  The work of S. Kichenassamy (1926-2015) in Theoretical and Mathematical
Physics covers the spectrum of Relativistic Physics: from the clarification of
the postulational basis of the two theories of Relativity, to applications to
the measurement of proper time, image formation, collision theory, kinetic
theory and radiative transfer, or pulsar electrodynamics, to name a few. After
brief biographic remarks,we show, by following an argument first formulated in
1963, and developed in a series of papers, that the physical interpretation of
General Relativity requires the replacement of the strong equivalence principle
by the C-equivalence principle. By giving a mathematical status to the
observer, his insight yields new results regarding measurements actual
observers can make, both in Special and in General Relativity. Applications are
outlined. His contributions to Indology will be discussed elsewhere.
