Nebula theory can reasonably explain many observation facts, but there are also some difficulties. On the other hand, computer theoretical simulation shows that if there is no angular momentum transfer in the process of nebula material contraction, the result will not be a central star and some surrounding asteroids, but a binary star. Even if the planet is formed in a binary system, it will fall into the star or be thrown into space before long, and it is impossible to exist around the star for a long time.
It seems that nature has given the primitive nebula two possibilities for development: matter maintains its original angular momentum and evolves into a binary star; Or they just parted ways in the process of evolution, producing a central star and planets orbiting it.
The long process of producing intelligent creatures
The evolution of living things is a very slow process, and the time required can be completely compared with the evolution of the sun. The study of fossils found that as early as 3.5 billion years ago, there was an advanced single-celled organism called blue-green algae on the earth. According to the theory of stellar evolution and the analysis of ancient rocks and meteorites on the earth, it is known that the formation of the sun and the earth was earlier than the appearance of this creature10 ~1500 million years ago. After the formation of the solar system, human beings have existed on the earth for about 5 billion years.
Now imagine compressing every 5 billion years into a simple ratio of 1 "year". By this measure, 1 week is equivalent to 1 billion years in real life, and 1 second is equivalent to 160 years. It has been about two years since the birth of BIGBANG to the birth of the solar system. The earth was formed in the third year of 65438+ 10. In March and April, an ancient single-celled organism, blue-green algae, appeared. Since then, life has been slowly evolving. In September, the first batch of nucleated large cells appeared on the earth, and multicellular organisms may have appeared in late June at 65438+ 10. By the end of 1 1, plants and animals took over most of the land and the earth began to become active. 65438+February 18 dinosaurs appeared, and these mighty behemoths only ruled the earth once. Beijingers appear on New Year's Eve 1 1, and Neanderthals appear at the party on New Year's Eve before midnight 10 minutes. Modern people only appear five minutes before the arrival of the new year, while the recorded history of mankind begins 30 seconds before midnight. Major events in modern life accelerated one after another in the last few seconds of the old year, and the population on the mainland tripled in the last second before midnight.
It can be seen that most of the time after the birth of the earth is pregnant with life, and only for a short period of time does life take on the form of advanced creatures.
The harsh conditions for life on this planet.
Now we see that the birth of intelligent life requires that stars must be able to emit light and heat stably for at least 5 billion years. The lifetime of a star is closely related to its mass. The thermonuclear reaction of massive stars can only last for millions of years, which is far from enough for the evolution of life. Only stars with similar mass to the sun are suitable candidates. There are about 654.38+00 billion such stars in the Milky Way, and there are about 40 billion single stars besides binary stars. Do all single stars have planets? Unfortunately, we know very little about other planetary systems, but we have gradually discovered through observation that there may be planets around some stars. Considering the objective existence of the solar system, even the big planets have their own satellite systems, it may be optimistic to assume that all single stars have planets.
The existence of planets does not mean that there is life, let alone advanced creatures. The key is that the distance from the planet to the parent star must be just right, no matter how far it is. Due to the limited level of understanding, we can only discuss life forms similar to the earth's environmental conditions, especially assuming that liquid water must exist. There are eight planets in the solar system, but only the earth is obviously in the so-called biosphere, where there are conditions for the formation of life. Venus and Mars are located at the edge of the biosphere, and it has been proved that there is no life on their surfaces.
It is very rare for a planet to have all the conditions that life must meet. The earth in the solar system is a unique lucky one. Detailed calculations show that among the above 40 billion single stars, at best, only 65.438+0 million are surrounded by planets that can make life evolve to an advanced stage.
Another limitation is that extraterrestrial life should have similar chemical composition to life on earth. Astronomical observations show that, with a few exceptions, the distribution of chemical elements in the whole universe is quite uniform, so there is every reason to believe that the materials needed to form all organic molecules can also be found on distant planets. In fact, many complex organic molecules have been found in many places. Therefore, it can be considered that as long as life can be formed in theory somewhere, it will actually form life. In this way, there will be 6.5438+0 million planets in the Milky Way, but life on each planet should be at different stages of evolution.
Can we find aliens?
If we are happy with the big number 1 10,000 and think that finding aliens is not a problem, it will be too early to be happy. For advanced extraterrestrial beings, it is only meaningful to establish contact with them. As far as human beings know, radio signals are the only feasible way to establish this connection, so it is necessary to further explore how many planets are inhabited by civilized creatures who can send such signals. If they have been sending such signals since their existence, there should be 1 10,000 planets broadcasting by radio. But in fact, not to mention algae, even humans 100 years ago did not have this ability. On the other hand, life forms whose technology has been destroyed and whose life itself has been destroyed will not do so. Please don't forget that almost at the same time as transmitting radio signals, mankind has also developed large-scale nuclear weapons, which is enough to completely destroy all living things on earth. Will aliens be dominated by irrational warmongers and destroy themselves? This possibility is not completely ruled out.
Again, let's be optimistic. Aliens have the ability and reason to solve the problems we are worried about, assuming that they have lived in a peaceful and prosperous environment 1 10,000 years. As science and technology are extremely developed and their lives are fully affluent, they are bound to think of and fully capable of spending huge sums of money on groundbreaking research of great significance, including trying to establish contact with similar people outside. They kept sending powerful radio signals to the outside world for 6.5438+0 million years. In this way, among the above-mentioned 6.5438+million planets, a small part is broadcasting this signal, and the proportion of this part is 6.5438+million years divided by 4 billion years, which is 0.025%. This means that only 2.5 million people are sending signals at present. If they are evenly distributed in the Milky Way, the distance between them is about 4600 light years. It takes 4600 years for the signal sent by human beings to reach the nearest alien. If they receive and give an immediate reply, we will have to wait patiently for another 4600 years to receive their reply! The contact object of Ozma project is only a dozen light-years away from us, so it really doesn't make much sense. In order to make the project meaningful, it is necessary to monitor whether every sun-like star within 4600 light-years is sending out meaningful signals.
More realistically, it is believed that human history is only 4000 years. If aliens have the ability of radio broadcasting for only 4000 years, then there is only one planet that broadcasts signals to the outside world today! As a result, at best, there is another civilized creature in the whole galaxy besides the earth sending signals, and our efforts to monitor such signals with radio telescopes in the galaxy are completely futile!
Readers may be deeply disappointed with this conclusion. So how different will the actual situation be from what is estimated here? There are many uncertainties in the above discussion. Are there planets around every star? Can life only be born in an environment like the earth? Besides, in fact, we don't know how long an intelligent creature can live. Can they live forever? I'm afraid these questions will not be answered clearly for a long time. However, how did primitive people think of today's large passenger planes, color TVs, fast electronic computers and flights to the moon? As long as human beings can live in a peaceful and prosperous environment, the development of science will gradually answer these questions. But for now, even if aliens exist, we can't communicate with them effectively for the time being. Therefore, I'm afraid it's not credible to link UFOs with whispers' spacecraft.
Why don't aliens appear?
Now we finally know that there are no aliens on the earth, because they were killed by gamma rays before they reached the earth.
Dr James Annis of Fermi Accelerator National Laboratory in Illinois said that the reason why aliens haven't reached the Earth is that our Milky Way galaxy didn't provide opportunities for life in space until recently.
Anis said that until hundreds of millions of years ago, our galaxy was often exposed to gamma ray bursts: the collision between dead stars and black holes released a lot of deadly rays. Only now, these collisions have become rare, and alien life may appear and travel to places quite far away from their planet.
Anis hopes that the theory he put forward in the British weekly New Scientist can solve one of the most famous debates about the existence of extraterrestrial life-Fermi Paradox. This paradox is named after Enrico Fermi, an Indian physicist who won the Nobel Prize. It is said that Fermi put forward this paradox in the 1950s. The point is: if aliens do exist, where are they?
This question is very convincing, because it is based on two facts of our galaxy: first, the galaxy is very old, with an age of about 65.438+0 billion years; First, the diameter of the Milky Way is only about 654.38+ million light years. Therefore, even if aliens can only travel in space at one thousandth of the speed of light, they can cross the universe in about 654.38 billion years-much shorter than the age of the universe. So, where are the aliens?
Fermi obviously takes this reason as evidence that there are no aliens at all. Now Annis claims that he has discovered a loophole in Fermi's inference: aliens are likely to exist, but recently the burst cycle of gamma rays has become longer and longer, thus providing enough time for aliens to travel between stars.