Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/12/84; site desint.UUCP Path: utzoo!watmath!clyde!burl!ulysses!allegra!bellcore!decvax!tektronix!hplabs!sdcrdcf!trwrb!desint!geoff From: geoff@desint.UUCP (Geoff Kuenning) Newsgroups: net.space Subject: Re: Light Sails Message-ID: <349@desint.UUCP> Date: Sat, 2-Mar-85 23:41:05 EST Article-I.D.: desint.349 Posted: Sat Mar 2 23:41:05 1985 Date-Received: Thu, 7-Mar-85 05:32:46 EST References: <722@mordor.UUCP> <133@uwvax.UUCP> Organization: his home computer, Manhattan Beach, CA Lines: 40 In article <133@uwvax.UUCP> derek@uwvax.UUCP (Derek Zahn) writes: >Got time for a dumb question? Too bad. I can't figure how the energy of >photons from the sun is going to be converted into motion of the light sail. >I jus' don't get it. If the photon is reflected, there can be no change in >momentum of the sail (momentum being conserved), unless the photon loses >energy. In this case, what is the mechanism that causes the transfer of >momentum, and how efficient could it possibly be? If the photon is absorbed, >it seems much more likely that the energy would be converted to heat. >Somebody help, for I is baffled. Wahl, Ah cain't resist demonstrating that I still remember my physics, despite senility... (a) Momentum is a vector. If the photon is reflected, it's momentum is reversed. To compensate for this, the sail must receive *twice* the photon's momentum in the opposite direction. In other words, before the collision the system has a momentum of 1 frobozz away from the sun. After the collision, the system must have the same momentum; since the photon has 1 frobozz of momentum directed *towards* the sun, the sail must pick up 2 frobozzes away from the sun to produce a net of 1 frobozz away. This is why reflective collisions are preferable. (b) Momentum and energy are two different things. Momentum must still be conserved in this collision; in this case the system (including the absorbed photon) must retain 1 frobozz away from the sun. This means that the photon will be slowed dramatically. Think of two frictionless clay lumps sliding on a surface, equal in weight. After the collision, the mass is doubled, so the velocity must be halved to conserve momentum. In the light sail case, things are a lot less equal, but it still works. The energy of the photon goes two places: a tiny bit becomes kinetic energy of the light sail, and the rest becomes heat. The photon was high-energy because energy is equal to (m*v**2)/2; the light sail only needs a little of this because its velocity is so low. -- Geoff Kuenning Unix Consultant (213) 545-4413 ...!ihnp4!trwrb!desint!geoff