[I]f our solar system is not unusual, then there are so many planets in the universe that, for example, they outnumber the sum of all sounds and words ever uttered by every human who has ever lived. To declare that Earth must be the only planet with life in the universe would be inexcusably bigheaded of us.
Death By Black Hole, p. 229
Regarding whether aliens tried to contact us before we had radio:
For all we know, the aliens have already done this and unwittingly concluded that there was no intelligent life on Earth. They would now be looking elsewhere. A more humbling possibility would be if aliens had become aware of the technologically proficient species that now inhabits Earth, yet they had drawn the same conclusion.
Death By Black Hole, p. 235
FM signals and those of broadcast television...[travel] out to space at the speed of light. Any eavesdropping alien civilization will know all about our TV programs (probably a bad thing), will hear all our FM music (probably a good thing), and know nothing of the politics of AM talk-show hosts (probably a safe thing).
Death By Black Hole, p. 172
Regarding radio signals that have been sent out into space in order to contact alien life:
In any case, the leading edge of our "on purpose" radio signals is 30 light-years away and, if intercepted, may mend the aliens' image of us based on the radio bubble of our television shows. But this will happen only if the aliens can somehow determine which type of signal comes closer to the truth of who we are, and what our cosmic identity deserves to be.
Death By Black Hole, p. 245
My only hope is that every other [alien] civilization isn't doing exactly what we are doing because then everybody would be listening, nobody would be receiving, and we would collectively conclude that there is no other intelligent life in the universe.
Death By Black Hole, p. 237
I'd bet almost anything that life from another planet, if formed independently from life on Earth, would be more different from all species of Earth life than any two species of Earth life are from each other.
Death By Black Hole, p. 306
[I]f an alien lands on your front lawn and extends an appendage as a gesture of greeting, before you get friendly, toss it an eightball. If the appendage explodes, then the alien was probably made of antimatter. If not, then you can proceed to take it to your leader.
Death By Black Hole, p. 107
On how to prove an alien abduction:
Grab something off the shelf that's on the spaceship—an ashtray, it doesn't matter what. Because I can tell you, if they flew here from another galaxy, no matter what you've pulled off the shelf, it'll be unlike anything we have on Earth.
The Amazing Meeting, Keynote Speech, 2008