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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