If the earth stopped rotating, where could we survive?

Produced by | Science Popularization China

Author | Coco

Producer | China Science Expo

When you were a kid, you looked at the night sky and saw the moon looming behind the hills, but by late at night it was already overhead. The teacher would tell us, "That's because the earth is rotating all the time. The alternation of day and night, the rising of the sun and the setting of the moon are all due to the rotation of the earth."

Image source: Veer Gallery

Is it really that simple? If you dig deeper, you may find that you can't actually answer the following two seemingly simple questions.

1. Why does the Earth rotate? Do other planets also rotate? Are there any celestial bodies that do not rotate?

2. What would happen to the Earth if it stopped rotating? Would anyone be able to survive such a catastrophe?

This time, let us persist in finding out how many secrets we don’t understand are hidden in the rotation of the Earth.

1

Why does the earth keep rotating?

Let me first state the conclusion. The existence of celestial bodies that do not rotate is only possible in theory, but in reality all celestial bodies are undergoing some form of rotation.

There are two main reasons for this: first, the birth process of a celestial body will inevitably have a certain initial angular momentum; second, a celestial body with zero angular momentum is in an unstable state, and this situation cannot be maintained for a long time even if it appears for a moment. It sounds a bit confusing, but it doesn't matter. Let's explain the above two reasons in plain language.

From a formal point of view, the formation of planets can be roughly divided into two situations. The first is that the materials that form celestial bodies gather together, constantly collide and rub, and finally form planets. It is like making cotton candy, the melted sugar threads are ejected from the nozzle of the machine, gradually entangled to form a large lump. The second is that the celestial body that already existed exploded for some reason, ejecting certain materials, and these materials formed celestial bodies again. It is like a flying disc thrown into the air being hit by a shotgun, ejecting countless small particles.

An image of the formation of the solar system. Image source: Public domain

From these two formation methods, no matter which one, the sum of the angular momentum of each part of the celestial body is unlikely to be exactly zero. In other words, the celestial body will naturally "rotate" (that is, the celestial body has a certain initial angular momentum when it is born). And according to the law of conservation of angular momentum, this rotation cannot be stopped. It is for this reason that the celestial body has been rotating since the moment of its birth.

Now let's look at the second reason. Assuming that there are indeed celestial bodies with zero initial angular momentum, this "perfect state" cannot be maintained for a long time. This is because there is gravity from other celestial bodies everywhere in the universe, and the rotation and revolution of these celestial bodies may cause changes in the gravitational state. It is like a boat that loses power in a stormy sea. Even if it is stationary at the beginning, it will eventually drift with the current.

The space probe captured a complete rotation of Eros in early December 2000 (period is about 5 hours and 16 minutes) | Image source: NASA

Combining the above two points, for celestial bodies in the universe, not rotating is relative, while rotating is absolute. This is also in line with the Marxist philosophical view of "relative stillness and absolute motion".

Some people have raised questions - the moon always faces the same side towards humans, doesn't this mean that the moon does not rotate? In fact, this is a specious fallacy.

We can use our right fist as the Earth, and the palm of our left hand as the side of the Moon facing the Earth. When you try to move your left hand around your right hand while keeping your palm facing your right fist, you will find that your left hand actually has to keep rotating. (In fact, the Moon's rotation and revolution are synchronized, a phenomenon called tidal locking or captive rotation.)

Schematic diagram of the moon's rotation. Note the difference in the orientation of the moons on the left and right sides. The moon on the left is always rotating, and only one side is facing the earth. 丨Image source: Wikipedia

2

What would happen to the Earth if it stopped rotating?

The Earth has been rotating since its birth. In the billions of years, the Earth's rotation period has been changing, but the state of rotation has never changed. If we let ourselves go and imagine that the Earth suddenly stops rotating, what consequences will occur?

We know that the angular velocity of the Earth's rotation is the same everywhere, but the linear velocity is different, with the largest velocity at the equator and the smallest velocity at the poles. The linear velocity of the Earth's rotation near the equator is about 465 meters per second, so if you are at the equator and lie flat at home for a whole day, you can also achieve "traveling 80,000 miles a day while sitting on the ground." Even in mid-latitude areas like Beijing, the linear velocity of rotation can reach 370 meters per second.

Assuming that the earth stops within 0.1 seconds, all objects not attached to the surface will maintain their previous state of motion and fly out of the earth under the action of inertia. According to calculations, objects attached to the surface near Beijing at this time will be subjected to an acceleration equivalent to 370g (g represents the acceleration of gravity, 9.8m/s2), and even if they are not broken into pieces, they will be broken into pieces.

Of course, the impact on high latitudes will be relatively small, so how small is it? If the latitude reaches 80 degrees, after the earth stops rotating, the human body will still fly out in the direction of rotation, and the linear speed will still reach 60 meters per second, and there will be no chance of survival.

However, if the Earth does not stop rotating in an extremely short moment, but stops slowly, the situation will be slightly better. For example, if we assume that the Earth stops rotating within 1 minute, the acceleration experienced by people in Beijing in the direction of the Earth's rotation will be less than 1g. Although it is still very scary, if there is a proper buffer, there is still hope to save life or even be unscathed.

Many people would say that since the surface of the earth is not safe, then I will just wait in the sky by plane until the earth stops rotating. This is of course a good idea, but we must know that the atmosphere on the surface of the earth is originally synchronized with the rotation of the earth, that is, it has the same state of motion.

Once the earth stops rotating, the atmosphere will still move in the direction of rotation due to inertia, which means that you will face a super storm with a speed of nearly 1,440 meters per hour over Beijing. No matter what kind of aircraft it is, it is probably unable to withstand such violent airflow. So what will happen on the sea? Once the earth stops rotating, even the Bohai Sea, which is not at a low latitude, will set off supersonic waves. The entire surface of the earth will be swept by an unprecedented super tsunami. However, the more terrifying thing is yet to come.

If we were to be more rigorous and take the law of conservation of energy into account, a giant like the Earth actually contains a huge amount of kinetic energy when it rotates. If the Earth stops rotating, this kinetic energy will be converted into internal energy, which means that the Earth will generate heat when it stops rotating.

According to estimates, the internal energy converted when the Earth stops rotating can raise the Earth's temperature by hundreds of degrees. On the surface where the kinetic energy is the greatest, the temperature rise may reach thousands of degrees. At this time, the surface of the Earth will undoubtedly be a scene of purgatory, and even the entire Earth may melt into liquid.

In short, if the earth really stopped rotating tonight, no matter where you are, everyone will be shattered in an instant, and the earth itself will disappear from the universe. However, even if the period of the earth's rotation will change, the rotation itself will still exist forever, and the earth's cessation of rotation is just a thought game.

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