= Spacetime interval
{{wiki=Spacetime#Spacetime_interval}}
In the <Galilean transformation>, there are two separate invariants that two <inertial frame of reference> always agree on between two separate events:
* <time>
* <length>, given by the <Pythagorean theorem>
However, in <special relativity>, neither of those are invariant separately, since space and time are mixed up together.
Instead, there is a new unified invariant: the spacetime-interval, given by:
$$
c \Delta t^2 - (\Delta x ^2 + \Delta y^2 + \Delta z^2)
$$
Note that this distance can be zero for two events separated.