According to the theory of relativity, which of the following always remains constant ?

The theory of relativity, or simply relativity, generally encompasses two theories of Albert Einstein: special relativity and general relativity. Concepts introduced by the theories of relativity include: Measurements of various quantities are relative to the velocities of observers. In particular, space and time can dilate. Space time: space and time should be considered together and in relation to each other. The speed of light is nonetheless invariant, the same for all observers.