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The dreaded derivation, Re: your starship-design email of 8/20

Steve VanDevender writes:
 > We have an observer P whose spacetime location is [ t x y z ].  We want
 > to find possible paths that correspond to light rays passing from S(t')
 > to P, or from P to S(t').  We can characterize this relationship with
 > the equation:
 > (P - S(t'))^2 = 0

Sigh.  While editing that last letter, I omitted the explanation that I
use shorthand U^2 = U dot U for vector quantities; I use that convention
frequently in my notebooks, but it would probably help you to know that
explicitly when reading the above-quoted section.