space@mit-mc (03/02/85)
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 -------
lew@ihlpa.UUCP (Lew Mammel, Jr.) (03/03/85)
The discussion of photon momentum in relation to the radiometer reminds me of a PSSC (Physical Sciences Studies Committee) film that I saw in high school. This one featured Gerald? Zacharias of MIT. He showed a radiometer spinning and hypothesized that it was the difference in photon momentum transfer that was driving it. He then did a series of demonstrations with a toy gun, shooting plastic bullets against suspended steel plates, the conclusion of which was that elastic collisions transfered twice the momentum of perfectly inelastic collisions. He then leaned over the radiometer and said, "... so it should spin this way." , pointing his finger in the opposite direction of the actual spin. After a pause he looked up at the camera and deadpanned, "Clearly this is the wrong explanation." After giving the correct explanation, he demonstrated actual photon momentum transfer with an intense beam directed against a bit of metal foil suspended in a high vacuum. I'd have to say this film achieved its point seeing that I remember it so well after 20 years! As subtle an effect as it is, the photon momentum accounts for almost 10% of the pressure at the sun's center - a radiation pressure capable of exploding the earth! (warning - this is my calculation) Lew Mammel, Jr. ihnp4!ihlpa!lew