To think that the physical universe starts and stops on Einstein's dime is folly. Once his relativity theories came out, that's it? The exploration of the universe must now die for eternity? Such a short-sighted vision. (Advanced propulsion means nothing if we adhere to sub-FTL limitations.) Free your mind.
Never say never. Human understanding of the universe around us changes and grows with each passing year. What we understand now could be totally different from what we learn a hundred years from now.
i.e ion thrusters. Also take a look at quantum computing. Sure it's not necessarily relevant to this research but quantum physics started from the study of atomic energy and studies of outer space radiation.
I am pretty fascinated by quantum computing, actually. In on itself such a computer would be of immense help in any kind of physics research, due to its immense computational capabilities.