Martin Perl (1927 - 2014)
October 6, 2014
About a hundred years ago, there were eighty seven fundamental particles. This was the number of chemical elements that had been discovered at that time, although you might add the electron, discovered by J. J. Thomson in 1897. Ernest Rutherford's discovery of the atomic nucleus in 1911, and the 1913 atomic model of Niels Bohr, simplified things considerably, since the chemical elements could be reduced to two particles, the nucleus (albeit of variable charge) and the electron.
Rutherford's 1917 discovery of the proton solved the problem of the variable charge of the nucleus, so all matter was then thought to be composed of two elementary particles. This majestic simplification became somewhat more complicated with the neutron's discovery in 1932. The neutron solved the problem of nuclear stability, so this increase in the number of elementary particles from two to three was not unwelcome.
We've discovered a plethora of subatomic particles since that time, so what first appeared to be a simplification of nature has become quite complicated. Not every subatomic particle discovered was elementary. Many of these were composites of other particles, just as atoms were discovered to be composites of electrons, protons and neutrons. There were so many particles in this subatomic zoo that in the 1960s theorists were looking for another simplification.
One proposed simplification was the bootstrap model, championed by Geoffrey Chew. To show how far that theory has faded into obscurity, just search for "bootstrap model" on Google to see what you get. The reason for the bootstrap's fall was the predictive power of quantum chromodynamics, which was able to build mesons and baryons from the newer elementary particles, quarks and gluons.
Electrons are not made from quarks and gluons, so they are still very elementary. There are a few other electron-like particles, called leptons. The electron is stable, and many leptons will decay quickly to produce an electron. Leptons do not interact by the strong nuclear force, as do baryons, but they are subject to gravitation, the weak interaction; and, except for the uncharged neutrinos, electromagnetism.
The next lepton discovered after the electron was the muon, whose large mass caused it to be confused with a meson. The muon was discovered in 1936 by Carl D. Anderson. The first neutrino, the electron neutrino, was discovered by Clyde Cowan and Frederick Reines in 1956, followed by the muon neutrino in 1962. The tau lepton was discovered in the mid-1970s by Martin Lewis Perl and many collaborators from the Stanford Linear Accelerator Center and Lawrence Berkeley National Laboratory. Martin Perl died last Tuesday, September 30, 2014, at age 87.[2-5]
Martin Perl was born in New York City to Fay and Oscar Perl, Jewish-Polish immigrants from Russia to the United States.[2,4] He was an excellent student, graduating from high school at age sixteen, while his sister graduated at age fifteen and a half.[2,4] As Perl wrote in his Nobel Prize acceptance,
"Whatever the course, whether the course was boring or interesting to me, whether I was talented in mathematics or not talented in languages, my parents expected A's. This was good training for research, because large parts of experimental work are sometimes boring or involve the use of skills in which one is not particularly gifted."
Perl was encouraged by his parents to play sports, since that's what American boys did, and they wanted him to mix in. They thought that too much reading interfered with sports, but Perl enjoyed reading, and he enjoyed rainy days when he could stay inside and read. He wrote that in later life he felt very content on a rainy days.
His parents encouraged him to study engineering, since engineering had excellent career prospects at the time, but his college studies were interrupted by World War II, in which he served in the Merchant Marine.[2-3] At the end of the war, the draft was still in effect, so Perl was drafted into the US Army, where he served "a pleasant year at an army installation in Washington, DC, doing very little."
Although Perl was awarded a physics medal at high school graduation, he hadn't considered becoming a scientist. He chose chemical engineering as his profession, since the 1930s and 1940s were good times for chemistry, leading to synthetic materials, such as nylon. His early love of physics was quenched by his first college physics course, which was "all about pulleys and thermometers."
Perl received his chemical engineering degree from Brooklyn Polytechnic Institute, from which he graduated summa cum laude in 1948. After graduation, Perl worked as a chemical engineer in vacuum tube production for General Electric in Schenectady, New York, working in the production of television picture tubes, and doing development to reduce the grid emission in power tubes. While there, he took physics courses at Union College. I wrote about my short stay in Schenectady, living a few blocks from Union College, in a previous article (Charles Proteus Steinmetz, May 3, 2012).
His oldest son, Jed Perl, is quoted in the Los Angeles Times as saying, "He was bored... He actually did a little bit of sculpture at that point, in his early 20s." Vladimir Rojansky, one of his physics professors at Union College, encouraged him to study physics; so with the additional encouragement of his wife, Perl became a graduate student of physics at Columbia University in 1950 at age 23.
Perl became a student of Isidor Isaac Rabi, who did not frequent the laboratory. Perl learned experimental technique from senior graduate students, although he sometimes sought advice from Rabi's colleague, Polykarp Kusch. Perl dreaded visits to Kusch, who "had a loud voice which he deliberately made louder so that the entire floor of students could hear about the stupid question asked by a graduate student." As Perl wrote in his Nobel acceptance speech, he was not interested in amateur radio or in building Radios, as were many physicists of his era. This was especially strange, since he had worked with vacuum tubes at General Electric.
Perl received his physics Ph.D. from Columbia in 1955 for a thesis on the nuclear quadrupole moment of sodium. For eight years after that, Perl was at the University of Michigan doing particle physics research. One of his students there, Samuel C. C. Ting, later shared the 1976 Nobel Prize in Physics with Burton Richter for their discovery of the J/psi meson
Perl moved to the Stanford Linear Accelerator Center, under construction in 1963, to investigate leptons. As mentioned above, the muon is a lepton that's so much larger in mass than an electron (206.8 times heavier) that it was an oddity, and such strange behavior intrigued Perl. He undertook experiments on leptons at high energy, looking for new leptons in electron-positron collisions eventually discovering the tau lepton, which has 3477 time the mass of the electron. Detection was difficult, since the tau has a half-life of just 2.9x10-13 seconds.
As Perl said about these experiments,
"People wanted me to be cautious... We kept taking data and the evidence kept coming in. Every month or so we would get another handful - 10 to 20 - of these funny [particle collision] events. I gave a lot of talks. There would be all sorts of objections. I would take it all down. Some I had direct answers for, and if not, I went back and looked. We eventually eliminated every other explanation. Eventually most of the collaboration, about 30 people, became convinced there was no other explanation. And we published. Eventually other people began to find them, too."
For the discovery of the tau lepton, Perl shared the 1995 Nobel Prize in Physics with Frederick Reines. Reines detected the neutrino with colleague, Clyde Cowan. Cowan, who would have assuredly shared the prize, had died about two decades earlier. I wrote about the unfortunate time lag between discovery and Nobel award in a previous article (A Younger Man's Game? May 2, 2014). Perl retired as an emeritus professor of physics at Stanford University (Stanford, CA).
Perl's hobbies included the collection of Erector Sets, Lincoln Logs, similar construction toys, andbooks. He admitted that this was compensation for not having them as a child.[2,4] As reported in the Los Angeles Times, he also did his own plumbing repairs, which is not an unusual activity for an experimental physicist. Perl published more than 200 scientific papers, and he was a member of the National Academy of Sciences and a Fellow of the American Physical Society..
Perl was socially active, being against the Vietnam War. He founded Scientists and Engineers for Social and Political Action in 1969. He was, as are many high-energy physics experimentalists, against string theory. His son, Joseph Perl, is a researcher at SLAC. Says SLAC Director, Chi-Chang Kao,
"... Martin was a wonderful colleague, and it was a privilege to know him. His love for scientific discovery never diminished, and neither did his affection for SLAC and all who work here. One would frequently find Martin in the SLAC cafeteria at lunchtime, inviting staff to sit and talk with him. His enthusiasm was as infectious as his smile."
I'll end this article with two memorable quotations by Martin Perl.
"...it is better to be second in publishing a result, than to publish first with the wrong answer."
"Life is much harder for the young women and men who are in science in present times. But they are smarter and better trained than I was at their ages; they know more and have better equipment. I wish them good fortune."
- Geoffrey F. Chew, "Hadron bootstrap: triumph or frustration?" Physics Today, vol. 23, no. 10 (October, 1970), pp. 23-28.
- Glennda Chui, "Stanford's Martin L. Perl, winner of 1995 Nobel Prize for discovery of tau lepton, dead at 87," Stanford Report, October 1, 2014.
- Geoffrey Mohan, "Martin L. Perl dies at 87; Nobel winner discovered tau lepton," Los Angeles Times, October 3, 2014.
- Martin L. Perl - Biographical, Nobel Media AB, Nobel Prize Web Site.
- Martin L. Perl, Winner of 1995 Nobel Prize for Discovery of Tau Lepton, Dead at 87, Martin Perl's Web Site, October 2, 2014.
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