According to Einstein's special theory of relativity, the speed of light (c) represents an insurmountable barrier to any object that has real mass (leaving only the elusive tachyon possibly exempt).
No ordinary material thing can be accelerated from sub-light speeds up to the speed of light or beyond, the theory says, for two reasons. First, pumping more kinetic energy into an object that is already moving at high speed has the main effect of causing a relativistic mass increase rather than a substantial increase in speed. This strange phenomenon becomes so pronounced that at speeds sufficiently close to the speed of light, an object's relativistic mass would become so great that for it to approach still nearer to c would take more energy than is available in the entire universe. Second, faster-than-light (FTL) speeds would lead to violations of the fundamental principle of special relativity, which is that all inertial reference frames are equivalent. In particular, FTL communication would enable simultaneity tests to be carried out on the readings of separated clocks which would reveal a preferred reference frame in the universe – a result in conflict with the special theory.
Not all is lost, however, for there is the general theory of relativity to consider. General relativity doesn't rule out faster-than-light travel or communication, but only requires that the local restrictions of special relativity apply. In other words, although the speed of light is still upheld as a local speed limit, the broader considerations of general relativity suggest ways around this statute. One example is the expansion of the universe itself. As the universe expands, new space is created between any two separated objects. Consequently, the objects may be at rest with respect to their local environment and with respect to the cosmic background radiation, but the distance between them may grow at a rate greater than the speed of light. Other possibilities, more directly relevant to interstellar travel, include wormhole transportation and the Alcubierre warp drive.