In a
nuclear photonic rocket, a
nuclear reactor would generate such high temperatures that the light from the reactor would provide thrust. Think of a nuclear light-bulb, with a reflector. The big advantage is that nothing material leaves the spacecraft, so only nuclear fuel is depleted. This has the highest-known
specific impulse of any rocket that might be made with known technology. The reactor would be constructed of
graphite and
tungsten.
A variation is the photon rocket from the german rocket scientist Eugen Saenger[?], using antimatter annihilation as a light source and an electron-mirror as a reflecting medium. None of his mirror designs seem to work in reality though.
See also: spacecraft propulsion