What is light? Is it some kind of tiny particle emanating from the light source and striking on the retina of our eye, or does some kind of wave remain a contentious issue. Let us begin with assuming light to be a wave. If light is a wave, what type of wave is this? Water wave requires water as the medium; waves of air striking our ear drum appears as sound to us. Of course air cannot be the medium for the propagation of light waves, since light comes to us from stars, the sun and the moon where there is no air, also light passes through transparent medium air, glass, water, etc. Hence the medium for the propagation of light must be present everywhere in the universe within and without the body. Physicists proposed a hypothetical medium to explain the propagation of light and called it Aether (Earlier it was called Aether, now it is Ether. I preferred to write it as Aether to differentiate it from the chemical compound Ether).
Two Americans Michelson and Morley (1887) ventured to measure this hypothetical medium. The idea for the measurement is simple; they assumed that if Aether is omnipresent, this means that our earth (including all heavenly bodies) is rotating relative to Aether.
We know that a boat travelling downstream will take less time than the boat travelling the same distance upstream, but the to and fro motion of the boat across the stream will take the same time. If we assume light pulse like a boat in the Aether medium, obviously light pulse would take lesser time when the pulse is travelling along with the Aether wind and longer time when moving opposite to the wind; to and fro motion of the light pulse across the Aether wind will take the same time. But to everyone’s surprise no such time difference was measured. Michelson and Morley experimentally disproved the idea of Aether. The absence of Aether led us to think that light propagates without any medium.
Einstein now put forward the two postulates known as the ‘Theory of Relativity’:
1) Speed of light remains constant for whoever measures it. This means that the speed of light will remain same for an observer at rest with respect to the light source, moving towards the light source or away from it.
2) The laws of physics must be the same no matter from which frame of reference frame, the observation is taken place, whether the reference frame is at rest or moving relative to the light source.
These two postulates spliced hitherto two quiet different physical entities Space and Time into one Space-Time Continuum. Since we are three dimensional objects; having length, breadth and height; to us space appears three dimensional. Addition of time makes the space-time continuum four dimensional. This means that a difference of two points (length) in one frame of reference includes the difference in position as well as difference in time. The same two points measured relative another frame – moving with constant velocity relative to the first one – will have different values of positions and times. This is the corollary of the Relative Theory and is conspicuous only when the relative speed between the two reference frames are very high – approaching the speed of light (three lakh kilometre per second, this length can engulf the earth seven and a half times in one second).