Tachyons travel faster than the speed of light, not ordinary matter, so even with a tachyon propulsion system, you cannot reach the speed of light. The most viable form of interstellar would be to create an artificial worm hole. We don't have that technology, though. The fastest system possible now is called Orion. It has small fusion bombs that explode behind the space craft. The craft has a plte behind itself in order to protect it from the explosion. The energy from the fusion bomb propels the space ship to speeds as high as 1/10 the speed of light. It can't be built because there is a treaty that bans the use of nuclear weapons in space. Antimatter-matter annialation will someday become a viable propulsion system, when we find a way to produce large quantities of antimatter. Antimatter rockets could go as high as 1/2 the speed of light. Light sails and ramjets are also possible, but are much more difficult because of their size. Ramjets need huge ion collecters, nearly the size of a moon, in order to collect enough hydrogen to create enough energy to travel through space. Light sails have to be kilometers in diameter, but can only be a few atoms thick, or their mass would be to high.