Friday, October 23, 2009

Felix Bloch

One hundred and four years ago today, Felix Bloch (1905-1983) was born in Zurich, Switzerland. Bloch received his PhD in physics in 1928 from the University of Leipzig working under Werner Heisenberg, and then immigrated to the United States after Hitler came to power in Germany. He worked for a time at Los Alamos on the Manhattan Project, and had a long career in the Physics Department at Stanford University.

Bloch is most familiar to readers of the 4th edition of Intermediate Physics in Medicine and Biology because of his contributions to our understanding of nuclear magnetic resonance. He shared the 1952 Nobel Prize with Edward Purcell for “their development of new ways and methods for nuclear magnetic precision measurements”. In Chapter 18 on Magnetic Resonance Imaging, Russ Hobbie and I present the Bloch Equations (Eq. 18.15), which govern the magnetization of a collection of spins in a static magnetic field. Essentially all of MRI begins with the Bloch equations, so they are part of the essential toolkit for any medical physicist. Bloch’s most cited paper is “Nuclear Induction” (Physical Review, Volume 70, Pages 460-474, 1946). The abstract is reproduced below:
"The magnetic moments of nuclei in normal matter will result in a nuclear paramagnetic polarization upon establishment of equilibrium in a constant magnetic field. It is shown that a radiofrequency field at right angles to the constant field causes a forced precession of the total polarization around the constant field with decreasing latitude as the Larmor frequency approaches adiabatically the frequency of the r-f field. Thus there results a component of the nuclear polarization at right angles to both the constant and the r-f field and it is shown that under normal laboratory conditions this component can induce observable voltages. In Section 3 we discuss this nuclear induction, considering the effect of external fields only, while in Section 4 those modifications are described which originate from internal fields and finite relaxation times."
At the moment, you can download a reprint of this historic article at

Bloch also appears in Chapter 15 of Intermediate Physics for Medicine and Biology, because of his contribution to the development of the Bethe-Bloch formula (Eq. 15.58) governing the stopping power of a charged particle by interaction with a bound electron. He is also known for his fundamental contributions to solid state physics, including his seminal calculation of the electron wave function in a periodic potential, derived when he was only 23. You can download a Biographical Memoir about Bloch by Robert Hofstadter at

I have an indirect connection to Felix Bloch. When in graduate school at Vanderbilt University in the 1980s, I had several classes from Ingram Bloch, who—if I recall correctly—was Felix’s cousin. At that time, Ingram Bloch was teaching many of the graduate classes, so I took classical mechanics, two semesters of quantum mechanics, and general relativity from him. I remember spending days working on his infamous “take-home” exams. They weren’t easy. With two physicists in the family, the Blochs made quite an impact on 20th century physics.

P.S. Right now, has the 4th Edition of Intermediate Physics for Medicine and Biology on sale at 40% off. I have no control over if and when amazon reduces prices on books, so the price may go back up anytime.

P.P.S. Last night I finished Steven Strogatz's book The Calculus of Friendship: What a Teacher and a Student Learned About Life While Corresponding About Math, mentioned in the July 3rd entry to this blog. In a word, the book is charming.

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