Einstein’s letters illuminate a mind grappling with quantum mechanics

Back in the days before the internet — with no e-mail, no texting, no Twitter — people wrote letters. Even famous people, like Einstein.

And famous people’s letters were most likely to have been saved — and in Einstein’s case, published. For more than 30 years now, the Princeton University Press has been publishing Einstein’s letters (and his papers, and talks, and whatever else he wrote). His letters reveal nuances about his genius — and some downsides to his personality — that seldom show in his formal papers and lectures.

This month Princeton released the latest volume of Einstein’s papers, covering the period May 1925–June 1927, while Einstein was at the University of Berlin. It was an especially exciting time in science, as it corresponded with the infancy of quantum mechanics, midwifed by Werner Heisenberg in 1925 while at the University of Göttingen in Germany. Einstein also faced new challenges to his theory of relativity during this time, and his letters convey his despair at lack of progress toward his goal of a unified theory of gravity and electricity.

During that quest, quantum mechanics arrived as an unwelcome distraction. Heisenberg ignited a flurry of quantum activity when he devised novel mathematics for describing the mechanics of electrons and other subatomic particles — work that extended the earlier quantum ideas of Max Planck, Niels Bohr and Einstein himself. Shortly thereafter, the Austrian physicist Erwin Schrödinger formulated a competing version to Heisenberg’s (which although appearing very different conceptually, turned out to be equivalent mathematically). Einstein liked the Schrödinger approach, but did not think very highly of Heisenberg’s.

“Heisenberg has laid a big quantum egg,” Einstein wrote to physicist Paul Ehrenfest in November 1925. “In Göttingen they believe it (I don’t).”

Schrödinger had applied the math of waves to electrons. But Heisenberg treated the electron as a particle, describing its energy states with mathematical expressions known as matrices. Matrix algebra had been studied by mathematicians for decades — news to Heisenberg, who figured it out for himself. He then collaborated with physicists Max Born and Pascual Jordan (who did know about matrices) to develop the new quantum mechanics for describing the subatomic world. Making sense of that world required physicists to relinquish ordinary notions of space and time, Heisenberg insisted. “Atoms would certainly not exist,” he wrote to Einstein, “if our space-time concepts were even only approximately correct for very small spaces.”

Einstein expressed interest in the Heisenberg-Born-Jordan approach and a related formulation by British physicist Paul Dirac. But he resisted accepting the matrix math as the correct language for describing nature.

“The Heisenberg-Dirac theories certainly drive me to admiration, but to me they don’t smell of reality,” he wrote to physicist Arnold Sommerfeld in August 1926. To Schrödinger, Einstein wrote that he much preferred the wave picture of the electron that Schrödinger had developed. “I am convinced that with your formulation of the quantum conditions you have found a decisive advance,” he wrote. “I am also convinced just as much that the Heisenberg-Born path is misguided.”

In December 1926, Einstein wrote to Born (a close friend) that he respected the matrix approach, but could not accept one of its revolutionary implications: that nature’s behavior was governed by statistical laws. “The theory delivers much but it hardly brings us closer to the Old One’s secret,” Einstein wrote (“Old One” referring to God). “In any event, I am convinced that He is not playing dice.”

During this time, Einstein struggled with the challenge of unifying his theory of gravity — general relativity — with the electromagnetic forces involved with electrons and light. He believed in the “essential unity of the gravitational field and the electromagnetic field,” while realizing the theoretical account of that unity had not been properly framed. In a paper published in September 1925, he announced that he had succeeded in finding “the true solution.”

Einstein soon recanted. In elaborating his equations of general relativity to include electromagnetism, he still believed he was on the right path, but was short of the destination. “These equations do not determine the mass and charge of electrons and protons; hence they still need amendment in order to express the entire laws of nature,” he wrote to astrophysicist Arthur Eddington in January 1926. Einstein acknowledged questions about whether the solutions to his equations would correspond to reality. “As long as these questions cannot be answered,” he wrote Eddington, “one cannot know whether the general theory of relativity … fails in the face of quantum phenomena.”

Meanwhile, his special theory of relativity, published in 1905, had been called into question by new experiments in California. Einstein’s special theory held that light traveled at the same velocity regardless of the motion of its source (or any observer measuring it). If so, the “ether” in which light waves supposedly vibrated must not exist. Otherwise the velocity of light measured on Earth would depend on the Earth’s direction of motion through the ether. Albert Michelson and Edward Morley’s famous experiment in Cleveland in 1887 had found no evidence of any such ether, crucial support for Einstein’s theory. But in 1925, physicist Dayton Miller reported new experiments contending that the ether affecting light’s velocity really did exist.

Miller had repeated Michelson and Morley’s experiment in Cleveland, finding only a slight possible hint of an ether effect there. But then at a higher altitude — Mount Wilson in California — Miller claimed to find substantial signs of the ether.

Miller’s results induced Edwin Slosson, director of the young publication Science News-Letter (which later became Science News), to write Einstein in June 1925 requesting comment. Einstein replied that if subsequent work confirmed Miller’s results, “then the special relativity theory, and with the general theory in its present form, falls. Experiment is the supreme judge.”

But Einstein did not believe that Miller’s result would stand. In December 1925, he wrote to his friend Michele Besso that failure to control temperatures properly probably led to Miller’s erroneous results. “I didn’t take them seriously for a single moment,” Einstein declared.

In January 1926, Einstein cabled a journalist saying that there was virtually no chance that Miller’s experiments were correct; they indicated some unknown source of error rather than a true effect. “If you, dear reader, wanted to use this interesting scientific situation to make a bet, I recommend you bet that Miller’s experiments will prove faulty” or having nothing to do with an ether, Einstein advised. “I myself would be quite happy to put my money on that.”

Soon other experiments failed to confirm Miller’s (his analysis of the data contained errors), and the special theory of relativity emerged victorious, as it remains today. As does Einstein’s legend.

That legend is not without tarnish though, as his letters occasionally reveal indications of misogyny. Most disconcerting to Einstein fans would be a letter from October 1925, in which Einstein berates his ex-wife, Mileva, for threatening to embarrass him with her memoirs. “Does it not enter your mind at all that no one would care one bit about such scribblings if the man that they were about had not, coincidentally, accomplished something special? If someone is a nobody, there is nothing to object to, but one should be truly modest and keep one’s trap shut. This is my advice to you.” Einstein then contends his remarks showed how good he was being to her — otherwise he would not be dispensing such sound advice. “Not only children need a smack now and again, but so do adults, and most especially women.”

Like most humans, Einstein was a mixed bag. His bag was filled with greatness but not free from flaws. As we all now know because, like few other people in history, his letters have been so carefully preserved for posterity to analyze, and admire, and sometimes criticize.