Bathed in cosmic rays

This stream of high-energy particles is all around us. How will they affect us?

At present, we often think that space is very far away, but it is actually closely connected with us: at this very moment, “fragments” from the depths of space are constantly passing through our bodies. These “fragments” are brought about by cosmic rays.

Radiation from high altitude

In 1912, an Austrian physicist discovered cosmic rays. He was Victor Francis Hess.

An amazing thing is that he received his doctorate from the University of Graz, Austria in 1905, when he was only 23 years old. After that, he accepted a teaching position from the University of Vienna. At that time, Madame Curie discovered radium, which had a great impact on the scientific community. So Hess decided to study the hottest subject in physics: radiation. He was fascinated by an unsolved problem at the time: an isolated electroscope, even if there is no radiation source around, it will always detect the charge. A reasonable guess is that the minerals on the earth will produce ionizing radiation, causing the electroscope to have an electric charge. If this is the case, then the electroscope readings will decrease in places higher than the ground.

Based on the above speculation, scientists believe that assuming that the ionizing radiation generated by surface minerals is uniformly distributed, then after 10 meters of elevation, the radiation amount will become 83% of the surface radiation. At an altitude of 1000 meters above the ground, the radiation amount should be only 0.1% of the surface radiation.

But the facts are just the opposite of what was estimated. In 1910, the French scientist Theodor Wolfe used electroscopes to measure the bottom and top of the Eiffel Tower. He found that more ionizing radiation was measured at the top (about 300 meters above the ground).

So, will the source of ionizing radiation come from the sky instead of the earth? Beginning in 1911, Hess took his instrument in a hot air balloon for 10 lift-off tests in 3 years. As the hot air balloon rose, he found that the amount of radiation detected would rise rapidly. When he reached an altitude of 5,000 meters, the reading turned out to be more than twice the value detected on the surface. Later, he published a paper announcing the discovery of a very penetrating radiation from outside the atmosphere.

Hess needs more data. He took a risky night lift-off, and the amount of radiation detected was no different from that during the day. Hess also carried out a liftoff on April 17, 1912. On that day, there was a solar eclipse, and most of the sun’s rays were blocked by the moon. But the amount of radiation he detected did not attenuate. This basically eliminates the possibility that the source of radiation is the sun.

Therefore, Hess declared in 1913 that the radiation does not come from the sun or minerals from the earth, but from the deep space of the universe. More than ten years later, Hess’ discovery was confirmed by American physicist Robert Millikan, and the term “cosmic ray” was first proposed by Millikan. In 1936, for his work on cosmic rays, Hess won the Nobel Prize in Physics that year.

“East-West Effect”

Millikan initially believed that the radiation detected by Hess might be gamma rays, but his hypothesis turned out to be wrong. In 1927, scientists discovered that the intensity of cosmic rays changes with the latitude of the observation ground: the equator has the lowest radiation intensity, and the two poles have the highest radiation intensity. If cosmic rays will be affected by the earth’s magnetic field, it is easy to explain this phenomenon: because the earth’s two poles have the strongest magnetic field, charged particles are easily attracted to the vicinity of the two poles. Therefore, they may be a kind of charged particles, rather than a kind of photon.

In 1930, scientists also detected that cosmic rays coming from the west were more intense than those coming from the east. This is because the charged rays are deflected under the influence of the earth’s magnetic field. According to the left-hand rule (the rule for judging the force direction of charged particles in a magnetic field) learned in high school physics textbooks, positively charged particles are more likely to enter from the west, and negatively charged are more likely to enter from the east. The above phenomenon called the “East-West effect” shows that cosmic rays are not only charged, but most of them are positively charged. This means that most of them are protons or atomic nuclei.

By the end of World War II, scientists had basically figured out the composition of cosmic rays, and discovered that the intensity of cosmic rays reaching the earth from all directions was equal.

Roughly, atomic nuclei constitute 99% of cosmic rays. Among them, about 90% are protons, or hydrogen nuclei; about 9% are alpha particles, or helium nuclei, composed of two protons and two neutrons; the remaining 1% are nuclei of heavier elements. In fact, the composition ratio of various elements in cosmic rays is exactly the same as that of the universe itself!

Since 99% of cosmic rays are atomic nuclei, what about the remaining 1%? Scientists have discovered that most of this 1% are ordinary electrons, which makes scientists feel strange. There are basically as many electrons as there are protons everywhere in the universe. So, why don’t electrons account for half of the cosmic rays? This question cannot be answered so far.

Extremely high energy

Many cosmic rays have extremely high energy, and they cannot come from within the solar system. The sun’s radiation is usually very weak, unless there is a violent activity, such as a coronal mass ejection-the sun ejects a cloud of coronal matter outwards within a few minutes to a few hours, and the energy of the radiated particles can reach several trillion electron volts. Even so, cosmic rays are much stronger than the sun, with energy ranging from 100 meV to 10 gigaelectron volts.

Astronomers believe that cosmic rays may come from violent astronomical events that occurred in distant space, such as supernova explosions. However, there are still some cosmic rays whose energy is incredibly high, and it is difficult to make a reasonable explanation for them.

When we talk about the energy of cosmic rays, we are actually talking about their speed. If the energy of a cosmic ray is 100 meV, it means that its speed is roughly 43% of the speed of light, which is extremely fast. However, many cosmic rays have energies of 10 gigaelectron volts, which means their speed is 99.6% of the speed of light.

These ultra-high-energy cosmic rays, we still don’t know where their crazy speed comes from. Astronomers once believed that some colliding galaxies, such as the Tentacle Galaxy in the constellation Corvus, are the source of these cosmic rays, but after analysis, they found that their energy is still not high enough. Today, active galactic nuclei are considered the most likely source. 10% of the galaxies in the universe have their cores active. There are supermassive black holes in the nuclei of active galaxies, which can accelerate the particles surrounding them to astonishing speeds and eject them.

Another bold idea is that mysterious dark matter particles will decay into a pair of high-energy protons, one of which will fall into a black hole, and the other will travel through the universe, so such protons become ultra-high-energy cosmic rays.

We are always in the rain of cosmic rays

A common sense is that things that move faster usually produce more destructive power. However, the cosmic rays that enter the earth’s atmosphere will continuously collide with the molecules contained in the air as they pass through the atmosphere. The collision will produce smaller atoms and other particles, and the resulting “debris” will pour down to the ground at the speed of light, like rain. But as they gradually approach the surface, their energy will gradually decrease. The cosmic rays that reach the ground will not cause significant harm to humans and all other lives.

Among these debris, there is a particle called a muon. A muon is a negatively charged elementary particle with a mass about 208 times that of an electron. Studies have shown that about 240 muons pass through your body every second. Radiation-derived particles such as muons may cause damage to the genetic material inside the cell nucleus. In 2016, a study published in the journal Nature showed that there is indeed a connection between cancer incidence and cosmic ray activity.

However, under normal circumstances, only about 1/10 of the natural radiation that people experience each year comes from cosmic rays. The remaining natural radiation comes from the air, the earth and food. And those who live in high altitude areas will be exposed to more cosmic rays. But if you choose to live underground, most of the particles from cosmic rays will not hit you.

When we fly in an airplane, we suffer from additional cosmic rays. So, how will the health of those pilots and crew members be affected? Investigations show that they are exposed to more than twice as much radiation as other people, and the probability of cancer due to radiation is about 1% higher than that of ordinary people.

Will cosmic rays ruin space dreams?

In the 1940s and 1950s, many people worried that after rising to a high altitude, any creature might not be able to withstand too much cosmic rays, which might hinder people’s flight to space. In order to find out whether this is really the case, the U.S. Air Force used a German V-2 rocket to send some animals into the outer atmosphere of the earth. At first, animals such as fruit flies and mice were sent. Later, the researchers sent primates to the outer atmosphere.

On June 11, 1948, the monkey entered the outer atmosphere for the first time. In 1954, researchers began to use high-altitude balloons for testing. After dozens of test flights, the animals returned to Earth unharmed. In 1955, researchers finally confirmed that cosmic rays are non-lethal to future astronauts, although some studies have shown that cosmic rays increase the risk of astronauts suffering from cancer.

From 1968 to 1972, during the Apollo moon landing program, the astronauts who participated in the moon landing experienced an experience they had never had before-they took the risk and left the protection of the earth’s magnetic field. The result is unexpected, even peculiar. Every astronaut sees a flash of light in his field of vision every 2.9 minutes. NASA doctors put forward an explanation at the time, and it was later confirmed: this flash is produced when powerful cosmic rays pass through the eyes of astronauts.

At this point, when you read this article to try to understand cosmic rays, thousands of particles from cosmic rays have passed through your body. Unless, you are willing to run to the underground mall to avoid them temporarily.

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