# Re: BOOKS: Pournelle's *A Step Farther Out*

Anders Sandberg (asa@nada.kth.se)
17 Feb 1999 19:47:47 +0100

Jonathan Reeves <JonathanR@mail.iclshelpdesks.com> writes:

> >> Exactly. It will never appear to pass c (either in the original
> >> inertial frame or in a Lorentz diagram), but this does not mean it is
> >> not travelling ftl relative to it's start point.
>
> >Huh? This seems to be a contradiction, how can something move FTL
> >relative to a point when in that point's frame it is moving sublight?
>
> To an observer in that frame, it will appear to slow down as it
> approaches the event horizon - just like falling into a black hole. The
> people in the ship however will not experience this and if they keep
> accelerating they will move into a different inertial frame to the one
> they started off in. Thus, they will be moving ftl relative to a point
> in that original frame (although no one there will be able to see them
> doing it)

I think you mix up things a bit. If I jump into my spaceship and blast off from Earth (I have an urgent meeting at Aldebaran) with a constant acceleration (as measured with an accelerometer onboard) you will see my ship recede with a speed (as measured from Earth, for example by observing the rate I pass regular milestones along my way) that initially increases linearly with time, but gradually the increase slows down and after a long while I appear to move near c. No FTL there.

Properly speaking the ship is in no inertial frame - it is defined as a frame of reference experiencing no accelerations - but it is possible to speak of instantaneous inertial frames for each moment. However, in neither the ship frames or the Earth's frame is anything seen moving beyond c.

All this is standard relativity textbook stuff. You will find a treatment in (say) Rindler's Essential Relativity. It might also be worth looking at the relativity FAQ,
http://www.corepower.com/~relfaq/relativity.html and especially
http://www.corepower.com/~relfaq/acceleration.html http://www.corepower.com/~relfaq/FTL.html

```--
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Anders Sandberg                                      Towards Ascension!
asa@nada.kth.se                            http://www.nada.kth.se/~asa/
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