Robert Frederick Christy (May 14, 1916 – October 3, 2012) was a Canadian-American theoretical physicist and later astrophysicist who was one of the last surviving people to have worked on the Manhattan Project during World War II.

A graduate of the University of British Columbia (UBC) in the 1930s where he studied physics, he followed George Volkoff, who was a year ahead of him, to the University of California, Berkeley, where he was accepted as a graduate student by Robert Oppenheimer, the leading theoretical physicist in the United States at that time.

When Oppenheimer formed the Manhattan Project's Los 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 into supercriticality, a great simplification of earlier concepts of implosion requiring hollow shells.

In 1960 Christy turned his attention to astrophysics, creating some of the first practical computation models of stellar oscillations.

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.

[4] Christy was educated at Magee High School, and graduated in 1932 with the highest examination score in the province of British Columbia.

He was awarded the Governor General's Academic Medal, and, importantly in view of his family's limited ability to pay, free tuition to attend the University of British Columbia (UBC).

At the award dinner he met the second-place winner, Dagmar Elizabeth von Lieven, whom he dated while at UBC.

[8] George Volkoff, a friend of Christy who was a year ahead of him at UBC, was accepted as a graduate student at the University of California, Berkeley, by Robert Oppenheimer, who led the most active school of theoretical physics in the United States at that time.

[9] For his thesis, Oppenheimer had him look at mesotrons, subatomic particles called muons today, that had recently been found in cosmic rays.

He published two papers on mesotrons with Kusaka in the Physical Review,[10][11] which formed the basis of his 1941 Doctor 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.

[14] This brought him to the attention of Eugene Wigner, who hired him for the same money that IIT was paying him but as a full-time research assistant, commencing in January 1942.

[20][21] In early 1943, Christy joined Oppenheimer's Los Alamos Laboratory in New Mexico,[22] where he became an American 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.

[25] The Water Boiler was an aqueous homogeneous reactor intended as a laboratory instrument to test critical mass calculations and the effect of various tamper materials.

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 by Emilio Segrè's group in April and May 1944 of high levels of plutonium-240 in reactor-produced plutonium meant that an implosion-type nuclear weapon was required, but studies indicated that this would be extremely difficult to achieve.

[29] Christy worked with Klaus Fuchs, Paul Stein and Hans Bethe to develop a suitable initiator design, which became known as an "urchin".

[30] The Gadget used in the Trinity nuclear test and the Fat Man used in the atomic bombing of Nagasaki used Christy pits.

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

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.

The drawback to working at Caltech was that neither Lauritsen nor Fowler was a theoretical physicist, so a heavy workload fell on Christy.

[38]In 1956, Christy was one of a number of scientists from Caltech who publicly called for a ban on atmospheric nuclear testing.

The 1963 Partial Nuclear Test Ban Treaty that Christy advocated put an end to one of his most unusual projects.