Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site utastro.UUCP Path: utzoo!watmath!clyde!cbosgd!ihnp4!mhuxn!mhuxj!mhuxr!ulysses!allegra!bellcore!decvax!genrad!panda!talcott!harvard!seismo!ut-sally!utastro!ethan From: ethan@utastro.UUCP (Ethan Vishniac) Newsgroups: net.space Subject: Re: Light Sails Message-ID: <1044@utastro.UUCP> Date: Thu, 28-Feb-85 10:12:44 EST Article-I.D.: utastro.1044 Posted: Thu Feb 28 10:12:44 1985 Date-Received: Sun, 3-Mar-85 07:35:23 EST References: <722@mordor.UUCP> <133@uwvax.UUCP> Organization: U. Texas, Astronomy, Austin, TX Lines: 29 > 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. > > derek *** REPLACE THIS LIGHTSAIL WITH YOUR WARPDRIVE *** Momentum is a *vector* quantity. When a photon is reflected it has a momentum which is the negative of its original momentum. The momentum transmitted to the sail is twice the original momentum of the photon. Of course, for a sail of finite mass this implies that there is actually a small loss of energy for the photon and its recoil momentum is not exactly equal in magnitude to its original momentum. [From conservation of energy]. "Don't argue with a fool. Ethan Vishniac Borrow his money." {charm,ut-sally,ut-ngp,noao}!utastro!ethan Department of Astronomy University of Texas Austin, Texas 78712 *Anyone who wants to claim these opinions is welcome to them*