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, having reached technological development it will inevitably have to emit radio waves into space in the form of TV, radio, data signals to and from satellites in orbit, etc. This is because radio waves are the simplest way to communicate. Other forms such as light or laser are in fact usable only as point-to-point communication while radio waves can transmit in broadcast. Moreover, if the planet has an atmosphere, the radio waves bounce off the ionosphere and are also received on the other side of the planet, contrary to what happens with light which instead "escapes" into space. Isaac Asimov in his capacity as scientist-popularizer in 1979 wrote the book "Extraterrestrial Civilizations" in which he estimated that only in our galaxy (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 drew up the famous Drake Equation with which he estimated that at least 600 planets in our galaxy should host technologically advanced civilizations.
Now then the question: "If the universe is teeming with aliens, where are all of them?". 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 the following I would therefore like to expose my theory which I have called "Radio Bubble Hypothesis" precisely as a solution to the Fermi paradox. Let's assume that an alien civilization reaches technological development, as has already happened with the human race today, and then begins to spread radio waves in space. In our case the first radio station dates back to 1920, so we have been broadcasting in 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 with a diameter of 180 light years (90 + 90) around our planet. A hypothetical listener with a radar antenna who was 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 be extinguished by natural disasters such as an asteroid impact, by self-destruction due to the misuse of technology, such as runaway nanotechnology, malicious artificial intelligence, by an accident in a particle accelerator or by other disasters created by technologies of the future. 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 1000 light-year wavefront actually contains information, the rest of 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 consideration the development of four intelligent civilizations in our galaxy. [A] is us in 2920, while [B], [C] and [D] are alien civilizations. The [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 virtually 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 gone extinct and therefore there will be no one who can listen to the radio broadcasts of aliens [C]. After 15.000 years the radio front of bubble [C] will also have reached our Earth at the center of bubble [A] but even here, as we were 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 come to develop technology are extinguished relatively quickly after reaching the "peak of intelligence".
2. Once the radio wave front of a technological civilization reaches the home planet of a neighboring civilization, it will not be able to listen as it will have gone extinct millennia earlier.
There are indeed three counter-deductions to this hypothesis. The first is that technologically advanced civilizations, at some point in their development, may begin transmitting with more advanced systems than radio waves (e.g. quantum entanglement waves or the like). The second is that we are the only intelligent inhabitants of our galaxy and therefore do not receive transmissions. The third is that technological civilizations at some point evolve into ultra-intelligent life forms that will have already discovered all the secrets of the universe and therefore will no longer need to transmit anything outside their home planet.
Of the three the most interesting seems to me the second 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