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Since space and time are now reduced to the subjective role of the elements of the language a particular observer uses for his or her description of natural phenomena, each observer will describe the phenomena in a different way. To abstract some universal natural laws from their descriptions, they have to formulate these laws in such a way that they have the same form in all coordinate systems, i.e. for all observers in arbitrary positions and relative motion. This requirement is known as the principle of relativity and was, in fact, the starting point of relativity theory. It is interesting that the germ of the theory of relativity was contained in a paradox which occurred to Einstein when he was only sixteen. He tried to imagine how a beam of light would look to an observer who travelled along with it at the speed of light, and he concluded that such an observer would see the beam of light as an electromagnetic field oscillating back and forth without moving on, i.e. without forming a wave. Such a phenomenon, however, is unknown in physics. It seemed thus to the young Einstein that something which was observed by one observer to be a well-known electromagnetic phenomenon, namely a light wave, would appear as a phenomenon contradicting the laws of physics to another observer, and this he could not accept. In later years, Einstein realized that the principle of relativity can be satisfied in the description of electromagnetic phenomena only if all spatial and temporal specifications are relative. The laws of mechanics, which govern the phenomena associated with moving bodies, and the laws of electrodynamics, the theory of electricity and magnetism, can then be formulated in a common ‘relativistic’ framework which incorporates time with the three space coordinates as a fourth coordinate to be specified relative to the observer.

Excerpted from Page 167 of ‘The Tao of Physics’ by Fritjof Capra