Friends who often pay attention to the universe should have heard of the "Fermi Paradox". Fermi is a scientist. He once asked this question. The universe is so big, there are hundreds of billions of galaxies and stars, and there are many rocky planets. These planets It is very likely to develop extraterrestrial life, so why have humans not found aliens yet? Or why haven't aliens come to Earth yet?

Many people may say, not to mention the entire universe, the Milky Way is too big for human beings, and even if there are aliens, they can't fly over.

Take the Milky Way as an example. The diameter of the Milky Way is about 200,000 light-years. It is not too big or too small, but the Milky Way is as old as the universe, with a history of more than 10 billion years. If the emergence of civilizations is very average, if a galaxy has once appeared alien civilizations, for example, it should have appeared in billions of years, as long as there is a civilization that has the idea of ​​conquering the galaxy, what will they do?

It is very simple, they will continue to send spaceships to colonize the local area, and then build more spaceships in the local area, in the same way that we humans conquer the earth, we humans do not conquer the earth alone, but generation after generation. One generation, say, conquer Asia, Europe, reproduce there, and then conquer other places.

If a civilization conquers the universe in this way, the result is a viral spread, so fast that you will find that it doesn't take much time for a civilization to take over the entire galaxy.

We can simply calculate it, the diameter of the Milky Way is 200,000 light-years, that is to say, it takes 200,000 years to travel through the entire Milky Way if it travels at the speed of light. % or 1% is still quite possible.

And to reach 1% of the speed of light is really not too difficult in terms of current human technology. Although it cannot be achieved now, it is believed that it will be achieved soon. If a civilization conquers the Milky Way at 1% the speed of light, it will take 20 million years to span the entire Milky Way. 20 million years sounds like a long time, but compared to the Milky Way’s tens of billions of years of history, it is not that long.

And as long as any civilization appeared billions of years ago, there would be enough time to conquer the galaxy, so according to normal logic, the galaxy should have been occupied by a civilization long ago, but why we see the opposite situation , the entire galaxy is almost completely empty.

Also, a civilization probably doesn't need to conquer the galaxy at all. As early as the 1950s, it was proposed that if we humans explore the entire galaxy in the future, we can send a probe. This probe can replicate itself, just like a virus. When the probe reaches a planet, it will use the local Resource replication, which can show exponential growth, can easily occupy the entire galaxy. Therefore, even if the aliens do not fly over in person, they should build some probes like this.

In response to this problem, many scientists have given three explanations.

The first is that humans may be the only unique civilization in the universe, but as we discover more and more new planets recently, fewer and fewer scientists accept this view.

The second explanation sounds a bit scary, saying that there is a modern civilization that rules the entire universe, just like we just said why no aliens were found, just like you found a house in a house without a bacteria. It's as strange. Because it stands to reason that as long as there is one bacteria, it can quickly multiply and spread throughout the room.

So this explanation is that the reason why there are no bacteria is because someone is cleaning. It is possible that our universe has been occupied by a super civilization a long time ago. Things like nature reserves protect some planets like the earth, of course other planets and even stars, and it is naturally difficult for humans in "natural reserves" to see their "masters".

There is also a third explanation. There may be many alien civilizations and intelligent creatures in the universe, but these civilizations have encountered some bottlenecks. It is very likely that there is a general law in the universe, that is, all civilizations have not developed beyond interstellar navigation and colonization. The stage of space will destroy itself. All alien civilizations are likely to encounter a very high threshold in the process of development. This threshold is difficult to pass. This threshold is like a filter, which makes all civilizations self-destruct before they start real interstellar travel. !