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

[Hal Finney]

Robert Bradbury writes:

> The relevant papers in ArXiv are:
> gr-qc/0505099<http://arxiv.org/abs/gr-qc/0505099>and
> gr-qc/0505098 <http://arxiv.org/abs/gr-qc/0505098>
>
> [Disclaimer: most of the math in the papers is beyond my capabilities.]
>
> However, it appears that even in the physorg article they point out that you
> have to have the mass performing the acceleration going at 57.7% (the 3^-1/2
> number in the abstracts) of the speed of light to get the repulsive
> acceleration.  The mass being accelerated also has to be in a relatively
> narrow acceleration cone in front of the mass performing the acceleration
> (think of it as surfing on a wave).
>
> Now, yes, in particle accelerators we probably have particles going at
> 57.7%the speed of light and the theory could be tested in them.  But
> nowhere do I
> see anything addressing how to get a relatively large mass up to 57.7% the
> speed of light and I presume that a proton or electron beam hardly has the
> mass sufficient to generate a field which could accelerate something like a
> human body (or even a small (few kg) interstellar probe).

I agree that the spin that Felber has put on his article is puzzling,
about this being useful for making an interstellar probe.  Nevertheless
the result is an amazing and beautiful piece of physics.  If you go to
this link: http://arxiv.org/e-print/gr-qc/0505099 you can download a
compressed file with several animations showing how it works.

Basically if you have a large mass approaching a test particle at greater
than 1/sqrt(3) of the speed of light, the particle will be accelerated
away from the mass.  And if it is going faster, the acceleration
extends even farther out.  So if you could get yourself in front of
a fast-moving star, you would be pushed away as the star approached,
exactly the opposite of what you would expect from ordinary gravitation.

It also implies that if a small body were launched straight towards the
sun at a large fraction of the speed of light, the sun's gravitational
field would decelerate it as it approached.  I haven't tried to read
the paper carefully enough to calculate how much it would be slowed
down or how close it would have to get.

One of the nice things about this effect is that tidal forces on the
accelerated body should be relatively small, if the speed is high enough.
So a space probe could be accelerated at even hundreds of g's and the
people inside would feel nothing.  It would feel to them like they were
in free fall.

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.  He suggests that this has "obvious
cosmological consequences", hinting that this could act as a force
of gravitational repulsion that could drive galaxies apart.  I don't
think this is necessarily true, though, as new effects come into play
at cosmological distances.  Still it is a curious point and we will see
if other physicists pick up on it.

The other issue is that as Robert points out, the whole thing doesn't
seem practical since there are no stars around going anywhere close to
this fast.  Felber suggests that his effect may actually be one reason
for this, that a star moving so fast would push on bodies ahead of it
and thereby be slowed down.  However I don't agree with this explanation,
for two reasons.  First, there are no stars going even much slower than
the threshold for his effect to kick in.  And second, due to symmetry,
his equations imply not only that there is a repulsion ahead of the star,
but there would be lessened attraction behind the star, which effects
should cancel each other out.

I doubt that we will really be using this effect to power space probes
any time soon, but it's still interesting that such simple and previously
unexpected phenomena lurk within the standard equations of relativity.

Hal