Why aren't Aliens Already Here? (Closer to Truth)

source: Closer To Truth    2016年7月18日
It seems absurd in a universe with 100 billion galaxies, each with billions of stars and planets, that we are the only intelligent life. So where are all the others?
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Lawrence Krauss - Why aren't Aliens Already Here? 3:58
Jill Tarter - Why aren't Aliens Already Here? 15:24
Jared Diamond - Why aren't Aliens Already Here? 8:29