John Lighton Synge FRSC FRS, mathematician and physicist, whose seven-decade career includes significant periods in Ireland, Canada, and the United States, is born in Dublin on March 23, 1897. He is a prolific author and influential mentor and is credited with the introduction of a new geometrical approach to the theory of relativity.

Synge is born into a prominent Church of Ireland family. His uncle, John Millington Synge, is a famous playwright. He is more distantly related to the winner of the 1952 Nobel Prize in Chemistry, Richard Laurence Millington Synge. He is a great-great-great-grandson of the mathematician and bishop Hugh Hamilton.

Synge attends St. Andrew’s College, Dublin and in 1915 enters Trinity College Dublin (TCD). He is elected a Foundation Scholar his first year, which is unusual as it is normally won by more advanced students. While an undergraduate at TCD, he spots a non-trivial error in *Analytical Dynamics of Particles and Rigid Bodies*, a textbook by E. T. Whittaker, who had recently taught there, and notifies Whittaker of the error. In 1919 he is awarded a BA in Mathematics and Experimental Physics, and also a gold medal for outstanding merit. In 1922 he is awarded an MA, and in 1926 a Sc.D., the latter upon submission of his published papers up to then.

In 1918, Synge marries Elizabeth Eleanor Mabel Allen. She is also a student at TCD, first of mathematics, then of history, but family finances forced her to leave without graduating. Their daughters Margaret (Pegeen), Cathleen and Isabel are born in 1921, 1923 and 1930 respectively. The middle girl grows up to become the distinguished Canadian mathematician Cathleen Synge Morawetz.

Synge is appointed to the position of lecturer at Trinity College, and then accepts a position at the University of Toronto in 1920, where he is an assistant professor of mathematics until 1925. There he attends lectures by Ludwik Silberstein on the theory of relativity, stimulating him to contribute “A system of space-time co-ordinates,” a letter in *Nature* in 1921.

Synge returns to Trinity College Dublin, in 1925, where he is elected to a fellowship and is appointed the University Professor of Natural Philosophy (later to be known as “physics”). He is a member of the American Mathematical Society and the London Mathematical Society. He is treasurer of the Royal Irish Academy in 1929. He goes back to Toronto in 1930, where he is appointed Professor of Applied Mathematics and becomes Head of the Department of Applied Mathematics. In 1940, he supervises three Chinese students, Guo Yonghuai, Qian Weichang and Chia-Chiao Lin, who later become leading applied mathematicians in China and the United States.

Synge spends some of 1939 at Princeton University, and in 1941, he is a visiting professor at Brown University. In 1943 he is appointed as Chairman of the Mathematics Department of Ohio State University. Three years later he becomes Head of the Mathematics Department of the Carnegie Institute of Technology in Pittsburgh, where John Nash, Jr. is one of his students. He spends a short time as a ballistic mathematician in the United States Air Force between 1944 and 1945.

Synge returns to Ireland in 1948, accepting the position of Senior Professor in the School of Theoretical Physics at the Dublin Institute for Advanced Studies. This school had been set up in 1940, and had several outstanding members, including Erwin Schrödinger, a contributor to quantum mechanics, who is also a Senior Professor.

Synge makes outstanding contributions to different fields of work including classical mechanics, general mechanics and geometrical optics, gas dynamics, hydrodynamics, elasticity, electrical networks, mathematical methods, differential geometry, and Albert Einstein‘s theory of relativity. He studies an extensive range of mathematical physics problems, but his best-known work revolves around using geometrical methods in general relativity.

Synge is one of the first physicists to seriously study the interior of a black hole, and his early work is cited by both Martin David Kruskal and George Szekeres in their independent discoveries of the true structure of the Schwarzschild black hole. Synge’s later derivation of the Szekeres-Kruskal metric solution, which is motivated by a desire to avoid “using ‘bad’ coordinates to obtain ‘good’ coordinates,” has been generally under-appreciated in the literature, but is adopted by Chandrasekhar in his black hole monograph.

In pure mathematics, Synge is perhaps best known for Synge’s theorem, which concerns the topology of closed orientable Riemannian manifold of positive sectional curvature. When such a space is even-dimensional and orientable, the theorem says it must be simply connected. In odd dimensions, it instead says that such a space is necessarily orientable.

Synge also creates the game of Vish in which players compete to find circularity (vicious circles) in dictionary definitions.

Synge retires in 1972. He receives many honours for his works. He is elected as a fellow of the Royal Society of London in 1943. He is elected as a fellow of the Royal Society of Canada, and in 1943 is the first recipient of the society’s Henry Marshall Tory Medal, as one of the first mathematicians working in Canada to be internationally recognised for his research in mathematics. In 1954 he is elected an honorary fellow of Trinity College Dublin. He is president of the Royal Irish Academy from 1961 until 1964. The Royal Society of Canada establishes the John L. Synge Award in his honour in 1986.

Synge dies on March 30, 1995, in Dublin, exactly one week after his ninety-eighth birthday.