Robert Frederick Christy
Robert F. Christyc. 1959
Acting President of theCalifornia Institute of Technology
In office 1977–1978
Preceded by	Harold Brown
Succeeded by	Marvin Leonard Goldberger
Personal details
Born	Robert Frederick Cohen (1916-05-14)May 14, 1916 Vancouver, British Columbia, Canada
Died	October 3, 2012(2012-10-03) (aged 96) Pasadena, California, U.S.
Citizenship	Canadian American
Scientific career
Alma mater	University of British Columbia University of California, Berkeley
Known for	Christy pits
Awards	Eddington Medal(1967) BronzeAcademic Medal of the Governor-General of Canada(1932)
Fields	Physics
Institutions	Illinois Institute of Technology Metallurgical Laboratory Los Alamos Laboratory California Institute of Technology
Thesis	Cosmic-ray burst production and the spin of the mesotron (1941)
Doctoral advisor	Robert Oppenheimer
Signature

Robert Frederick Christy (May 14, 1916 – October 3, 2012) was a Canadian-Americantheoretical physicist and laterastrophysicist who was one of the last surviving people to have worked on theManhattan Project duringWorld War II. He briefly served as acting president ofCalifornia Institute of Technology (Caltech).

A graduate of theUniversity of British Columbia (UBC) in the 1930s where he studied physics, he followedGeorge Volkoff, who was a year ahead of him, to theUniversity of California, Berkeley, where he was accepted as a graduate student byRobert Oppenheimer, the leading theoretical physicist in the United States at that time. Christy received his doctorate in 1941^[1] and joined thephysics department ofIllinois Institute of Technology.

In 1942 he joined the Manhattan Project at theUniversity of Chicago, where he was recruited byEnrico Fermi to join the effort to build the firstnuclear reactor, having been recommended as a theory resource by Oppenheimer. When Oppenheimer formed the Manhattan Project'sLos Alamos Laboratory in 1943, Christy was one of the early recruits to join the Theory Group. Christy is generally credited with the insight that a solid sub-critical mass of plutonium could be explosively compressed intosupercriticality, a great simplification of earlier concepts of implosion requiring hollow shells. For this insight the solid-core plutonium model is often referred to as the "Christy pit".

After the war, Christy briefly joined the University of Chicago Physics department before being recruited to join the Caltech faculty in 1946 when Oppenheimer decided it was not practical for him to resume his academic activities. He stayed at Caltech for his academic career, serving as Department Chair, Provost and Acting President. In 1960 Christy turned his attention to astrophysics, creating some of the first practical computation models of stellar oscillations. For this work Christy was awarded theEddington Medal of theRoyal Astronomical Society in 1967. In the 1980s and 1990s Christy participated in the National Research Council's Committee on Dosimetry, an extended effort to better understand the actual radiation exposure due to the bombs dropped on Japan, and on the basis of that learning, better understand the medical risks of radiation exposure.

Robert Frederick Cohen was born on May 14, 1916, inVancouver, British Columbia,^[2] the son of Moise Jacques Cohen, an electrical engineer, and his wife Hattie Alberta née Mackay, a school teacher. He was named Robert after his maternal great uncle Robert Wood, and Frederick after Frederick Alexander Christy, the second husband of his maternal grandmother. He had an older brother, John, who was born in 1913. Moise changed the family surname to Christy bydeed poll on August 31, 1918. On November 4, Moise was accidentally electrocuted at work.^[3] Hattie died aftergoitre surgery in 1926. Christy and his brother were then cared for by Robert Wood, their grandmother Alberta Mackay, and their great aunt Maud Mackay.^[4]

Christy was educated atMagee High School, and graduated in 1932 with the highest examination score in the province ofBritish Columbia. He was awarded theGovernor General's Academic Medal, and, importantly in view of his family's limited ability to pay, free tuition to attend theUniversity of British Columbia (UBC). At the award dinner he met the second-place winner, Dagmar Elizabeth von Lieven, whom he dated while at UBC.^[5] He received hisBachelor of Arts (BA) degree in mathematics and physics with first class honors in 1935, and hisMaster of Arts (MA) degree in 1937,^[6][7] writing a thesis on "Electron attachment and negative ion formation in oxygen".^[8]

George Volkoff, a friend of Christy who was a year ahead of him at UBC, was accepted as a graduate student at theUniversity of California, Berkeley, byRobert Oppenheimer, who led the most active school of theoretical physics in the United States at that time. This inspired Christy to apply to the University of California as well. He was accepted, and was awarded a fellowship for his first year. At Berkeley he shared an apartment with Volkoff,Robert Cornog,Ken McKenzie and McKenzie's wife Lynn McKenzie.^[9] For his thesis, Oppenheimer had him look atmesotrons, subatomic particles called muons today, that had recently been found incosmic rays. They were so-called because they were more massive thanelectrons but less massive thanprotons. With the help ofShuichi Kusaka he performed detailed calculations of the particle'sspin. He published two papers on mesotrons with Kusaka in thePhysical Review,^[10][11] which formed the basis of his 1941Doctor of Philosophy (PhD) thesis.^[12]

Christy could have graduated in 1940, but could not then be a teaching assistant, and this would have left him jobless and without income.^[13] In 1941, Oppenheimer found him a post at thephysics department atIllinois Institute of Technology (IIT).^[14] In May 1941, he married Dagmar von Lieven. They had two sons: Thomas Edward (Ted), born in 1944,^[15] and Peter Robert, born in 1946.^[16] IIT paid Christy $200 per month to teach 27 hours per week for 11 months per annum. To keep abreast of developments in physics, he attended seminars at theUniversity of Chicago.^[14] This brought him to the attention ofEugene Wigner, who hired him for the same money that IIT was paying him but as a full-time research assistant, commencing in January 1942.^[17]

Enrico Fermi and his team fromColumbia University arrived at the University of Chicago in January 1942 as part of an effort to concentrate the Manhattan Project's reactor work at the newMetallurgical Laboratory.^[18] Fermi arranged with Wigner for Christy to join his group,^[19] which was building anuclear reactor, which Fermi called a "pile", in thesquash court underStagg Field at the University of Chicago. Construction began on November 6, 1942, and Christy was present whenChicago Pile-1 wentcritical on December 2.^[20][21]

In early 1943, Christy joined Oppenheimer'sLos Alamos Laboratory in New Mexico,^[22] where he became anAmerican citizen in 1943 or 1944.^[23][24]Hans Bethe, the head of T (Theoretical) Division, detailed his physicists to assist with the projects at the laboratory. With his knowledge of reactors, Christy's assignment was to helpDonald W. Kerst's Water Boiler group.^[25] The Water Boiler was anaqueous homogeneous reactor intended as a laboratory instrument to testcritical mass calculations and the effect of varioustamper materials. It was the first reactor to useenriched uranium as a fuel, and the first to use liquid fuel in the form of solubleuranium sulfate dissolved in water. Christy estimated that it would require 600 grams (21 oz) of pureuranium-235, a figure he subsequently revised to 575 grams (20.3 oz). When the reactor went critical on May 9, 1944, with 565 grams (19.9 oz), the accuracy of Christy's figures raised the laboratory's confidence in T Division's calculations.^[26]

The discovery byEmilio Segrè's group in April and May 1944 of high levels ofplutonium-240 in reactor-produced plutonium meant that animplosion-type nuclear weapon was required, but studies indicated that this would be extremely difficult to achieve.^[27] By August 1944, the calculations had been made of how an ideal spherical implosion would work; the problem was how to make it work in the real world wherejets andspalling were a problem. Christy worked inRudolf Peierls's T-1 Group, which studied the theory of implosion. He suggested the possibility of using a solid plutonium core that would form a critical mass when compressed. This was an ultra-conservative design that solved the problem of jets by brute force. It became known as the "Christy pit" or "Christy gadget", "gadget" being the laboratory euphemism for a bomb.^[28] However the solid pit was intrinsically less efficient than a hollow pit, and it required amodulated neutron initiator to start the chain reaction.^[29] Christy worked withKlaus Fuchs, Paul Stein and Hans Bethe to develop a suitable initiator design, which became known as an "urchin".^[30]The Gadget used in theTrinity nuclear test and theFat Man used in theatomic bombing of Nagasaki used Christy pits.^[6]

Later in life, Christy agreed to give a number of bothoral history and video interviews in which he discussed his role in the Manhattan Project and latter interests.^[31][32]

After the war ended, Christy accepted anassistant professorship at the University of Chicago, at a salary of $5,000 per annum, twice what he had been making before the war. He moved back to Chicago in February 1946, but suitable housing was hard to find in the immediate post-war period, and Christy and his family shared a mansion withEdward Teller and his family.^[33]

Before the war, Oppenheimer had spent part of each year teaching atCalifornia Institute of Technology (Caltech). Christy was one of Oppenheimer's Berkeley students who made the trip down toPasadena, California, each year to continue studying with Oppenheimer.^[34] After the war, Oppenheimer decided that with his additional responsibilities he could no longer continue this arrangement. The head of the W. K. Kellogg Radiation Laboratory at Caltech,Charles Lauritsen therefore asked Oppenheimer for the name of a theoretical physicist that he would recommend as a replacement. Oppenheimer recommended Christy.Willy Fowler then approached Christy with an offer of a full-time position at Caltech at $5,400 per annum, and Christy accepted. He remained at Caltech for the rest of his academic career. The drawback to working at Caltech was that neither Lauritsen nor Fowler was a theoretical physicist, so a heavy workload fell on Christy. This was recognised by a pay raise to $10,000 per annum in 1954.^[35]

Christy joined Oppenheimer, Lauritsen andRobert Bacher, who joined the faculty at Caltech in 1949, inProject Vista, a detailed 1951 study on how Western Europe could be defended against theSoviet Union.^[36] Christy was distraught at the outcome of the 1954Oppenheimer security hearing. When he encountered Teller, who had testified against Oppenheimer, at Los Alamos in 1954, Christy publicly refused to shake Teller's hand.^[37] "I viewed Oppenheimer as a god", he later recalled, "and I was sure that he was not a treasonable person."^[2] Asked about his relationship with Teller in 2006, Christy said:

I've seen him from time to time. Our relationship has remained cool. Since that time, I have disagreed with him in a number of areas. For example, theStrategic Defense Initiative. I have disagreed with him, but I have not argued with him publicly, because Teller operates at a much different level than I do. He's a confidant of presidents; I'm not. As I say, I merely disagree privately, and that's the way it is.^[38]

In 1956, Christy was one of a number of scientists from Caltech who publicly called for a ban on atmospheric nuclear testing. The 1963Partial Nuclear Test Ban Treaty that Christy advocated put an end to one of his most unusual projects. He worked withFreeman Dyson onProject Orion, the design of a spacecraft propelled by atomic bombs.^[36]

During a sabbatical year atPrinceton University in 1960, Christy began an investigation ofCepheid variables and the smallerRR Lyrae variables, classes of luminousvariable stars. At the time it was a mystery as to why they varied. He used the knowledge of thehydrodynamics of implosion gained at Los Alamos during the war to explain this phenomenon. This earned him theRoyal Astronomical Society'sEddington Medal for contributions to theoretical astrophysics in 1967.^[36]

Christy was appointed vice president andProvost of Caltech in 1970.^[39] Under Christy and PresidentHarold Brown Caltech expanded itshumanities and addedeconomics to allow (or perhaps to compel—undergrads were required to take 25% of their units in "humanities") students to broaden their education. He had David Morrisroe appointed as vice president for Financial Affairs, and they steered Caltech through the financially stringent 1970s. The first women were admitted as undergraduates in Fall 1970.^[40]

WhenJenijoy La Belle, who had been hired in 1969 but refused tenure in 1974, filed suit with theEqual Employment Opportunity Commission, Christy pressed for the case to be settled and La Belle to be given tenure. The EEOC ruled against Caltech in 1977, adding that she had been paid less than male colleagues. La Belle received tenure in 1979.^[40]

In 1970 he became romantically involved withInge-Juliana Sackman, a fellow physicist 26 years his junior.^[41] He divorced Dagmar in early 1971,^[39] and married Juliana on August 4, 1973.^[42] They had two daughters, Illia Juliana Lilly Christy, born in 1974, and Alexandra Roberta (Alexa) Christy, born in 1976.^[43]

Christy briefly became acting President of Caltech in 1977 when Brown left to becomeSecretary of Defense. Christy returned to teaching afterMarvin L. Goldberger became president in 1978. He became Institute Professor of Theoretical Physics in 1983, and Institute Professor Emeritus in 1986.^[36]

Christy died on October 3, 2012. He was survived by his wife Juliana, their two daughters, Illia and Alexa, and his two sons, Peter and Ted.^[2] He was buried at Mountain View Cemetery inAltadena, California.^[44]

• Christy, I.-Juliana (2013).Achieving the Rare: Robert F. Christy's Journey in Physics and Beyond. Singapore: World Scientific.ISBN 978-981-4460-24-8.
• Hoddeson, Lillian; Henriksen, Paul W.; Meade, Roger A.;Westfall, Catherine L. (1993).Critical Assembly: A Technical History of Los Alamos During the Oppenheimer Years, 1943–1945. New York: Cambridge University Press.ISBN 0-521-44132-3.OCLC 26764320.
• Rhodes, Richard (1986).The Making of the Atomic Bomb. New York: Simon & Schuster.ISBN 978-0-684-81378-3.OCLC 13793436.

• 1983 Audio Interview with Robert Christy by Martin Sherwin Voices of the Manhattan Project
• "Robert Christy tells his life story".Web of Stories. Archived fromthe original on March 2, 2014. RetrievedFebruary 23, 2014.
• Interview with Robert F. Christy Caltech Oral Histories, Caltech Archives, California Institute of Technology.

• 1916 births
• 2012 deaths
• American astrophysicists
• 20th-century American astronomers
• Members of the United States National Academy of Sciences
• Presidents of the California Institute of Technology
• Manhattan Project people
• University of Chicago faculty
• University of British Columbia Faculty of Science alumni
• UC Berkeley College of Letters and Science alumni
• Fellows of the American Physical Society
• Canadian emigrants to the United States

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