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*To*: <starship-design@lists.uoregon.edu>*Subject*: RE: starship-design: Casimir-Foreward balloon*From*: Steve VanDevender <stevev@efn.org>*Date*: Tue, 25 Jan 2000 15:24:51 -0800 (PST)*In-Reply-To*: <000901bf6758$02e1f910$0401a8c0@broadsword>*References*: <3.0.1.32.20000125103357.03284c74@pop.xs4all.nl><000901bf6758$02e1f910$0401a8c0@broadsword>*Reply-To*: Steve VanDevender <stevev@efn.org>*Sender*: owner-starship-design@lists.uoregon.edu

L. Parker writes: > Well, that is the problem. The total momentum (and energy) must be defined > in the terms of the system, which as you point out is rather large in this > case. I seem to remember that it WAS possible to violate conservation > locally as long as you didn't violate it in terms of the larger system. The > physics for this are beyond me I'm afraid. So far as anyone knows momentum and energy are always conserved in relativistic physics. If you take a group of particles and keep track of them through all their interactions with each other, their total momentum and energy remains the same. If they interact with other particles you didn't initially include, you have to go back and include those other particles in your entire analysis until you have a closed system -- one that does not interact with any parts other than its own. The way this balloon thing or my simpler one-sided mirror example work is that the system is defined to be the balloon or mirror plus all the photons that interact with it considered over a period of time. Starting out you have the balloon or mirror sitting "at rest" (by definition, for our purposes), and a bunch of photons initially uniformly distributed in direction and with no net momentum. At the end you have a moving balloon or mirror, and a now non-uniformly-distributed bunch of photons whose net momentum is now opposite to that of the balloon or mirror. The physics are really quite simple -- and when you get to chapters 7 and 8 of _Spacetime Physics_ you'll see how.

**References**:**starship-design: Casimir-Foreward balloon***From:*Timothy van der Linden <Shealiak@XS4ALL.nl>

**RE: starship-design: Casimir-Foreward balloon***From:*"L. Parker" <lparker@cacaphony.net>

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