(01-04-2018, 12:03 PM)The Eleventh Doctor Wrote: First, you have to realize just how big the universe is.  Take all the grains of sand on the beach.  Now consider your chances of finding a specific grain of sand -- without knowing which grain of sand you're looking for.  You could spend years, decades, even centuries just looking in the wrong area.  

Then you have to realize that on the off chance some alien civilization discovered earth--that our government has been able to keep it quiet.  And not just our government, but potentially other governments as well.  Discovery of Alien life would probably propel a nation up in the international standings.  And it could destabilize governments who have been hiding that information.  I mean alien life forms have just as much the chances of landing in the United States as they do... anywhere else in the world.  Iraq.  Iran.  North Korea.  Pakistan.  China.  India.  Anywhere in Europe, including Russia and anywhere in Africa.    So this wouldn't just require a national conspiracy, but an international conspiracy.  

Even then, there's always the risk some alien life form crashes in the middle of a city.  Accidents happen.  And even if there were rules regarding contact with less technologically advances civilizations, you'd always have people breaking those rules.  Which is too big of a risk for a global conspiracy.

They'd also need some way of disusing themselves as humans  OR actually look like us (which is an astronomically low probability itself)

And still you risk some rogue scientist from some nation deciding a Nobel Prize and putting his name in the history books is totally worth the risk to expose the already giant conspiracy.

It's not impossible that alien life lives among us.  But it's astronomically low that a significantly technologically advanced alien race discovered our planet, was covered up by our world's governments, never exposed, could perfectly disguise themselves as humans, have a rule against visiting insignificantly technologically advanced civilizations that is adhered to at an absurd level, who can disguise themselves as humans, and never once had an accident.