THE INERTIA OF LIGHT. VERIFICATION OF NEWTON’S SECOND LAW BY A CONFINED FLOW OF RADIATION IN A REFLECTIVE CAVITY

Abstract

In 1904, the Austrian physicist Fritz Hasenöhrl examined by means of mental experiments the black body radiation in
a reflecting cavity. By calculating the work required to keep the cavity moving at constant velocity in opposition to the
radiation pressure, he calculated for the radiation energy a value equivalent to E =3/8mc², relation corrected in 1905 to
E =3/4mc². This relation establishes an equivalence between mass m and radiation energy E and was finally corrected to
the present known form E = mc² by Einstein. The conclusion from these deductions is that light has mass and inertia.
Based on a thought experiment inspired by Hasenöhrl’s, in which we accelerate a reflecting cavity containing an internal
radiation flux, we conclude that, under certain conditions of motion, light verifies Newton’s 2nd Law of Inertia.