Wednesday, June 08, 2005

Catch my gamma rays if you can.

NIAC just released their new grants. One of them is focused on how to harvest antimatter. Why antimatter ? well because everybody knows that if you use some antimatter with its matter equivalent, they annihilate with each other releasing a large amount of energy. Why do we care ? well, the story goes that with a lot of energy, you can go anywhere in space. In essence, once you get antimatter, you're ready for some Star Treck action.

The main problem is that the last statement is just plain wrong. In order to go anywhere in space, you need to eject mass from your spaceship. You know, inertia, if something goes one way, the rest goes the other way. Hence, in order to have a rapid spaceship, you have to efficiently eject mass as rapidily as possible in the opposite direction. The only way we know how to rapidily eject matter is to heat it up. The warmer the heat, the higher the ejection speed of the fuel and therefore, the higher the speed of your rocket. This is why chemical rockets are less efficient than nuclear rockets.

In the case of the "antimatter" devices proposed, the large amount of energy released is just massless photons, high energy photons, but massless photons nonetheless. How do you suppose large masses of matter can be heated up and ejected when the fuel is made up of massless photons ? Well if they remain photons, it won't help therefore you need a conversion process enabling matter to pick up the energy of the massless photons instead. And here lies the problem, the cross section of gases to high energy photons is very very small. In effect, physically, it is so poor that it does not compete seriously with any existing and tested nuclear rocket concepts.

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