The Drake equation quantifies our chances of detecting a signal from an advanced civilization in space. However, it misses a crucial possibility: most technological civilizations that ever existed might be dead by now.
There are two obvious reasons to suspect that this might be the case. First, as soon as we developed advanced technologies, we also developed the means for our own destruction through catastrophic nuclear, biological or chemical wars, or through a global change of the Earth’s habitat. Second, thanks to the Kepler satellite data we now know that more than a quarter of all stars host a habitable, Earth-like planet. This naturally raises a paradox. In 1950, the physicist Enrico Fermi went to lunch with his research group where they discussed the high likelihood that we might not be alone, and asked: “where is everybody?” The simplest answer is: “dead”.
But this does not mean that we cannot prove they existed. On Earth, we find evidence for advanced ancient civilization that are not around anymore, like the Mayans, by the artifacts they left behind. Similar to the work of archaeologists who dig into the ground, we can search for advanced technological civilizations by digging into space. This constitutes “space archaeology”. What should we expect to find?
It is prudent to start with our back yard, which is nearby and easier to reach. The first target would be technological equipment floating through the Solar system, since within the first century of our technological revolution we already sent Voyager 1 and 2 and New Horizons spacecraft out of the Solar System.
The simplest way to find alien equipment is through its reflection of sunlight. As it turns out, the first interstellar object that originated from outside the Solar System and was detected near the Earth is
Oumuamua. This 100 meter size object showed weird properties, such as an extreme axis ratio, an excess push without a cometary tail, a high reflectance and an origin in the rest frame of the Milky Way. The Vera Rubin Observatory would be far more sensitive to detecting interstellar objects than the Pan STARRS telescope that discovered Oumuamua. It could find a new interstellar object every month. We can take a close-up photograph of all interstellar objects to find out whether they are artificial or natural in origin.
Another approach is to search for artificial meteors, namely technological equipment that collides with the Earth at a high speed, indicated that it was unbound to the Sun, and is detected as it burns up in the Earth atmosphere. If the object is bigger than a few meters, it could leave a debris meteorite, providing the best opportunity for us to put our hands around alien equipment. Finally, we can search the surface of the Moon for an interstellar technological debris that crashed on it. To find traces of technological equipment that crashed on the lunar surface a billion years ago with a letter from an alien civilization saying “we exist”. Without checking our mailbox, we would never know that such a message arrived.
In the above examples, both the Moon or the Earth serve as fishing nets. But Jupiter and the Sun could potentially serve as a gravitational fishing net which traps interstellar objects.
Extending the search to the outskirts of the Solar System, one can search for artificial lights that originate from powerful spacecrafts. A city like Tokyo could potentially be detected with the Hubble Space Telescope out to the Kuiper belt. An artificial light source can be distinguished from an object reflecting sunlight by the way it dims as it recedes from us. An artificial source of light will dim like a light bulb inversely with distance squared whereas reflected sunlight will dim inversely with distance to the fourth power.
Beyond the Solar system one could search for artificial light or heat redistribution on the surface of a planet. The nearest star to the Sun is the dwarf star, Proxima Centauri, whose mass is only 12 percent of that of the Sun. The habitable zone around this faint star is twenty times closer than the Earth-Sun separation. As it turns out, Proxima hosts an Earth-size rocky planet at that distance. But since this planet is so close in, it is tidally locked like the Moon is to the Earth, and so it faces the star with the same side at all times. The permanent dayside is hot and illuminated whereas the permanent nightside is cold and dark. An advanced civilization would attempt to cover the dayside with photo-voltaic cells that would generate electricity to light up and warm up the night side. As the planet moves around the star, the varying level of emitted light from its surface could inform us whether a global engineering project of this type took place on it. We could also search for the unusual reflectance and color expected from solar cells on the dayside.
But terraforming may have other consequences such as industrial pollution. Our archaeological dig could include a search for artificial molecules, such as CFCs, that outlive the industries that produced them. Such molecules as well as any other megastructures may survive long after the civilization that produced them died.
At even greater distances, we could search for light beams sweeping across the sky at frequencies ranging from the radio to X-rays. Such beams could be used for communication or propulsion. In particular, propulsion systems using the technology of light sails, would inevitably produce short transients as the light beams leak across the boundary of the sails. Whereas radio frequencies are ideal for transport of massive cargos at modest speeds between nearby planets such as Earth and Mars, infrared or optical lasers are ideal for acceleration of lightweight probes near the speed of light.
In addition, one could search for a swarm of satellites or megastructures that block a significant fraction of the light from distant stars, as envisioned by Freeman Dyson in his concept of “Dyson Spheres”. However, such a gigantic engineering project may be rare or non-existent as it faces many engineering challenges.
If we recover anything artificial through our archaeological dig into space, the natural question to ask is: “are we the smartest kids on the block?” If not, we can learn a lot from what we find and short cut our own evolution by thousands, millions or maybe even billions of years.
When reading the morning newspaper, it is difficult to avoid the thought that our intelligence bar is not particularly high and difficult to surpass. We fight among ourselves in “lose-lose” situations, we do not promote long-term benefits in favor of short-term manipulations and we have been broadcasting our existence to the Galaxy in radio waves for over a century without worrying whether there are any predators in outer space. One even wonders whether we had been ignored by predators because we appear so incompetent. But as far as we are concerned, the key challenge to improving our awareness of other civilizations is whether we are intelligent enough to adequately interpret their signals or a piece of their technological equipment. One fact is clear. If we assign a zero prior probability for evidence coming our way, as some scientists did in the case of `Oumuamua by stating “it’s never aliens”, we will indeed never find any evidence for aliens.
How can our civilization mature? The same way kids do, by leaving home into the neighborhood, meeting others and comparing notes with them. In other words, we can develop a balanced perspective on our current technological accomplishments by searching for relics of extraterrestrial intelligence. Since our own technological development accelerates exponentially with an e-folding time of a few years, it is difficult to imagine the face of a much more advanced technology crafted by a civilization that had lived for a cosmic timescale, lasting billions of such e-folding times.
Discovering a piece of advanced technological equipment that was developed by an extraterrestrial intelligence may resemble an imaginary encounter of ancient cave people with a modern cell phone. At first, they would interpret the phone as a shiny rock without realizing that it is a communication device. One may therefore wonder whether we are able to recognize technologies that were not already developed by us. After all, these technologies might feature subtle purposes - like the cell phone communication signals which a cave person would miss.