There is one thing to say, “The Wandering Earth 2” was shot really well. In the face of the epic narrative and Chinese-style positive energy, the set of Marvel Thanos snapping his fingers suddenly seemed too childish and boring.
“The Wandering Earth 2” movie poster
However, if you like it, you like it. Since you have said all the good things, as a nuclear engineer, it is meaningless for us to add icing on the cake, so we have to pick up a magnifying glass and find fault with it, picking out its “little problems”.
Friendly reminder, the following content involves slight spoilers , friends who have not watched the movie, you can share this article to Moments first, and come back to read after watching the movie.
Nuclear fusion in “The Wandering Earth 2”
Have you noticed that Da Liu likes nuclear fusion very much ? The main settings of “Wandering Earth 2” are almost all related to nuclear fusion:
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Solar helium flash is a kind of nuclear fusion;
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The planetary engine used to push away the earth also relies on heavy nuclear fusion to generate a thrust of 150 trillion tons, which is so large that only the Eurasian continent and the American continent can bear it;
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Many of the 3,614 nuclear bombs detonated on the moon were hydrogen bombs based on the principle of nuclear fusion;
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The point is, these nuclear bombs also triggered fusion in the core of the moon, shattering the moon and saving mankind!
Nuclear fusion is the process by which multiple lighter nuclei combine to form a heavier nucleus .
For example, the nuclear fusion on the sun can be generally considered as the fusion of four hydrogen nuclei into a helium-4 nucleus (consisting of two protons and two neutrons).
Since the mass of the helium-4 nucleus is lighter than the sum of the four hydrogen nuclei, the lost mass is converted into energy, so the nuclear fusion inside the sun can release huge energy, making the sun continuously emit light and heat.
As for the subsequent helium flash, three helium 4s fused into a carbon 12 nucleus.
The energy of the sun comes from nuclear fusion in the core of the sun | Screenshot of the “Wandering Earth 2” trailer
Deuterium-tritium fusion occurs in hydrogen bombs in nuclear bombs and in controlled nuclear fusion devices . Deuterium and tritium also belong to the hydrogen element, but a deuteron is a proton plus a neutron, and a tritium is a proton plus two neutrons.
Deuterium-tritium fusion will form a helium 4 and a neutron, which can also release a lot of energy, so the hydrogen bomb explosion is particularly powerful, far exceeding the atomic bomb.
Deuterium-tritium fusion process, generate a helium 4 and a neutron, and release energy
In the big wave of nuclear fusion in “The Wandering Earth 2”, the most reliable ones are the more than 3,000 nuclear bombs made by humans , which can really explode, are genuine, and are innocent.
The heavy nuclear fusion of the planetary engine and the more amazing lunar nuclear fusion can be described in one sentence: it is difficult to move God!
Well, I believe many people will think that my brain is locked up by sophons (the TV series “Three-Body Problem” is also very popular now), but under the current laws of physics, I can only draw such a conclusion.
planetary engine
The planetary engine in “The Wandering Earth 2” | Screenshot of the movie trailer
Let’s talk about the 12,000 extremely spectacular planetary engines first. Without them, the earth would not be able to set off to wander.
Planetary engines are probably the most powerful engines ever described in science fiction. Liu can imagine this thing, and the movie can make it convincing, the author really admires it!
In terms of scientific principles, planetary engines basically make sense . Planetary engines are fueled by rocks, and the major component of rocks is silicon. Silicon nuclei can be fused to generate iron nuclei and release energy at the same time.
The judge asked, why not use deuterium-tritium or hydrogen as fuel? These can all be extracted from sea water. The reason is that the planetary engine consumes too much, and the seawater on the earth is not enough to burn… So, it is better to use stones, which are everywhere, inexhaustible and inexhaustible.
The original novel did not elaborate on the structure of the planetary engine, but only mentioned that its height reached 11 kilometers and its bottom diameter reached 50 kilometers. Trucks pour rocks in from the feed port, and the nozzle on the top of the engine will shoot a huge jet of plasma upward.
The movies “The Wandering Earth” and “The Wandering Earth 2” have reproduced these huge engines on the screen, and they are full of details. The author speculates that the planetary engine should originate from the current magnetic confinement controllable nuclear fusion device, namely the Tokmak .
The planetary engine under construction in “The Wandering Earth 2” | movie poster
Even deuterium-tritium fusion, which has a lower threshold, requires harsh conditions. This is because atomic nuclei are all positively charged, and in order to initiate fusion, they need to be brought close to a distance of 10 -15 meters, which is smaller than a hundred-billionth of a hair. As the so-called same- sex repulsion , the charge carried by the nucleus will produce a huge repulsion force in such a short distance.
To overcome this repulsive force, extremely high temperatures are required . Realizing deuterium-tritium fusion on the earth requires a high temperature of about 100 million degrees Celsius, even higher than the center of the sun.
The temperature at the center of the sun is only about 15 million ℃. It can achieve fusion because the core pressure is extremely high and the mass is extremely large. Even though the temperature is low and the probability of fusion is low, its huge mass makes the total power of fusion still large.
At a high temperature of 100 million degrees Celsius, no matter can remain solid and become plasma, and no container made of any material can hold these plasmas, so a magnetic field must be used to confine the plasma The body is in direct contact with the container wall.
The tokmak device confines the plasma in a circular vacuum chamber for fusion. The International Thermonuclear Test Reactor (ITER) currently under construction is a tokmak, capable of achieving fusion energy that is 10 times the input energy, but there is no controllable nuclear fusion device that can achieve continuous output of nuclear fusion energy.
The International Thermonuclear Test Reactor under construction | ITER
If the planetary engine achieves continuous and controllable silicon nuclear fusion, it still needs to overcome a big problem, that is, how to convert the energy into the momentum of the plasma jet.
The fuel density in the vacuum chamber of the current Tokmak device is very low. Even if the temperature reaches hundreds of millions of degrees, the pressure is not high. The current highest pressure is only 2.05 atmospheres. With this little pressure, it is impossible to produce a high-speed jet like a planetary engine.
“The Wandering Earth 2” movie poster
Therefore, the planetary engine must achieve nuclear fusion under extremely high density and extremely high pressure . To control such a high-density plasma, an unimaginably strong magnetic field is required, and the feed and jet flow cannot destroy the stability of the plasma. . This level of difficulty is unimaginable with the current scientific and technological capabilities, and perhaps future humans will be able to do it.
But can this kind of silicon nuclear fusion really be realized on earth?
heavy nuclear fusion
Considering that the planetary engine burns stones, that is, the fusion of silicon nuclei, the conditions for this relatively heavy nuclear fusion are more stringent than those for deuterium-tritium fusion, because the greater the atomic number, the greater the charge it carries, and the greater the repulsion force. powerful.
Hydrogen nuclear fusion can occur at 15 million degrees Celsius inside the sun, or even lower, but for helium flash to occur, that is, from three helium 4 fusions to one carbon 12, it needs 100 million to 200 million degrees Celsius, and Accumulate a large amount of helium 4 like a star in its later years.
For the real sun, it will take at least billions of years to achieve this condition, so there is no need to worry about the solar helium flash crisis within 100 years like the setting of “The Wandering Earth”.
For the fusion of two silicon nuclei (14 protons + 14 neutrons), the repulsive force between the charges is much greater than that of helium fusion, so you can imagine how difficult it will be.
The direct fusion of two oxygen nuclei (8 protons + 8 neutrons), whose atomic number is much smaller than that of silicon, requires a high temperature of 1.5 billion ℃ and a high density of 10 million tons per cubic meter.
How high the conditions are needed to achieve the fusion of two silicon nuclei , there are few literatures in this area, and some articles say that 3 billion ℃ is required, which is a reasonable guess.
Therefore, even in the interior of a star, the fusion from silicon nuclei to heavier nuclei is not simply adding two silicon nuclei, but continuously absorbing helium 4 nuclei, increasing the atomic number by 2 for each absorption. It takes 7 additions to fuse nickel 56 nuclei. This nucleus is very unstable and will quickly decay into cobalt-56 and then into stable iron-56 (26 protons + 30 neutrons).
The Process of Iron 56 Formed by Internal Fusion of Massive Stars
But such a process cannot be realized by our sun, and a star with a mass more than 8 times that of the sun is needed to finally produce iron 56.
In other words, the planetary engine in the wandering earth needs to simulate the internal environment of a massive star several times larger than the sun, so that it is possible to burn “stones” and achieve silicon fusion. But the problem is that there is not so much helium on the earth. Helium resources have always been scarce, and they are not as easy to find as stones.
Therefore, the planetary engine must achieve a high temperature of 1.5 billion to 3 billion degrees Celsius and an unimaginably high density , so that the silicon nucleus and oxygen, magnesium, or itself can be defused.
Do you think this condition can be realized on earth?
Even if it is assumed that it can be realized, there will be a basin of cold water next, that is – the fusion of silicon into iron does not release a lot of energy !
This is because the specific binding energy of atomic nuclei is different.
The specific binding energy is the energy required to completely disassemble a nucleus into nucleons, on average, per nucleon. The higher the specific binding energy, the more stable the nucleus.
Specific binding energies of different nuclei
From hydrogen or deuterium-tritium fusion to helium 4, the specific binding energy increases extremely fast, so a lot of energy is released, but from helium 4 and then all the way up to iron 56, the specific binding energy increases very slowly, and the released energy is very limited.
For this part of the limited energy to achieve the extreme conditions at the core of a massive star, even if it can be done, it may not be worth the loss .
Nuke the Moon
After talking about the “burning stone” planetary engine, let’s talk about blowing up the moon in “The Wandering Earth 2”.
This brain hole is really big, and people have to admire the screenwriter’s imagination. After all, the original novel just pushed away the moon instead of blowing it up.
But to blow up the moon, it is obviously not enough to rely on 3614 nuclear bombs. It is also mentioned in the movie that the energy produced by these nuclear bomb explosions is only one billionth of the energy required to destroy the moon. Because although the moon is much smaller than the earth, it is still very large, with a diameter of 3476 kilometers. Even if there are tens of thousands of nuclear bombs, it will only scratch the moon’s itch and stir up a cloud of dust.
Therefore, the setting in the movie is to arrange these nuclear bombs in a huge crater to form an array, and detonate the nuclear bomb array precisely to trigger fusion of the moon’s core, and then blow up the moon.
Detonate the moon | “The Wandering Earth 2” trailer screenshot
These nuclear bombs only function as a “trigger” or “fuze”, which is a bit like the principle of a hydrogen bomb: an atomic bomb is used as a fuze to trigger the fusion of deuterium and tritium.
However, during the detonation process of the hydrogen bomb, the atomic bomb does not directly rely on high temperature to initiate fusion, but uses the X-rays generated during the explosion to compress and heat the fusion fuel to achieve nuclear fusion conditions.
The moon’s shell is also mainly made of stone, which is not conducive to the propagation of X-rays. Therefore, the author speculates that the shock wave generated by the nuclear explosion may be used to achieve instantaneous compression of the center of the moon to trigger lunar nuclear fusion.
Reasoning up to this point, everything is reasonable, but then it is outrageous. Because the moon, like the earth, has an iron core, that is, the main component of the moon core is iron 56.
As mentioned earlier, the volume of the moon is not small, and it is larger than Pluto, so the differentiation of elements will occur inside it. In the early days of the moon’s birth, the inner matter was in a molten state, so the heavy matter kept sinking and the light matter kept rising. Because metals such as iron are relatively heavy, they gradually “sink” toward the center to form a metallic core, mainly iron, with small amounts of sulfur and nickel.
internal structure of the moon
Iron 56 has a great reputation in the universe, its specific binding energy is the highest, and its structure is more stable than any other atomic nucleus (except hydrogen). Fusion from iron 56 upwards will no longer release energy, but needs to absorb energy , and requires unimaginable extreme conditions- for example, when the shell of a massive star collapses inward and hits the core at the moment of a supernova explosion, or when two neutron stars merge Time.
Elements with a higher atomic number than iron on Earth, such as gold, silver, and uranium, are mostly formed under such extreme conditions.
Schematic diagram of neutron star merger moment | University of Warwick/Mark Garlick
With more than 3,000 nuclear bombs, can such conditions be formed? Obviously not!
Even if it is really wrong, with a very small probability, a little bit of iron 56 at a certain point undergoes fusion, because this fusion is an energy-absorbing reaction, it will not produce a violent explosion, the shock wave of a nuclear explosion The energy is absorbed directly.
Some people say that lunar nuclear fusion can also be regarded as a supernova explosion, because there is also an iron core before the supernova explosion. This idea is completely wrong.
A supernova is a big explosion at the end of a massive star’s life. The reason for the explosion is not because of the fusion of iron cores, but because the massive stars have burned up all the things that can be fused, and the remaining iron cores are not fused, and they lose their gravitational resistance. The force of the collapse, under the huge gravitational force, the outer shell of the star crashed down on the inner iron core, which triggered a big bang.
Schematic diagram of a supernova explosion
Therefore, the energy source of the supernova explosion is the gravitational potential energy of the star itself, rather than the fusion of the iron core. A planet as small as the moon does not need fusion to resist gravitational contraction at all, so supernova explosions are impossible.
So, want to blow up the moon? Even if these more than 3,000 nuclear bombs were handed over to God, he could only spread his hands: It is really impossible!
Then again, as a sci-fi movie, “The Wandering Earth 2” requires a huge imagination, and some unreasonable things in science are inevitable. As long as it can meet the needs of the plot and promote the development of the plot, it makes sense logically!
Just like the author, although I picked so many problems, I was still very interested in watching the movie in the cinema, and I still couldn’t get enough of it after watching it. It can definitely be called the pinnacle of science fiction movies.
Not much to say, this is the second brush!
Author: Sagittarius A
Edit: Steed
Cover image source: “The Wandering Earth 2” movie stills
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