What are the past, present and future?

Alexander Waugh in the Wall Street Journal:

PT-AN459_books__DV_20100107191822 Sean Carroll is a formidable theoretical physicist from the California Institute of Technology, and “From Eternity to Here” is his first work of popular science. He outlines, in the simplest possible terms, all that is known about the arrow of time. That is to say, all that we think we know about the arrow of time, for Mr. Carroll's greatest virtue, aside from the clarity of his prose—an absolute “must” when dealing with matters as complex and counterintuitive as quantum gravity, black holes, tachyons and dark energy—is his honesty in delineating precisely what is known, what is unknown, what is subjective, what is hypothetical and what is purely theoretical.

Many popular-science writers try, to their discredit, to blur these lines, usually out of simple fear of revealing the depths of man's (and thereby their own) ignorance. But Mr. Carroll is not afraid to leave his readers with a general impression that practically nothing is known or properly understood about time, space, our universe or its place relative to anything outside of it. This is one of his book's greatest strengths, for only by admitting to our own lack of understanding can we find the pluck to ask the simplest questions. “Why does the arrow of time flow from the past toward the future—why not the other way round?” The question seems trivial, even a trifle babyish, but Mr. Carroll keeps on asking it and, in his brave attempt to provide a full and coherent answer, takes his readers on a fascinating and refreshing trek through every known back alley and cul de sac of quantum mechanics, relativity, cosmology and theoretical physics.

More here.