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

Jonathan Reeves (JonathanR@mail.iclshelpdesks.com)
Wed, 17 Feb 1999 15:39:34 -0000

This message is in MIME format. Since your mail reader does not understand this format, some or all of this message may not be legible.

Content-Type: text/plain

Anders Sandberg writes:

```>> >Jonathan Reeves <JonathanR@mail.iclshelpdesks.com> writes:
>>
>> >> The energy needed to accelerate it from it's _starting_ point
>> increases,
>> >> but not the energy it needs to accelerate itself.
>> >> An object/vessel which is capable of generating it's own thrust
```
will
```>> not
>> >> need to output more power to maintain a constant acceleration the
>> >> further it gets from it's origin.
>>
>> > True. But an observer sitting at the origin will not see it pass c,
>> > and neither will any other observer moving in an intertial frame
```
with
>> > a relative velocity to the origin less than c. A spaceship >> > accelerating at constant acceleration (as measured by the crew) will
>> > describe a hyperbolic path in a Loretnz diagram; it will never break
```>> > c.
>>
>> 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?

Jon Reeves

Content-Type: text/html
Content-Transfer-Encoding: quoted-printable

Anders Sandberg writes:
>> >Jonathan Reeves = <JonathanR@mail.iclshelpdesks.com> writes:
>>
>> >> The energy needed to accelerate it = from it's _starting_ point
>> increases,
>> >> but not the energy it needs to = accelerate itself.
>> >> An object/vessel which is capable = of generating it's own thrust will
>> not
>> >> need to output more power to = maintain a constant acceleration the
>> >> further it gets from it's = origin.
>>
>> > True. But an observer sitting at the = origin will not see it pass c,
>> > and neither will any other observer = moving in an intertial frame with
>> > a relative velocity to the origin less = than c. A spaceship
>> > accelerating at constant acceleration = (as measured by the crew) will
>> > describe a hyperbolic path in a = Loretnz diagram; it will never break
>> > c.
>>
>> 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)

Jon Reeves