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