PHYSICS 115 



i: Vi (v/c) ? . This is the Fitzgerald-Lorentz shrinkage, now no longer a physical 

 reality, but an appearance due to physical limits of observation. The velocity of light 

 plays the part of an infinite velocity, and any speed surpassing that limit becomes 

 meaningless. 



A notable consequence, already pointed out by Einstein, 1 is that the inertia of a body 

 depends upon the energy contained in it. This implies a fundamental identity of mass 

 and energy. Thus, radiation must have a " mass." If a body in space were to emit 

 radiation in a single direction, the recoil pressure of that radiation (the reality of which 

 has been demonstrated by Poynting and Barlow), 2 would exert a force equal to E/c, 

 where E is the energy emitted per second. If we call M the " mass " of the radiation 

 E, its momentum is Me. This momentum generated per second equals the force E/c. 

 Hence M = E/c 2 . Thus c 2 = 9Xio 20 represents in ergs a mass of i gram, and the solar 

 system is filled with this new type of " radiant matter." 



Once we admit the materiality of radiant energy, we are obliged to consider the ques- 

 tion of it berrg subject to gravitational force. Einstein, 3 indeed, goes so far as to 

 answer the question in the affirmative and boldly predicts the observation of the 

 gravitational deflection : of a beam of light from a star by the sun's gravitational field, 

 a deflection which, though only amounting to 0.83", ought to be unmistakable. Here, 

 however, the relativity theory is on dangerous ground, for any such gravitational 

 effect on beams of light contradicts its own second fundamental assumption, which 

 postulates the absolute constancy of the velocity of light. This absolute constancy 

 is all that remains of the properties of the luminiferous aether, which adherents of the 

 relativity theory sometimes profess to have completely eliminated. As it stands, it 

 really represents a modus vivendi by means of which the substance of the undulatory 

 theory and the aether principle may be saved in spite of the overwhelming negative 

 evidence which has accumulated against the latter. Should this attempt fail, there is 

 always the alternative of assuming that the velocity of light is added to that of the 

 source, as it is in the electromagnetic theory devised by Ritz. 4 This, of course, means 

 the total abandonment of the aether as a luminiferous medium, and the revival of the 

 emission theory of light. 5 



Quanta of Energy. The " materialising tendency " of present-day physics, to which 

 Sir O. Lodge drew attention in his Becquerel Memorial Lecture (1912), is nowhere more 

 evident than in the new theory of radiation propounded by Max Planck, 6 according to 

 which radiant energy is emitted in discrete quantities proportional to the vibration 

 frequency of the emitting atomic or molecular element. The formulation of some such 

 theory was necessitated by a breakdown of the Maxwell-Hertz electromagnetic theory 

 of radiation when molecular oscillators are considered. Imagine a closed vessel imper- 

 vious to heat, and filled with gas. Let a hot and a cold body be introduced into the 

 vessel. Heat will pass from the hot body to the gas and to the cold body until the tem- 

 perature is equalised, and the entropy has become a maximum. But that final state 

 will also be reached if the gas is removed and the inner walls are perfect reflectors. 

 Then the transfer of heat is made exclusively by radiation, which behaves as if it had 

 a temperature, like the gas. Equilibrium is again established, just as if the entropy 

 had become a maximum, whereas we can hardly speak of the entropy of a vacuum, 

 since it is the logarithm of a probability of collocation in space of discrete elements. 

 Thus we are led to attribute entropy to radiation itself. 



The only approach to a mathematical theory of the process is through the concep- 

 tion of molecular oscillators and resonators. If a number of these are contained in 

 the vessel, their energies will be distributed as among the molecules of a gas, and, since 

 each resonator has two degrees of freedom (corresponding to electric and magnetic 



1 Ibid., 18, p. 639 (1905). 



2 Proc. Roy. Soc., A 83, p. 534 (1910). 3 Annalen der Physik, 35, 5, p. 898 (1911). 

 4 Annales de Chimie et de Physique, 13, p. 145 (1908). 



* See Laue, " Das Relativitatsprineip " (Vieweg, Braunschweig, 1911). 

 6 Deutsche Physikalische Gesellschaft, Verhandlungen, 13, p. 136 (1911). See also his 

 "Theorie der Warmestrahlung" (Barth, Leipzig, 1906). 



