In theories of quantum mechanics, time is basically taken for granted; it simply ticks away in the background, just as it does in life. Like a clock at a sports game, it provides an invisible framework in which events unfold. That's not the case in Einstein's general theory of relativity. To describe the universe on the largest scale, Einstein had to weave time and space together into the very fabric of the universe. As a result, in general relativity, there is no invisible framework, no clock ticking outside the universe against which to measure events.