British astrophysicist Stephen Hawking believes that extraterrestrial life exists, but feels it may be dangerous for us to contact them or let them know we are here.

"I imagine they might exist in massive ships, having used up all the resources from their home planet. Such advanced aliens would perhaps become nomads, looking to conquer and colonise whatever planets they can reach. It would be 'too risky' to attempt to make contact with alien races. If aliens ever visit us, I think the outcome would be much as when Christopher Columbus first landed in America, which didnt turn out very well for the Native Americans."

Maybe Prof. Hawking has recently seen the 1996 movie "Independence Day". In that story, alien nomads arrive in a huge mothership (about a quarter the mass of the moon) and begin an attack to conquer the Earth for its resources.

Of course, it may be too late about that aspect of keeping quiet and not letting anyone know we're here...
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