A little over 80 years ago, humanity first began broadcasting radio and television signals with sufficient strength to leave the Earth's atmosphere and move deeper into interstellar space. If someone living in a distant star system is vigilant about these signals, he will not only be able to catch them, but will immediately identify their sender as an intelligent species. In 1960, Frank Drake pioneered the search for such signals from other stellar systems using large radio dishes, leading to the SETI initiative: the search for extraterrestrial intelligence. But over the past half century, we have developed far more effective communications for the entire globe than radio and television signals. Does this mean that searching for aliens in the electromagnetic spectrum no longer makes sense?
This question, of course, is extraordinarily speculative, but it gives us the opportunity to look at our own technological progress and consider how it might take place elsewhere in the universe. In the end, if someone from a society in which the signals of drums and fires communicate, finds himself deep in the forest, he may come to the conclusion that intelligent life does not exist around. But give him a phone and he can contact relatives. Our conclusions can be as biased as the methods we use.
The mechanism of electricity began to be understood only in the late 18th century, thanks to the work of Ben Franklin. The power of electricity began to power our wires and other devices only in the 19th century, and the phenomena of classical electromagnetism began to be understood only in the second half of this century. The first transmissions of electromagnetic signals took place only in 1895, and radio broadcasting took us into the interstellar medium only by the 1930s.
The speed of light is also very limited: if our radio signals fly through interstellar space for only 80 years, this means that only civilizations within a radius of 80 light years can pick up these signals and only civilizations within a radius of 40 light years can pick up the signal and send back a response that would have come today. If the Fermi paradox poses the question "where is everyone?", The answer is "not within 40 light years of us." But what can this say about intelligent life in the Universe? Never mind.
Although our galaxy may contain hundreds of billions of stars and about two trillion galaxies in the observable universe, there are fewer than 1,000 stars within 40 light-years of Earth.
In addition, the electromagnetic signals traveling from the Earth into interstellar space decrease, rather than increase. Increasingly, television and radio broadcasting signals are carried over cables or transmitted via satellite rather than broadcasting towers on Earth. A century will pass, and, most likely, the signals that we have sent throughout the 20th century will stop leaving the Earth altogether. Perhaps an alien civilization will conclude that this blue, watery planet with life has reached a certain stage of development, and then it was destroyed, and the signals stopped sending.
In other words, to draw conclusions about what is and what is not, according to a certain form of the electromagnetic signal is a completely wrong strategy.
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If we were to look at the Earth up close in visible light, we would doubtless assume that it is inhabited: the glow of cities at night is an unmistakable sign of activity. But this light pollution is a relatively new phenomenon. We are constantly learning and investing money, effort and time to get rid of it. There is no reason to believe that by the end of the 21st and 22nd centuries the Earth will look the same as it is now, and not the way it looked billions of years before: dark, in places illuminated by auroras, thunderstorms or volcanoes.
But if you are looking for non-electromagnetic signals, then what? Everything in the Universe is limited by the speed of light, and any signal created on another planet must somehow manifest itself so that we can notice it. These signals fall into four categories:
- Electromagnetic signals, including any form of light of any wavelength that might indicate the presence of intelligent life
- Gravitational wave signals, which - if belonging to intelligent life - we can detect with sufficiently sensitive equipment from anywhere in the Universe
- Neutrino signals - which, although extremely scattered over long distances - could be an unmistakable indicator under certain conditions
- Finally, macroscopic space probes, robotic, computerized, autonomous or inhabited, that approach the Earth
Surprisingly, our fantastic imaginations are focused almost exclusively on the fourth possibility, which is the least likely.
When you think about the enormous distances between the stars, how many stars have potentially habitable planets (or even satellites), and how many resources are needed to physically send a space probe from one planet to another planet, another star, this method of communication seems completely insane. … It is much easier to build a detector that could explore different regions of the sky and find signals that would absolutely indicate the existence of intelligent life.
In terms of the electromagnetic spectrum, we know how our living world reacts to the seasons. In winter and summer, our planet "glows" in different ways. Along with the change of times, the colors in different parts of our planet also change. With a sufficiently large telescope (or an array of telescopes), one could discern individual signs of our civilization: cities, satellites, airplanes, and so on. But perhaps the best thing we could find is changes in the natural environment, consistent with what only an intelligent civilization would create.
We haven't done that yet, but perhaps large-scale modifications to the planet are what we should look for. Remember, the civilization we find is unlikely to be a technological baby like us. If she survived and survived all the disasters, she will be tens or hundreds of thousands of years older and more advanced than us. Just remember how we were just 200 years ago.
Perhaps as our gravitational wave technology becomes advanced enough to detect the first signals from the universe, we will begin to discover more subtle manifestations of activity in space. Perhaps we can identify a planet with tens of thousands of satellites in orbit from its unique gravitational wave imprint. This area is very young now, so it has a long way to go. But these signals do not disappear the way electromagnetic signals do, and there is no way to hide them. Perhaps in a hundred or two years this will be our main instrument for space exploration.
But there is another option. What source of energy will a sufficiently developed civilization use? Maybe nuclear. More likely, it will be fusion energy, a special type of it that is different from what flows in the cores of stars, and emits a very, very specific neutrino signature as a by-product. And these neutrinos will directly indicate that energy is born not in a natural, but in a technogenic process.
If we can predict what the signature is, understand it, build a detector for it and measure it, we can find a fusion civilization anywhere, and we don’t have to worry if it is transmitting radio signals or not. As long as it generates energy, we can find it.