We have already discussed the invariability of the speed of light. Some other concepts in relativity are invariant, or unchanged by relative motion or position, including the charge of an electron, proper length and time, and rest mass.
Spacetime intervals, defined as {\Delta}s^2=(c{\Delta}t)^2-{\Delta}x^2 for 1D motion in a reference frame S, is also an invariant quantity. This means that {\Delta}s^2={\Delta}(s^{\prime})^2, which follows that (ct^{\prime})^2-(x^{\prime})^2=(ct)^2-(x)^2.
The following chart describes the values of {\Delta}s^2:
| Value of {\Delta}s^2 | Description |
| {\Delta}s^2\gt0 | Physical interaction between 2 reference frames possible. Time-like interval, but simultaneity impossible. |
| {\Delta}s^2=0 | Only signals at v=c like radio waves can interact between reference frames. |
| {\Delta}s^2\lt0 | Space between 2 reference frames too large for any physical interaction. |