Einstein abandons the ether, which he declares to be the totally unnecessary conception. Einstein makes two postulates which are sufficient to explain all phenomena now known. The first has been stated, the other is that the velocity of light is the same when measured in any system. By measures of this velocity, we can, therefore, not determine whether the system is moving or at rest. Clothed in a more mathematical form, such as has been given by Minkowski, we may state the principle as follows: If instead of the distance measured in the direction of the motion of the system, and of the time measured by a clock standing still, we substitute a quantity denoting a new length and a new time, then all the equations of electro-dynamics and presumably all those of physics admit of a so-called linear transformation of the variables and to the variables and . Under this transformation, the equations remain, therefore, absolutely unchanged. It is accordingly impossible by any observations to determine whether the time measured by the clock is or or whether the distance measured by the scale is or . As has already been said, this proposition is of the most startling nature and results in connecting the notions of time and space in a most unexpected manner. In fact we may briefly sum up by saying that we can not tell where a point is until we know when, and we can not tell the time when until we know the place where! If we accept this principle it may be necessary to totally abandon the hypothesis of the ether. Certain writers, such as Ritz in France, have established a system of electrodynamics in which the conceptions of the ether and of the magnetic and electric fields have totally disappeared. Ritz, for instance, bases his whole theory upon the so-called retarded potentials of Lorentz, by means of which the action of any electric charge, fixed or in motion, is calculated at any other time and place by means of definite integrals. This conception has been vigorously maintained: in England I may mention the name of Mr. Norman Campbell, who in a recent article in the Philosophical Magazine, as in his excellent modern treatise on electromagnetic phenomena, has vigorously assailed and even ridiculed the school of those whom he calls the "etherealists," as making use of a totally useless and hindering conception.
In 1900 Professor Poincaré had already asked the question, "does the ether exist?" This I may characterize as now the question of the hour. To sum up what I believe to be the state of the case, certain phenomena concerning radiation and the distribution of energy in the spectrum have led to the necessity of certain assumptions which seem difficultly explained on the ether hypothesis. Sir Joseph Thomson also, in order to explain certain phenomena connected with the emission of electrons from metals under the action of ultra-violet light and other phenomena with which he is particularly competent to deal, has propounded the hypothesis that a wave of light is not uniform but is somewhat of a fibrous nature. I find it difficult to see how such a hypothesis
