[extropy-chat] New antigravity solution will enable space travel near speed of light by the end of this century

[Russell Wallace]

On 2/14/06, "Hal Finney" <hal at finney.org> wrote:
>
> Then third, take a movie of that and show it in reverse.  Since
> relativistic physics is time-reversal symmetric this is kosher, all
> the physics will still work.  It shows a small body heading rapidly
> away from the star, accelerating faster and faster as it goes.  (One of
> the curiosities of time-reversal is that although velocities reverse,
> acceleration stays the same.  Imagine a movie of a ball being thrown up
> in the air and falling back down.  Reverse the movie and everything still
> looks OK, it is still accelerated downwards.)  So fast-moving bodies get
> a push away from the star whether they are heading directly towards or
> directly away from it.


Oh! Yes, that makes sense, thanks.

I've been trying to come up with an intuitive explanation for why
> this effect happens.  I'm not sure I have it, but I *think* it can be
> thought of as due to gravitational time dilation.  Time slows down
> as you get deeper into a gravitational field.  Normally it is to an
> almost undetectable degree for non-exotic objects, but I think maybe
> when the test body is moving at close to the speed of light, this tiny
> time dilation is enough to make a significant difference in terms of
> how close to the speed of light it is going.
>
> The article points out that this effect is well known in the case of
> a black hole.  From the external reference frame, objects falling into
> a black hole slow down as they approach the event horizon and in fact
> never cross it.  His paper basically shows that the same kind of slowdown
> happens to a lesser degree with every gravitating object.  As I explained
> above, a slowdown of an infalling object is equivalent to an antigravity
> "push" from an approaching object, if you just switch reference frames.
> That's what is happening here.
>

Right... though, correct me if I'm wrong, but doesn't the slowdown of an
object falling into a black hole, only happen in the frame of reference of a
distant observer? From the viewpoint of the falling object, no slowdown
occurs?

Thinking about it a bit more, even if there was a star moving at
relativistic speed, it wouldn't be much use for transport - position
yourself ahead of the star, and it crashes into you! (You won't be punted
ahead of it - consider that cosmic rays, for example, crash into Earth
without losing much of their velocity.) Or behind it, and it flies away
before it has time to impart much velocity.

But _could_ it account for the current acceleration of the expansion of the
universe? After all, the reason the expansion was expected to decelerate is
gravity, but if that becomes antigravity for objects receding faster than a
certain speed, would that produce the observed results?

- Russell
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