The Italian physicist Enrico Fermi (1901–1954) was once asked if intelligent extraterrestrial life exists. Fermi replied, “Where are they?” The so-called Fermi Paradox 3 about extraterrestrial intelligence can be simply summarized. If technological advancement on Earth follows its current trajectory, in a few centuries or millennia we will be an interstellar space-faring species. After that point in human history, even if it took our spaceships a century to get to the nearest stars, humans could populate the entire galaxy in about ten million years. Since the Milky Way has been forming stars for about ten billion years—a thousand times that long—a human-like civilization would develop into such an advanced civilization very quickly compared to the age of the galaxy. If even one such advanced civilization existed in our galaxy, then evidence of its existence should be abundant in our astronomical observations. Since such evidence has not been observed, it is reasonable to think that such a civilization does not exist.
The Voyager spacecraft carry gold records with sound recordings from Earth that, one day, might be listened to by alien ears. (NASA)