The theory of relativity represents a most strikingly original conception of time and space, which was suggested by Einstein in order to correlate with all our past experience certain observations made in recent years. It is therefore extremely comprehensive in its scope; it demands from us a radical revision in our notions of time and space; it throws new light on the nature of mass and energy, and finally, it furnishes a totally new conception of the old problem of gravitation. The starting point of the theory is the familiar observation that motion is always relative: that is, to define the motion of any object we must always use some point of reference. Thus we speak of the velocity of a train as 40 miles per hour with respect to the earth's surface, but would find it impossible to determine its absolute speed, or motion in space, since we know of no star whose position can be spoken of as absolutely fixed. These and similar considerations have led to the conclusion, pointed out by Newton and others, that it is impossible by any mechanical experiments on the earth to measure its velocity in space. However, the results of observations on the phenomena of light and electricity led to the revival of the same problem under another form. As well known, there was evolved from these discoveries, the theory that light and electrical energy are of the same nature, and are in each case manifestations of wave-disturbances propagated through a hypothetical medium, the ether, with a velocity of 186,000 miles per second. The problem therefore arose as to whether the earth and all stellar bodies move through this ether. In that case it ought to be possible to measure the velocity of the earth with respect to this medium, and under these conditions we could speak, in a sense, of absolute motion. A large number of experiments has been tried with this end in view. The most famous of these, and the one which stimulated the subsequent development of the theory of relativity, was that carried out by Michelson and Morley in 1887. To understand the significance of this experiment we shall refer briefly to an analogous observation which is quite familiar. Does it take longer to swim to a point 1 mile up a stream and back or to a point 1 mile across stream and back? The experienced swimmer will answer that the up-and-down journey takes longer. If we assume that the swimmer has a speed of 5 miles an hour in still water and that the current is 3 miles an hour, we find that, while it requires five-eighths hour to make the up-and-down journey, it takes only one-half hour for the trip across stream and back. The ratio between the times required for the two journeys is thus five-fourths, and if this is written in the form $$\frac{1}{\sqrt{1 - (\frac{3}{5})^2}}$$ it shows how the result depends upon the square of the ratio of the speeds of the swimmer and the current.