>> 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, futurist, physicist, beth selling author. his best new book "physics of the impossible: a scientific exploration into the world of phasers, force fields, teleportation and time travel." seth shostak, the senior adviser of the seti institute. david brin, astronomer and futurist, and our friend the well-known actor dan aykroyd who believes that alien life exists. we wanted to hear directly from the renowned british scientist, former guest on this program stephen hawking about his controversial claims. so i sent him some questions.