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*To*: starship-design@lists.uoregon.edu*Subject*: Re: starship-design: The speed of now*From*: Steve VanDevender <stevev@efn.org>*Date*: Wed, 27 Aug 1997 10:47:46 -0700*In-Reply-To*: <199708271634.SAA26572@zmit1.ippt.gov.pl>*References*: <199708271634.SAA26572@zmit1.ippt.gov.pl>*Reply-To*: Steve VanDevender <stevev@efn.org>*Sender*: owner-starship-design

Zenon Kulpa writes: > > From: wharton@physics.ucla.edu (Ken Wharton) > > > [...] > > That value is D = [t ^ 2 - x ^ 2]. The distance between any two > > events, measured in terms of D (where t is the time-separation in > > seconds and x is the distance separation in light-seconds) will be > > identical in all frames. > > > As far as I remember it should be D = x^2 - t^2 > (or rather: D = x^2 + (it)^2, where i = Sqrt(-1)). > Or am I wrong? This is partly a matter of convention. You can choose either metric ((t^2 - x^2) or (x^2 - t^2)) as long as you're consistent. _Spacetime Physics_ by Taylor and Wheeler uses (t^2 - x^2), which I happen to like because it's more computationally convenient for some purposes. _Gravitation_ by Misner, Thorne and Wheeler uses (x^2 - t^2) (well, the 4-d version (x^2 + y^2 + z^2 - t^2)), and also includes a large conversion chart showing the sign choices used by various authors, which also indicates there apparently hasn't been much widespread agreement on sign conventions. The (x^2 + (i*t)^2) form has fallen into disfavor. There's a sidebar in _Gravitation_ where the authors explain why they don't like it. It's another attempt to make things computationally convenient but is somewhat conceptually misleading.

**References**:**Re: starship-design: The speed of now***From:*Zenon Kulpa <zkulpa@zmit1.ippt.gov.pl>

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