Section 1? Old quantum theory

One or two dark clouds

Our story begins with the birth of modern physics.

Since Galileo invented the telescope to open the door to the universe, our physics has been developing out of control, showing a thriving situation, and it was not until Newton appeared that our physics gradually became mature and perfect.

Newton devoted a lot of efforts to physics, discovered and perfected various physical theories, which played a vital role in the development of physics today, and gradually formed a modern classical physics system composed of thermodynamics, energy conservation law and statistical physics. Since then, people have lived in the world of classical physics.

Looking back at the beginning, the classic physical picture constructed by people was so beautiful and intoxicating that it made an indelible contribution to the development of mankind. At that time, people thought that the classical physical system could explain almost all physical phenomena, the basic laws of the world had been discovered, and physics had reached its own limit and end, and it was impossible to make any breakthrough.

However, by the end of19th century, all kinds of new physical theories had been surging in the warm world of classical physics, which seriously impacted the glorious building of classical physics, among which the most famous one was the "two dark clouds" hanging over physics.

Time flies to 1900, which is a turbulent year.

That year, China was under the rule of the Qing government. In China, Eight-Nation Alliance is waging a fierce war with the Boxer Rebellion. In the west, it is experiencing a period of great development after the industrial revolution.

1900 In April, it was still foggy over London. Lord Kelvin gave a speech at the lecture of the Royal Institute in the thick fog.

Kelvin, who is over 70 years old, said in his speech with a strong Irish accent: The building of classical physics is close to perfection. Physicists in the future only need to repair the edges and corners, only need to improve the precision after the decimal point of the constant by several digits, and the rest is just to do some work to check for leaks and fill gaps. Of course, the keen Lord said nothing. Finally, the Lord paused and whispered, of course, there are still disturbing "two dark clouds" floating in the clear sky, which makes our physics look pale and powerless.

Lord Kelvin may not have thought that a speech in this foggy season will go down in history from now on, and "two dark clouds" will leave a strong stroke in physics from then on.

It is these "two dark clouds" that finally triggered a storm that swept the physics community. Kelvin can find two dark clouds and worry about them, which proves that he is far-sighted.

"Two dark clouds", one is Michelson-Murray experiment and the other is blackbody radiation. The former, in order to verify the existence of the legendary "ether", Michelson designed a set of quite wonderful experimental instruments-Michelson interferometer. However, the experimental results strongly deny the existence of ether. This result puts classical physics in a dilemma, and the physics community suddenly falls into confusion. By what, why is the speed of light constant at 300 thousand kilometers per second?

In the end, "this dark cloud" made scientists give up the ether theory and re-examine the nature of light, thus producing the theory of relativity.

In addition, the latter dark cloud refers to the inconsistency between the blackbody radiation experiment and the theory. At the time of Kelvin's speech, there was no clue to solve this inconsistency. The final result of this "dark cloud" is to push quantum mechanics onto the stage of world science and begin to exert its powerful power.

A long time ago, people began to notice that there was a certain connection between heat and radiation of an object. For example, if a piece of metal is heated on a fire, its color will change with the increase of temperature and become dark red. As the temperature rises, it will turn orange. At extremely high temperature, if it is not vaporized, you can see blue and white. That is to say, there is a certain proportional relationship between the light emitted by the thermal radiation of an object and its temperature. The question is what is the functional relationship between the light emitted by an object and its temperature.

In order to find this relationship, many physicists have devoted a lot of research. In theory, physicists have assumed an ideal object-blackbody, as the standard object for thermal radiation research. Black body is a pure black object, which can absorb all external radiation without any reflection, and the absorption rate is 100%.

Until the end of 19, scientists studying this theory have not made any progress.

So is light a particle or a wave? Without solving this problem, physics cannot continue to develop, and when Kelvin described the "second dark cloud" on the stage, people didn't know what the final result of this problem would be. However, the arrival of the new century 1900 also brought a new era of physics. Max Planck, the protagonist of quantum mechanics, will open a new situation in physics.

Second, the energy quantum theory

Planck, a German, was born in 1858. At first, Planck's research interest was originally concentrated in the field of classical thermodynamics, but in 1896, he showed great interest in blackbody radiation.

On the basis of the failure of other scientists, Planck decided to abandon all traditional theories and find new ones to study blackbody radiation. Finally, he found that in order to establish the blackbody radiation formula, it is necessary to make an assumption that light is not continuous when it emits or absorbs energy, but one after another! Only in this way can the blackbody radiation formula be derived.

This decision seems simple, but it is a very remarkable discovery, because it is completely contrary to all physical concepts at that time. If it is true, it is tantamount to subverting the cornerstone of physics and making the whole physics have to be rebuilt.

For example, when water is heated to the boiling point 100 degrees Celsius, we take it for granted that the water temperature will reach 50 degrees Celsius, 60 degrees Celsius, 70 degrees Celsius and 99 degrees Celsius at a certain moment. In short, the water temperature must pass all the values before 100 degrees Celsius, and at a certain moment it is exactly equal to that value, and it will continue to slowly rise to 100 degrees Celsius.

People have never doubted the function of continuity and smoothness, because it is the fundamental basis of calculus and the physical system of Newton and Maxwell. But now Planck stands up and says that it is possible for the water temperature to reach 100 degrees directly without passing a certain value before 100 degrees. How is that possible? This is really a strange statement.

Planck's theory shows that light also has a minimum unit, and these minimum units are combined to form light, just like when we go up stairs, we have to go up at least one step at a time, and it is impossible to go up1/2,3/4 steps, where each step is a minimum unit. And the same is true of light, one by one according to the smallest unit. Between these two smallest units, it is a forbidden area that we can't know. In other words, light cannot be infinitely subdivided. There is a minimum energy unit, and light is the expression of the combination of these energy units.

And crucially, we can calculate the exact number of the smallest unit from Planck's formula, which is about equal to -34 joules per second of 6.63× 10. This unit is very small, as small as astronomical figures, followed by more than 30 zeros, but such a small number has become the most important constant in science and is named Planck constant.

Planck also found that his blackbody radiation formula can be deduced theoretically if he makes the following assumptions: for radiation with a certain frequency ν, an object can only absorb or emit in hν as the energy unit, and H is called Planck constant. In other words, an object can only absorb or emit electromagnetic radiation in quantum form, and the energy of each quantum is E=hν, which is called the action quantum.

From the point of view of classical mechanics, the concepts of energy discontinuity and single discrete are absolutely not allowed. Planck's hypothesis that a single quantum harmonic oscillator absorbs and radiates radiant energy is a new one, which seriously impacted classical physics and was strongly opposed by physicists at that time.

Planck's energy quantum theory tells people that the vibration of energy quantum can occur not only in light, but also in any other matter, and any matter is continuous and discrete, which subverts the traditional classical mechanical concept.

With the passage of time, people gradually realize that Planck's quantum theory is correct and the hypothesis is well-founded and gradually accepted by people. Planck's quantum-to-energy theory has changed people's thinking, changed our understanding of the world and subverted all previous physical theoretical phenomena, which is a leap in the history of human development.

Please remember 1900, 14 One day in February, Planck read out his famous paper "Energy Distribution in Black Body Spectrum" at the German Physical Society. This day was later recognized as the birth day of quantum mechanics, and the Planck Institute was established to commemorate his outstanding achievements. At present, the Planck Institute in Germany is the largest, most prestigious and most successful autonomous scientific organization funded by the government in the world.

The quantum concept put forward by Planck is a great discovery for the modern scientific community. From then on, an invisible ghost began to wander in people's hearts, which was no less than Newton's law of universal gravitation. Quantum theory has opened up a brand-new subject field, and any scientific development in later generations basically involves quantum related theories, which also marks the birth of quantum mechanics.

Although Planck put forward the energy quantum theory, most scientists don't care about his energy quantum theory, because his theory is not as good as classical mechanics, and even most scientists oppose his results, thinking that his theory is just a hypothesis without any experimental basis. Even Planck himself thinks there is something wrong with his theory In this way, the energy quantum theory was completely cooled for 15 years and did not play a decisive role in physics. It was not until Bohr succeeded in 19 15 that energy quantum theory entered the stage of scientific development. It is no wonder that neither he nor others fully understood the related concepts of quantum at that time, which led to such a result, which is also inevitable.

Nowadays, most physicists have regarded quantum mechanics as the basic theory to understand and describe nature. Quantum theory strongly impacted the original classical physics theory and pushed physics to a deeper micro-world, thus laying the foundation of modern physics.

Until now, physicists' theory about quantum correlation has been constantly supplemented and improved. With the unremitting efforts of scientists, the scope of quantum related theories has been continuously expanded, and some unsolved mysteries have been continuously verified, which has also spawned some branch theories, such as string theory.

After Planck discovered quantum theory, he warned people that quantum mechanics is so powerful that we must use it carefully and never use it unless we have to. It can be seen that Planck 100 years ago had a strategic vision. Today, we almost forgot Planck's warning and used quantum theory on a large scale.

Third, Einstein's photoelectric effect

When Planck put forward the quantum theory of energy, the reaction of physics to Planck's quantum theory of energy was extremely cold, and even many scientists opposed Planck's quantum theory. In the following five years, no one paid attention to Planck's energy quantum theory, so Planck's energy quantum theory was put on hold for five years. It was not until 1905 that Einstein further popularized Planck's energy quantum theory.

Einstein believed that not only the energy exchange between the blackbody and the radiation field is quantized, but also the radiation field itself is composed of discontinuous photons, and the energy of each photon and the frequency of the radiation field satisfy ε=hν, that is, its energy is only related to the frequency of photons, but not to the intensity (amplitude).

Assuming that a beam of light is aimed at the metal surface, it is essentially to project a photon stream with energy ε=hν on the metal. If the frequency of the irradiation light is too low, that is, the energy of each photon in the photon flow is very small, when it irradiates the metal surface, the electron absorbs this photon, and its increased energy is still less than the work function required for the electron to leave the metal surface, so the electron cannot leave the metal surface, and therefore the photoelectric effect cannot be generated. If the frequency of irradiation light is high enough to make electrons absorb enough energy to overcome the work function and leave the metal surface, photoelectric effect will occur.

In this way, Einstein introduced the energy quantum theory into his own experiments and successfully explained the photoelectric effect. He assumed that light travels in space like particles, which are later called light quanta or photons.

When Einstein put forward the theory of light quantum, he was also strongly opposed by physicists. ? Even Planck, who put forward the concept of energy quantum at that time, thought that Einstein's theory of light quantum "went too far" and deviated from the development direction of physics, so people denied it.

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Fourthly, Bohr model.

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Until 19 13, Danish physicist niels bohr skillfully combined Planck's and Einstein's quantum theory and Rutherford's model, which were greatly doubted at that time, with the spectral experiments that were seemingly irrelevant and belonged to the chemical category at that time, so that the quantum theory was accepted by the public.

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19 10, Rutherford found that the nucleus is positively charged and the electrons circulate around the nucleus, but this discovery is extremely impossible in theory, because the negatively charged electrons will lose energy bit by bit, and the electrons that lose energy will eventually hit the nucleus. The whole process happened very quickly, even in the blink of an eye. Only by creating new theories can this phenomenon be explained, but it is a thing.

Bohr was a student of Rutherford, and he didn't give up his research because of Rutherford's difficulties. With keen insight and intuition, he realized that the classical theory of atoms would no longer be established at this level, and only Planck's quantum concept was the breakthrough point to solve the problem, so he used the concept of quantization to study the movement of electrons.

Bohr found that the motion process of electrons in the stationary orbit is also discontinuous and arbitrary, and its orbit is quantized and can be divided into small units. The smallest unit is the forbidden area where electrons cannot appear. An electron can only switch between different positions according to a certain quantum unit, so that you can see it in different positions at any time. Just like a superb magician, he will magically change positions on the stage, but you can't see every step.

Bohr also thinks that the nucleus has a certain energy level. When an atom absorbs energy, it will jump to a higher energy level or excited state. When an atom releases energy, it will jump to a lower energy level or ground state. Whether the atomic energy level jumps depends on the difference between the two energy levels. According to this theory, it is in good agreement with experiments. But Bohr's theory also has its limitations. For larger atoms, the calculation results have great errors. Bohr still retains the concept of orbit in the macro world. In fact, the coordinates of electrons appearing in space are uncertain. The more electrons gather, the greater the probability of electrons appearing here, and vice versa.

This is the concept of atomic model put forward by Bohr, which fully absorbs and develops Planck's quantum hypothesis. The previous quantum theory was collectively called the old quantum theory.

Bohr published his own theory in the form of a paper, which had a far-reaching impact on the development of quantum in later generations and added a strong stroke in the history of quantum physics.

Bohr's atomic model is the symbol of the formation of the old quantum theory. Although the old quantum theory explained some phenomena, there were serious defects and deficiencies in dealing with practical problems logically. His theory was also opposed by many physicists, including Einstein, who thought it was too mysterious. Most physicists don't agree with Bohr's theory. Nevertheless, Bohr's theory is still of great significance to the development of physics and is an epoch-making important document in the history of quantum physics.

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