# Re: starship-design: FTL idea - Isaac

```Timothy van der Linden wrote:
>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)

I have a feeling I may have made an arithmetic error or a typo when
I made up that concrete example.

>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!

Time is relative.  As has already been stated in IMO confusing
examples (not confusing to me, but confusing to someone unfamiliar
with special relativity), different observers won't agree on what
order things occured in with events outside each other's light
cones.  Despite the claims of someone on this list, this does _not_
_always_ imply a causality loop violation can be accomplished.
It only does so if relativity is valid and space is convex/flat.

The question of whether or not the message ship B receives occured
before or after it was sent is not well defined.  In some frames of
reference, it looks like ship B received the message before it was
sent from ship A (this includes ship B's frame of reference).  In
other frames of reference, it looks like ship B receives the message
after it was sent (this includes ship A's frame of reference).

However, _everyone_ in every frame of reference agrees that ship
A receives the message before it sends its message.

>Can you explain why the direction of the FTL-beam is so important?

Because if ship B directed its beam away from ship A, 1) ship A
would never receive the beam, and 2) the beam would be going
forward in time in ship A's frame of reference (at a speed much
less than 1.8c).

>>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.

Yes, but the first beam "originates" from the Foo _receiver_.

>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.

Except they _can_ change their future.  Ship A can link the Foo receiver
and Foo transmitter to a computer, which is programmed to transmit
A's computer is programmed to say the opposite of what the Foo

>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.)

It's not a matter of free will.  It's a matter of us being able to
actually make machines which behave reliably, like a computer program
which outputs the opposite of its input.

>>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?

I'll try to find an easy to find book which you can reference.  Maybe
there's even something on a web page.
--
_____     Isaac Kuo kuo@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
__|_)o(_|__
/___________\ "Mari-san...  Yokatta...
\=\)-----(/=/  ...Yokatta go-buji de..." - Karigari Hiroshi

```