From: Adrian Tymes (wingcat@pacbell.net)
Date: Fri Feb 28 2003 - 11:50:01 MST
--- scerir <scerir@libero.it> wrote:
> 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's called "friction". As in, the chair *would* turn
the other way, but it transmits that force to the
ground, which transmits it to the Earth. Technically,
the Earth does turn the other way - but by
conservation of momentum, the Earth's change in course
is immeasurably small, even if you did a whopping
gigantic number of them. Meanwhile, all you perceive
is yourself spinning.
> It is the same
> effect that happens when a cat falls down and lands
> on its feet - by
> twisting its torso.
Twist to get at least one paw on the floor, while the
rest of the body twists the other way. Once at least
one paw is on the floor, friction can keep it from
twisting away while the cat twists the rest of its
body into place. Fast, yes, but that doesn't violate
any known laws of physics. (Now, how cats can wander
behind a couch and disappear...nah, that's probably
just some poorly-understood-by-us application of
quantum teleportation. ^_-)
> 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'll believe it when they build a device that can
provably do this. Not until then. (Because, frankly,
it doesn't matter until it can be applied.)
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