Stephen Hawking might be a brilliant mathematician, he might understand the cosmos enough to be able to visualise what goes on inside a black hole but the evidence of todays announcement shows that he shouldn't be allowed to give an opinion without some idiot filters in place.
His declaration of the existence of alien life and it's possible or probable hostility is so incredibly irresponsible that it beggars belief that someone hasn't pulled the plug on his voice box!.
Firstly, the world is full of people who will be gullible enough to listen to this rubbish. The press has splurged it in the papers and on the web and to anyone who might take Hawking as an authority on such matters could imagine that we are in imminent danger of invasion from an awful alien menace bent on stealing our metals and impregnating our women.
Secondly, there are so many reasons why what he suggests is complete hogwash that anyone with a modicum of sense would see that this is nothing more than the ramblings of a chap who doesn't get out enough!
Just ask yourself this. If dangerous alien races are so common and interstellar travel is so easy why are we not overrun by aliens? The fact is that its really really hard to travel from star to star and no one would do it without vast incentives.
This leads us to what might incentivise aliens to come all the way here to rob us. What could they want? Water?, air? metal?, oils and hydrocarbons?, diamonds?, gold?
All of these things are available in truly vast quantities right out there in space. What's more, they are cheaper to obtain if they dont have to mine the stuff and drag it up out of a gravity well too!
When you cut to the chase and think of the only things that aliens might want from us they boil down to two things. Creativity and manpower.
They won't need our knowledge of physics because they would already know more than us just by the fact that they could get here. The only other creativity would be our literature and art which might hold a great price on the interstellar market.
If they only want us for our manpower then that too would be a good thing because the human race needs to break the bounds of the earth just to get a sniff at the aforementioned vast resources that are out there for the taking but as idiot governments run by idiot risk averse lawyers are shutting down space programs right and left these days then its likely that the best we can hope for is to hitch a ride with the Vogons. If they stole the whole human race to use as slaves we would eventually escape in small numbers and breed enough to fill the galaxy on the back of their technology. BRING IT ON!!