On small longitudinal material waves accompanying light waves

All experiments on the pressure of light agree in showing that there is a flow of momentum along the beam. This How is manifested as a force on matter wherever there is a change of medium. When the light is absorbed, the momentum is absorbed by matter. When the beam is shifted parallel to itself there is a torque on the matter effecting the shift. The momentum would therefore appear to be carried by the matter and not merely by the ether. Though there is an obvious difficulty in accepting this view when the density of the matter is so small as it is in interplanetary space, it appears to be worth while to follow out the consequences of the supposition that the force equivalent to the rate of flow of momentum across a plane perpendicular to a beam of light acts upon the matter bounded by the plane. This rate of flow per square centimetre is equal to the energy density or energy per cubic centimetre in the beam. Of course, in experiments, only the average of the rate of flow during many seconds and the average energy per cubic centimetre in a length of beam of millions of miles is actually measured. But on the electromagnetic theory of light which suggested the experiments and which gives the right value for the pressure, this pressure is equal to the energy density at every point of a single wave. Let us suppose that we have a train of plane polarised electromagnetic waves of sine form, the magnetic intensity being given by H = H1 sin2π/λ(x–vt ), where H1 is the amplitude of H. The magnetic energy per cubic centimetre at any point is μH2 /8π, and as the electric energy is equal at each point to the magnetic energy, the total energy is μH2 /4π.