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Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!ihnp4!zehntel!dual!mordor!space@mit-mc
From: space@mit-mc
Newsgroups: net.space
Subject: Photon momentum
Message-ID: <787@mordor.UUCP>
Date: Fri, 1-Mar-85 17:30:27 EST
Article-I.D.: mordor.787
Posted: Fri Mar  1 17:30:27 1985
Date-Received: Sun, 3-Mar-85 03:09:21 EST
Sender: daemon@mordor.UUCP
Lines: 22

From: BIESEL@RUTGERS.ARPA

About that dime-store gizmo with the black and silver vanes: it is a common
misconception that it rotates because of the difference in momentum exchange
between the light and dark sides. T'aint so. Careful observation will show
that the wheel turns in a direction that implies that there is a greater
force per unit area on the dark side of the vanes than on the light side.
In fact, the propulsive force is thermal: the dark paddle gets warmer than
the light side; this in turn heats the remaining air molecules in the bulb
differentially. The consequent minute pressure differences between light
and dark sides of the vanes make the whole thing spin. Note that the bulb
contains a rather poor vacuum. Too much air and the viscous drag keeps the
thing from turning, too little air, and the heat engine doesn't work any more.
In a very good vacuum you'd have an approximation of your light sail;
unfortunately it would not work, because the forces generated by photon
momentum exchange are orders of magnitude less than the radiant energy in
the photon stream which the little heat engine is using, and would not
suffice to overcome the friction in the system. (unless you had *very*
good bearings, were fanatical about balancing the thing, etc.)

	Biesel@rutgers.arpa
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