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If time machines were possible, argues science-fiction writer Arthur C. Clarke, then somebody somewhere would have built one by now. Plus, if such contraptions existed, we’d have been overrun by tourists and historians from the 28th century long ago. In today’s “Science Fact & Fiction” lecture, the audience can pose such theories to Dr. PaulNahin, who’s explored such philosophical and scientific conundrums in Time Machines: Time Travel in Physics, Metaphysics, and Science Fiction. Nahin appears on a double bill with Dr. Leroy Dubcek, author of Voyages: Learning Science Through Science Fiction at 7:30 p.m. at the Air & Space Museum’s Einstein Planetarium, 6th & Independence Ave. SW. FREE. (202) 357-2700. (Greg Kitsock)