I don't believe this, Stephen Hawking went slightly cracked. There is no evidence that whether aliens exist or not! Then how this intelligent mind so confident to say about aliens that they are dangerous. Mine one is experimental in nature - if aliens rather visit our earth they will not be coming in groups but singly first to see what there is on earth. We too are developed with highly function able war material to give them tit tat reply if they do something wrong. rather if they visit here and interestingly we would be able to capture them, who fantastic it would be, un likely if we sit like dumbs rather doing nothing else. Am against Stephen Hawking.