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RE: anti matter
By joset00 on 12/3/2006 9:27:33 PM , Rating: 2
@ AzureKevin:

True. But, there's a bigger catch, hidden in E=m*c^2.

Matter & Anti-matter produce high-energy photons, during anihilation; photons are massless particles, no matter how energetic they are.
The push-per-unit-time they'd produce would be insignificant, compared to nuclear devices or, even, to throwing stones, all other things being equal.
Analogy: Throw a 10Kg (or pounds) from a small boat, in mid-ocean & measure the reaction; do the same with small stones (same 10Kg/£)...


RE: anti matter
By Visual on 12/4/2006 5:00:29 AM , Rating: 2
if you throw the small stones *fast enough*, you can get just as good a result.
light might be massless, but to counter that it's helluva fast ;)
it still has momentum. momentum preservation would indicate that if you manage to direct that light backwards, you'd gain momentum forwards.
in fact, light is the best possible "reaction mass" you could use, looking at the impulse per energy ratio, i.e. impulse per mass of fuel, specific impulse.

RE: anti matter
By joset00 on 12/4/2006 9:31:44 PM , Rating: 2

Not necessarily.
You'd spend more energy than available throwing small stones "fast enough", in a closed system.
Moreover, matter/anti-matter anihilation (M/AMA) is, indeed the most eficient way of 'turning' mass into energy (E=m*c^2, where m=0, for the photon's rest mass); however, it would take a ship forever/all of the ship's mass to reach very close to c; what would be the point? Even a small mass portion left (say, you & your toothbrush), would refrain the whole system (ship) to reach such a speed.
(As for momentum, p=mv in Newtonian Mechanics, etc, just try to imagine the photons' impact, whenever one's X-rayed; acceleration would be tremendously faint/the 'ship' would be tremendously massive...).


RE: anti matter
By Visual on 12/6/2006 8:45:50 AM , Rating: 2
ok these boards arent the most convenient place for an argument, what with the lack of reply notifications and such... so this is gonna be my last post on the subject.

put simple, newtonian mechanics just doesn't apply here. p=mv, no longer. p=gamma*m0*v is still valid where m0>0, but gives 0/0 indefinite for photons. a more general formula, working for both massive and massless particles, is
as you can see, for a set total energy E, the maximum momentum would be that of a massless particle, i.e. the photon. so light propulsion is the most efficient one possible. in other words, throwing a stone, or any other reaction mass, will always gain you less momentum than annihilating it and emmiting all of the energy behind you.

as to your speech about not being able to reach c - it is both corrent and unrelated to what is being discussed. we won't reach c, we can reach arbitrarily close to it though, and doing it with light propulsion is the most efficient way.


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