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From: lew@ihlpa.UUCP (Lew Mammel, Jr.)
Newsgroups: net.physics
Subject: Compton effect & lightsail
Message-ID: <145@ihlpa.UUCP>
Date: Sat, 9-Mar-85 21:31:39 EST
Article-I.D.: ihlpa.145
Posted: Sat Mar  9 21:31:39 1985
Date-Received: Sun, 10-Mar-85 08:10:51 EST
Distribution: net
Organization: AT&T Bell Laboratories
Lines: 29

Mike Augeri posted a derivation of the momentum transfer to a lightsail
which I claim is all wet.  He based it on the Compton effect, but I don't
see why that should apply here.  Specular (mirror like) reflection is
all that is necessary - as long as you are in the mirror (or lightsail)
frame of reference. Also, please note that Mike's derivation took no
account of the speed of the lightsail.

I think it bothers some people that the photon doesn't lose energy
in the lightsail's rest frame. How can the lightsail gain if the
photon doesn't lose?  Let's resort to a classical analogy to explain
this.

Suppose we are accelerating an aircraft carrier (in space!) by shooting
BBs at its deck. In the ship's rest frame we can assume no loss of energy
by the BBs. This is because the energy gained by the ship is:

	M/2 * dv * dv

where M is the ship's mass and dv is its incremental speed.  In the
"stationary" or shooter's frame, the ship gains energy:

	M * v * dv   +   M/2 * dv * dv

so when we integrate we can ignore the second order term.  Hence
when calculating in the ship's frame we can ignore the transfer
of energy.  This seems paradoxical, but I believe I'm on solid
(certainly familiar!) ground here.

	Lew Mammel, Jr. ihnp4!ihlpa!lew