From: Hal Finney (hal@finney.org)
Date: Fri Feb 28 2003 - 12:50:33 MST
Scerir writes:
> In the 28th Feb. 2003 issue of Science, there is an advance article
> on a soon-to-be-published paper that theoretically shows the
> possibility of having a "motion" without propulsion (i.e. not using
> Newton's Third Law). Written by Jack Wisdom of MIT, the highly
> mathematical and complicated paper reveals a motion similar to when
> one is sitting on a swivel chair and by thrusting one's arm in
> specific directions, one can make the chair turns. It is the same
> effect that happens when a cat falls down and lands on its feet - by
> twisting its torso.
>
> This new paper deals with the same idea and involves with a curved 4D
> spacetime. By stretching and retracting the "limbs" of a body, Wisdom
> showed that one can "swim" through a curved spacetime. Interestingly
> enough, and just like the swivel chair case where one can change
> one's orientation but not spin, in this "swim" one can change one's
> position but not the overall velocity.
I haven't read this yet, but a few comments in advance:
I remember an article in Scientific American several years ago about how
you could change your body's orientation even if you were floating in
space, by waving your arms and legs around properly. You'd think it's
impossible, by conservation of angular momentum, but it's not; in fact,
that law is what allows it to happen. For example, you can swing your
arm in a circle, and by conservation of angular momentum your body will
start rotating in the other direction. I think some satellites use this
method to control their orientation, by spinning internal flywheels.
It does sound plausible that you could do something similar for position
in curved space. I'll bet the problem in practice is that space around
here is curved too slightly, it is too close to flat. The strong-seeming
1G acceleration fields around earth are microscopic when expressed in
geometric coordinates based on the speed of light. You'd need to swim
up to a neutron star or a black hole to get any considerable amount of
spacetime curvature which would be necessary for "traction".
Now, I know what you're thinking: how about conservation of momentum?
If you move from point A to point B to point C, don't you have a velocity?
and therefore a momentum? But you started with zero momentum. How does
that work?
Well, I'm not sure. I suspect what happens is that in curved space,
any global coordinate system is going to have some disagrements about
velocities, as measured from place to place. Therefore you can move
around without breaking the rules because you change which rules apply
as you move. You change the local coordinate system by which you are
judging your velocity. Your velocity is always zero as measured locally,
always non-zero as measured remotely, and that's what allows you to move.
That's just a guess. In any case this is an amazing result, like a
real-life version of the mythical Dean Drive (or Mike Lorrey's Drive)
which was supposed to produce propulsion without reaction. However if
my intuition is correct, this drive will not produce acceleration,
rather the (pseudo) velocity will be constant and proportional to the
curvature of space. Hence it would not be practical for space tr You'd
crawl along at one nanometer per second or whatever, and eventually you'd
reach the moon, but no one would care any more by the time you got there.
because no matter how long you left it on, you wouldn't speed up.
Hal
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