Interstellar archaeology: the search for non-terrestrial artifacts

In the summer of 1950, Enrico Fermi had a lunchtime discussion at Los Alamos National Laboratory about extraterrestrial technological civilizations, during which he asked, “where is everybody?” In retrospect, this appears to be a naïve proposition for a scientist, similar to asking “where are our neighbors?” without looking out the windows or venturing to the backyard. Surely, we must first construct telescopes that engage in the search for interstellar objects from outside the Solar system before concluding anything.

We now know, based on Kepler satellite data, that a substantial fraction of all Sun-like stars hosts an Earth-size planet roughly at the same separation. Since most of these stars formed billions of years before the Sun, the dice of “intelligent life as we know it” was rolled tens of billions of times, in the Milky Way alone, long before we came to exist. There are trillions of galaxies in the observable volume of the Universe, all the way back to the cosmic dawn when the first galaxies formed. The exquisite level of uniformity of the cosmic microwave background informs us that there is no “cliff” out to 4,000 times the size of our cosmic horizon – implying there should be at least 64 billion times more galaxies beyond the trillions we can in principle see with our telescopes.