Answer: Invariance of the speed of light in vacuum
Special relativity was proposed on 1905 by Einstein, who developed his theory based on the following two postulates:
1. The laws of physics are the same in all inertial systems. There is no preferential system.
2. The speed of light in vacuum has the same value for all inertial systems.
Focusing on the first postulate, it can be affirmed that any measurement on a body is made with reference to the system in which it is being measured.
In addition, it deals with the dilation of time stating that time passes at different rates in regions of different gravitational potential. That is, the greater the local distortion of space-time due to gravity, the slower the time passes.
On the other hand, following what relativity establishes, bodies within a gravitational field follow a curved space path.