The Einstein group can be obtained by factorizing the squared spacetime invariant interval
2.
The invariant interval can be seen as a non-positive definite distance function on spacetime.
3.
Two invariant interval ", which can be either space-like, light-like, or time-like.
4.
Different observers may calculate different distances and different time intervals between two events, but the " invariant interval " between the events is independent of the observer ( and his velocity ).
5.
This establishes that the quantity c ^ 2 t ^ 2-x ^ 2 is an invariant : it takes the same value in any inertial coordinate system and is known as the invariant interval.
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