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1905 is a very important year in the history of physics. the theories born in this year laid the foundation of physics throughout the 20th century, and all these theories were put forward by the same person, Albert Einstein (1879-1955).

Albert Einstein in 1905, Einstein published four papers in succession. Each of these four papers is of epoch-making significance-the first one explains the photoelectric effect and puts forward the concept of photons, which is a great development of quantum theory; the second one explains Brownian motion and provides an important proof of the existence of atoms; the third one puts forward the special theory of relativity, which is formally born. The fourth part reveals the deep essence of the mass-energy relationship. The mass-energy equation is popular all over the world with its concise and beautiful form and has become synonymous with the theory of relativity. Later, 1905 was called the year of Einstein's Miracle.

This year, Einstein was just 26 years old. It has been five years since Planck put forward the idea of quantization of energy, but in these five years, quantum theory has not developed at all, and well-known professors from European universities are still busy mending the buildings of classical physics. No one can realize what energy quantization means, and Planck's work is almost uninterested. Even Planck himself is mired in deep self-doubt, and there is a serious inherent contradiction in his derivation of the formula of blackbody radiation, which makes him feel that the quantization of energy may be an expedient and difficult to get into the hall of elegance, so he has been trying to find out how to go back to the framework of classical physics to derive the formula of blackbody radiation.

A year ago, no one would have thought that the most talented physics genius in Europe would be a clerk in the Swiss Patent Office. At this time, Einstein, who did not join any academic organization, only organized a book club called the Olympia Academy of Sciences with a few friends who loved science and philosophy.

Several young people are not in the academic circle, they have daily work to support their families, but from the bold name "Olympia Academy of Sciences", it can be seen that they are a group of ambitious young people. They squeeze out weekends or off work to get together and discuss topics they are interested in-philosophy, physics, math and literature-while reading.

The reason why Einstein's academic career started from the patent office was not that he was unwilling to enter the academic palace, but that no university could accept him. Einstein often skipped classes when he was in college, which left a bad impression on the teacher.

It's not that he doesn't like learning, but that what the teacher says is out of date and can't meet his needs, so he skips class and goes outside to study by himself. He read through the works of Kirchhoff, Hertz, Boltzmann, Lorentz, Maxwell and other physics masters, and understood the most cutting-edge contents of physics, but this did not help him much in his graduation examination. after all, the focus of the teacher's examination was not within the scope of his reading, which led to his poor graduation grades.

In 1900, the year Planck proposed energy quantization, Einstein graduated from college, and he wanted to stay as a teaching assistant, but his teacher rightly turned down a student who was always skipping classes. Then he sent cover letters to European universities and even secondary schools, but did not respond. After wasting two years, he found a job as a patent office technician with the help of his good friends in the university, and finally did not become an unemployed young man.

After the stability of his work and life, Einstein no longer had to worry about supporting his family. He could calm down and study his beloved physics. Even if he could only do research in his spare time, it was rare for him. He always keeps a keen eye, tracks the frontier progress of physics, and studies all directions of physics.

"Light" is a focus that Einstein always pays close attention to. Whether it is the speed of light or the nature of light, it is what he thinks about. During this period, he not only learned about Planck's solution to the problem of blackbody radiation, but also thought about another strange problem related to light-the photoelectric effect. Einstein also won the 1921 Nobel Prize in Physics for the photoelectric effect.

In 1887, the German physicist Hertz confirmed the existence of electromagnetic waves for the first time through experiments, and then he proved that light waves are electromagnetic waves, which fully verified Maxwell's electromagnetic theory. However, Hertz not only verified the classical electromagnetic theory, but also found an abnormal experimental phenomenon-photoelectric effect.

The photoelectric effect, as the name implies, is the effect of electricity produced by light. Metals are made up of atoms, and atoms are made up of nuclei and electrons.

Hertz found that by irradiating some metal plates with ultraviolet light, the electrons in the metal can be knocked out, and a voltage is applied to the two opposite metal plates, and the electrons generated will form an electric current. This phenomenon aroused the interest of many researchers and soon obtained a large number of experimental results, but the electromagnetic wave theory encountered serious difficulties in explaining these experimental results.

It has been found that the key to whether or not to produce electrons lies not in the intensity of light, but in the frequency of light. Ultraviolet light can easily extract electrons from metal, but visible light cannot. At that time, people were puzzled about this, because according to the classical wave theory, the intensity of the wave represented its energy, and as long as the light intensity was enough, the electrons could get enough energy to get rid of the shackles of the metal surface. therefore, light of any frequency should be able to produce electrons, but the experimental result is that no matter how strong visible light can produce electrons, which is completely contrary to the theoretical prediction.

Nearly 20 years have passed since the photoelectric effect was discovered, but this problem is still unsolved. As the saying goes, newborn calves are not afraid of tigers. In the face of such recognized scientific problems, young Einstein did not flinch. His keen intuition told him that the classical electromagnetic theory mainly describes the overall properties of macroscopic light. Blackbody radiation and photoelectric effect are essentially related to the production process of microscopic light, since Planck solved the problem of blackbody radiation through energy quantization. Then the problem of photoelectric effect should also be enlightened.

Why is the frequency of light so important in the photoelectric effect? Einstein stared closely at Planck's quantum formula of energy:

Estrangh v

According to this formula, the energy carried by the energy quantum is only related to the frequency of light. When the light hits the metal surface, the energy quantum is constantly hitting the metal surface, so what does the energy quantum mean? Is it a short wave? Is the tiniest vibration? Or something? According to Planck's paper, Planck did not give a clear picture of the quantum of energy, and this image should be crucial.

Lost in thought, Einstein, as if the old monk had settled in, remained motionless, and no one could see that his genius brain was running at a high speed. Gradually, there seemed to be a picture in front of him: the quantum energy of light was like bullets fired into the metal, and the electrons hit by the bullet got the energy of the bullet and broke free from the shackles inside the metal.

Got it! Einstein slapped the table and sprang to his feet. whether the electron can be shot out depends entirely on how much energy the bullet can provide to the electron! He walked around the room excitedly, then sat down at the table and picked up the draft paper and quickly deduced it. Soon, a formula sprang up on the paper:

Electron kinetic energy = h v-electron escape work

This formula means that the quantum of energy provides the electron with the energy of h v, which becomes the kinetic energy of the electron in addition to helping the electron break free from the shackles on the metal surface (the electron escapes work).

At this point, the image of the quantum of energy is completely clear in Einstein's mind. This small portion of electromagnetic radiation energy is not a small wave, but individual particles, which are inseparable and can only be absorbed or emitted by the whole. Einstein named these energy point particles photons, which were later renamed photons.

Einstein's photon theory explains the photoelectric effect very well. Because the energy of each photon is fixed h v, when the light irradiates on the metal surface, the energy absorbed by electrons mainly depends on the energy of a single photon rather than the intensity of light, which is only the density of photon flow.

Because the frequency of visible light is low, the energy of photons is not large enough to overcome the escape work of electrons, so it is impossible to shoot electrons. But the ultraviolet ray frequency is high, the photon energy is high, so it is very easy to hit the electron.

It was bold for Einstein to put forward the photon hypothesis because there were not enough experimental facts to support his theory.

It was not until 1916 that Milligan, an American physicist, made a comprehensive verification of his theory. Interestingly, when Millikan was doing the photoelectric effect experiment, he wanted to overturn Einstein's photon theory, so he had been doing the experiment for 10 years, and he kept improving the accuracy of the experiment for 10 years, only to find that the higher the accuracy of the experiment, the more it can prove the correctness of Einstein. In the end, he had to admit Einstein's theory and measured the Planck constant more accurately by the way.

After Einstein made it clear that light has particle properties, he further put forward the momentum formula of photons according to the theory of relativity.

P=h/ λ

In the formula, p is the momentum of the photon; λ is the wavelength of light; h is the Planck constant.

In 1923, the American physicist Compton and his student Wu Youxun confirmed that photons do have momentum through the verification experiment of Compton effect, which provided irrefutable evidence for the particle nature of light.

There has been a long debate about the nature of light in history. At the end of the 17th century, there was a long-term debate between the particle faction represented by Newton and the volatility school represented by Huygens.

In the 18th century, people discovered the interference and diffraction of light, and the wave theory gained the upper hand. In the late 19th century, with the advent of electromagnetic theory, people made it clear that light is electromagnetic waves, and particle theory was completely abandoned. As a result, a few years later, Einstein put forward the photon theory again and made clear the particle nature of light, so how should we treat the fluctuation of light this time? In fact, Einstein has given the answer in two formulas of photon theory. Let's take another look at these two formulas:

Electron energy (photon energy = Planck constant × frequency of light)

P=h/ λ (photon momentum = Planck constant / wavelength of light)

These two formulas look simple, but in fact they are not. Because Einstein connected particles with waves through these two formulas: the energy and momentum of particles are calculated by the frequency and wavelength of waves. In other words, Einstein endowed light with the properties of both particles and waves, and light has wave-particle duality!

The discovery of wave-particle duality is a major breakthrough in man's understanding of the nature of light, and the resulting "butterfly effect" will bring about a great leap in man's understanding of the material world, which will be made by the French scientist de Broglie 18 years later. At this time, de Broglie is only a 13-year-old boy, he is immersed in history and literature, determined to become a historian in the future.

Wen Yuan: "telling Quantum Science to teenagers" author: Gao Peng part of the picture source network copyright belongs to the original author. Editor: Zhang Runxin this article comes from Wechat official account: Origin Reading (ID:tupydread), author: Gao Peng

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