Comments on Modern Physics

The state of physics, at the threshold of the modern era, in 1880, is briefly reviewed. The classical description of a physical event, as observed in Galilean frames, is outlined. The following experiments are discussed: aether drift (Michelson and Morley), propagation of radio waves (Hertz;Lodge), magnetic fields generated by motion of charge with respect to the aether (Roentgen; Trouton and Noble), and dependence of electronic charge to mass ratio on velocity (Kaufmann). The postulates of relativity by Poincaré and by Einstein are stated. The effects of the Lorentz transformation on time and space, the nature of the four-vectors, potential, momentum and frequency, and the tensor electromagnetic field are reviewed. The Fourier spectrum of a physical event, which is localized in space and time, is described in order to illustrate the uncertainty principle the inherent limitation on the accuracy of determining a region in space-time that relates, through Planck's constant of proportionality, to limitations on the determination of energy within a given time and momentum within a given space.