Antimatter Propulsion System Is there a possibility ( in theory ) to build a antimatter propulsion system, if so how can we control the collision of matter-anitmatter, will humans be able to control this force just like electricity ?
 A: The basic tragedy of space travel is expressed by the Tsiolkovsky rocket equation, which says that the amount of reaction mass you need grows exponentially with your $\Delta v/v_e$, where $v_e$ is the exhaust velocity. The advantage of antimatter propulsion is high energy density, but energy density doesn't have any direct, major effect on the amount of reaction mass needed. For a lot of purposes within the solar system, solar power provides plentiful energy for something like an ion drive (which has been successfully tested).
The real advantage of antimatter propulsion would be if you wanted to send a probe to another star and have it get there within a human lifetime. For this type of mission, you can't use solar power or beamed energy, and you need  a relativistic $\Delta v$, which means that the energy density of chemical or fusion or fission reactions is not high enough. In this situation, the rocket equation is going to kill you unless the exhaust velocity is also highly relativistic. You can't achieve that simply by reacting the matter and antimatter in a chamber and letting reaction mass fly out through a nozzle -- the chamber would be destroyed. So the design you're probably going to end up with is a heat engine that generates electric power to run a particle accelerator, which shoots ions out as reaction mass at relativistic speeds.
There are many, many problems with carrying this out. Antimatter is ridiculously hard to produce in any significant quantity. It's hard to store safely. No way do you want ton quantities of antimatter anywhere near the planet you live on, because a disaster (or, say, a hijacking) would probably wipe out all life.
Assuming you get the engine running, the second law of thermodynamics says that the heat engine can't be 100% efficient, and the waste heat is likely to melt the entire vessel almost instantaneously.
The matter-antimatter reactions will produce a witch's brew of high-energy particles. Matter-antimatter annihilation (such as proton-antiproton annihilation, Amsler 1997) makes a lot of high-energy gamma rays, e.g., $\gtrsim 67$ MeV gammas from decay of $\pi^0$'s, and the energy of those gammas is hard to collect, because they're very penetrating. You also get muons and neutrinos from the charged pions that are produced. The neutrinos are lost energy. The energy from the charged particles (muons and undecayed charged pions) is the only energy in the process that's easy to collect in a controlled way. There will also be 511 keV gammas from electron-positron annihilation.
Amsler, http://arxiv.org/abs/hepex/9708025
