Children of immigrants in Brooklyn
Martin Lewis Perl was born in New York City on June 24, 1927. His parents, Oscar Perl and Fay Rosenthal, had come to the United States as children around 1900 from the Polish part of the Russian Empire; as Jews, he wrote, their families left to escape poverty and antisemitism. His father grew up on Manhattan’s Lower East Side and his mother in Brownsville, Brooklyn. Both finished their schooling at high school. His father worked as a clerk and salesman in a printing and stationery firm and then founded his own printing and advertising company, Allied Printing, which, though often precarious, carried the family into the middle class and through the Depression.
The Perls lived in good Brooklyn neighbourhoods with good schools, and school was serious business. Perl’s parents regarded teachers as higher beings and expected top marks in every subject, whether the subject bored him or not, which he later called good training for the dull parts of experimental work. His sister, Lila Perl, became a well-known writer.
Books, construction sets and earning a living
Young Martin read everything, carrying home six books at a time from two public libraries within walking distance. He borrowed Lancelot Hogben’s Mathematics for the Million and Science for the Citizen again and again, and copied out the calculus chapter he could not yet understand. He loved mechanics: cranes, ships, trains, and the construction sets he played with at a cousin’s house. He graduated from James Madison High School at 16, in 1942, with the physics medal.
Yet he never thought of becoming a scientist. For the children of immigrants, he explained, the point of education was to earn a good living, and a Jewish boy of the time was expected to become a doctor, dentist, lawyer or accountant. Engineering, his choice, was unusual, because antisemitism was still common in engineering companies. He enrolled in chemical engineering at the Polytechnic Institute of Brooklyn, interrupted his studies to serve as an engineering cadet in the U.S. Merchant Marine and then in the Army, and graduated summa cum laude in 1948.
“What you are interested in is called physics”
Perl went to work for General Electric as a chemical engineer in its electron tube division in Schenectady, New York, speeding up production of television picture tubes. To understand the tubes he took evening courses in atomic physics and calculus at Union College, where a professor, Vladimir Rojansky, told him that what he was interested in was called physics, not chemistry. At 23, with one year of introductory physics behind him, he entered the doctoral programme at Columbia University in 1950.
His adviser was I. I. Rabi, the Nobel laureate who had invented the molecular beam resonance method. From Rabi, Perl said, he learned to choose his own problems, to work on fundamental questions and above all to get the right answer and check it thoroughly. Rabi made him wait weeks for French physicists to confirm a similar measurement before he would let Perl publish his thesis result on the sodium nucleus. Rabi also steered him toward particle physics. Perl received his PhD in 1955.
A lepton hunter
He chose Michigan deliberately, over departments with stronger reputations in particle physics. He later passed on the reasoning to his own students as a two-part rule: choose the group or department where you will have the most freedom rather than the most powerful one, and remember that in a small or new group you will get the credit for what you accomplish. The launch of Sputnik in 1957, which opened the doors of federal research funding, gave him and Jones the chance to start their own programme.
Perl spent eight years at the University of Michigan studying collisions of pions and neutrons with protons, first with bubble chambers and then with spark chambers he and Lawrence Jones built with their own research grants. In 1963 he moved to the new Stanford Linear Accelerator Center, SLAC, then under construction in California.
There he became fascinated by a puzzle. The electron and its heavier cousin the muon behaved almost identically, yet the muon weighed about 207 times as much. Why should nature make a heavier copy of the electron, and might there be more? Perl insisted there was no good reason to assume only two such particles, called charged leptons, existed. When SLAC built SPEAR, a ring that collided electrons with their antimatter twins, positrons, he saw a practical way to look for a third.
The third family
Perl’s group joined Burton Richter’s group and a team from Lawrence Berkeley Laboratory in building the Mark I detector. Perl proposed a search for a very particular kind of event: collisions that produced just one electron and one muon of opposite charge, with energy missing and nothing else. No known process could do that. By 1974 the data contained 24 such events, far more than errors in identifying particles could explain. Perl challenged everyone in the collaboration to find a flaw in his analysis. No one could.
In 1975 the team announced the anomalous events, calling the presumed new particle “U”, for unknown. Many physicists were sceptical. By 1976 the sample had grown to 139 events, and their energies matched the decay of a heavy new lepton. In 1977 experiments at the German laboratory DESY confirmed the discovery. Perl named the particle tau, from the Greek word for “third”. The tau weighs about 3,500 times as much as the electron and lives for less than a trillionth of a second.
Why the tau matters
The tau was the first particle of a third generation of matter. Ordinary matter is built from the first generation, the electron and the up and down quarks; the muon belongs to a heavier second generation. The tau showed that nature repeats the pattern a third time, and physicists soon found the third-generation bottom and top quarks to go with it. Experiments at CERN later showed that there are exactly three such families. This three-family structure is a central feature of the Standard Model, the theory that describes all known particles. It also matters for one of the deepest questions in science: theory shows that the small difference between matter and antimatter that allowed our matter-filled universe to exist can only arise in the Standard Model if there are at least three families.
Perl’s work also became a model of method. He showed how to find something genuinely new by looking for a signature no known process could fake, and by counting and challenging every possible source of error before believing a result.
Pinning down the tau
Once the tau was accepted, Perl set about measuring its properties. True to his style, he hunted relentlessly for decays the Standard Model forbade, such as a tau turning into an electron and a photon, and found none. When younger colleagues proposed building a precise vertex detector for SLAC’s new Mark II experiment to measure the tau’s fleeting lifetime, he told them emphatically to go and do it. The detector measured the lifetime to better than 20 per cent, at the predicted value, confirming that the electron, muon and tau obey the same rules apart from their masses.
In the early 1990s he championed a dedicated machine, a tau-charm factory, to produce huge numbers of taus and test the theory more precisely. The proposal failed to win approval at SLAC in 1993, and a later attempt to build it in southern Spain also failed. In 1996 he published Reflections on Experimental Science, a collection of essays, papers and memoir about how experimental research is really done. He also served on the advisory board of Scientists and Engineers for America, which promotes sound science in government.
Jewish and American
Perl wrote frankly about growing up as the son of Jewish immigrants: parents determined to reach the middle class, anxious that their children be one hundred per cent American, and certain that education was the way up. He named the antisemitism that still shut Jewish engineers out of many firms in the 1940s. His Nobel autobiography is a portrait of that Brooklyn world of free libraries, demanding public schools and practical careers, and of the good fortune, as he put it, of coming of age as American universities and federal support for research expanded.
An independent mind to the end
After the tau, Perl kept hunting for things others thought did not exist. In the 1990s he automated Robert Millikan’s famous oil-drop experiment and searched about 40 million drops for particles with a fraction of an electron’s charge; he found none. In his last years he designed laboratory experiments with atom interferometers to look for traces of dark energy, working with young physicists at SLAC and, as a visiting professor, at the University of Liverpool. He came to SLAC nearly every day. His colleagues remembered a mentor who urged younger physicists to be thorough and systematic, to question conventional wisdom and to pursue new directions.
He received the Wolf Prize in 1982 and shared the 1995 Nobel Prize in Physics with Frederick Reines for pioneering experimental contributions to lepton physics. He was married to Teri Perl and later divorced; their children included the art critic Jed Perl. Martin Perl died at Stanford Hospital on September 30, 2014, aged 87.
Sources & further reading
Martin L. Perl, Biographical, NobelPrize.org · Martin Lewis Perl (obituary), Physics Today · Nobel laureate Martin Perl dies at 87, Physics World · Obituary: Professor Martin Perl, University of Liverpool · Martin Lewis Perl, Wikipedia · Researched from the sources above and checked against them. Spotted an error? Tell us.
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