What happens when a needle hits the earth at the speed of 300 thousand kilometers per second? Humans were almost sent away.

In Liu's science fiction novel "Three-body", there is a weapon called "light particle" in higher civilization, which refers to an object with small volume and mass but close to the speed of light, which can easily destroy the earth and even stars. After all, this is science fiction, which sounds a bit wild, but in fact it's all based on physics.

Asteroids almost sent humans away.

65 million years ago, an asteroid with a diameter of 10 km crashed into the Yucatan Peninsula in the Gulf of Mexico at a speed of 20-40 km per second, surpassing the dinosaurs that dominated the earth for hundreds of millions of years. The impact caused a huge roar to sweep the global coastline, and the smoke and a lot of sulfur dioxide ejected from the earth filled the air, blocking the sun. Eventually, plants gradually withered after losing photosynthesis, and the earth's food chain collapsed, and about 95% of biological species became extinct.

This is the closest of the five mass extinctions on earth, but it happened before the rise of mankind. However, humans were almost sent away by asteroids, which was called the Tunguska Big Bang.

1On the morning of June 30th, 908, residents near the Tunguska River in Siberia, Russia were awakened by a loud explosion. The shock wave produced by the explosion shattered the windows of all the surrounding houses, and all the residents who got up early in the street were knocked to the ground.

According to the description of the residents at that time, there was a fire that split the sky in two, just like the end of the world. Only 1 min after the explosion, the entire tunguska coniferous forest was ignited, and the area within 2000km of Fiona Fang, Fiona Fang was razed to the ground, and more than 80 million trees were burned. According to the damage caused by the explosion, scientists predict that the power of this explosion is 1000 times that of Hiroshima atomic bomb, which is about 10 megaton TNT.

The cause of the Tunguska Big Bang has always been a mystery. In 2008, the research team of Siberian Federal University in Russia gave the most reasonable answer so far on the basis of collecting a lot of evidence and computer simulation. The paper describes that the explosion originated from an asteroid with a diameter of nearly 200 meters, which rushed into the earth's atmosphere at a speed of 20 km/s, and exploded at 10 km above the earth due to intense friction with the atmosphere, bursting out with huge energy.

Fortunately, although the asteroid entered the atmosphere at a very close angle to the earth, it finally only passed through the stratosphere. This explains why there were no craters at the site of the Tunguska Big Bang, and no related meteorites were found, only a very small amount of debris. The explosion that just passed through the earth is so powerful that it is hard to imagine what would happen if an asteroid hit the earth.

Back to the topic, what would happen if an advanced alien civilization hit the earth with light particles and fired a needle at the speed close to the speed of light? According to Einstein's theory of relativity, the earth will become interstellar dust.

kinetic energy

A friend asked, an asteroid is so big and powerful. What kind of splash can a needle dig out? The key to solve this problem lies in a physical concept: kinetic energy.

Whether it is an asteroid or an embroidery needle, the amount of energy released depends not only on the quality, but also on the speed. Like a bullet. If you just throw it with your hands, you won't hurt anyone. If the Heksulubo meteorite 65 million years ago crashed into the Yucatan Peninsula at the speed of 1cm/s, then almost nothing would happen on earth.

In classical physics, the kinetic energy formula of object motion is:

(Classical physical kinetic energy formula)

M is mass and v is speed. According to the kinetic energy formula, we can calculate that the kinetic energy of the Hik Sulubo meteorite is about 4.2 10 23J, which is equivalent to137.6 billion times of the Hiroshima atomic bomb and 2 million times of the Tsar's hydrogen bomb. If the asteroid that produced the Tunguska Big Bang hits the earth, it will release 55,000 times as much energy as the Hiroshima atomic bomb.

If an embroidery needle (assuming only a small mass of 1g) hits the earth at the speed of light (300,000 km/s), its kinetic energy is about 4.510/3j, which is about the power of Hiroshima atomic bomb. With this ability, is the so-called light particle strike a joke?

Theory of relativity

In fact, according to Einstein's special theory of relativity, any object with mass can't reach the speed of light, but can only approach the speed of light infinitely, so there is no needle flying at the speed of light. In addition, classical physics is only applicable to the low-speed range. When the moving speed of an object is at a high speed (greater than110 light speed), especially when it is very close to the light speed, classical physics will produce very large errors.

(m0 is the mass of pointer 1g, and m is the dynamic mass of pointer).

As described in the mass-velocity relation in relativity, the faster the relative velocity of an object, the greater its mass. When the speed of an object is infinitely close to the speed of light, its mass will become infinite. On the other hand, if the speed of the needle is much less than the speed of light, substituting the speed v into mass-velocity relation's formula, which is about 1 under the root sign, you will get m=m0, so the moving mass can be ignored at low speed, and the energy can be calculated by classical physics.

When the pointer is infinitely close to the speed of light, we use the kinetic energy formula of relativity. M0c^2 is the inherent energy of the magnetic needle at rest. When the speed v of the needle is infinitely close to the speed of light c, it will be infinitely small under the root sign, so the kinetic energy of the needle is infinite, which will destroy the earth and even the whole universe.

Of course, if you want to accelerate the needle to infinitely close to the speed of light, you need infinite energy, which is so great that the whole universe is not enough. Therefore, it is a question of advanced civilization gaining infinite energy and then striking with light particles.