<div class="text-justify">Einstein's theory of special relativity did not represent a traumatic rupture with classical physics, but it did end the mechanical vision of the world by introducing fields as necessary entities. Another break with the mechanistic vision has to do with the concepts of space and time. The Newtonian conception of the universe considers that space and time are absolute, which means that they are the same for all observers regardless of their relative movement. Einstein showed that the measurements of space and time in special relativity depend on the relative movement of the observers. In addition, it turned out that space and time are, in fact, intertwined.

<center>![Spacetime.jpg](https://steemitimages.com/DQmeewBVEQHDzCdaHygAnTVuWEjkKpLy8wfXdhSwgJyRbPk/Spacetime.jpg)</center>

An intuitive approach to this reality we saw when we considered the mental experiment in which we made measurements of length of a mobile platform. The measurements of the meter must be made at the ends of the platform at the same instant in time. Due to the postulate of the constancy of the speed of light, a person at rest on the platform and a person who sees the platform in motion will not agree on when the measurements will be simultaneous.

Events occur not only in space, but also in time. In 1908, the German mathematician Hermann Minkowski suggested that in the theory of relativity, time and space can be seen as unified to form the four dimensions of a four-dimensional world called spacetime. Four-dimensional spacetime is universal because an "interval" measured in this world would prove to be the same for all observers, regardless of their relative motion at a uniform velocity.

The spacetime interval is a kind of "distance" between events. But not the distance that separates events in space, nor the distance that separates them in time, but the distance that separates them using a measure that includes both space and time.

Somewhat more formally, in general, the spacetime interval between two events in coordinate systems that are in uniform relative motion will be the same. Therefore, the space interval and the time interval will vary in the different systems of computation, but not the interval of time-space. This is nothing more than affirming that in the theory of invariance the interval of spacetime is an invariant property.

In summary, while for Newton both length and time and simultaneity were invariant, they are not invariant for the theory of invariance. In this, however, these invariant properties become relative and what was relative for Newton, the speed of light (in a vacuum), becomes an invariant (a constant) and a new property is introduced in the interval of Spacetime that is also.

Minkowski's approach is a "geometric" approach to relativity: it starts from the existence of a four-dimensional spacetime and, therefore, four-dimensional coordinate systems that are also in uniform relative motion, and in which we can use the expressions we have seen for the relativity of time and length, which are usually known as Lorentz transformations, to pass the coordinates in one system to another.

This geometric approach is very common when studying relativity and becomes the main one when it is generalized to coordinate systems that are in relative movement of any kind, that is, when there are also accelerations. This is what we will explore next: the bases of the so-called general theory of relativity.</div>