). and we are subject to a translatory motion of about the same magnitude: hence we should be able to detect a change in the velocity of light due to our change of motion through the ether. These considerations give rise to Michelson and Morley's experiment.
Michelson and Morley attempted to detect motion relative to the supposedly fixed ether by the interference of two rays of light, one travelling in the direction of motion of the earth's velocity, the other travelling across this direction of motion.
No change in the initial interference bands was, however, observed when the position of the instrument was changed, although such an effect was easily within the limits of accuracy of the experiment. Many modifications of the experiment likewise failed to demonstrate the presence of an "ether-wind."
To account for these negative results as contradicting deductions from Fizeau's experiment, Fitzgerald and, later, independently, Lorentz suggested the theory that bodies automatically contract when moving through the ether, and since our measuring scales contract in the same ratio, we are unable to detect this alteration in length; this effect would lead us always to get the same result for the velocity of light. This contraction-hypothesis agrees well with the electrical theory of matter and may be attributed to changes in the electromagnetic forces, acting between particles, which determine the equilibrium of a so-called rigid body.
Thus Michelson and Morley's experiment seems to prove that the principle of relativity of mechanics also holds for radiation effects, that is, it is impossible to determine absolute motion through the ether or space: this implies that there is no unique system of co-ordinates. It disagrees with Fizeau's result and seems to indicate the existence of a "moving ether," i.e. an ether which is carried along by moving bodies, as was upheld by Stokes and Hertz. Lord Rayleigh pointed out that if the contraction-hypothesis of Lorentz and Fitzgerald were true, isotropic bodies ought to become anisotropic on account of the motion of the earth, and that consequently, phenomena of double refraction should make their appearance. Experiments which he himself conducted with carbon bisulphide and others carried out by Bruce with water and glass produced a negative result.
II. The "Special" Theory of Relativity
Einstein, in the special theory of relativity, surmounts these difficulties by doing away with the ether (as a substance) and assumes that light-signals project themselves as such through space. Faraday had already long ago expressed the opinion that the field in which radiations take place must not be founded upon considerations of matter, but rather that matter should be regarded as singularities or places of a singular character in the field. We may retain the name "ether" for the field as long as we do not regard it as composed of matter of the kind we know. Einstein arrives at these conclusions by critically examining our notions of space and time or of distance and simultaneity.
We know what simultaneity (time-coincidence of two events) means for our consciousness, but in making use of the idea of simultaneity in physics, we must be able to prove by actual experiment or observation that two events are simultaneous according to some definition of simultaneity. A conception only has meaning for the physicist if the possibility of verifying that it agrees with actual experience is given. In other words, we must have a definition of simultaneity which gives us an immediate means of proving by experiment whether, e.g. two lightning-strokes at different places occur simultaneously for an observer situated somewhere between them or not. Whenever measurements are undertaken in physics two points are made to coincide, whether they be marks on a scale and on an object, or whether they be cross-wires in a telescope which have been made to coincide with a distant object to allow angular measurements to be made; coincidence is the only exact mode of observation, and lies at the bottom of all physical measurements. The same importance attaches to simultaneity, which is coincidence in time. It is to be noted that no definition will be made for simultaneity occurring at (practically) one point: for this case psychological simultaneity must be accepted as the basis: the necessity for a physical definition arises only when two events happening at great distances apart are to be compared as regards the moment of their happening. We cannot do more than reduce the simultaneity of two events happening a great distance apart to simultaneity referred to a single observer at one point: this would satisfy the requirements of physics.
Einstein, accepting Michelson and Morley's result, introduced the convention in 1905 that light is propagated with a constant velocity (=