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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*