Have aliens visited earth?

[wacki]

[ QUOTE ]
There are probably 100,000,000,000 galaxies, each containing an average of 100,000,000,000 stars. Yet a cathedral with three grains of sand in it is more densely packed than the universe.

I'll let others do the math.

[/ QUOTE ]

* For 1,000,000 civilizations in the galaxy
o the average distance between them will be ~ 150 ly!!!
o two-way communication will take at least 300 years!
* If the lifetime of a technical civilization is less than 3000 years
o Average distance is so large that civilizations will die, on average, before two-way communications can be established!

http://instruct1.cit.cornell.edu/cou...o101/lec35.htm