Can the "ladder to heaven" in "The Wandering Earth 2" really be built?

The movie "The Wandering Earth 2" is three hours long, and its huge capacity is not given for nothing. The first disaster that is used to set the atmosphere alone would have supported the entire film if it were somewhere else. In this first disaster, the unlucky one is the "space ladder", which is easy to have bad luck in various science fiction works.

The so-called space ladder, sometimes simply called the space elevator, is a hypothetical way to enter space. Ordinary buildings and ladders are not strong enough to withstand compression. They will collapse before they are piled up a few thousand meters high. They cannot rise directly into space like the pea vine in the fairy tale. However, the tensile strength of many materials is far better than their compressive strength. If humans build a ladder with very tensile materials in synchronous orbit and hang it from the ground, they can make the ladder relatively still with the ground, and then they can climb into space along this ladder.

In "The Wandering Earth 2", humans built a ladder near Libreville, the capital of Gabon - this is in line with the laws of physics. Libreville is almost on the equator, and a standard ladder needs to be built on the equator. This is because the space station is above the equator. Building a ladder on the equator can ensure synchronization of the upper and lower positions, and can rely on the acceleration generated by the centrifugal motion of the earth's equator to maintain the force balance of the ladder with the earth's gravity. If you choose to build a ladder at other latitudes, you will need additional means to ensure that the ladder will not be torn apart due to uneven force. In the film, a group of candidate astronauts took the space ladder to the space station for training. When they were tortured to death by the acceleration of up to 9 g (g is the acceleration of gravity, 9 g can be simply understood as 8 people as heavy as you are pressing on you), the ladder was attacked by terrorists, and three terrorists were lurking in the cabin. As a result, the two protagonists could only fight with the terrorists in the cabin while watching the drones and fighter jets outside the window perform a dogfight fireworks show.

There were dozens of astronauts in the car, none of them were cowards, and their physical fitness was good enough, so why was it so difficult to fight three unarmed terrorists? It turned out that in order to protect the astronauts under high-g conditions, everyone was locked tightly in a roller coaster-like safety device. Most people couldn't get out at all and could only stare blankly in their seats. Only a few people were able to unlock the safety device by chance and fight the terrorists.

However, if this journey is handed over to a qualified aerospace engineer, it probably won't be so strenuous. Because the space elevator actually does not require such high-g conditions, nor does it require such complicated and strict safety devices - if it insists on being like in the movie, it will completely lose the meaning of the elevator.

What is the meaning of the ladder?

Imagine you have to go up five floors. If there were stairs, it would be simple: take one step, one step, and then take another. Each step requires very little energy, and briefly uses the support force to overcome gravity and push the body up a few dozen centimeters. Even if you are very weak, it doesn't matter. As long as you have enough strength to take one step, you can eventually climb to the fifth floor - at most it will take a little longer.

But if there were no stairs, it would be difficult. Once a person's feet leave the ground, they can no longer provide upward force, so they must release all the energy to reach the top in a few tenths of a second. Considering that they need to overcome air resistance during the ascent, more energy is required. Superman can jump to the fifth floor in one jump, but ordinary people cannot produce such a large amount of power. Although both go from the first floor to the fifth floor, the total amount of gravitational potential energy that needs to be overcome is the same, but one is very easy and the other is as difficult as climbing to the sky.

The principle of the ladder is of course different from that of the stairs, but the actual effect is almost the same. If there were no ladder, humans would need rockets to go to the sky, and the rocket would need to continuously release gas to generate thrust, so that the rocket could rise to the sky by relying on the reaction force of the thrust, which could cause it to explode. On the contrary, with a ladder, this huge energy can be divided into many small parts, with low power requirements and less waste. Theoretically, if there really is a staircase connecting to space, you can walk up one step at a time. The world record for long-distance stair climbing by humans is 12 hours to climb 13,145.65 meters of stairs. If a person can climb like this for 12 hours a day and rest for 12 hours, it will only take a little more than a week to enter space (100 kilometers above sea level is the boundary between the atmosphere and space). (Of course, this requires air in the stairwell, and food and water reserves at intervals)

It is also worth mentioning that one of the important reasons why rockets are difficult to build is that they must carry their own fuel, and in order to carry this fuel on the road, more fuel is needed. Although this cycle is not endless (a lot of fuel is burned on the way and does not need to be carried the entire way), it also means that sending one ton of objects into space often requires 10 tons or more of fuel. The elevator does not have this problem - just build a generator on the ground and power the motor of the car throughout the journey.

Therefore, a real space elevator can ascend to the sky as slowly as a high-speed train, which is its biggest advantage. We don't know how far the target space station of the astronauts in the film is from the ground, but it doesn't seem to be very far away - it fell down not long after the explosion. If the height of this space station is similar to the height of the International Space Station in reality, that is, about 400 kilometers, then using the speed of high-speed rail, it will only take a little over an hour to reach it, which is completely acceptable. Using such a large acceleration of 9 g is purely tormenting - in fact, the acceleration of a manned rocket during launch in reality is only 3-4 g.

What's more, even if the purpose is to torment these candidate astronauts who have no medical insurance, they should at least use a car designed for high g - the car in the movie also seems to have some problems.

The first problem is the appearance. As an object that needs to withstand 9 g acceleration, the top and bottom of the car are flat. This is a problem.

The reason why rockets grow into rocket shape is largely to reduce air resistance and improve energy efficiency. Air resistance is proportional to the square of speed, so it only needs to be considered at high speeds. Normal rockets must reach super-high speeds of several thousand to more than ten kilometers per second, and need to withstand huge air resistance. It is definitely not cost-effective to make a flat box rocket.

There is no speed requirement for the space elevator's cabin, and it can go up slowly. In that case, it is fine to make it a flat box. However, it is not possible to make it both flat and high-speed. The film only shows the fire generated by the bottom of the cabin hitting the atmosphere when going down, but it is safe when going up. This is very strange. After all, visually there is no obvious difference in the speed of going up and down, so there should be huge resistance and fire.

Of course, visual judgment is inaccurate, and it is possible that the car is actually moving at a very low speed - but this is even more strange, the purpose of high g is to reach the highway as quickly as possible, and it only takes 0.3 seconds to accelerate from 0 to 100 kilometers at 9 g. Using high g but not high speed will cause a lot of disadvantages without any benefits. This is obviously unreasonable.

The second problem is more serious and deserves more elaboration: the astronauts' seats were facing the wrong way.

Now, everyone knows that astronauts have to go through acceleration training before going to space. Training can make people less prone to dizziness, vomiting or coma, but no matter how much training, there are still physical and physiological limits in front of them. The human body evolved in an environment of 1 g. The mechanical strength of all parts of the human body is only adapted to this environment, and the most vulnerable link is probably the blood. Even in an environment of 1 g, the failure of blood pressure control takes many lives every year, and it is even more difficult in a high-g environment.

When facing upward acceleration, the blood will become very "heavy" and flow toward the soles of the feet of people who are standing or sitting, which will cause cerebral ischemia. Generally speaking, ordinary people cannot withstand 5g upward for more than one or two minutes, and then they will fall into a coma. Contemporary fighter pilots can withstand a maximum of about 9g for a short period of time, but in addition to long-term training, they also need the help of special pressurized suits, which use many air bags in the legs and abdomen to pressurize the blood and sometimes require chest pressure adjustment to assist breathing. Judging from the smooth postures of the people in the film when fighting, it doesn't look like they are wearing pressurized suits (of course, it may also be that pressurized suits have become super advanced 20 years later).

This is not the worst. What is even more terrifying is that when the ladder descends, the negative acceleration generated by the deceleration movement before it touches the ground will cause blood to flow back to the brain. This will prevent the cerebral blood vessels from bursting and causing death on the spot - the human body is extremely alert to the increase in cerebral blood pressure. Once the carotid sinus detects abnormal blood pressure, it will quickly stop, but the acceleration does not listen to it. All it can do is to continuously reduce the heart rate and arterial pressure. As a result, the abnormal blood cannot maintain the oxygen content difference between the artery and vein, and will first fall into a coma due to lack of oxygen. In short, people's tolerance for negative acceleration when the ladder descends is even lower.

There is no other way for pilots to maneuver in the air, but the astronauts' journey to space is very simple. Forcing them to resist 9 g acceleration is not only unnecessary, but also there is no safety margin in case of any failure, which will definitely not be approved by engineers. Besides, the solution to this problem is actually ridiculously simple: just let people lie down and fly to space.

When lying down/prone, the human brain and heart are basically at the same height, and the impact of the heavier blood is greatly reduced. This does not mean that there are no problems. The human respiratory system will still be under considerable pressure, the blood oxygen concentration will decrease, the vision will also be significantly reduced, and the limbs will not be able to move freely, but it is still no problem to survive a dozen g for one or two minutes. If you have to choose one of the two, then "lying down to accelerate and lying down to decelerate" is better, because this posture puts the least pressure on the retinal blood vessels.

Of course, a group of people lying in the car not only looks silly, but also takes up a lot of space. It is understandable that ordinary low-g elevator cars choose roller coaster-like seat design. However, ordinary cars should not force people to take 9 g. Such illegal overload operation will disintegrate even if no one deliberately sabotages it.

Author: Fan Gang

Review｜Zhou Xiaoliang Senior Engineer, Beijing Jiaotong University Physics Laboratory