Einstein in a photo when he was starting to get famous
At the time, much was discussed and investigated about the nature of light and its propagation, as well as the possibility of the existence of a subtle medium (in which light would propagate), which was called “ether”. The ether would permeate the entire universe, opening the possibility for the existence of a privileged, absolute referential, according to which it would be possible to study and describe, in principle, all movements.
In the year 1881, Michelson found no experimental evidence to support the hypothesis of the existence of aether. Several repetitions of the experiment would be carried out between 1881 and 1930, all with negative results.
In the year 1898, Jules Henri Poincaré exposed his dissatisfactions and doubts in an article on the measurement of time, in which he wrote that the simultaneity of two events or the order of their succession and the equality of two intervals of time must be defined in such a way. so that statements of natural laws are as simple as possible. In 1904, Poincaré pointed out that clocks, in different systems of reference, must maintain what we can call local time, but no observer can know whether he himself is at rest or in absolute motion.
The new Einsteinian mechanics, later known as the Special Theory of Relativity, was based on just two postulates, which, in current language, can be stated as:
1 – The laws of nature, or of Physics, are the same for all inertial references.
2 – The speed of propagation of light in a vacuum is constant, independent of the movement of the source and has the same value for any inertial reference frame.