Science has always been so mysterious and interesting that it contains the logic of seeking truth and cognition.
Scientists in ancient Greece believed that the reason why people can see the objects around them is because their eyes can emit tentacle-like light, and they can see wherever they shine. Of course, this is an unscientific conjecture of the ancients, but it is also quite similar to modern active radar search and ultrasound imaging. We now know that light is a generous gift of the universe, not laboriously emitted by the human eye, which is only responsible for receiving photons from the light source.
The greatest light source on earth is the sun, which has been tirelessly pouring light and heat into the vast universe since time immemorial. The earth receives only one-sixth billionth of its energy, so the seasons will change, everything will rise and fall, there will be wind, rain, thunder and lightning, and the rivers and seas will churn.
The infinite energy comes from the nuclear fusion reaction in the inner center of the sun. Two lighter atoms are tightly pressed together by the huge mass of the sun. Under the combined action of high temperature, they merge into a heavier atom, but the mass of this atom is less Less than the sum of the previous two atomic masses, the lost mass, according to the famous Einstein mass-energy equation, becomes energy and is released in the form of photons.
But these poor photons, in the sun with frequent nuclear explosions, have to constantly collide with other particles, going through hundreds of revolutions and thousands of times, before they can squeeze onto the surface of the sun. At this time, it has been 5,000 years since the thermonuclear reaction that gave birth to it. year.
The sun spews flames thousands of kilometers high, throwing photons into the depths of the universe. Some of them reach the earth after about eight minutes of flight, they pass through the dense atmosphere, sprinkle on mountains and rivers, sprinkle on streets and buildings, and finally enter our retina, triggering the electrical response of visual cells, forming the brain The images make us feel light and warm. Every ray of sunshine we see today was born in the era of Yao and Shun five thousand years ago.
"Light" as we know it
When the sun completely falls into the horizon, the scattering of sunlight in the atmosphere disappears, and the starlight is clearly visible. At this time, looking up at the sky, the starlight is bright. Except for a few planets in the solar system that reflect sunlight, the rest are all luminous stars. Their light-emitting mechanism is the same as that of the sun, but the distance is very far, and the farthest is even billions of light-years, which means that the starlight you see far exceeds the history of human beings, and may even be older than the sun. Possibly, by the time you see starlight, the star that gave it off is long dead, and starlight is its last echo in the universe.
Although ancient starlight cannot bring light and heat to the earth, it has played an important role in navigation, archaeology and technological progress. For example, people in ancient times sailed on the sea and did not dare to leave the coastline too far. Later, people found that the position of the North Star is unchanged (the North Star is directly above the North Pole, and does not change its position in the starry sky with the rotation of the earth), then according to the height of the North Star, you can infer your own latitude: if the North Star is overhead, then observe The observer is near the North Pole at high latitudes, and if not far above the horizon, the observer is near the equator at low latitudes. As long as the latitude does not change during navigation, the course must be a spherical circle line coaxial with the equator.
Because the earth has been autobiography, latitude is easy to obtain, longitude is difficult to obtain. Finally, the superb watchmaker John Harrison made a pocket watch with high accuracy. People can know the longitude of the place based on the time of departure and arrival, supplemented by the calculation of the position of the stars. Once the latitude and longitude can be obtained, ocean voyage is no longer dangerous, and the era of great sailing has really come.
The regular star movement has faithfully witnessed the historical changes, which has provided great help for archaeologists to determine the age. The Chinese believe in the unity of heaven and man, and strange celestial phenomena often herald great opportunities. Therefore, when recording major events, special star images are also recorded, which is used to determine the exact date of the event and even the observer's geography. location offers the possibility.
Starlight also proved general relativity for Einstein. In the 17th century, Newton proposed universal gravitation. In the 20th century, Einstein was not satisfied with what he knew about gravity, but was diligently exploring why. He believes that the essence of gravity is the distorting effect of the mass of an object on space-time. If space-time is compared to a flat sponge mattress, and the sun is compared to an iron ball, then the space-time around the sun is like a mattress dented by an iron ball. , the passing objects are affected by the depression and have a tendency to move towards the center of the depression, forming the external manifestation of gravity, which is the intuitive discussion of the famous general theory of relativity.
After the theory is put forward, people will doubt it. How can it be proved? He thought of starlight and the sun, and the distortion of starlight by the sun is a powerful proof of general relativity! To explain more intuitively, we can illustrate with a fictitious detective story:
"Bang!" Gunshots sounded, and a bullet passed through the glass in the living room, leaving a hole in it and a bullet hole in the wall of the living room. Soon, the police rushed to the scene and extended the connection between the bullet holes in the wall and the glass outwards, pointing to the woods diagonally behind the opposite building, where the police frowned. Although the trajectory of the line pointed to that location, the prisoner could not be caught there. At this time, the detective "Sherlock Holmes Einstein" came. He took out a pen and paper, substituted the ballistic data into the relativity formula to start the calculation, and then touched the mustache and told the police that the gunman was hiding directly behind the building opposite.
Right behind? The police expressed disbelief, "Can a bullet turn a corner?"
"The building opposite is of great quality. The gunman's original intention was not to hit the old Zhang's house, but the building distorted time and space. When the bullet passed by, the trajectory changed, and the glass in the living room was damaged." The detective explained the principle. According to the detective's reasoning, the police found the gunman according to the formula, and the story ended.
The schematic diagram of the story "Bending Bullet" and the schematic diagram of the 1919 solar eclipse observation.
Of course, the story is fictional, but the science is real. Back to the experiment, the building opposite is the sun, and the gunman is a star behind the sun. The light emitted by the star behind the sun would not be directed towards the earth, but when passing through the space-time distorted by the sun, it no longer propagates in a straight line. Eventually it made its way to Earth. Now it is enough to prove that the star behind the sun can be seen from Earth, and general relativity can be proved correct. The problem is that the sun is strong, and the faint starlight is submerged in it. It is impossible to distinguish it at all. Only when a solar eclipse occurs, can you observe it. In 1919, there was a solar eclipse on the earth. Two observation groups went to Brazil and Africa respectively. Not only did they photograph the star behind the sun, but the refraction angle data was consistent with the general theory of relativity, which was inconsistent with Newton's gravitational calculations. on the altar of science.
The Story of the "Photon Family"
Light in a broad sense, however, contains wider spectral lines, from infrared all the way to gamma rays. Visible light only accounts for a small part of it, and the rest are collectively referred to as invisible light. It is light, but invisible, this seemingly contradictory physical word contains philosophical speculation. What we see may not exist, such as a mirage. What we cannot see does not necessarily exist, such as beautiful flowers, if they are irradiated with ultraviolet rays, they can show more beautiful patterns, and the eyes of bees can see ultraviolet rays, so they can distinguish different flowers.
Infrared has the longest wavelength, the lowest frequency, and the lowest energy, while gamma rays have the highest energy. Take a simple example to illustrate the relationship between wavelength and energy: fix one end of a long rope, hold the other end and shake it up and down, the rope will move in waves. If you sway lazily and slowly, the distance between the crest and the trough is longer, that is, the wavelength is longer and the frequency is lower; if you sway quickly up and down, the distance between the crest and the trough is shorter, that is, the wavelength is shorter. short, high frequency. Of these two methods, the energy you pay is obviously much greater in the latter, because you will soon be tired and weak. So where did the energy go? becomes the undulations of the rope. So waves with shorter wavelengths and higher frequencies carry more energy. We can also see this from the color of the flame. Without the interference of metal ions, the higher the flame temperature (higher energy), the more the color tends to be blue-violet (higher frequency and shorter wavelength).
The king of destruction in the photon family is the highest frequency gamma photon. It has high energy and strong penetrating power. When the photon family irradiates the object, some cause vibration and heat (such as infrared rays), and some let the electrons fly from the lower orbit to the upper orbit to continue running. Only γ and x photons can knock out electrons in molecules or atoms. called ionizing radiation. In nature, there are a small amount of gamma photons from the universe and natural radioactive elements, which are called background radiation. Because animals and plants have a certain ability to repair themselves, they are harmless. But if the gamma photon dose exceeds the standard, the speed of repair cannot keep up with the speed of destruction, and animals and plants will become sick or even die.
In 1999, workers at a uranium conversion plant in Japan violated operating procedures in order to go home early, resulting in high doses of gamma rays and neutron radiation. For the employee closest to the radiation, the DNA of his body cells was all damaged by high-energy particles. The government department knew that he would surely die, and they still invested in the latest medical technology to rescue him. They may also want to conduct a treatment test on him for severe radiation damage, but he is still in the hospital. Died tragically after 83 days. Starting from a fertilized egg, the human body has been in a constant process of metabolism. Old cells are constantly dying and falling off, and new cells are constantly being produced and updated. DNA is the "blueprint" for cell replication. Now that the "drawings" are all damaged, it is impossible to produce normal new cells. In addition, its tragic situation can be imagined. Another employee who was a little further away was also tortured by radiation during the 220-day treatment, and the rescue failed. Microscopic examination of their dead cells revealed only one type of cell that was not affected by the radiation: cardiomyocytes. This also leaves a direction for scientific breakthroughs.
The lethality of gamma photons is amazing. It is also one of the killing weapons of nuclear weapons. When a nuclear bomb explodes, a large number of gamma photons and neutrons are produced. How to block gamma photons? Under the condition of a certain thickness of the protective material, the denser the material, the better, so considering the cost performance, the lead element with a large atomic weight is generally used for the barrier. Theoretically gold is also possible, but no one can afford it.
X-rays were discovered by German Professor Roentgen (1845-1923), so they are also called Roentgen rays. Their energy is slightly lower than that of γ-rays, and they are also composed of high-energy photons, but they can still cause ionizing radiation, so long-term or large doses are not acceptable. receive irradiation.
A schematic diagram of the electromagnetic spectrum. In the entire spectrum, "visible light" is only a very small segment.
Gamma rays and x-rays are two ferocious members of the photon family, but they are also contributing to humanity. For example, in medicine, gamma rays are used in radiation therapy to precisely kill tumor cells in the body; X-rays are used in medical imaging to help doctors see the details of lesions inside the human body. If gamma rays are the workhorse of radiotherapy, then x-rays are the backbone of radiology. There is also a demand for non-destructive testing in the industry, so these two members can be used according to actual needs.
Less energy than x-rays is ultraviolet light. But it also has certain dangers. It has been reported that children mistakenly entered the room sterilized by ultraviolet light and stayed there for a period of time, causing irreversible damage to the cells of the fundus. Ultraviolet rays in sunlight can cause melanosis, which is the enemy of beauty girls, but moderate exposure to ultraviolet rays can promote the body to produce vitamin D and maintain bone health.
No matter how much energy is smaller than ultraviolet light, it is the visible light we introduced earlier. Visible light is harmless to the body. However, according to the previous introduction, I believe everyone can infer that blue-violet light energy is stronger, so long-term watching TV, computer Or mobile phones, pay attention to blue light protection, long-term blue light will cause harm to the macula under the eyes.
In the light family, the least energy is infrared light, but it has a special ability, that is, the thermal effect. If gamma and x photons are kings of destruction, then infrared photons are gentle masseurs. Macroscopically static matter, the molecules inside vibrate and rotate all the time, and only stop completely when the temperature drops to absolute zero (-273.15 degrees Celsius). Infrared can intensify the vibrations of molecules, which rub against each other to generate heat. The longer the wavelength of infrared light, the deeper it can penetrate into the interior of an object. Therefore, the deep thermal effect of far infrared is more obvious. If the wavelength is longer, it will run out of the scope of light discussed in this article and enter the microwave band. You must think of microwave ovens, and its heating effect is from the inside out.
The light from the stars illuminates the earth day after day, subtly affects the earth, and makes life gradually sprout. Although there are times when the sun's rays rush in violently and high-energy cosmic rays arrive unexpectedly, the earth has gradually developed a series of protective mechanisms, such as the atmosphere, magnetic field and ozone layer, to protect the prosperity of all things. Finally, 1.8 million years ago, in the cave of Xihoudu, Ruicheng County, Shanxi Province, the first beam of light controlled by human beings was revealed, which opened the prelude to the light of human beings.
