In many textbooks, the interval
$$ I = -(c\Delta t)^2 + (\Delta x)^2 + (\Delta y)^2 + (\Delta z)^2 $$
is taken for granted as the same for two events in any reference frame.
Is it possible to derive this just from the two postulates,
- That the laws of physics are the same in all reference frames
- That the speed of light c is constant for all reference frames?