Since two observers with coinciding origins at t=t'=0 both measures a light wave to be moving at c they will both claim that r^2= (ct)^2 and thus that r^2-(ct)^2 = 0. Thus(adsbygoogle = window.adsbygoogle || []).push({});

r'^2 - (ct)^2 = r^2 - (ct)^2 = 0

which is really just stating that 0=0 for events following the spherical wave off light. I just wondered what the argument now is that

r'^2 - (ct)^2 = r^2 - (ct)^2

for any spacetime point (event) and not just in the trivial case following the spherical wave of light?

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# Why does observers agree on the spacetime interval?

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