What are the equations of motion of classical physics?

Abstract

Action at a distance in Newtonian physics is replaced by finite propagation speeds in classical physics, the physics defined by the field theories of Maxwell and Einstein. As a result, the differential equations of motion in Newtonian physics are replaced in classical physics by functional differential equations, where the delay associated with the finite propagation speed (the speed of light) is taken into account. Newtonian equations of motion, with post-Newtonian corrections, are often used to approximate the functional differential equations of motion. Some mathematical issues related to the problem of extracting the ``correct'' approximate Newtonian equations of motion are discussed.