robertcr@tektronix.UUCP (Robert Cram ) (03/16/84)
I was just reading an article about the Dean Drive. The supposed space drive that defies conservation laws. The article (Analog 6/76) suggest that Newton's 2nd Law has an additional term in it for the third derivative. According to the article, there is a "critical action time" during which a system is non-Newtonian and "cannot accept energy". By playing around with this, one can supposedly levitate without using reaction mass. This seems far fetched to me, but is there sound theoretical or experimental work that flatly says that there can be no non-zero coefficent of the third derivative? Is there an upper limit on its value?
MJackson.Wbst@Xerox.ARPA (04/13/84)
The Dean Drive is well-known to be bogus. It is a simple nonlinear, nonsymmetric oscillator (the internal mass is pushed gently one way for a long time and strongly the other way for a short time). It slides along the floor just as you could slide a chair in "jerks" without touching the floor. The apparent effects depend on the existence of friction to negate the effect of the gentle forces, so that the strong forces dominate the average. If you suspended it from the ceiling it would hang straight down; if you put it in free space it would stay there. A non-zero coefficient for the third derivative would imply that momentum is not conserved. Conservation of momentum is pretty well established. Mark
JGA%MIT-MC@sri-unix.UUCP (04/17/84)
From: John G. Aspinall <JGA @ MIT-MC>
The Dean Drive came up in a discussion on the SPACE and SF-Lovers
digests a while ago. Marvin Minsky submitted an amusing anecdote
about the subject to the lists. With his permission, here is his
story.
From: Marvin Minsky@MIT-AI (Sent by MINSKY@MIT-AI)
Date: 01/17/82 00:25:30
Subject: Dean Machine History
Shortly after the Dean drive was described in Astounding, John Campbell
published a picture of it. I examined the picture with a lens and
managed to read the brand name of the bathroom scale used to measure the
loss of weight of the machine. My college roommate, Roland Silver, and
I conjectured correctly that this scale had a "diode" in it that coupled
the platform and the reading device. So we went to Sears Roebuck in
Porter Square, Cambridge and bought that very scale.
When you stand on it it reads your weight fine, but if you pump your
arms up and down -- just as did the dean machine itself -- then the
weight fluctuates a lot -- with the mean weight (and even the maximum)
far below the real weight.
So then Clause Shannon and John Pierce and I wrote a sharp detailed
letter to Campbell about this.
John Campbell didn't print our letter, but he sent me (knowing I was the
instigator) a long letter that I still have here, denouncing
establishment scientists for their reactionary and unimaginative
rigidity and general intolerance.
Suitably chastened, I dropped the matter and continued with my
reactionary, establishment-bound studies.
Anyway, this incident jibes with Pournelle's account about Cambell
seeing the machine which "jumped around a lot" on a bathroom scale. I
checked out all the other scales, too, and finally found one that reads
high when you bounce. But these were much less common. So, possibly,
Dean was hoist by this pitiful petard. But I maintained that this was
extremely unlikely since, obviously, he was all too familiar with
flakey, vibrating, weighing mechanisms.
-- marvin
P.S.. I should add that much as we hated him, we loved him greatly too,
and for all he did for all of us. And same for G. Harry Stine.crummer%AEROSPACE@sri-unix.UUCP (04/20/84)
From: Charlie Crummer <crummer@AEROSPACE> The Dean Drive first appeared in Analog in the late '50's. Dean actually applied for a patent. The drive does depend on reaction mass, at least as described in the patent disclosure. This hare-brained contraption got all the way to NASA where an experimental apparatus was set up and conservation of momentum was verified to a considerable degree of accuracy. The apparatus proposed by Dean does not, in fact, levitate. Speculation as to additional terms in Newton's 2nd law are probobly irrelevant. The most accurate investigation of this type of phenomenon must be carried out with the theory of relativity. --Charlie
jerry@oliveb.UUCP (Jerry Aguirre) (04/23/84)
Granted that no amount of pushing and shoving will result in a net
change of momentum unless there is something outside to push against.
Does the same restriction apply to angular momentum? Given an object
suspended in space with no internal motion. Is it possible to spin up a
gyroscope, precess it a bit, brake it back down, and result in a net
rotational motion? The restriction here is that you must restore the
state of no internal motion. Is this guaranteed to cancel out all spin
imparted to the object?
Jerry Aguirre
{hplabs|fortune|ios|tolerant|allegra|tymix}!oliveb!jerrygwyn@Brl-Vld.ARPA (04/27/84)
From: Doug Gwyn (VLD/VMB) <gwyn@Brl-Vld.ARPA> It is supposedly possible to change one's orientation by an appropriate series of internal motions, but not one's angular momentum (conservation of which is a direct consequence of rotational symmetry of the fundamental laws).
crummer%AEROSPACE@sri-unix.UUCP (05/13/84)
From: Charlie Crummer <crummer@AEROSPACE>
Date: Fri, 27 Apr 84 16:01:48 EST
From: Doug Gwyn (VLD/VMB) <gwyn@Brl-Vld.ARPA>
Subject: Re: Dean Drive Possible?
It is supposedly possible to change one's orientation by an appropriate
series of internal motions, but not one's angular momentum (conservation
of which is a direct consequence of rotational symmetry of the fundamental
laws).
If by change in orientation you mean the angle swept out by some orientation
vector during the orientation change then this angle is proportional to the
time integral of the angular momentum of the system with respect to some
reference.
When the angular momentum is constant the orientation is changing at a constant
rate unless the orientation vector is aligned with the axis of rotation in
which case the proportionality constant is zero. When the direction of
something, e.g. a telescope, on a satellite is changed it is sometimes done,
at the expense of a compensating change in the opposite direction of something
else, by "gyro torquing". Is this the "internal motion" you mean?
--Charliegwyn@Brl-Vld.ARPA (05/14/84)
From: Doug Gwyn (VLD/VMB) <gwyn@Brl-Vld.ARPA> No, what I had in mind was more like this: A space-suited person in outer space holds two massive dumbbells. He takes turns holding them asymmetrically at different distances and waving them around. After a while the dumbbells are in their original positions w.r.t. the person but he is now facing some direction different from the one he started out facing. I am not sure that this scenario is actualizable; it was told to me during a discussion of the Dean Drive and freely-falling cats. Supposedly someone determined that it was possible for a cat to reorient itself without reacting against the air. I would be skeptical except that I haven't been able to come up with a quick counter-proof (the problem is that the inertia tensor is not constant).
bill@utastro.UUCP (William H. Jefferys) (05/14/84)
> When the angular momentum is constant the orientation is changing at a constant > rate unless the orientation vector is aligned with the axis of rotation in > which case the proportionality constant is zero. When the direction of > something, e.g. a telescope, on a satellite is changed it is sometimes done, > at the expense of a compensating change in the opposite direction of something > else, by "gyro torquing". Is this the "internal motion" you mean? What is being referred to is the same effect that allows a cat to right itself in the air after being dropped (with no net angular momentum) from a reasonable height. Athletes can also change their orientation in midair, starting and ending with zero angular momentum. There was a good article on this a few years back, probably in Scientific American, which showed precisely how it can be done. -- Bill Jefferys 8-% Astronomy Dept, University of Texas, Austin TX 78712 (USnail) {ihnp4,kpno,ctvax}!ut-sally!utastro!bill (uucp) utastro!bill@ut-ngp (ARPANET)
jlg@lanl-a.UUCP (05/14/84)
iiii It IS possible to change orientation without effecting angular momentum or any other momentum. By change of orientation I mean 'face the other way' or 'turn around' relative to distant reference points. Scientific American had an article on this a couple of years ago (I don't remember the date). The main application cited in the article was Diving (off a diving board, olympics, that kind of stuff). A diver does a twist by counter-rotating various parts of his body and then bringing such relative motions to a stop again, but now his body as a whole is in a different orientation. The net angular momentum of his body never changed - and could have been ZERO.
merlyn@sequent.UUCP (05/16/84)
> Message-ID: <715@sri-arpa.UUCP> > Date: Mon, 14-May-84 02:41:38 PDT > From: Doug Gwyn (VLD/VMB) <gwyn@Brl-Vld.ARPA> > > No, what I had in mind was more like this: > A space-suited person in outer space holds two massive dumbbells. > He takes turns holding them asymmetrically at different distances > and waving them around. After a while the dumbbells are in their > original positions w.r.t. the person but he is now facing some > direction different from the one he started out facing. > I am not sure that this scenario is actualizable; it was told to me > during a discussion of the Dean Drive and freely-falling cats. Supposedly > someone determined that it was possible for a cat to reorient itself > without reacting against the air. > > I would be skeptical except that I haven't been able to come up with a > quick counter-proof (the problem is that the inertia tensor is not constant). How about this... (no dumbells even!)... In space, start your hands at your side. Then, make a large circle forward, up, over your head, backward, down, and back to your sides. If you end up in the same orientation after you complete the circle, you have some pretty strange hands or strange ideas about what happens in space. Really, wouldn't a large massive wheel with a center of rotation aligned with the center of mass of a spaceship affect the rotational acceleration of the ship based on the rotational acceleration of the wheel? Sounds pretty straightforward to me. So, suppose the rotational velocity of the ship is zero, as well as the rotational velocity of the wheel. Then, rotate the wheel one turn (ship relative). The ship will be facing a different direction (rotational position), and still have zero kinetic rotational energy, as will the wheel. I'm not a physics expert, but I do know that it is possible to spin yourself around in space. (Not change linear momentum, however... unless the Dean Drive works.) -- A particularly personal and original observation from the thought-stream of Randal L. ("(null)") Schwartz, esq. (merlyn@sequent.UUCP) (Official Legendary Sorcerer of the 1984 Summer Olympics) Sequent Computer Systems, Inc. (503)626-5700 (sequent = 1/quosine) UUCP: {decwrl,ogcvax,pur-ee,rocks34,shell,unisoft,vax135,verdix}!sequent!merlyn