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starship-design: Thoughts on antimatter and fusion...

L. Parker writes:
 > While reading a recent post a thought occurred to me. Forgetting for the
 > moment that ANY drive that requires reaction mass is inherently unsuited for
 > Interstellar space...
 > Everyone has always assumed that the epitome of reaction drives is the
 > Antimatter Drive. The basis for this belief has been the supposition that
 > since antimatter/matter reaction is the ONLY way to get 100 percent
 > conversion of mass to energy, then it would naturally be the ONLY way to get
 > nearly 100 percent efficiency out of the engine.
 > Here is the thought:
 > The key here is getting 100 percent conversion of mass to energy, NOT the
 > antimatter/matter reaction. What we are actually looking for is ANY method
 > of converting a given amount of mass into 100 percent energy, preferably in
 > a (somewhat) controlled manner. So perhaps we should expand our thinking a
 > little. There might be some method (don't ask me, I'm just brainstorming) to
 > persuade matter to give up its all without resorting to antimatter.
 > Maybe there is some way of causing a stick of beryllium to turn into a
 > coherent beam of gamma radiation without wasting an enormous amount of
 > energy in the process. This would constitute a photon drive, every bit as
 > efficient as antimatter/matter conversion.

Actually, I think I've said much the same thing in the past.  The
theoretically maximally efficient reaction drive would convert fuel mass
entirely to photons.  In fact, matter/antimatter reactions aren't 100%
efficient at converting mass to photons, partly because it's hard to get
a complete reaction and even if the reaction is complete not all of the
reaction products are photons.

I'm not expert enough at particle physics to know if there's any deep
reason why converting a quantity of mass (as opposed to equal quantities
of matter and antimatter) entirely to photons would be difficult or
impossible.  I suspect that some of the more subtle conservation laws in
particle physics may prevent 100% complete conversion.  In fact, there
are things that probably also come into play like the charge-parity
asymmetry which makes matter more prevalent in the universe than