| A | B |
|---|
| Inertial reference frame | Reference frame at rest or moving with constant velocity |
| Speed of light | Invariant |
| Space and time | Independent of a frame of reference |
| Relates measurements made in different inertial frames of reference | Lorentz transformations |
| Rest energy | mc^2 |
| Total energy of an object | K + mc^2 |
| Photon | Massless particle |
| Proper time | Time interval between two events occurring at the same point in a particular reference frame |
| Proper length | Distance between two points at rest in a particular reference frame |
| Time dilation | Time interval between events in two different reference frames with a relative velocity between them |
| Length contraction | Distance between points in two different reference frames with a relative velocity between them |
| General relativity | Includes accelerating reference frames |
