Cocktail Party Physics writes up a very nice summary of a panel of some of the world's top physicists discussing time at the recent World Science Festival in New York City, describing the differences between relativistic and quantum versions of time, why it appears to move in only one direction and what might have come before the big bang. Sean Carroll, who has a forthcoming book on the subject, was a panelist and had this to say on that:
Scientists have gotten used to the idea that when people ask us 'What happened before the big bang?' we give St. Augusta's answer: we say there was no such thing as before the big bang. But in very recent times, beyond Einstein, we're realizing that we have absolutely no justification for saying that that's true. We have to more beyond Einstein to understand what happened at the big bang. And the answer might be that the universe can into existence at the big bang: there's nothing before. Or it might not. There could be something before the big bang...cosmologists, people who are working on quantum gravity, are very interested in what we've learned since Einstein to answer these questions and go back and answer St. Augusta's question.
Incidentally, the World Science Festival was co-founded by former IdeaFestival presenter, theoretical physicist and a favorite author of mine, Brian Greene, who has a few words to say on the subject in The Fabric of the Cosmos. I highly recommend it.
Have a wonderful time if you're celebrating the Fourth of July this weekend!