Light travels from spacetime point to spacetime point in Minkowski space. What is the value of the invariant interval between and
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The value of the interval between any two points light travels between is always zero. This is because the speed of light is c in all inertial frames, so:
c 2 = ( d t d x ) 2 ,
in all frames. This rearranges to:
0 = − c 2 d t 2 + d x 2 ,
which is to say that the invariant interval is always zero for any "lightlike interval."