 |
Feliks BlohSwiss physicist, Nobel laureate
Date of Birth: 23.10.1905
Country:  Switzerland |
Content:
- Biography of Felix Bloch
- Scientific Contributions
- Move to the United States and Later Career
- Personal Life
Biography of Felix Bloch
Swiss physicist, Nobel laureateEarly Life and Education
Felix Bloch, a Swiss-American physicist, was born in Zurich, Switzerland in the family of Gustav Bloch, a wholesale grain trader, and Agnes (nee Mayer) Bloch. He attended the Cantonal Gymnasium in Zurich and graduated in 1924. Despite his initial interest in mathematics and astronomy, he enrolled in the Federal Institute of Technology in Zurich as an engineering major. However, after taking his first physics course, Bloch decided to pursue a career as a theoretical physicist rather than an engineer. From 1924 to 1927, he studied at the Federal Institute, where his teachers included Peter Debye and Erwin Schrödinger. He then studied at Leipzig University under Werner Heisenberg. Bloch received his doctorate in 1928 in Leipzig for his dissertation on electron conductivity in metals. In this dissertation, which is now recognized as laying the foundations of solid-state physics, he formulated the theorem that determined the form of electron wave functions in metals (Bloch functions).
Scientific Contributions
After completing his doctoral dissertation, Bloch became the recipient of several scholarships that allowed him to work with Heisenberg, Niels Bohr, Enrico Fermi, and Wolfgang Pauli. During this period, he made his major contributions to theoretical physics. Bloch derived the empirical law of German physicist Eduard Grüneisen concerning the dependence of metal conductivity on temperature, now known as the Bloch-Grüneisen relation. Due to his contributions to the theory of superconductivity and the theoretical understanding of magnetic systems, several theorems and effects are named after him, such as Bloch's theorem in superconductivity and Bloch's law, which describes the temperature dependence of magnetization in ferromagnetic materials (materials like iron with atomic structure that allows them to be easily magnetized), and Bloch walls, which are transition zones between regions of ferromagnetic material with different magnetic orientations. In 1932, Bloch extended the work of Bohr and Hans A. Bethe on the stopping power of moving charged particles in matter, resulting in the Bethe-Bloch formula for this effect.
Move to the United States and Later Career
When Hitler came to power in 1933, Bloch, being of Jewish descent, left Germany and settled in the United States. He became an adjunct professor at Stanford University in 1934 and two years later assumed a full professorship. During this time, he conducted important research on the quantum theory of electromagnetic fields. He then investigated the newly discovered neutron, predicting that its magnetic moment (a measure of its magnetic field strength) could be determined by scattering slow neutrons on iron and that the neutron beam would become polarized after scattering on an iron target. These predictions were confirmed the following year. Bloch then returned to experimental research. In 1939, he and Luis W. Alvarez measured the magnetic moment of the neutron using the cyclotron at the University of California, Berkeley as a neutron source. During World War II, as a member of the Manhattan Project to develop the atomic bomb, Bloch studied the properties of uranium isotopes. He later became an assistant head of the group engaged in military radar developments at the Harvard University Radio Research Laboratory.
After the war, Bloch returned to Stanford University. Here, he applied the radio wave techniques he had learned while working on radar during the war to the study of nuclear magnetic moments. Physicists trying to understand the behavior of atomic nuclei needed to know the relative magnetic moments of different types of nuclei with high precision. In the 1930s, Isidor Isaac Rabi developed a method for measuring nuclear magnetic moments, but his method required evaporating the sample and was not very accurate. In 1946, Bloch proposed a method that was highly accurate and did not damage the sample. Using the NMR method, Bloch placed the sample of the material under study in the magnetic field of a powerful electromagnet, causing the nuclei of the sample to precess (similar to the effect of gravity on a spinning top, causing it to wobble). The frequency or rate of precession of the nuclei depended on the strength of the magnetic field and the magnetic moment of the nucleus. Thus, if the field strength is known and the precession frequency is determined, the magnetic moment can be calculated. To determine the precession frequency, Bloch excited the sample with a much weaker magnetic field controlled by radio signals, which fluctuated (changed direction) at a frequency corresponding to the frequency of the controlling radio waves. When the frequency of the exciting field became equal to the precession frequency of the nuclei, the orientation of the nuclei's spins abruptly changed to the opposite direction - this easily detectable effect is known as nuclear magnetic resonance (NMR). The known frequency of the radio signals corresponding to this resonance frequency is equal to the precession frequency of the nucleus. Knowing the precise precession frequency of a given nucleus in a field of known strength allowed the magnetic moment of that nucleus to be determined with extraordinary precision. The Bloch method provided nuclear physicists with accurate and highly desired information, and the sample was not damaged at all. Moreover, using this method, magnetism could be measured in a completely new and simple way: once the magnetic moment of a given nucleus is known, it can be used to determine the strength of the magnetic field.
At the same time, Edward M. Purcell (who also worked on radar during the war) was investigating the same problem. Independently and simultaneously, he developed a method for measuring nuclear magnetic moments that was almost identical to Bloch's method. Using NMR, Purcell discovered that hydrogen emitted a signal in the radio frequency range (a discovery that led to the development of radio astronomy). Researchers using NMR found that the resulting magnetic moment of an atomic nucleus in a molecule changes due to the magnetic fields of surrounding electrons. It is in these changes that the key to the structure of molecules lies. NMR quickly became one of the most important analytical tools in chemistry. Furthermore, NMR measurements do not affect the sample at all and can be performed on living organisms without causing harm. The techniques and computational methods used in computerized tomography (CT scanners were developed by Allan Cormack and Godfrey Hounsfield) were combined with NMR observation methods in the 1970s, resulting in the development of scanning NMR devices that allowed the observation of specific chemical reactions inside the human body. These devices proved to be of great significance for scientific research and became a powerful tool in medical diagnostics. Diagnostic scanning NMR devices became available for medical use in the mid-1980s.
Bloch and Purcell were awarded the Nobel Prize in Physics in 1952 "for their development of new methods for nuclear magnetic precision measurements and discoveries in connection therewith." During the presentation of the laureates, Erik Holmberg, a member of the Royal Swedish Academy of Sciences, noted that "the methods of Purcell and Bloch provide a great simplification and generalization" of Isidor Isaac Rabi's molecular beam method, "allowing them to be applied to solid, liquid, and gaseous substances." Holmberg continued, "Since each kind of atom and its isotopes have a strictly defined and characteristic nuclear frequency, we can search for and explore all possible types of atoms and isotopes present in the object under investigation... without having any appreciable influence on the specimen." The application of their physical research to astronomy, chemistry, and medicine is an outstanding example of how fundamental research has an impact far beyond the area in which it was conducted. Most of Bloch's research after 1946 was related to the applications of NMR or, as he originally called it, "nuclear induction." In 1954-1955, he took a two-year leave of absence from Stanford to become the director-general of CERN (European Organization for Nuclear Research) in Geneva, Switzerland. In 1963, he assumed a professorship in physics at Stanford. Retiring in 1971, Bloch returned to Zurich, where he passed away on September 10, 1983.
Personal Life
In 1940, Bloch married Lore C. Misch, a physicist and fellow German refugee. They had three sons and a daughter. He became a U.S. citizen in 1939. Bloch was a member of the National Academy of Sciences, the American Academy of Arts and Sciences, the Swiss Academy of Natural Sciences, and the American Physical Society, of which he served as president in 1965.
