In 1950, the renowned Italian-American physicist Enrico Fermi discussed the issue of UFOs and extraterrestrial life with colleagues after lunch. He suddenly posed a question: If there are extraterrestrial civilizations, where are they?
In fact, humanity is continuously searching for extraterrestrial civilizations through various means, whether by observing with optical telescopes or listening with radio telescopes. The results are consistently the same: no evidence of the existence of extraterrestrial life.
A research team at the University of Nottingham estimates that there are 36 active civilizations in our galaxy.
At Least 36 Civilizations Exist in the Milky Way
Are humans truly the only advanced intelligent life forms in the universe? This notion seems inconsistent with common sense, as scientists have calculated that, based on certain probabilities, extraterrestrial life could still exist in the universe.
Modern cosmology tells us that the age of the universe is approximately 13.8 billion years. Throughout this long history, the universe’s space has been continuously expanding. The observable diameter of the universe has reached about 28.5 billion parsecs (93 billion light-years), and it is estimated that there are around 2 trillion galaxies in the observable universe.
Among them, there are 200 billion galaxies similar to the Milky Way, and each galaxy contains about 200 billion stars similar to our Sun.
Although the specific conditions of each star system differ, on average, each star has at least five rocky planets with a mass no less than Mercury orbiting around it. With this data, the existence of habitable rocky planets is a necessary condition for the emergence of advanced civilizations, leading to the conclusion that there are at least 2 million trillion rocky planets throughout the universe.
Researchers estimate that the average distance between civilizations is around 17,000 light-years.
Of course, having a rocky planet is not sufficient; a suitable environment is also necessary for life to thrive. Taking our Solar System as a reference, each star system must have a habitable zone where the distance between the planet and its star is just right for surface temperatures to range between -50°C and 50°C. If the star has a larger mass and higher temperature, the habitable zone will be farther away from the star, and vice versa.
We can assume that in each star system, there is at least one rocky planet located within the habitable zone. After this screening, there remain at least 400 trillion habitable planets in the universe, with environments differing from that of Earth.
At this point, the objective conditions are basically satisfied, but this does not mean that there must necessarily be life in a suitable natural environment. In fact, the emergence of life on Earth can be viewed as a random event with a very low probability.
However, despite this low probability, it is not impossible. Therefore, given the vast number of planets in the universe, such low-probability events will also occur in large quantities.
According to Christopher Conselice, the lead researcher, the team utilized various assumptions and limitations regarding the development of life on planets, similar to Earth, to analyze the potential emergence of intelligent civilizations in the Milky Way.
According to a study from the University of Nottingham, Christopher Conselice, an astrophysics professor, and his team developed the Copernican principle with limitations regarding life in the galaxy. They considered the star formation history of the Milky Way, the age and metallicity of stars, and the likelihood of a star having an Earth-like planet within the habitable zone.
Based on existing data, researchers constructed a corresponding computer model and found that after several rounds of filtering, the Milky Way has at least 36 civilizations currently existing, with a maximum of 211 civilizations, and their civilizations are fundamentally much more advanced than humans. The researchers calculated that the average distance between these civilizations is approximately 17,000 light-years.
According to Christopher Conselice, these civilizations could be as far as 17,000 light-years from Earth, making it currently very difficult to connect with one another.
But Why Can’t We Find Civilizations Outside Our Universe?
There seem to be only two hypotheses that can reasonably explain this strange phenomenon. The first is that the universe is still very young, and the second is that all civilizations are limited by the speed of light.
It has been 13.8 billion years since the universe came into being, but stars did not exist from the beginning; they gradually appeared after the Big Bang. The early universe had a high density of matter, and the first stars were also very massive, along with temperatures much higher than today.
Due to their large size and high temperature, their internal nuclear fusion rates were also rapid, resulting in shorter lifespans, often only a few tens of millions of years. When these stars died, they created supernova explosions.
During this process, heavy elements such as phosphorus, calcium, and iron were produced, which are essential materials for the formation of rocky planets.
Long-lived stars with moderate mass were essentially born about 5 billion years ago, allowing rocky planets around them to develop stably. For early life, this process would take about 2 billion years, and it would take another 4 billion years to evolve into advanced intelligent life.
According to this hypothesis, the universe is still in the early stages of the emergence of intelligent life, and humans are the first instance of intelligent life.
The Copernican principle limits that intelligent life forms develop within less than 5 billion years, or after about 5 billion years—similar to Earth, where a communicative civilization formed after 4.5 billion years.
If the first hypothesis suggests that other advanced civilizations in the universe have yet to emerge due to timing issues, the second hypothesis is somewhat more hopeless.
The speed of light is the fastest speed known to humanity in the universe, thus making it our ideal speed. However, Einstein’s general relativity tells us that as an object’s speed approaches the speed of light, its moving mass will approach infinity, and the energy required to maintain that speed will also be nearly infinite. Therefore, only objects with zero rest mass can achieve the speed of light, which is clearly impossible.
Even if one day we achieve the speed of light, the vastness of the universe will still present insurmountable distances. The stars in the night sky appear densely packed, but in reality, they are very far apart. The distance from our Solar System to the nearest star, Alpha Centauri, is 4.2 light-years; even if we travel at the speed of light, it would take nearly 10 years to go back and forth, a duration that is not short for a human lifetime.
Currently, humanity has only been generating signals like radio from satellites for a short period, and our “technological” civilization is only about 100 years old.
According to research from the University of Nottingham, the closest civilization to our Solar System may be 17,000 light-years away. If we were to travel at the speed of light, it would take 17,000 years to reach there—a duration that is likely a figure that all intelligent civilizations cannot surpass.
