstein would probably be pleased that so many physicists are enthusiastically devoting their careers to pursuing his dream.

*Ed. Note: As the World Year of Physics draws to a close, we are ending our series of history columns concentrating on Einstein. We are also changing authors. For five years, "This Month in Physics History" has been written by associate editor Jennifer Ouellette; upcoming columns (as well as this one) will be the work of science writer Ernie Tretkoff.*

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