Session Q19: Invited Session: The Scientific Legacy of Edward Purcell (1912-2012)

Purcell and NMR

In 1946 and 1947 the author carried out research at Harvard University under the guydance of Professor Edward M.Purcell. The results were communicated in a frequently cited paper,often referred to as BPP, after the authors Bloembergen, Purcell and Pound. The same material appeared in my 1948 Ph. D. Leiden thesis ``Nuclear Magnetic Relaxation.'' Some personal interactions with Ed Purcell and Bob Pound in these early and in later years at Harvard will be recounted.   

Purcell and the Development of Radioastronomy

``Join me for a ride on an electron, as we fly through electric and magnetic fields.'' With those words, Ed Purcell began a course in Electromagnetic Theory. I had a front row seat. Ken Bainbridge recommended I take the course and get to know Ed. Ed's wisdom and lucidity of thought soon gave the course, and Ed, a special place in my learning experience. I did not take notes in class. I was in awe at Ed's ability to present the subject with such clarity and simplicity. Ed's broad scope of interests and ability to present simple solutions to complex issues quickly led to my identification of Ed as the supreme mentor on any and all subjects. While working with Norman Ramsey to obtain an external beam from the Harvard Cyclotron, I consulted with Ed on the subject of available options. He suggested I scatter the beam off a target that could be remotely positioned, and catch it in a tunnel shielded from the magnetic field of the Cyclotron. When I had a problem with implementation I would, `` Ask Ed.'' During a visit to the Lab by Fermi, he commented on the simplicity of the solution. He was not surprised to learn that Purcell provided critical guidance. When I suggested Meteorology as a subject for my Oral Test, Purcell said it was not a science and I should pick another subject. I argued it was a science. Ed asked that I loan him some books on the subject and the Oral would be in two weeks. When I walked into the room for the Oral, I noticed that Ed had invited all seven authors of the books I had loaned to him. A simple Purcell answer to a problem. When I asked about his recommendations concerning doctorate dissertation topics, he said the selection must be based on my interests not his. I provided a brief summary: Mathematics, Quantum Mechanics, Meteorology, and Astronomy. Within two weeks, Ed proposed I look into the Hydrogen Line. After a joint review of the papers by Van deHulst and Shklovski we concluded that: van de Hulst had clearly shown the line was undetectable, Shklovski had claimed the line was detectable, however, there was an error in his calculations. We noted, however, that the topic fit my interests. Purcell had a simple solution. I would proceed with a ``negative thesis,'' the goal to measure the level of non-detectability. As with all other joint ventures, Ed was always there when I needed help. His guidance was always simple and embarrassingly obvious. For my doctorate oral exam, Ed invited Van deHulst as the only other participant. Purcell began by asking if I had any questions. I asked Van deHulst why the line was detectable. Purcell then announced that I had my doctorate and could leave at any time, while he and Van deHulst worked on the answer to my question. Many of us became terribly alone on March 7, 1997, when we learned we could no longer ``Ask Ed.''   

On small things in water moving around: Purcell's contributions to biology

I went to see Purcell after finishing my course work for the Ph.D. (1961) to ask whether I might join his group. ``But I don't have any graduate students,`` he said. ``Why is that?'' I asked. ``I can't think of anything to do,'' he replied. That was a wipe out. After I had finished my Ph.D. with Ramsey on the hydrogen maser (1964), Ed and I came up with an idea that led to work on sedimentation field-flow fractionation (\textit{PNAS} 1967). We had hoped this method would be useful for biology, but problems of adsorption of proteins to surfaces stood in the way. Then I moved over to the biology department and got interested in the motile behavior of bacteria (1968). Here was a subject that I thought Ed would really enjoy. There were wonderful movies made by Norbert Pfennig of experiments done by Theodor Engelmann in the 1880's. We found a 16-mm projector and looked at these movies on Ed's office wall. Ed's first comment proved seminal, ''How can such a small cell swim in a straight line?'' We thought about how cells count molecules in their environment and wrote ``Physics of chemoreception,'' (\textit{Biophys. J}.,1977). In the meantime, Ed gave a memorable lecture at Viki Weisskopf's retirement symposium, his classic ``Life at low Reynolds number'' (\textit{Am. J. Phys.} 1977). Ed really wanted to understand what it would be like to swim like a bacterium! He wasn't very interested in what the literature had to say about such a problem, he wanted to think it through for himself. My role was straight man. I very much enjoyed the ride.   

Purcell's Work Helping the Government

I worked closely with Ed Purcell from 1956 through 1975 or so, largely through our joint membership on and consulting with the President's Science Advisory Committee (PSAC) and working with the ``Land Panel'' on reconnaissance satellites. Purcell's work with the government had begun long before, with his 5-year service at the MIT Radiation Laboratory, and his advisory role had included, in particular, important work of the Technological Capabilities Panel (TCP) of the predecessor to PSAC. I will try to capture the flavor of Ed's contributions and the context of the times in which he was involved. His style and impact are well characterized by this quote from the book of Eisenhower's Science Advisor and PSAC Chair, James~R.~Killian, ``When Eisenhower was later to speak in memorable tribute of `my scientists' he was surely recalling among others this quiet, modest, lucid man. Robert Kreidler [one of Killian's staff], in an interview I had with him in preparing for this memoir spoke almost with awe of his [Purcell's] impact on PSAC, `Ed Purcell did not speak often,' he said, `but when he did there would be enormous silence in the room, because everybody knew that whatever he said was going to be worth listening to with careful attention.''' I give some examples why it was so worthwhile listening to Ed Purcell.   

Purcell the Teacher: In and Out of the Classroom

As a high school student Edward Purcell read articles by K.K. Darrow from his series, ``Advances in Contemporary Physics.'' Many years later, Purcell, referring to those articles, said they remind us ``that great teaching does not require a classroom{\ldots}.'' Many of Purcell's choices were motivated by his devotion to teaching: the undergraduate courses he preferred to teach at Harvard, the textbooks and pedagogical papers he wrote, and the professional activities he engaged in. He delighted in explaining complex phenomena in simple ways -- a mark of a great teacher.