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

[Hal Finney]

Russell Wallace writes:
> On 2/13/06, "Hal Finney" <hal at finney.org> wrote:
> > A couple of other interesting points.  The article points out that
> > this implies that a stationary mass will repel objects receding from
> > it at greater than 1/sqrt(3) * c....
>
> Repel objects _receding_? I don't get this part - I thought the repulsion
> was of _approaching_ objects, and had assumed it was something akin to frame
> dragging and that by the same token, receding objects would be attracted
> (more strongly than normally, that is). If both approaching and receding
> objects are repelled, is there a layman-understandable explanation of how
> this can be the case?

It's just symmetry, I think.  First, imagine a fast-moving star
approaching a small body, and pushing it along, accelerating it away
from the star.  This is the basic result of the paper.  Second, look
at it in the frame of reference of the star.  It sees a fast-moving
small body approaching it, which then slows down, i.e. decelerates,
i.e. accelerates in the direction away from the star.  Clear so far?

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.

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.

Hal