Re: starship-design: FTL idea

[TLG.van.der.Linden@tip.nl (Timothy van der Linden)]

Hello Steve,

> > I still don't understand why FTL creates causality problems. FTL doesn't
> > mean going back in time, it just means that you can get a places before
> > others can. For that matter you can't kill your mother before you are born
> > even if you can travel FTL.
>
>Since you can do it, Timothy, do a Lorentz transformation on a FTL
>worldline.

I know what a world line, but am not sure what you mean with "Lorentz
transforming it".
But I think I know what you mean, so following that thought...

>Note that for certain relative velocities the FTL worldline
>swings from traveling forward in time to traveling backward in time, or
>even for a particular sublight relative velocity appears to represent
>infinite speed.

I could indeed imagine someone observing this.

>If you have a collection of events connected by worldlines representing
>velocities less than or equal to c, then all observers at any relative
>velocity to those events will agree on the time ordering of the events,
>if not the time intervals between them.

True. (Assuming they remember that what they see is not what actually happens.)

>If there are events connected
>by FTL worldlines, they can no longer agree on the time ordering, and
>hence they will not agree on the causal order of events.

Why not? Please give me an example.

> > >P.S: If two objects APPROACH, each one travelling .99C, what is their
> > >combined velocity of approach? Is this done the same way as the regular
> > >velocity addition?
> > 
> > Ken gave you the relative velocity, which is what a person on one of the
> > objects would measure. For an observer to wich one objects is coming from
> > the right with 0.99c and one from the left with 0.99c, it is just a matter
> > of normal addition: The objects close eachother with 1.98c (If they are 1.98
> > lightsecond apart, it will take 1 second untill they collide.)
>
>Relative velocity is a frame-dependent notion.  Ken's answer is correct
>in the frame of either object given the measured velocities in a third
>frame.  Your answer is correct in the frame in which both objects are
>seen to travel at 0.99c in opposite directions.

Yep, I wasn't implying that Ken's answer was wrong, I just wanted to give
Kyle another look at the problem.


Timothy