Otto Stern turned atomic beams into powerful experimental tools, was involved in one of the seminal experiments of quantum theory, left Germany with the rise of the Nazis, and won the Nobel Prize for the discovery of the magnetic moment of the proton.
Otto Stern, a brilliant experimental physicist and son-in-law Nobel Prize in Physics, was born on February 17, 1888, the eldest child of the wealthy grain merchant Oskar Stern and his wife Eugenia, née Rosenthal, in the town of Zorau in Upper Silesia – now Zhori, Poland. Otto was one of five children in the family: two sons and three daughters.
In 1892, the family moved to Breslau, now Wrocław, Poland. From 1897 to 1906, Stern studied at the Johannes Humanist Gymnasium, graduating with a high school diploma.
After his matriculation exams, he studied physical chemistry for 12 semesters at the University of Breslau. His financial independence, thanks to his family's wealth, allowed him to study for periods at the universities of Freiburg and Munich. In 1912, he completed a doctorate in physical chemistry at Breslau.
Conversations with Einstein
Stern established close contact with Albert Einstein and joined him in Prague and then in Zurich. He later recounted their conversations to his young colleague, the Jewish-Austrian physicist Otto Robert Frisch:
"We talked about the newest theories. In the pubs there was unbearable noise – music, dancing, singing and shouting, and sometimes even fights. The quietest places we found were the cafes in the brothels. We sat at a table in the corner, talked, argued and wrote on napkins and tablecloths. No one bothered us. Strangers could have said: Einstein and Stern don't come out of brothels!"
In 1913, Einstein moved to the Federal Institute of Technology in Zurich, and Stern continued to work alongside him as a young researcher.
At the outbreak of World War I in 1914, Stern was drafted into the German Imperial Army and served, among other things, as a meteorologist. He was later transferred with other scientists to the laboratory of Nobel Prize winner in chemistry Walter Nernst at the University of Berlin, where research was conducted for the War Ministry.
In Berlin, Stern worked alongside Jewish physicists. Max Born and James Frank, both future Nobel laureates. Under the influence of Frank, a skilled experimentalist, Stern gradually moved from theoretical pursuits to experimental work.
Beams of invisible particles
After the war, Max Born was appointed director of the Institute for Theoretical Physics at the University of Frankfurt and invited Stern to serve as his assistant. Born wrote of him: "I managed to find in Otto Stern a Privatdozent of the highest quality, a kind-hearted and cheerful man, who quickly became our good friend."
Stern began to investigate experimentally the theory of molecular motion developed by James Clerk Maxwell in the 19th century. According to the theory, gas molecules are in constant random motion and their velocities are distributed in a calculable way.
Stern developed a facility based on Molecular beamsHe created a narrow stream of atoms or molecules moving through a vacuum, and thus could measure the properties of particles that were not visible to the naked eye. In 1920 he used the method to examine the distribution of molecular velocities and provide experimental confirmation.
Stern–Gerlach experiment
In 1921, Stern moved to the University of Rostock. At that time, he devised an experiment designed to test whether the direction of atomic angular momentum could take on a continuum of values, as expected in classical physics, or only discrete values, in accordance with the old quantum theory.
In 1922, Stern and physicist Walter Gerlach performed the experiment that has since become known asStern–Gerlach experimentThey heated silver in a furnace and created a beam of neutral silver atoms. The beam passed through a non-uniform magnetic field and struck a plate that served as a detector.
According to the classical prediction, a continuous distribution should have formed on the plate, reflecting a variety of directions of the magnetic moments. Instead, the beam of atoms split into two separate paths.
In the terms of that time, the result was considered direct evidence of spatial quantization. It was only after the concept of theSpin In 1925, the modern meaning of the splitting became clear: the silver atoms appeared in two discrete quantum states.
There is a famous story associated with the experiment. The layer of silver that had accumulated on the detector was too thin to be seen clearly. Stern was a heavy smoker of cheap, sulfur-rich cigars. According to historical evidence, when the researchers blew on the plate, sulfur compounds from the mouthwash reacted with the silver, forming a dark layer of silver sulfide. The thin mark suddenly became visible, and the split appeared before their eyes.
This is how a bad cigar helped reveal one of the most important findings in the history of physics.
From quantum skepticism to the Nobel Prize
From 1923 to 1933, Stern was professor of physical chemistry and director of the laboratory at the University of Hamburg. In the early 1930s, Stern, Otto Frisch, and Emanuel Estermann used sophisticated molecular beam methods to measure the magnetic moment of the proton—a magnitude much smaller than the magnetic moment of the electron.
Frisch said that in 1913, Stern and Max von Laue swore that if “Bohr’s absurdity”—the early quantum model, which deviated radically from classical physics—proved to be true, they would retire from physics. Stern’s own experimental discoveries finally convinced him of the correctness of the quantum picture.
When the Nazis came to power in 1933 and began to expel Jews from German science, Stern was initially eligible for exemption from some of the anti-Semitic measures due to his service in World War I. Despite this, he resigned from his position in August 1933 and left Germany.
His resignation was also linked to the dismissal of his longtime colleague Emanuel Estermann and the demand to remove Einstein's portrait from his office. After his departure, the Hamburg institute lost the central figure who gave it its scientific power.
His colleague James Frank was one of the first German scientists to resign on principled grounds. On April 17, 1933, he wrote to the Minister of Science and Education that the reason for his resignation was the government's treatment of German Jews. Frank, as a veteran of World War I, was also not obliged to retire immediately, but he refused to fire his Jewish colleagues and students.
In a letter to the university rector, Frank wrote: "We, Germans of Jewish origin, are treated as foreigners and enemies of the fatherland. It is to be expected that our children will grow up knowing that they will never be allowed to prove that they are worthy Germans."
Stern did not return to Germany. He was invited to serve as a professor at the Carnegie Institute of Technology in Pittsburgh, Pennsylvania, later Carnegie Mellon University. On March 8, 1939, he became an American citizen, and during World War II he served as an advisor to the United States War Department.
In 1943 he was awarded the Nobel Prize in Physics "for his contributions to the development of the molecular beam method and for his discovery of the magnetic moment of the proton." In 1945 he was elected to the National Academy of Sciences of the United States.
Scientific strength in a small number of articles
In 1946, Stern retired and moved to Berkeley, California. There he renewed his ties with the Jewish-Italian physicist Emilio Segre, winner of the Nobel Prize in Physics. Segre had studied with Stern in Hamburg in 1930 on a Rockefeller Foundation scholarship and on the recommendation of his teacher Enrico Fermi.
The two physicists fled European fascism for the United States. In Berkeley, they met regularly and discussed discoveries in astrophysics and particle physics until Stern's death.
In 1973, Segre published a memoir about his teacher and friend: "Stern was one of the greatest physicists of the twentieth century. He wrote a relatively small number of papers, but what power there is in those he wrote!"
Segre described Stern's extraordinary scientific determination, his ability to identify fundamental problems and focus on them, and also his love in his later years for fine cuisine, good cigars, and regular visits to the cinema.
Otto Stern suffered a heart attack while watching the film and died in Berkeley on August 17, 1969, at the age of 81. He left behind a legacy of experiments that gave tangible form to the quantum world – a world that had before him been largely a collection of bold theoretical ideas.
Questions and Answers
Question: What did the Stern–Gerlach experiment show? Answer: The experiment showed that a beam of silver atoms passing through a non-uniform magnetic field splits into discrete states instead of forming a continuous distribution. It provided direct evidence for the quantization of angular momentum.
Question: Did Stern and Gerlach discover spin? Answer: Not directly. The experiment was conducted in 1922, while the concept of spin was only proposed in 1925. In retrospect, it turned out that the result of the experiment matches the behavior of the spin of the electron in the silver atom.
Question: What did Otto Stern receive the Nobel Prize for? Answer: The prize was awarded to him for developing the molecular beam method and for discovering the magnetic moment of the proton, and not specifically for the Stern-Gerlach experiment.
Question: Why did he leave Germany? Answer: Stern, who was Jewish, resigned from the University of Hamburg in 1933 due to Nazi policies, the dismissal of his Jewish colleagues, and the violation of scientific freedom.
For the original publication: Opening the original publication
More on the subject on the science website
- We came to banish darkness: a conversation with Prof. Zeev Sternhal
- to be in two places at the same time
- A new method to bend and split light rays of different colors by a non-linear optical process
- Fundamental concepts in quantum physics: Bell's inequality - which was the basis of the Nobel Prize
- Basic concepts in quantum physics: what is spin?
