For Einstein, linking space and time means that neither distance nor time are absolute; the perception of time depends primarily on the speed of whoever is measuring it. The adventure of Langevin’s traveler gave us proof that time can be modified, which has since been verified by numerous experiments. Often these results are unexpected, putting our credulity to the test, as they relate to events that do not occur in everyday life and cannot be observed by the naked eye.
Generalizing the theory in 1915, general relativity is in fact a study of the laws of gravitation. Its perspectives are revolutionary. It affects all acceleration phenomena, especially in very intense force fields.
This theory was so unexpected because its effects could not be observed during Newton’s time, when the world was hardly subject to the effects of acceleration and could not imagine the subtle changes exerted on a body propelled at high speed, subject to extremely intense acceleration or gravitational fields.
Einstein’s theory, like any law of physics, is only an approximation. But it is sufficient to describe the current structure of the universe. That said, relativity is now a mature science, whose maturity allows it to set down its own laws and respond to its critics.
The complex geometry in four dimensions that binds three-dimensional space and time has influences on the matter contained in space, which results in the concept of curved space.
While for Newton light travelled in a straight line, Einstein showed that it was “sensitive” to the presence of matter and “changed trajectory” near a massive body. The reason is that the photon, despite having no charge and no rest mass, is sensitive to electromagnetic and gravitational fields. Einstein demonstrated more generally that the gravitational force that draws two bodies together is a distortion of space-time caused by the matter it contains. The trajectory of the bodies becomes a geodesic. By way of analogy with a sheet that dips when a ball is placed on it, all the objects in the universe have an effect on the average curvature of space-time.
In light of General Relativity, the universe takes the form of a curved space in four dimensions which is called “Riemannian”. If the inertia of bodies can distort space-time, ultimately this inertia is transformed into energy. Therefore, around any massive body we can imagine that space deforms and curves; parallel lines merge, no longer obeying the rules of classical Euclidean geometry. On a large scale, space is curved with a radius of curvature linked to the density of the matter it contains. There is a critical density determined by nucleosynthesis, after which point the universe closes in on itself completely. As we will see, it is important to determine the density of the universe to clarify how it will evolve.