if civilizations take billions of years to evolve, only to vanish virtually overnight, then sadly, we've next to no chance of hearing from them. they are simply too far away in space and time to reach. but there is one last possibility. that aliens who have avoided destroying themselves are already colonizing the universe. >> larry: we're back. in a new discovery channel documentary, astrophysicist stephen hawking said that communicating with aliens could be a threat to earth. hawking says it's likely that alien life exists, and that a visit from extraterrestrials might be similar to columbus' arrival in the americas. in other words, didn't turn out too well for native americans. joining us are dr. michio kaku, author of "physics of the