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*To*: starship-design@lists.uoregon.edu*Subject*: Re: starship-design: FTL idea*From*: TLG.van.der.Linden@tip.nl (Timothy van der Linden)*Date*: Wed, 20 Aug 1997 17:50:54 +0100*Reply-To*: TLG.van.der.Linden@tip.nl (Timothy van der Linden)*Sender*: owner-starship-design

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

**Follow-Ups**:**Re: starship-design: FTL idea***From:*Steve VanDevender <stevev@efn.org>

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