Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/5/84; site mordor.UUCP 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 -------