Joel Achenbach is probably the best writer at the Washington Post, and today he presents a fascinating interview with physicist Brian Greene (no relation, I hope, to Brian Austen Greene of Beverly Hills 90210 fame, and, boy, do I need to stop watching F/X before coming to campus):

His reputation as a leader in the field of "string theory" must inevitably compete with his reputation as a good-looking guy who's comfortable on television. He recently hosted, with boyish enthusiasm, the three-part PBS series "The Elegant Universe," a madcap romp through the funhouse of modern physics, based on his bestseller by the same name. Invariably Greene is compared to the late Carl Sagan, for like Sagan, Greene makes science sound like the coolest thing humankind ever invented. There's an unspoken refrain of, Check this out, this will really blow your mind . . .

His new book focuses in particular on time, the most baffling dimension of them all. Length, breadth, depth -- no one's got a real problem with those. Time is the brain-boggler.

"I do believe in time," the scientist says. "I just think our intuition about it is wrong."

Just as we hit 18th Street, and seem to be heading into the Starbucks across the street, we take a sharp turn to the right, northward. Or perhaps we also go straight. And left. And stop completely. And return backward. Such multiplicity would be possible under the "many worlds" scenario of quantum mechanics, the hypothesis that there is no singular reality, that not only do subatomic particles exist in probability waves, but all their possible trajectories come true, causing the universe to splinter continually into parallel universes.

Greene finds that theory too extravagant, and in any case, in the version of reality that culminates in this story, we go to the right.

We head up 18th, up Mass. Ave. to Dupont Circle, down 19th. Greene doesn't do much sightseeing -- he's trying to explain that the past is as real as the present.

"I don't think that the past is gone. I think the past feels gone," Greene says. "There you were at a party, New Year's Eve, you were experiencing that moment. I would say you are still experiencing that moment."

Greene isn't just saying that somewhere on some distant place in the universe, an astronomer can see the light finally arriving from some event in our past. That's not controversial. That's simply speed-of-light stuff. When we see the Andromeda galaxy, we're looking about 2 million years into the past, because it takes that long for the light to reach us across the enormous distances of space.

Greene's point is more radical: That there is no such thing as "now." That just as there is no center of the universe, there is no location in the "loaf" of spacetime that's more special than any other . This is an implication of Einstein's theory of relativity (his "special" theory, if you can stand the irony).

"This is really a question of what's real. You're saying what's real to me is 'now' " -- the former judo competitor is hacking at the air in a fashion that might alarm other pedestrians -- "but she" -- a woman walking by -- "would slice through the spacetime continuum at a different angle."

This article will a) blow your mind and b) make you want to stay very, very still for a while.