The emergence of quantum physics at the beginning of the twentieth century was extremely important in explaining some phenomena that had hitherto been inconsistent between classical theory and experimental results. As some questions persisted unanswered, they were solved by a new theory: the Theory of relativity, by Albert Einstein.
This theory consists of two parts: the Restricted Relativity Theory (or Special Relativity Theory), published in 1905, in which phenomena are treated in relation to necessarily inertial General Relativity Theory, published in 1915, in which phenomena are treated in relation to non-inertial references. We will focus our attention only on the study of the Theory of Restricted Relativity.
It is noteworthy that the theory of relativity does not invalidate Newtonian mechanics, just as quantum physics does not invalidate classical electromagnetic theory. Einstein's theory only correctly explains the behavior of phenomena when the order of magnitude of the speed of motion is comparable to the speed of light in a vacuum, something classical mechanics cannot explain.
The Theory of Restricted Relativity was built on two postulates:
- The laws of physics are the same in any frame of reference. There is no privileged reference.
- The speed of light in a vacuum has the same value (c = 300000 km / s) relative to any inertial frame.
Note: The second postulate differs from the Newtonian Mechanics formulation of velocity composition, since, according to the theory of relativity, no velocity composition can be higher than the velocity of light in a vacuum.