More than 13 billion years have passed since the birth of the universe. There are only about 2.5 × 10^11 stars in the Milky Way, and the number of stars in the entire observable universe is about 7 × 10^22. A natural idea is that human beings are not unique intelligent life in the universe, and there will be other higher civilizations in the entire universe.
But the question is, why haven't we found aliens?
Of course, the places that humans can explore are insignificant relative to the entire universe. But the universe has a long history and a large number of stars. We have reason to believe that the existence of a certain alien civilization is far higher than that of human beings. They (at least the message they sent) should have reached the earth. But so far, no one has found evidence of the existence of aliens.
In 1950, physicist Enrico Fermi discussed this problem with colleagues on the way to lunch. Fermi asked: "Where are they?" (Where is everybody?) "They", naturally refers to aliens. This famous problem is now called the Fermi paradox . It is fully expressed as : the remarkable scale and age of the universe means that higher civilizations should exist. However, this assumption has not been supported by sufficient evidence.
There are many studies and explanations for Fermi's paradox: perhaps the existence of intelligent life is beyond our imagination; perhaps other intelligent life behaves differently than ours. Two scientists from Russia and Ukraine, Bezsudnov and Snarskii, built a model of intelligent biological expansion in the universe in their own research, trying to explain the problem.
Expansion of cosmic civilization
In this study, the automaton models established by the two scientists are described below:
The entire universe is simulated with a finite-size two-dimensional discrete grid, each of which may have one or more civilizations, or no civilization;
The time in the simulated universe is discrete, with a minimum time unit of 1;
Each grid has n at each time;
In the T0 time after the birth of a civilization, civilization will expand outward. The law of expansion is: centering on the lattice of the original civilization, each time unit expands four sides up and down and becomes one. a new square; the civilization will then contract inward at the same rate until it eventually disappears;
If two civilizations come into contact (when the expansion occurs in a grid, only the first contact is calculated), then the time for their expansion increases by Tb time units;
When two civilizations come into contact, the communication between them will not be interrupted; if civilizations A and B contact at a certain time, if B contacts C again, then even if A and C are not in direct contact. Civilization A will also gain the expansion time of T b.
Let us explain. The researchers hypothesized that each civilization has a specific life span and will experience the process of development - peak - recession - extinction or collapse .
When the development of technology encountered limitations, civilization began to decline. Because of the lack of new technologies and resources, this civilization gradually terminated its contact with the outside world. In the above model, scientists have also proposed a new rule : when different civilizations meet in space and time, they will communicate with each other and promote each other's development, thus extending the life of their civilizations.
The T0 in the above model is the life of an isolated civilization . When it communicates with the new civilization, the time to promote each other to extend its life is set to Tb (for convenience, it is assumed that all civilizations are the same).
For example, as shown in the figure, at some time t1, there are several civilizations in the universe, among which civilizations No. 2 and No. 5 are shrinking, and other civilizations are expanding. By t2 = t1 + 1, civilizations 3 and 4 were in contact during the expansion process, and the expansion time of civilizations 3 and 4 would each increase T b . When t3 = t2 + 1, the civilization No. 1 and No. 3 civilization also came into contact, and the civilizations No. 1, No. 3 and No. 4 each gained the expansion time of Tb. In other words, the final civilization No. 1 received the total time bonus of Tb, while the civilizations No. 3 and No. 4 each received a time bonus of 2Tb.
It can be seen that there are only three parameters in this model: T0, Tb and n. The researcher thus established such an automaton. In their simulation, the size of the universe is 104×104, the range of T0 is 4 to 20, and the range of n is 3 × 10^-8 to 3 × 10^-5. After adjusting the value of Tb and performing a simulation of 320,000 time units, the scientists discovered some interesting phenomena.
What happens when the universe is a "dark forest"
In the "Three-body" of science fiction writer Liu Cixin, the "dark forest" theory was proposed . This theory points out that because of the chain of suspicion and technological explosion, higher civilizations are hostile to lower civilizations, and any civilization that exposes itself will be destroyed .
Although the model presented in this paper does not consider the mutual elimination or hostility of civilizations, we can consider a situation in which civilizations do not interfere with each other.
At this time, there is no mutual exchange and promotion between civilizations; for the whole model, it is the case of Tb=0.
In this state, the duration of each civilization is determined : after the civilization is created, it first expands and then shrinks, lasting T0 time units, and eventually this civilization will disappear. If you think of time as the third dimension (see the figure below) except for the two dimensions of the above grid diagram, then in this model, there is the ratio of the volume of civilization to the volume of the entire simulation process (that is, the cylinder with color in the figure) The ratio of the total volume to the volume of the entire space is:
Since T0 is much larger than 1, we can make the following estimates:
And because V0 is given, it can be considered that the volume of life in the universe is proportional to the power of n and T0, namely:
From the above analysis, it can be estimated that α≈1, β≈3. A more precise calculation indicates that α = 0.91 ± 0.005 and β = 2.79 ± 0.02.
In this case, life will multiply independently. Obviously, under this parameter, the universe can never be filled with life. For a civilization, there is no other civilization in the process of production, development, and disappearance, which is probably a matter of course.
Mutual promotion between civilizations
Although the "dark forest" is a very imaginative theory, it is quite normal for two civilizations to interact with each other (rather than to strike each other) in terms of human values. This situation of mutual communication among different civilizations is the case of Tb > 0.
For different Tb, several scientists have simulated separately, as shown below:
The horizontal axis on the way is the time axis of the universe, and the vertical axis is the degree to which the universe is filled with civilization at a certain moment (ie, V / V0 calculated in the previous section).
When Tb = 0, the simulation results are very similar to the previous calculations. The whole universe has been in a state of steady development, and civilization has continuously produced and disappeared.
When Tb = 28, you can see that the universe is filled more than in the former case. And at some point, the value of V/V0 will suddenly fluctuate. This is because the generation of civilization is random, and at some point, several closely related civilizations will meet, which will allow them to continue to develop for a long time.
During this time, the balance of “self-destruction” in the previous case was broken, and the universe was covered by the star of life. But after that, because they did not have enough opportunities to encounter new civilizations, these civilizations eventually disappeared and the universe returned to its original state.
When Tb = 38, the image is completely different from the previous two cases. It can be seen that at some point in time, the civilization in the universe suddenly surged and eventually occupied the entire universe. This is because the value of Tb is greater than the first two cases, and civilization will have more time to expand itself, thus encountering more new civilizations in the process of expansion. When Tb is large enough (for example, Tb = 38), the time for civilization to expand is long enough, the probability of not meeting a new civilization during the expansion process, and the ultimate contraction is almost zero. At this point, it can be considered that civilization will continue to expand. Until the entire universe is occupied by civilization.
Real world universe
This study by two scientists suggests some ways to explain Fermi's paradox. The Tb value and the value of n in the model can significantly affect the possibility of civilization and civilization encounter. Perhaps because the value of Tb in the universe is too small, human civilization does not have enough chance to explore other civilizations; it may also be because human civilization has produced too short a time to reach out to aliens.
Although this research is very attractive, it is important to know that it is an extremely simplified model and does not fully represent the real universe. When humans explore the universe and try to find clues about the existence of alien civilizations, the problems encountered are far more complicated than this model.
For example, a well-known distributed computing project, SETI@Home, is an attempt to find evidence of the existence of an alien civilization by analyzing the radio signals collected by the Arecibo radio telescope from the universe. But unfortunately, this project has not been discovered until today. First of all, alien civilizations do not necessarily use electromagnetic waves for information transmission. Even if such electromagnetic waves exist, extracting them from the background noise of the universe is not as simple as imagined. The day when humans find alien civilization seems to be far away.
However, humans will not stop the pace of exploration. The hunger for the unknown is an important driving force for the development of the entire society. There are still many things we cannot understand in this universe; but the future provides us with endless possibilities - who can say that there will not be such a day, people can really find alien civilizations, and even They met and shook hands friendly.
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