Best not to book those ringside seats for the French Revolution — or any ones they may have coming up. While Einstein proved we could theoretically travel forward through time by traveling at the speed of light, the feat is near impossible; meanwhile, moving backwards is even more complex. And while those facts frustrate scientists, they're also comforting. Do we really want other eras to be accessible, and even malleable? Anyone who's seen "Back to the Future" knows the answer to that question.