We all understand the concept of not wasting time. “But what is time? To paraphrase Justice Potter Stewart, we know it when we see it — but certainly, a few years into the 21st century, our understanding of time must be deeper than that. By now, you’d think, science must have figured out why time seems to flow, why it always goes in one direction and why we are uniformly drawn from one second to the next. The fact is, though, the explanations for these basic features of time remain controversial. And the more physicists have searched for definitive answers, the more our everyday conception of time appears illusory.”