The question referred to the fact that it seemed strange to him that we did not receive any extraterrestrial transmission from space. If it is true that there are millions of planets out there similar to ours and if at least a small percentage of them have developed intelligent life, why do we not receive any radio transmission?
This inconsistency was later called the “Fermi Paradox”. It is clear that if intelligent life develops on a remote planet, once technological development has been reached, it will necessarily have to emit radio waves into space in the form of TV and radio transmissions, data signals to and from orbiting satellites, etc. This is because radio waves are the easiest way to communicate. Other forms such as light or laser can actually only be used as point-to-point communication while radio waves can transmit in broadcast. Furthermore, if the planet has an atmosphere, the radio waves bounce off the ionosphere and are also received from the other side of the planet, contrary to what happens with light which instead "escapes" into space. Isaac Asimov in his role as a scientist-popularizer back in 1979 wrote the book "Extraterrestrial Civilizations" in which he estimated that in our galaxy alone (which has about 75 billion stars similar to the Sun) there should be at least 650 million planets compatible with the life. Later the physicist Frank Drake wrote the famous Drake Equation with which he estimated that at least 600 planets in our galaxy should host technologically advanced civilizations.
So now the question: “If the universe is teeming with aliens, where are they all?” This is also the title of a recent book by the physicist Stephen Webb who, in spite of the humor on the cover, exposes with scientific rigor fifty solutions to the problem of the Fermi paradox and tries to explain why despite the SETI project he probes the sky from the 1974, using myriads of radio telescopes, until now no significant radio communications have been detected by intelligent civilizations.
In what follows I would therefore like to explain my theory which I have called the "Radio Bubble Hypothesis" as a solution to the Fermi paradox. Let's assume that an alien civilization achieves technological development, as has already happened with the human race today, and then begins to spread radio waves into space. In our case the first radio station dates back to 1920, so we have been transmitting into space for about 90 years. This means that we have been radiating radio waves into space for a radius of 90 light years since 1920. So we have created a radio bubble around our planet with a diameter of 180 light years (90+90). A hypothetical listener with a radar antenna 80 light years away would hear our radio broadcasts from 1930.
Let's admit then that a technological civilization lasts an average of a thousand years. It could become extinct due to natural disasters such as an asteroid impact, self-destruction due to the misuse of technology, such as runaway nanotechnology, malevolent artificial intelligence, a particle accelerator accident, or other disasters created by future technologies unimaginable to us. In this case the radio bubble produced would have an extension of a thousand light years (in our case from 1920 to 2920).
But we must consider that a thousand years in relation to the age of a galaxy that has more than 10 billion years is very little (one ten millionth of the total time). This means that when the bubble expands into space only the thin wave front of 1000 light years actually contains information, everything else in the bubble does not. In other words, the information is contained only on the surface of the expanding sphere while the interior is empty. The image below can help you understand what this hypothesis is based on.
It takes into account the development of four intelligent civilizations in our galaxy. [A] is us in 2920, while [B], [C] and [D] are alien civilizations. [C] and [D] are civilizations that appeared 10.000 and 6.000 years ago respectively and therefore their bubbles expanded earlier. However, only the first 1000 light years of these bubbles contain information, the interior is effectively empty. So when the wave front of the bubble [C] reaches the home planet of civilization [B] (it will happen in about 10.000 years) the civilization [B] will have become extinct and therefore there will be no one who can listen to the radio transmissions of aliens [C]. After 15.000 years the radio front of the bubble [C] will have also reached our Earth at the center of the bubble [A] but here too, since we became extinct 14.000 years earlier, there will be no one to listen.
This hypothesis therefore explains why nothing is heard from radio telescopes and the reasons, summarizing, are essentially two:
1. Intelligent civilizations that develop technology become extinct relatively quickly after reaching “peak intelligence.”
2. Once the radio wave front of a technological civilization reaches the home planet of a neighboring civilization, the latter will not be able to listen as it will have gone extinct millennia before.
There are indeed three counter-deductions to this hypothesis. The first is that technologically advanced civilizations, at a certain point in their development, could begin to transmit with more advanced systems than radio waves (for example quantum entanglement waves or similar). The second is that we are the only intelligent inhabitants of our galaxy and therefore we do not receive transmissions. The third is that technological civilizations will at some point evolve into ultra-intelligent life forms that will have already discovered all the secrets of the universe and will therefore no longer need to transmit anything outside their home planet.
Of the three, the second seems to me to be the most interesting and it is precisely on this that Nick Bostrom wrote an article entitled "Where are they?” which you can find translated into Italian on Futurology.it