I know of Project Daedalus, Project Icarus, and Project Orion. These basically utilize pulse propulsion. In Daedalus and Icarus helium 3 and deuterium are combusted by a laser to create thrust. And in Orion nuclear bombs explode behind the craft to get it moving. All of these designs only reach a small fraction of lightspeed. Are there any plausible proposals for spacecraft that could reach relativistic speeds and achieve significant time dilation? Will humans be forever confined to the solar system because it's too expensive and impossible from a practical stand point to use antimatter or huge lasers to reach such high subluminal velocities?