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

[kevinfreels.com]

It's awfully neat stuff, but none of it explains how someone could expect to
use it for space travel in the near future. I just don't see that as a
realistic possibility.


----- Original Message ----- 
From: ""Hal Finney"" <hal at finney.org>
To: <extropy-chat at lists.extropy.org>
Sent: Tuesday, February 14, 2006 1:16 PM
Subject: Re: [extropy-chat] New antigravity solution will enable spacetravel
near speed of light by the end of this century


> 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
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