Archive for July, 2001


Sender: Daniel Boese <>
From: Foxtaur <>
Subject: Re: FTL, light cones, and conic sectioning?
References: <Cw_47.21147$> <9j2gug$ih1$>
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Date: Thu, 19 Jul 2001 18:23:06 GMT
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Organization: Bell Sympatico

George William Herbert <> wrote:
> Foxtaur  <> wrote:

>> Thinking about the thread asking about non-causality-violating FTL 
>>travel, I started pondering the past and future light-cones that describe 
>>what are and aren't "time travel". Visualizing the two cones touching at 
>>their tips, they suddenly reminded me of an old high-school model 
>>demonstrating the various types of conic sections - ellipses, parabolae, 
>>and hyperbolae.
>> Perhaps when an FTL drive is activated, it forms some sort of "barrier" 
>>along its path (and likely some distance to either side), which prevents 
>>any potentially causality-violating FTL trips...?

> We discussed this in the thread last week when I asked about
> non-causality-violating FTL.  You can do that, but if you
> do then someone can fly through your area in a highly accellerated
> reference frame and block "normal" FTL travel.  That may or may
> not be a problem; the universe could work that way, but it's hard
> to write a lot of fiction around.

 Hm... I'm not quite sure whether or not your near-C traveller would be 
able to affect "normal" travellers by turning on its FTL drive. With 
enough travellers jumping around and building their barriers, the combined 
effect would seem to be a large "wall" that defines a preferred reference 
frame for FTL travel.

 Say somebody pops from Sol to the ever-exampled Alpha Centauri. If they 
arrive after the light-speed photons of their departure, then they're just 
chopping their subjective time; if they arrive so soon that the photons 
from their arrival would return before they left, then they would have 
created a potential paradox. Let's say our rubber-physics FTL drive does 
neither option, and they arrive somewhen in the ~8.6-year window between 
both extremes. So far, all well and good... but let's now introduce Mr. 
Paradox, who's trying to destroy the universe by travelling through time.

 When our heroes arrive at A.C., Mr. Paradox immediately tries to use his 
FTL drive to jump back to Sol before they left... but the heroes' "drive 
wake" (for lack of any better technobabble) bounces his own ship's course 
later-time so that he arrives a few instants after they left. Disgruntled, 
he pops back to A.C., arriving just after his own departure. "Aha!", 
thinks the would-be Evil Overlord, "all I have to do is jump off to the 
side, away from their wake, then sneak back in underneath it..." And so he 
jumps to Wolf 359, and then aims for Earth... but this time, his /own/ 
wake, from the A.C.-to-Wolf-359 trip, bounces his course at the start of 
his "jump", as it was "tilted" at the same angles as the Earth-A.C. jumps 
(in 4-space, that is), as no drive-wake can intersect with any other. Back 
at Wolf, he jumps to a third nearby star, and waits until the edge of his 
wake is no longer between the time-space instant our heroes left, and his 
current location... but that instant turns out to be the same time the 
photons from that event arrive, putting him in its future light-cone, and 
once again preventing his travel.

 There are two or three other interesting points that I can think of. 

 First, as I said, this drive seems as if it would tend to create a 
preferred reference frame for FTL travellers, so that spending 6 hours on 
one planet delays your arrival-time on the next by an equal amount.

 Second, if you set your drive to jump from Earth to A.C. at just under 
light-speed, then jump to arrive back home 8.6 years later (just outside 
the light-cones), then you've effectively cut off Earth from A.C. for 
those 8 years - the only possible jumps would be to the moments between 
you arrived and left. But that begs the question - if two ships, unknown 
to each other, jump on courses in wake-free space on courses that would 
prevent each others' travel, would either arrive? And if so, which?

 Third, the "wake angle" seems to be effective in relatively flat space... 
but I haven't yet figured out what might happen with those sci-fi staples, 
space-warping wormholes and black holes.

 What are your own thoughts?

Thank you for your time,
The Rrangoon species is available at