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This time, watching Liu Cixin’s "Three-body" is indeed a good science fiction novel. However, even the best science fiction is still a science fiction, not to mention the "hardness" is different, the scientific background can always find a place that is not true. Of course, these can't be said to be bugs. After all, there is always room for some free writing of the book. Anyway, this is not a physics paper.
Moreover, a good sci-fi is easy to pull people into a dream. For example, when watching "Spherical Lightning", I often feel cold sweat. At this time, scientific knowledge can pull people out of the artistic conception created by the novel. Like the way I escaped from the ghostly plots such as Quantum Rose, I am chanting "I believe in ensemble explanation." Learn more about the background and let the licenses do less nightmares.

Three-body problem

Needless to say, from the book name, the three-body problem is one of the biggest backgrounds of the "Three-body". The three-body problem is an old problem of celestial mechanics in classical mechanics. From the time of Newton, it was the nightmare of physicists and mathematicians.

Let me talk about what is three-body. In physical language, there are N mass points in an inertial reference system, and the equation of motion for solving these N mass points is the N-body problem. The reference frame is the inertial reference frame, that is to say, it is not affected by the force outside the system, and all the forces are derived from the N particles within the system. In celestial mechanics, we usually only consider gravity.

In mathematical terms, classical mechanicsNThe body problem model is that there is in the three-dimensional straight spaceNThe mass of each particle is known and does not change. At the initial moment, the position and velocity of all particles are known. Each particle is only subjected to gravitation from other particles, and the gravitational force is described by the inverse inverse of Newton's square of the same distance. The solution is to solve the position of a certain particle at any moment.
N=2 is the two-body problem.N=3That is the three-body problem we have to say.
The case of N=2 has been basically solved as early as Newton. After learning middle school physics, everyone knows that the two particles move in a conic curve around a common centroid on a plane, which can be a circle, an ellipse, a parabola or a hyperbola.

However, the situation of the three-body movement is much worse. After conquering the two-body problem, Newton naturally began to study the three-body problem, and the result was very natural - the headache was unbearable. Newton’s self-reported approach to this type of headache is to use a cloth strap to entangle the head until it is dizzy—although this method does not cure the problem and there is not much creativity, but it still works.

In fact, the three-body movement has been simplified to the actual physical reality. For example, for the quality point, the rotation, the shape, we do not have to consider. But as long as the actual earth motion is studied, it is already much more complicated than the particle. For example, the earth is not a point, not even a sphere. It roughly looks like an ellipsoid that is fat out on the equator. Thus, under the gravity of the moon, the direction of the Earth's rotation axis is not fixed, and the North Star will not be that one forever. When considering the role of tides, the earth can not be seen as "hard", and the earth's rotation is therefore slower and slower.

However, even the extremely simplified three-body problem, Newton, Lagrange, Laplac, Poisson, Jacobi, Poincaré and other masters have given countless brains to the altar and failed to overcome it.

Of course, efforts are not completely in vain, and many effective approximation methods are being drummed out. For the solar system, perturbation theory is a very effective approximation of the problem. For the Earth and Moon system, the Earth and the Moon can be considered as a two-body system, and then the influence of the sun's gravity can be considered. The theoretical calculations of "the moon turns around the earth and the earth rotates around the sun" have been made very accurate, and the eclipse of the eclipse for thousands of years can be well predicted. For a comet that is disturbed by the gravitational pull of the planet, one can also calculate a very precise orbit for a period of time. For example, astronomers can predict that a comet hits Jupiter a few years in advance. Moreover, the stability of the solar system has also been largely proved. For example, the orbital changes of large planets are generally periodic and will not always change in one direction until the planetary system disintegrates.

From the mathematical method, the solution to the 2-body problem is to solve the differential equations, and the solution can be satisfactorily solved by the method of integration, and the analytical solution is obtained. Naturally, physicists and mathematicians also use this method to deal with the three-body problem.
In 1772, Lagrange simplified the 18 equations of the three-body problem to only six. However, progress has come to an end. The study at the end of the 19th century gave mathematicians a series of blows.
Bruns (1887), Poincaré (1889) and Panles (1898) gave a more rigorous proof than one, blocking many ways of seeking integration.
In 1941, Siegel simply proved the death penalty of algebraic integration, and it was impossible to declare enough algebraic points. Of course, the mathematical study of the three-body problem is not nothing but failure. It brings many new discoveries. For example, chaos theory is born from its ruins.

Of course, we only talked about Newtonian mechanics. If you take into account the revision of general relativity, it is even worse, and even the two-body problem has only an approximate solution. Moreover, the two-body problem of general relativity is also unstable. Because the gravitational wave loses energy, the two stars will collide sooner or later, although the time to wait may be longer than the life of the universe.

Let me explain how astronomers get these data from the three brothers. Stellar sights are directly measurable. Nanmen 2 is very close to us, and the distance can be measured very accurately by the triangular parallax method. In fact, South Gate II was the first star to be measured (except of the Sun), but since the British astronomer Henderson, who observed it in South Africa, had to wait until he returned to the UK to publish observations, the result was German astronomy in 1838. Home Bethel grabbed the first seat of the Cygnus 61 in Königsberg. With distance and visual brightness, absolute brightness can be easily calculated.

The surface temperature of a star can be determined by studying the spectrum of the star. According to the law of blackbody radiation (such as Stefan's law or Planck's formula), the brightness of a star is proportional to the fourth power of the surface temperature, and the brightness is proportional to the surface area of the star (that is, proportional to the square of the radius). Therefore, based on the data already measured, it is not difficult to find the radius of the three brothers.
It is a lot of trouble to measure the quality. The eldest and second of Nanmen 2 are a pair of closely spaced double stars with an average distance of 23 astronomical units (the average distance from the Earth to the Sun), and the cycle of detours is about 80 years. With orbital data (and of course more than these two data), the exact value of the mass of the South Gates A and B can be calculated from Newtonian mechanics.

The quality of the neighboring stars is hard to measure. The younger brother, who is next to the star, is too far away from his two brothers, about 0.2 light years, which is equivalent to more than 500 times the average distance between the two stars, and its quality is small. Therefore, it is impossible to determine the quality of the South Gate II A and B by measuring the gravity of the neighboring star. The quality of the neighboring star currently used is an estimate. It is estimated by the theoretical model and other stars of similar brightness and temperature, and has a very small mass, which is only slightly larger than one tenth of the sun.

The two stars A and B of South Gate II circumvent each other in an elliptical orbit. Below is an illustration of their orbits (an imaginary earth is added here).

Their orbits are fairly flat and the eccentricity is about 0.5. They are all in the "near point" in the picture. Of course, don't worry that they will collide. Their movement cycles are the same and they are synchronized. The nearest time is 11 astronomical units (slightly farther than the Sun to Saturn), reaching 35 astronomical units (farther than the Sun to Neptune) when they are farthest apart. Because their orbital planes are not directly facing us, from Earth observation, with South Gate 2A as the reference point, South Gate 2B will depict a very flat ellipse as shown in the figure below.

In order to better understand the scenery of the South Gate II, we will launch the space-time transmission machine (using the metamorphic earth engine of "Wandering Earth"? Too old and old), send our earth to the field to travel. .
The South Gate II A star is the boss, very similar to the sun. In fact, it is the star most similar to the sun in the 30 light years around the sun. If you really say "the sea memory confidant, the end of the world if you are neighbors", then it is very suitable for it. In this case, give us the feeling of going home, and place the earth on a circular orbit from an astronomical unit in South Gate II.

Ah, this will be a disaster. Don't forget, the quality of the South Gate II A is 10% larger than the Sun and the brightness is 51% larger. Nowadays, only a few greenhouse gases have been emitted, and the global warming caused by it has already been overwhelming. If the sun is half brighter, then the earth will be born without grass.
"The XXXth generation civilization was destroyed in the heat, because the position of time and space crossing was set wrong..." (really shameful)

No, we have to remedy it, move the earth farther away, and increase the distance from South Gate 2A to 1.23 astronomical units. Thus, the light provided to the Earth by Nanmen 2A is exactly the same as the sun, we can still have one A planet with a good climate and a prosperous life.
The orbital situation makes us quite satisfied, because the South Gate II B is quite close, but the distance in the recent time is still nearly nine times larger than the distance to the South Gate II A. Therefore, the orbital disturbance to the Earth is not large. It can be expected that the earth can live a fairly stable life in a circular orbit.

Now is the day, let us praise "everything grows by the sun." Looking up, South Gate II A looks slightly smaller than the sun we are familiar with (about 2.5% smaller), if we have not forgotten to drag the moon as it is, Then you won't see the solar eclipse here, it will only be a total solar eclipse or a partial eclipse. But because the surface temperature of South Gate II A is a little higher than the sun (the surface temperature of the sun is 5700K), its surface appears to be brighter. If you are careless, you won't think the sun has changed. One year is three or four months longer than we are now, but the difference is not too big.

If the time coincides, we can also see a bright "flying star" in the clear sky, this is the South Gate II B. Its brightness varies from 1/300 to 1/3000 of the "Sun" of South Gate II A, depending on how far it is from us. Of course, the teachers will emphasize to you that you can't look directly at the sun with the naked eye. They also emphasize that they should not look directly at the "flying star" with the naked eye. Because the brightness of the South Gate 2 B is still at the point of the stab, equivalent to 1000 to 100 times the brightness of the moon, enough to hurt your eyes.
When the "flying star" is the brightest (that is, the most recent time), you can see that it is not a "star", but a small sun, about one-tenth the diameter of the sun, you can see a small Round face. In the darkest time, there is no structure visible to the naked eye. It looks like a very bright star, but under the telescope, its round surface is still visible, and we are almost as big as Jupiter in the real world. But no matter what, its brightness is enough to ensure that it can be easily seen even during the day.

The sun is setting, let's take a look at the stars. Hey, how is it so bright, what stars can't see? It turns out that "Flying Star" has not yet gone downhill. The brightness of "Flying Star" reaches 1000 to 100 times that of the moon. The "night sky" under its illumination is still very bright, like our usual cloudy sky, and the brightness of the ground is no different from that in the office building. Since the surface temperature of the South Gate 2B is much lower than the sun, its color is orange-yellow. Under the shining of the "flying star", the sky and the earth are illuminated by the high-pressure sodium lamp beside the city street, bathed in a mild In the orange glow.

If you are interested in the operation of "Flying Star", we have to observe it for a long time. Its operation is like the operation of outer planets in the solar system. Every 80 years (the "earth year" converted to South Gate II A is of course only about 60 "years"). It runs on the celestial sphere relative to the background of the stars, and each " In the year, there will be two phenomena of "the flying stars do not move." Of course, this is not a precursor to a catastrophe. It is just that the direction of the earth is just facing or deviating from the South Gate II. The duration is only a few days, and the “retention” of planetary movements in and around the solar system is actually the same thing.

In fact, the South Gate II B is also a good place for humans to live in, although the brightness is only less than half of the sun. But as long as you are close, it will be fine. The orbit of Venus is a very good choice. Similarly, such an orbit is quite stable. On the above we can see a larger orange sun and a yellow, brighter and larger "flying star".

In Newton's classical mechanics system, the simplification of the three-body problem can be roughly represented by the following figure (here, the orbit calculation of the lunar rocket is used as an example of a practical application of a three-body movement, actually than the three-body movement. Still complicated)

After the 1950s, mathematicians added a new helper: computers. So, two new methods came out, one is to use the series to represent the integral (simple algebraic integration is not expected), the other is simply to use the numerical method to find the approximate solution.

The series solution has been very successful in theory. For example, in the restricted circular three-body problem, it has been proved that the required integral exists (but on the other hand, it has long been proved that the algebraic formula cannot be expressed). . These integrals can be expressed in power series and prove that the power series is convergent. But these power series converge too slowly, for example, for Lagrange points, in order to achieve acceptable accuracy, at least 10 ^ 8000 items must be taken! The number of particles in the entire universe is 10^80.
The joining of computers has made people less helpless about the three-body problem. Although there is no algebraic formula, the results of numerical calculations are not bad. For example, launching a spaceship to detect other planets is a typical three-body problem, and Voyager 2 said that it would be possible to go to Neptune. For another example, the movement of the solar system's 40 million-year-old planet is also calculated. At least for the time being, the planetary system of the solar system will not fall apart.

Let us look at the general status of the three-body problem:
1. The current research focuses on the restrictive three-body problem because it is simplified and has practical value.
2. For the restricted three-body problem, the existence of the understanding is proved by the series method (this is already a very large result). Moreover, the qualitative analysis of astronomical mechanics and astronomical observations (such as the multi-billion-year-old life on the earth) have proved the existence of stable solutions of the restricted three-body system.
3. The method of solving the two-body problem, that is, the method of algebraic integration, is confirmed to be impossible to solve the three-body problem.
4. The numerical calculation of the longer-term three-body problem with a computer was successful.
5. The algorithm for the three-body problem has much to improve. After all, the calculation of 10^80000 items is too horrible.

Going back to the "Three-body" novel, with the "human computer system" of "Qin Shihuang", it is still possible to calculate a simplified three-body problem. However, if the three suns in the novel are of similar quality and are similar in distance, the three-body problem they face cannot be reduced to a restrictive three-body problem, and the calculation is much more difficult. However, using a computer to calculate a relatively short-term forecast should be feasible. After all, the weather forecast doesn't have to know the specific weather next year. It's more accurate to know that the weather is good for a week (usually we only hear if it rains tomorrow). It is already very beneficial for the trio to know whether it is "dehydration" or "soaking". Constantly correct predictions with observations, at least for small "disruptive generations".
Of course, if the three-body civilization is only at the level of I/II civilization, it is impossible to solve the three-body problem by removing the stars. Then, the phrase "but the important thing is to change the world" is still correct to the point of cruelty, predicting that "Samsung volley" will not help to escape destruction.

So far, we have been using paper, pen and computer to discuss the three-body problem, using the deductive method. But don't forget that there is another, more important weapon in the scientific method: induction. We can use observations and experiments to see what the actual three-body experience looks like.
Because on our daily scales, the gravitational pull is negligible, and only on the astronomical scale, gravity shows its power, such as the earth pulling us on the ground. Therefore, it is impossible to implement a three-body system in an ordinary laboratory. We can only turn our eyes to the sky and examine what kind of examples nature has arranged for us.
Of course, as we have already seen, in the solar system, it has been fully demonstrated that the restrictive three-body problem has a stable solution. But what about the three bodies of the same magnitude? We can take a look at the stars.
There are no more than 100 billion stars in the Milky Way, and stars like the Sun are actually a minority. Stars generally like to be lively. The number of double stars is very large, and many of them are already billions of years old partners (such as the South Gate II A/B mentioned below), which is equivalent to experimentally demonstrating the stability of the two-body system.
The three-pointed star is not uncommon, but it is usually a pair of double stars and then a long-distance single star. Similarly, a large number of stars composed of stars are mostly composed of binary stars and single stars. It should be said that this also strongly implies that nature also believes that the three-body system is unstable. After all, the three bodies in the Milky Way are not ideal three-body systems. One star can collide and merge. Second, once a star is thrown too far, it may be out of the system and mainly by the gravitational pull of the Milky Way. Controlled. Through these two methods, the three-body system becomes a stable two-body system.
Of course, there is also a system of "quadrilateral polystars". The stars are of similar quality and the distance is similar. The most famous example is the quadrilateral polystar in the center of the Orion Nebula M42 (with a telescope of about 5 cm, the magnification can be distinguished by 50 to 100 times). It is worth noting that these quadrilateral stars are very young, such as the Orion quadrilateral, which is only a few million years old. For astronomy, this is completely infancy. No old quadrilateral polystars were found, indicating that nature believes that this configuration is also unstable and will eventually collapse.
In the central area of the Orion Nebula M42, the four bright stars in the center of the picture are Orion quadrilateral stars.

Interestingly, the value of N increases again. For example, an open cluster of N=100 level or a globular cluster of N=100,000 is a very stable mechanical system. These clusters are over several billion years or even billions of years. Abound. Of course, the result of too intensive is that there are many collisions, and there are a lot of bright stars merged in the center of the globular cluster.
The Pleiades Group (M45), which is about 50 million years old, is a fairly young open star cluster with about 100 member stars.

Globular cluster M13, over 10 billion years old, with approximately 300,000 members

Centaur alpha (South Gate II) triple star system

There is no doubt that the "living prototype" of the system in which the three-body inhabited by the three-body is located is the Centaurus alpha (South Gate II) triple-star system.

In the night sky of the earth, South Gate II is the third brightest star in the whole day (after Sirius and the old star), and the brightness is -0.27. However, for most Chinese, the chances of seeing South Gate II are small. South Gate II is too south (declination -60 degrees). Considering light pollution, people north of the Tropic of Cancer should not expect to see it on the horizon. (The place in the Red Bank should not receive the signal of the South Gate II. Of course, the novel can always introduce other mechanisms.)
With a small telescope, the South Gate II can be broken down into two bright stars. The angular distance between the two stars is about 14" in 2000, which is less than 1/120 of the apparent diameter of the sun we see. It is one-fourth of the resolution of the human eye. The third member of the South Gate II, the famous neighboring star, is not easy to see. Its brightness is very dark, which is equivalent to seeing the darkest star by the naked eye. One percent must be seen with a fairly large telescope. The distance between the neighboring stars and the two main stars is also very far, with a viewing distance of 2.2 degrees, more than four times the diameter of the sun.

Let's take a quick look at the physical values of these three stars and compare them to our sun.
To Earth's distance (light years), visual star magnitude, absolute star surface temperature, total luminosity (sun = 1) radius (solar = 1) mass (solar = 1)
A: 4.350.01 4.38 5800K 1.51 1.2 1.10
B: 4.35 1.34 5.72 5300K 0.47 0.84 0.91
C: 4.22 11.05 15.49 2700K 0.0017 0.19 0.11
(The luminosity and visual brightness are still very different. I am confused here.)

Obviously, the neighboring star is closer to us than the two big brothers. South Gate II is the star system known to be closest to the Sun (the possibility of being closer and not being discovered is unlikely, as in the Supernova Era, a superstar is actually hiding from astronomers), and the neighboring star is The star closest to us except the sun. "The sea memory confidant, the end of the world if neighbors", it got such a poetic Chinese name. Of course, we will see below that this name is really not suitable for it. It is not like the sun at all, and it is too far away from "confidant". The boss of South Gate II is very similar to the sun.



Going far, come back to our planet. Now, "Flying Star" has also gone down the mountain. This time, the real night has finally arrived, and the stars are full of night sky. Suppose you are familiar with the stars (now the children in the city are really pitiful, the Galaxy has never seen it before), the starry sky here will be quite familiar, no big changes. This is of course, after all, from the sun to the south gate two A, our position only moved 4.35 light years, not a big distance. Looking closely, there are a few changes in the position of the bright stars, such as Sirius. That's because these stars are also close to the sun (Sirius is 8.6 light years), and the changes brought about by moving 4 light years cannot be ignored. However, the biggest change is obvious, the brightest star of Centaur here is gone.

Oh, still looking for the South Gate II? The two guys have already gone down the mountain. In addition, there is a brilliant yellow bright star in Cassiopeia, which can be discharged into the top ten bright stars throughout the day. Don't be confused, this bright star is where we come: the gentle and great sun. If you are an astronomer and want to see if the sun has a planet, as an immigrant from there, I can tell you that the sun's largest planet is called Jupiter. From the South Gate II, it is the longest distance from the sun. Less than 4′′, the brightness is only 21, etc., only about one hundred millionth of the brightness of the sun. Direct observation may be somewhat difficult, but with the method of astrometric measurement, you should be able to find that the sun's running track is a little wavy, thus inferring the planet's presence.

The sun is a gentle star. It has maintained its current brightness of 5 billion years and will remain for 5 billion years. Its brightness will slowly increase in this 10 billion years, but it will not more than double. Since humans have recorded astronomical observations about the sun, in the visible region, its brightness has never changed by more than a thousandth. South Gate II A, B is also such a gentle star, stable and reliable, with a long life.

Wait, isn't it that the South Gate II is a three-in-one system? Where is the third place? Neighboring star - South Gate II C is too dark. Even on the Earth now on the South Gate IIA, the South Gate II C is only about 0.2 light years away (about 12,000 astronomical units), and it is still not bright. In the stars of the night sky, the neighboring star is just a very inconspicuous dim red star, which can barely be seen by the naked eye. Because the surface temperature is too low, its color is red. In fact, most of its energy is emitted in the infrared way we can't see. However, occasionally it will suddenly attract attention. The neighboring star is a shining star. Sometimes the brightness can suddenly increase several times in a few minutes, become a fairly bright star, and then quickly dim in a few minutes. In addition, ordinary people simply do not notice the difference between neighboring stars and other stars. Astronomers will soon notice that it is very close, and that the operation is "quite" fast - about 500,000 to 2 million years can be around the South Gate two A and B. (Speaking secretly, don't trust astronomers too much, look at it, just ask them to measure a simple cycle of data, the error can reach 4 times. Moreover, some people think that the neighboring star is just a passing, not the South Gate II a part of.)

Looking back now, although the South Gate II is a three-in-one system, the three-body is not complicated, because the younger brother is too far away (it is nearly 600 times the distance between the boss and the second child). This system is actually composed of a pair of very close binary stars plus a very distant single star. A single star does not have much effect on the movement of a double star, and in dealing with a single star's movement, a double star can be treated as a celestial body. So this three-body can be solved quite well with two two-body problems. Our imaginary earth can comfortably live.

In this way, Nanmen II should be a good place to produce life, even intelligent life. Unfortunately, this possibility does not exist in the observations and theories we currently have. The reason is very simple. According to our current understanding, life always comes from the planet (those more wonderful creatures, it is completely out of imagination, it will not be discussed). From the observations, no signs of the planets of the South Gate II A/B have been found. From the theory of planetary formation, the South Gate II A/B is a relatively close binary star. During the star formation stage, their gravitational forces will interfere with each other, so that dust and gas have no chance to condense to form a planet, and then these materials are swept away. . An example of the corresponding solar system is that in the asteroid belt inside Jupiter, a large planet cannot be formed, and Jupiter occasionally pulls some unlucky little guys into satellites or simply eats them.

Although the South Gate II A/B is bright enough, stable and longevity, and is as suitable for life as the sun, it has lost the stage of life. If humans want to fly out of the solar system and immigrate to the universe, the South Gate is also a good choice. But this will be a self-help tour, we have to bring a planet to the past.
By the way, if our sun really has a long life (as in "Wandering Earth"), can South Gate II be used as a refuge? It is a pity that if our current observations and theories about stars are correct, the conclusion is still not good. The stability period of the star is negatively correlated with its quality. The higher the quality, the shorter the life expectancy (the brethren who are overweight still lose some fertilizer). The quality of the South Gate II A is slightly larger than the sun, meaning that its life will be shorter than the sun. Moreover, from the observed indicators, the South Gate II is a bit older than the Sun. In this way, when the sun is not working, the South Gate II A has not been able to stand up.

And what happens when the sun (and South Gate II) arrives in their later years? The intensification of the internal nuclear reaction and the enthalpy of burning (not as convincing as the dramatic flash in the Wandering Earth) have caused the radius of the sun to expand hundreds of times beyond the radius of the Earth's orbit. The surface temperature drops below 4000K, but due to the large increase in area, the total brightness will exceed 1000 times that of the current sun. At this time, the sun becomes a typical red giant. Planets that are close to each other will be brittle and vaporized, and far away will not escape the fate of being burnt.

If the South Gate II A becomes a red giant star, even the planet next to the South Gate 2 B will be scorched (even if it is "dehydrated"). Moreover, the South Gate II A is close to the Roche radius after the expansion, and the South Gate 2B will take the opportunity to peel off its skin and take it for itself. At this time, the hot gas, dust and charged particles are largely ejected by the South Gate II A and then swallowed by the South Gate II B. This will be a spectacular sight, but for life it is devastating.

Of course, if human beings can avoid rapid destruction and support for such a long-term future, this kind of catastrophe may not be a problem at all. At that time, maybe we could have played with the stars at the fingertips.


Don’t dare to speak loudly, fear the heavens

We have been scared enough by aliens in science fiction. Rather than believing in good wishes, it is still more reliable. After all, we don't seem to believe that the trio people "have to have higher civilization and moral standards." Even Ye Wenjie is not sure.

Then, our instinctive reaction should be to "close the door to develop", and if the aliens come to the door, there is no way to accept the bad luck, at least we don't want to provoke them. Just like the signal sent by Ye Wenjie's first trio in the Red Bank, she should stop sending any signals. This idea, Li Bai has long said clearly: "The dangerous building is 100 feet high, the hand can pick the stars. Do not dare to speak loudly, fear the heavens."
It is a pity that even for an alien civilization with only human current technical standards, such as the Axirebo radio telescope, it is too late for us to start collectively “losing the sound”. We have exposed ourselves enough. It’s late, it’s too late to hide now.

The 305-meter-diameter Axirebo radio telescope is currently the world's largest radio telescope. The mirror surface is fixed, and a relatively large sky area can be observed by moving the receiving device suspended in the air.

The Asilrebo radio telescope is located in a small basin on the Caribbean island of Puerto Rico. Using the terrain, 40,000 aluminum panels are spliced together in this small basin to form a fixed parabolic antenna. Radio waves from the distant universe are reflected by the antenna and converge on the receiving device suspended above the antenna. The seemingly small receiving device has a height of 140 meters to the bottom of the antenna and weighs 900 tons. It also carries a large amount of liquid helium for cooling. The receiving device can be moved, so that it is not possible to observe only one point on the top of the head, but to observe a relatively large sky area.

This fixed-type radio telescope has a diameter of 305 meters and is currently the largest. The receiving area is equivalent to more than a dozen football fields. In contrast, the largest rotatable radio telescope is the new 110*100-meter telescope built by the National Radio Astronomy Observatory, located on the green shore of West Virginia (Ye Wenjie's “Red Shore” corresponds to this). Such large radio telescopes are also often referred to as planetary radar stations because they are often used for interplanetary radar observations, such as transmitting radio waves to Venus and receiving returned waves. Through this observation, you can determine the distance of Venus (by the way, verify the general theory of relativity), determine the rotation period of Venus, and draw a surface map of Venus.

As for the receiving sensitivity of radio telescopes, an example is the Pioneer 10 detector launched in 1972. Its transmitting antenna has a diameter of 2.7 meters and a transmitting power of only 8 watts (much smaller than ordinary 25 watt bulbs), and Battery power is decreasing and gradually decreasing. But before it completely lost contact in 2003 due to lack of electricity, it had already flown beyond 12.2 billion kilometers, and at this distance, several of the Earth's 60-meter-diameter radio telescopes remained connected. Because of this, the main difficulty in radio astronomical observation now lies in environmental noise. For example, in the Green Shore Observatory and its surrounding areas, ordinary cars are strictly prohibited, and cars can only use diesel engines. Because the spark plug of a gasoline engine produces a very wide frequency of electric waves, although it seems to be an insignificant little spark, it is enough to interfere with the work of the radio telescope.

In contrast, Axirebo's launch power is 1 megawatt (of course, much better than the 25 megawatts of the Red Bank). Coupled with good directionality and narrow frequency, the communication ability is strong. In Drake's words, if there is an Axirebo telescope at the other end of the Milky Way (100,000 light-years in diameter), and the direction and frequency are chosen, the Earth and it can fully realize radio communication.

When it comes to Drake, he is a pioneer in the detection of alien civilization. In 1960, it was he who launched the Ozma program at the Green Shore Observatory to detect the radio signals of alien creatures using a 26-meter radio telescope. The target of the detection is the constellation Thale Star (Tiancang V) and the Ou Xing Star (Tianyuan IV). These are two single stars similar to the sun, and the distance is about 11 light years. The total monitoring time is 200 hours. The result is conceivable. No alien signals were found, but humans received their own interference signals. Drake also worked with Karl Sagan to propose the famous Drake equation (also known as the Green Shore equation), describing the number of civilizations that may exist in the Milky Way.

If we set the direction of Axirebo's launch to South Gate II, and at this time there is an observer who uses the radio telescope to observe the direction of the sun, and the frequency used is also this frequency, then what will he find? In his radio telescope, the signal from Axirebo will be the strongest signal in the universe, a million times stronger than the sun signal. Humans don't need to be arrogant, even though we are not advanced, we can still express our existence strongly in the universe.

The above figure can be briefly explained that the radiation of the sun is basically in accordance with the black body radiation. For the 5700K solar surface, most of the energy of the radiation is concentrated in the visible region. In this region with a wavelength of 0.4 to 0.8 microns, more than 50% of the energy is concentrated. In the vast radio wave region with a wavelength of more than 1 mm, only 1% of the energy is not available. Of course, since blackbody radiation is too weak in the radio wave region, blackbody radiation in the solar radio wave radiation is very important, and the main radiation is radiation from charged particles in the solar atmosphere. When the sun is in a quiet period of the 11-year cycle (such as this year, the sunspot activity is pitiful), the quiet sun is very weak, and when the activity is intense, a large flare of radio radiation will increase the sun's radio wave radiation by a hundred times. the above.
In 1932, Jansky first discovered the radio radiation in the universe. It is easy to think that this radio source should of course be the sun, just like the sunlight makes everything eclipse when it is white. However, no. The first radio source found was the core of the Galaxy, which was more than 30,000 light-years away. In 1937, Rebe created the world's first radio telescope (9 meters in diameter), used it to observe the sky, drew the first radio map, and observed many radio sources in the universe. But his search for solar radio failed in failure because it was a quiet period of solar activity. It was not until 1942, when the sun "wake up", the British air defense units found that their radars were often subjected to sudden and intense interference. At first they thought it was a secret weapon of Germany, and later discovered that it was a solar radio.

For most ordinary stars similar to the sun, their radio is very weak. For example, South Gate II, the A/B two-star radio is so weak that it is almost impossible to observe. On the contrary, it is the weakest adjacent star in the visible region, because there will always be a change of light. In the case of a change, it is a bright star in a radio telescope. Therefore, the stars do not have much interference with the intelligent life that they want to communicate with radio waves (they should live next to the more stable stars).

The solar radiation of the sun is emitted almost uniformly in all directions and is distributed over all wavelengths. When the planetary radar station transmits signals, the orientation is very good, concentrated in a small angle, and the transmission power is also concentrated in a small wavelength range (the corresponding frequency range is also small). In this way, in a certain direction and a specific frequency, Axirebo overwhelming the sun is simply easy. If we want to communicate with the South Gate II technical level and our civilized communication, it is enough to send the antenna directly to the South Gate II. However, it takes nearly nine years for each dialogue to come and go.

It seems that it seems that it is not difficult to detect extraterrestrial wisdom and civilization, as long as they also send radio signals. However, don't forget how much we have made in order to make Asi Rebo's strength exceed the sun. First, the orientation of the launch is directional and is only sent to a small area in the sky. Second, the frequency is also limited to a small range. In this case, we must receive a radio signal sent to us by an alien civilization. Our radio telescope must first point in that direction at the right moment, and secondly, it should be received at the same frequency.

If you want to have a big grasp, such as monitoring the signal in 1000 light years, considering that there are millions of stars in 1000 light years, then to monitor so many stars, you have to monitor each star at the same time. A lot of frequencies are really a big project. However, the difficulty is mainly in the quantity, not technically. Of course, even if you monitor so many stars, data analysis is a big job to scare people. Now SETI@home is actually a project that launches a piece of analysis.

The current monitoring frequency, the most commonly used is the 21 cm wavelength, the corresponding frequency is 1420 megaHz, which is just between the single frequency (900 MHz) and dual frequency (1800 MHz) of the GSM mobile phone. The reason why this frequency is particularly favored is because it is the most abundant element in the universe - the radiation frequency of neutral hydrogen atoms, and the area with the least radio noise in the universe, and the Earth's atmosphere is also very transparent to this frequency. "Taking it for granted", this frequency deserves special attention from all civilizations that wish to make radio connections. In order to express respect for astronomers, this frequency is near the frequency exclusion zone in the international radio standard, and the operating frequency of the radio equipment must not be set in this band.

Of course, there may be this possibility. Aliens have long disdain to use the backward communication method of radio, but to use more advanced methods, such as neutrinos, gravitational waves or simply wormholes, etc. . Earth's human beings are backwards, only knowing that radiocommunication organisms are not considered to be eligible to join civilized clubs (such as "Country Teachers"), so of course everything is a matter of rest. But if they are willing to take us to play, then the Earth already has enough technology, if it is willing to invest in manpower and resources. Of course, we must be able to endure pure unilateral communication. At present, no one will be able to get a conversation for a thousand years.

Humans' monitoring of space civilization has continued, but now governments seem to have little open interest. Of course, many people think that the government is communicating with aliens in private. One of the reasons why NASA's application for funding was dismissed in the US Congress is that instead of spending so much money to find smart life in space, it is better to find a few smart people in Washington, although the truth is that the budget even buys half of the B-2 wings. Not enough. However, relying on money from other places, these plans are still in progress, and there have been many advances in technology. Now the scope of monitoring is much larger than the Ozma plan, the sensitivity is much improved, and the signal analysis capability is greatly enhanced. As for money, like Allen (the partner of Gates), he is willing to pay for a large radio telescope dedicated to this search.
Of course, until today, our search for alien civilization on the radio is still the cause of nothing, and we humans are hard to blame. There is no difference between a group of unspoken people in the dark and a group of nephews. So far, in general, we have followed the tenet of “Don’t dare to speak loudly, fear the world’s people”, instead of “singing their voices and asking for their friends”, the public has been sending radio signals to the outer stars only Once, in 1973, Axirebo to M13 (the globular cluster mentioned in the first post), the distance is as far as 24,000 light years, so for us who are "not satisfied with life," even if "long-year-old worry" Also can't manage it. Of course, as for the type of government and private non-public communication of the Red Bank, there is no way to explain it.

Of course, humans have also used other methods to express their existence to the outside world. For example, the pioneers 10/11 each brought a gold-plated aluminum plate with some lines. It is expected that hundreds of thousands of years will fly to a certain star. Near (about a few light years from the star). This kind of information is not so much a communication as it is a drift bottle.

So, are we humans better hiding in the universe? Regret to tell the monks worry about the sky, No! Although humans have been in radio communication for more than a hundred years, we have created too many noises to leak our existence. For example, from the high-power signal, the planetary radar station will inevitably have a leak signal. When the US-Soviet high-power radar (provided a 90-second warning before the intercontinental missile, so that there is time for everyone to die) is a moment. Constantly transmitting high power radar signals. In terms of life, microwaves can travel through the ionosphere. The signals transmitted by TV stations are also simultaneously broadcast to the universe at the same time, although aliens watching signals may complain about poor signal quality. Even if we turn into fiber-optic communications in time, the signals that have been sent out over the past few decades have long been overwhelming, and are expanding outward at the speed of light. They have already swept through South Gate II, Sirius and many more distant planets. If there are observers in the South Gate II, it can already be found that the Earth is not normal.

Any alien civilization that can come to the earth must be technically as powerful as a god. Of course, how they treat us depends entirely on their own decisions. However, even if it is a believer, it will not be ridiculous to drop the pie in the sky. It is absurd for human beings to stop progress because they do not know where the alien civilization is.

If human beings are not willing to perish, building a better world, and understanding the wider universe is an inevitable choice.
Ps: According to the plan, the FAST radio telescope will be built in Guizhou at the end of this year and will be completed in 2013. This is a multi-country cooperation project, technically mainly Sino-British cooperation. The FAST structure is similar to Axirebo, but the diameter is up to 500 meters and the sensitivity is much improved. This will also be part of the progress of human understanding of the universe.

Main reference:
Liu Cixin's "Three-body" (Ref. "Wandering Earth" "Country Teacher")
"China Encyclopedia - Astronomy"
"100 Billion Suns"
Carl Sagan's Exploration of Alien Planet
Axirebo Observatory