# Re: starship-design: FTL idea - Isaac

```Isaac,

>Like I said, I don't know exactly what your equation is calculating.
>You have a bunch of numbers but no generic equation.

Oh boy, I had copied the formula sometime in the past but it must have been
from an example where the two velocities where already opposite, hence the
necessary minus signs where left out. Sorry, I wish I had noticed that long
before.
(You using 1.42 instead of 1.45 in the first letter didn't help either)

OK, now we at least agree about the number. What I don't understand from
your example is why the FTL-beam only travels back in time when it goes back to
ship A. I would expect that it goes back in time regardless its direction
and thus also arrives at ship B before it was sent!
Can you explain why the direction of the FTL-beam is so important?

>One thing, though, is that the message is being sent backwards in
>time, so the sign of its velocity is the opposite of what you'd
>expect.  A message being sent backwards in time in the "south"
>direction has a velocity in the "north" direction.

That means that it appears as if the FTL beam originates twice from ship A.
Once at 9:44 and once at 10:00. The beam from 9:44 just travels a bit slower
and thus both messages arrive at the same time at ship B.
The people on ship A experience a strange phenomenon at 9:44, their radio
starts sending a message over which they have no control. They listen in on
the conversation and thus know what they'll say at 10:00. So know they know
about their future. Big deal, we know about our past, but can't change that
either.
So why should we think that we can actually change the future?
(I'm sorry this turns towards philosophy, I wasn't intending this when I
started this discussion. It may be a fundamental answer though.)

>Like I said, the easiest way to see how a message is being sent back
>in time is with space-time diagrams.

Drawing spacetime diagrams can be a pain even without doing it in ASCII
(especially for accelerated movement). I've been trying to draw some on a
paper, but how does one draw simultaneity lines for an object that goes FTL?
You seem to know how, can you describe in words or formulas?

Timothy

```