# Re: starship-design: The speed of now

```> 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?

> For light, D=0.  Thus my assertion that light "really" travels
> infinitely fast.
>
> For STL, D>0.  The time is always larger than the distance.
>
Rather (using my formula for D) - distance larger than time.
It would be intuitively natural:
you never have enough time to be in time... ;-)

> But for FTL, D<0. You also get D<0 for local time travel; set x=0 (you
> don't go anywhere) and set t<0 (you go back in time).
>
t^2 >= 0 always, also when t < 0.
In my version of the formula for D,
D < 0 always when x = 0 and t =/= 0.
Something is still wrong...

> Therefore the D
> associated with time travel is equivalent with the D associated with
> FTL.  It's very simple to take two FTL journeys, one out and one back,
> that gets you to return before you left.
>
> Just another way of thinking about it, I guess. Which, of course, was
> probably more confusing than illuminating...
>
This interpretation wouild be quite illuminating for me,
except for the problem with D < 0 for x = 0...

-- Zenon

```