Einstein’s Theory of Relativity was based on Maxwell’s theories of electromagnetic waves.
Let us now consider another example: a meteorite is approaching the Earth with a speed of 1000 k/s. Due to friction with the air, the meteorite glows and emits light, becoming visible to the naked eye. We know that light travels with a speed of 300,000 km/s, so the light that the meteorite emits has this speed.
Taking into account the theories of Classical Physics, if a person observes the meteorite fall from Earth, the light emitted by it should reach that person’s eyes with the speed of 301,000 km/s, that is, we would add the speed of light plus the meteorite’s speed. However, we say that this is not the case. In practice, light reaches the eye of the observer with a speed of 300,000 km/s.
Einstein proposed that nothing can move faster than light in a vacuum, that is, the speed of 300,000 km/s would be the maximum speed that a body or a wave could reach. According to Einstein, the speed of light, in a given medium, is constant for whatever frame of reference is adopted.
The conclusion that Einstein proposed was based on the theories of electromagnetic waves enunciated by Maxwell. Maxwell’s theory proposes that electromagnetic waves are composed of two fields, one electric and one magnetic. The changing electric field gives rise to a changing magnetic field. In turn, the changing magnetic field generates the changing electric field. We can see that the combination of these two fields creates an electromagnetic wave that propagates in a vacuum carrying energy.
If Classical Mechanics were valid and an observer traveled alongside a ray of light, that is, in the same direction, in the same direction and with the same speed, he would have the impression that the light was stopped. Based on these observations, Einstein concluded that the speed of light is the same for whichever frame of reference is adopted.