Gravitational Shift of Spectral Lines

Abstract

THE assumption that the equations of motion in a gravitational field can be deduced from a condition of the form δ∫ds=o is in itself little more than a very natural way of expressing the principle of least action. The greatness of Einstein's theory really lies in the suggestion, made apparently on purely a priori grounds, that a certain set of six relations between the coefficients in the formula for ds², which are true when no heavy body is near, still hold near one. These are found to make the coefficients determinate, whereas previously they were quite arbitrary, and the observed motions of the planets, including the advance of the perihelion of Mercury, are at once deduced.