Felix Bloch

Physicist

• Born: October 23, 1905
• Birthplace: Zurich, Switzerland
• Died: September 10, 1983
• Place of death: Zurich, Switzerland

Swiss American physicist

Twentieth-century Swiss-born physicist Felix Bloch shared the Nobel Prize in Physics in 1952 with Edward Purcell. Bloch’s experiments led him to determine the magnetic moments in nuclei, which in turn led to the discoveries of nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI).

Born: October 23, 1905; Zürich, Switzerland

Died: September 10, 1983; Zürich, Switzerland

Primary field: Physics

Specialties: Quantum mechanics; theoretical physics; nuclear physics

Early Life

Felix Bloch was born on October 23, 1905, in Zürich, Switzerland, and was raised in the Jewish faith. He was the second child born to parents Agnes Meyer Bloch and Gustav Bloch. He began school at age six, but it was not until his parents enrolled him in secondary school in 1918 in Zürich that his love for learning, especially the quantitative aspects of math and science, began. He was an excellent student and had particularly strong interests in mathematics and astronomy. Bloch graduated in 1924 with plans to become an engineer.

From 1924 to 1927, Bloch attended the Swiss Federal Institute of Technology (Eidgenössische Technische Hochschule) in Zürich. He spent his first year studying engineering; however, Bloch’s teachers, including physicists Erwin Schrödinger, Paul Scherrer, and Hermann Weyl, influenced him to study the new field of wave mechanics, which then shifted his focus to the study of theoretical physics. Upon completing the final examinations at the Institute in 1927, Bloch enrolled in the physics department at the University of Leipzig, Germany.

His mentor while at Leipzig was acclaimed quantum physicist Werner Heisenberg, originator of the uncertainty principle of particle motion. Heisenberg’s teaching greatly influenced Bloch, whose career breakthrough involved estimating the position of subatomic particles.

Bloch soon began research in theoretical physics. His doctoral thesis, for which he received his PhD in 1928, was an investigation of quantum wave mechanics in solids. This work led to advancements in the fields of conductivity and superconductivity.

After receiving his PhD, Bloch worked as a research assistant in Switzerland, the Netherlands, and Denmark. In 1932, he was back at the University of Leipzig as a lecturer and researcher, but he soon fled Germany for Copenhagen, Denmark, after Nazi leader Adolf Hitler obtained control of the German government in 1933.

During his stay at the University of Copenhagen, Bloch worked with the physicist Niels Bohr. He continued to conduct research in conductivity, collaborating with physicists Enrico Fermi and Hendrik Kramers. California’s Stanford University soon invited Bloch to join its faculty. Bloch, who was unfamiliar with the university, sought advice from several colleagues in Copenhagen, and on the recommendation of Bohr, he accepted the position in 1934, intending to stay only a short time. While there, Bloch worked with particle physics, focusing on the properties of the recently discovered neutron, and he became the first professor for theoretical physics at the university. Bloch was made the Max Stein Professor of Physics at Stanford in 1961 and taught there until his retirement in 1971. Bloch married German physicist Lore Misch in 1940.

Life’s Work

In 1939, in collaboration with physicist Luiz Alvarez, Bloch started to investigate the concept of the magnetic moment of neutrons in an atom, which, in the context of physics, refers to the estimated position of a subatomic particle when a measurement is taken.

According to Heisenberg’s uncertainty principle, it is impossible to know both the speed and the direction of a subatomic particle at any given time. Bloch was determined to develop a method for accurately determining the location of the neutron. Such information would reveal aspects of atomic structure and behavior.

Bloch hypothesized that iron would be the best element to use because it is easily magnetized, and if he could use a magnetic field to scatter and separate the charged particles in an atom, the magnetic moment of the neutrons could be accurately measured.

Bloch allowed the neutrons of the atom to act as radio wave transmitters in the presence of a magnetic field. He filled a test tube with iron nitrate and water and placed it between the poles of an electromagnetic field. The atoms scattered and divided into two groups according to the polarization of their charged particles. The neutrons, which have no charge, were unaffected and thus easier to observe. Bloch then focused radio waves on the test tube.

The neutrons transmitted the radio waves into a coil surrounding the test tube, which provided a rough reading of the locations of the neutrons. With this initial success, Bloch revealed aspects of atomic structure and behavior while also finding a way to estimate the location of the neutrons. Thus, nuclear induction was born. For his discovery, Bloch was awarded the 1952 Nobel Prize in Physics, along with Edward Purcell, who made the same discovery while experimenting at Harvard University.

By 1946, the technique was fully developed. It became known as nuclear magnetic resonance (NMR) spectroscopy. Bloch and his Stanford colleagues William Webster Hansen and M. E. Packard began to experiment with the possibilities of NMR.

Before long, the technology was used to study activity in the human body. This application of NMR is commonly known as magnetic resonance imaging (MRI). MRI is a safer medical technique than using X-rays and is far more effective in detecting elusive substances such as hydrogen and diseases such as cancer.

Once NMR was expanded into spectroscopic technology such as MRI, its applications became numerous. NMR can be used to identify unknown substances in chemistry, and astronomers often use it to detect hydrogen clouds in space.

Impact

Bloch is recognized as a pioneer in theoretical physics and magnetism. The application of the electromagnetic field in particle motion has since been standardized in the decades since Bloch’s innovation. His early work in the field of conductors and superconductors was also innovative. Superconductors are often applied in the construction of electromagnets for equipment such as MRI machines. Several concepts and instruments related to superconductors have been named for Bloch, including Bloch oscillations and Bloch transistors.

During World War II, Bloch assisted the US government as a researcher. He first worked in the Manhattan Engineering Department, which was commonly known as the Manhattan Project and was the program developed by the United States to build an atomic bomb. His specific role in the Manhattan Project was to research uranium isotopes that would be used in the bomb’s construction.

Bloch was then assigned to work at Harvard University’s Radio Research Library. While there, he researched counter-radar technology designed to disable enemy radar devices.

In 1954, Bloch took a leave of absence from Stanford University to become the first director-general of CERN (the European Organization of Nuclear Research) in Geneva, Switzerland, where he lived for several months before returning to Stanford in 1955 to concentrate on his research. Bloch published many important papers related to electromagnetism, nuclear physics, and group theory, including “The Magnetic Moment of the Neutron” (1938), “Nuclear Induction” (1946), “The Principle of Nuclear Induction” (1953), and “Spectroscopic and Group Theoretical Models in Physics” (1968).

In 1961, Bloch received an endowed chair at Stanford by his appointment as Max Stein Professor of Physics. Bloch retired from his post at Stanford in 1971 and returned to Zürich, Switzerland. He died on September 10, 1983, in his Zürich home.

Bibliography

Bloch, Felix. Fundamentals of Statistical Mechanics: Manuscript and Notes of Felix Bloch. London: Imperial College P, 2000. Print. Provides information pertaining to Bloch’s first-year graduate course on statistical mechanics that he taught at Stanford University. Includes samples of Bloch’s handwritten lecture notes and all of his problem sets beginning in 1933.

---. “Oral History Transcript: Felix Bloch.” Center for History of Physics/Niels Bohr Library & Archives. American Institute of Physics, 2012. Web. 7 May 2012. Features a transcript of a 1964 interview with Bloch, who discusses his family background, education, and influences on his career as well as the discovery and interpretation of quantum mechanics in the 1920s.

Hofstadter, Robert. Felix Bloch, 1905–83: A Biographical Memoir. Washington: Natl. Acad. of Sciences, 1994. Print. Provides information on Bloch’s early life and career in physics, both in Europe and the United States. Bloch’s experiments, discoveries, and published articles are described, along with a list of his publications, excerpted interviews, and personal correspondence.

Kevles, Daniel J. The Physicists: The History of a Scientific Community in Modern America. Cambridge: Harvard UP, 1995. Print. Examines the history of physics in the United States and provides information on discoveries beginning during the US Civil War through the 1990s. Discoveries are linked to the cultural, political, and social changes at the time of the discovery.