334 
PHYSICS: J. L ARMOR 
ON THE ESSENCE OF PHYSICAL RELATIVITY 
By Sir Joseph Larmor 
Cambridge, England 
Communicated, September 19, 1918 
In an important investigation recently reported, 1 Mr. Leigh Page has deter- 
mined an expression for the translatory force required to sustain an assigned 
varying velocity in an electrostatic system of a certain type, that namely which 
it has been usual to investigate as a model of an electron in analytical discus- 
sions. The expression contains terms involving the acceleration of the system 
and its time-gradients, but no term involving its velocity except in combina- 
tion with these other quantities. The application of his formula to which Mr. 
Page restricts himself is to challenge a result that an isolated radiating system 
is subject to retarding force from the reaction of its own radiation of amount 
equal to its velocity multiplied by rate of radiation divided by c 2 . The 
necessity for such refutation is based on the idea that on principles of relativity 
the velocity of an isolated body could have no meaning. But the formula 
obtained seems to leave this question as it was : for equally the acceleration of 
the isolated body could have no meaning; and moreover, though the thrust of 
the radiation is, in this case compensated, the velocity appears to be actually 
involved in the formula in the same manner as the impugned result would re- 
quire for this particular problem of a convected electrostatic system, for which 
the radiation is extremely transient and very slight in the absence of extraneous 
force. 
It has always seemed to me that this subject, which may be described as that 
of interaction of the aether with uniform motion, though of slight account 
phenomenally, is theoretically of high significance, in that it is destined perhaps 
to throw light on the nature of the forward momentum that is convected by 
radiation, and thence on the intimate dynamical nature of radiation itself and 
the physical function of the aether. I therefore propose briefly to indicate 
what I hold, provisionally of course, to be their present trend of knowledge. 
An adequate survey of this widely ramifying domain, in its various aspects^ 
is difficult to compress, even when concentration and avoidance of detail is 
the main aim, and is therefore reserved for a communication to the Physical 
Review. 
In the first place the plan now customary of regarding an electron as merely 
an aggregation of differential elements of electricity seems to be far too narrow 
a foundation for its internal properties, except by way of partial and pro- 
visional illustration. If electricity is to be explained as constituted of electrons, 
it can hardly be a step towards finality to explain electrons as constituted of 
electricity. On such a view the kinetic reaction of the inertia of an electron 
presents itself in theory under the artificial guise of a self-retarding force exerted 
