Simple understanding -the material concept of quantum field theory

The following article comes from WeChat public account: Hui Pu, author Dai Jin

Written article ｜ Dai Jin

1

Maternity

In the chemistry class of middle school, I have learned a "law of material immortality". When the gasoline was burned, the material did not disappear, but the carbon and hydrogen atoms in gasoline combined with oxygen in the air into water steam and carbon dioxide. Various chemical reactions are just the conversion of substances between different combinations.

The fever when gasoline burns comes from the photons radiated when the electronic energy level changes in the chemical reaction. Facing the fact that photon is radiated, people who believe that the immortal of material may also adhere to the narrow material concept, saying that photon is not considered material. But a pair of positive and negative electrons met and annihilated into two photons; this is obviously not like the component in the electronics re -combined to become photons; and it is more like electron extinguishing. The particles, except for positive electricity, are the same), as shown in Figure 1.

Figure 1 The annihilation of positive electrons

Maternal immortality is an illusion, just because in chemical reactions, atoms and internal electrons do not have enough energy to make particles other than photons. Since the invention of the particle accelerator in the 1950s, physicists have discovered that high -energy particles have collided with new particles and belong to home -often. A large number of new particles are found on the accelerator, which has achieved the great development of quantum field theory.

When the particles are generated or eliminated, the particles involved in the reaction generally are close to the speed of light and must use relativity. Most particles have quiet quality, which generates a particle, and the energy required for minimum limit is given by Einstein's quality formula.

E = mc2

Therefore, the quantum field theory is also regarded as the fusion of relativity and quantum theory.

Figure 2 LHC accelerator of European nuclear centers | Source: European nuclear center official website

The LHC accelerator of the European nuclear center is to find the famous Higgs particle accelerator, as shown in Figure 2. It consists of an underground tunnel like a 27 kilometer. A ring of high -energy particles is rotated and accelerated in the vacuum pipeline in the tunnel. Thousands of superconducting magnets help particles turn. It can accelerate the energy of the proton to 6.6tev (1012 electronic), which is 1 trillion times higher than the typical energy of 1 electronic il in the chemical reaction. A collision of two such protons can produce thousands of particles.

The quantum field theory is closely linked to the field of high energy physics. The so -called high -energy physics is that each particle is very high, not only higher than the chemical reactions, but also much higher than the nuclear reaction.

2

What is the field?

Why can particles be generated and disappeared out of thin air? To explain such a phenomenon, a theoretical foundation is needed. Let's introduce the concept of the field from the most familiar electromagnetic field.

In our middle school physics textbooks, the law of Kulun tells us that the power and charge between the two charges are proportional, and it is proportional to the square. Thinking about it, there is a problem with Kuron's law: if the two charges are in the exercise, this law seems to be saying that one charge can "perceive" the position of another charge at any time, and there are some unreasonable in the meditation.

Figure 3 The interaction between the two sports charge

Using the complete classic electric power theory of Maxwell equation, people found that the law of Kuron needs to be corrected in the case of two charges. One charging "perceive" another charging position has a small time delay. This delay is delayed. It is equal to the time when the electromagnetic wave reaches the other charge from one charge, as shown in Figure 3. The discovery of Maxwell's equations and electromagnetic waves made people realize that electromagnetic interactions spread at a limited (although very fast). Studies of classic electrodynamics prove that the energy transmission between the two charges is not manufactured by one side in one side, but flowing in the space.

The media interaction and the medium of electromagnetic waves are called electromagnetic fields. Whether it is the inside of the object or the vacuum of the air that removes the air, the electromagnetic field is ubiquitous. Electromagnetic fields carry energy and information, it has material attributes. Therefore, the acceptance of modern physics and the invisible vacuum is also a material form. The electromagnetic field is a property of this material.

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The quantization of the field and the birth of particles

Quantum field theory is a kind of quantum mechanics, but its first object is not a particle, but a field.

Describe the state of the particles, use three position coordinates, or three momentum components. The description field needs to be used as a field or strong field for space -time functions. For example, the electromagnetic field needs to be described by A (x, T) and φ (x, t). In relativity, A and φ form a vector in the four dimensions in time and space. The state of the particle only requires three numbers. The terms are called three degrees of freedom, and the field has infinite degrees of freedom. It is much more complicated in mathematics.

In quantum mechanics, the position of the particles may be uncertain, the state of the particles can be superimposed in different positions, and the position and momentum cannot be determined at the same time. In the quantum field theory, the field on a spatial point can also be superimposed with different strengths, and the change rate of the field and field over time (compared to the speed of the particles) cannot be determined at the same time. The quantum field theory, some like crystals, is a quantum mechanics on many space points. It's just that the crystal is only a limited multiple atoms distributed on the discrete crystal dot, and the field has the infinite degree of freedom in the continuous space.

This sounds very complicated, but the study of quantum field theory quickly produced a simple and important result: all fields fluctuated, such as electromagnetic waves in the electromagnetic field. In a column of waves, each point is performed near the balance point to perform periodic vibration, just like the vibration of the atom in the crystal. The dynamics of a wave of waves is like a quantum resonance. Its energy level is equal interval (e = (n+1/2) HF, n = 0, 1, 2, 3, ... of which H is Planck is the Planck is the Planck is the Planck is the Planck is Planck. Constant, F is the frequency of resonant), and the energy required for each level of energy is HF. This is exactly the energy of a particle in a column! The energy of the waves is quantized. Each energy is a particle, like a sound in the crystal. The energy level of quantum resonance is poor, and the energy of each particle in the duality of the wave particles is. It turns out that this is not a coincidence. Let's summarize the material concept of quantum field theory:

Each basic particle corresponds to a field, even in the vacuum, these fields are everywhere

In the vacuum, there is no substance that can be observed, because all the fields are in a state with the lowest energy state

The energy of the field is quantized, and the excitation of each energy adds a particle in the vacuum

The generation and elimination of particles are due to the results of different fields that exchange energy through interaction

The photon field is the electromagnetic field, and the electron also has its own field. The electromagnetic field is a vector in the four -dimensional space -time. The type of electronic field is a rotation, with four plural weights. The stimulus of the electronic field, including electron and electrons, electrons with positive electrons. As Fermiko, the quantum rules of the electronic field are different from the electromagnetic field. It needs to meet the principle of compatibility of bubbles. Electronics in the same state can only be stimulated by one at most, as shown in Figure 4.

Figure 4 The relationship between the energy level and particles on a wave of the quantum field

So far, particle physics has identified 17 basic particles, mainly divided into two categories. One is a narrow material particle, with 6 types of quarks, μons,

There are 3 neutrinos; these are the same as electronics, which is 1/2 of Fermiko, which is represented by a rotary field. The other is the particles that spread interaction between these material particles. There are rubber seeds with strong communication interaction, and W and Z -particles that spread weak interactions. They are the same as photons. Said the description of the Yangmirus field. In addition to these two categories, there is also a Higgs particle, which spin is 0, and its field is the scalar in four -dimensional time and space.

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Zero -point confusion and the fate of the universe

The minimum energy of quantum resonance is not 0, and particles cannot be absolutely static. According to the same principle, the quantum field is not allowed to be absolutely calm. In each volatility, there is zero vibration energy.

This problem has made quantum field theory into difficulties. First of all, there are zero -point energy in the infinite volatility mode. The total energy density of vacuum must be infinite. Of course, not all zero points can be positive. Fermiko's zero point can be negative, and positive and negative energy can be offset. In addition, there are different particles in vacuum, and these fields also have interactions, which will also affect the energy of vacuum. Quantum field theory cannot calculate the energy density of vacuum, but reasonable speculation, it should not be 0.

In the vacuum that can't be seen in anything, is the energy related to energy? Is the energy of vacuum physical significance?

There is a very interesting phenomenon that shows the vacuum energy, called Casimir effect. When the two metal boards are closer, if they are charged, you know you will be attractive or exclusive. But quantum field theory predicted that when they are not charged, they also have an attractiveness.

Figure 5 The electromagnetic wave vibration mode between the two metal boards

As shown in Figure 5, because the electric field cannot enter the metal, between the two metal plates, the vibration mode of the electromagnetic wave is limited, and only a series of resident waves can exist. Even on these resident waves, even if there are no photons, the existence of two metal plates also affects the zero -point energy of a part of the vacuum in the middle. Although the quantum field theory is not as short as the vacuum, it can accurately calculate the poor energy inside and outside and the attractiveness caused by poor can be. This calculation result is confirmed by experiments. Of course, the two boards will be very attractive to be very close (nano -level).

There are also more important: Einstein's narrow theory tells us that energy and quality can be converted to each other, and the energy in real vacuum can be converted into quality and can also produce gravitational gravity. The energy density of vacuum is the cosmic constant in Einstein's general theory of relativity. It has a decisive impact on the bending and evolution of the universe space.

This is very interesting, the physics of the most microscopic basic particles determines the fate of the most macro universe.

Middle school physics may make you feel that energy is a scalar; but in the narrow sense of relativity, energy and momentum form a vector in the four -dimensional space -time space and space. Energy is the amount of this vector in the direction of time. Energy density is more complicated. In a broad sense of relativity, it is a component of a 4X4 volume. The element of this tension on the diagonal line is pressure and energy density.

You may have heard of dark matter and dark energy. Dark substances are some non -glowing substances in the universe. In addition to being not easily seen, they have contributed as gravity as ordinary substances.

Vacuum energy is a dark energy, and its nature is very different. If the dark energy density is positive, it also contributes attraction, but the positive energy density will always be accompanied by negative pressure, and the net effect is exclusive. The negative dark energy density contributes a net attractiveness. Our universe is extremely vast and looks straight. For a long time, people think that the energy of vacuum is 0. Until the 21st century, astronomical observations confirmed that the universe was accelerating, which meant that the vacuum had a small positive energy density. This energy density is converted into quality. There are only a few hydrogen atoms per cubic meter, but it also exceeds the average total density of all substances (visible substances and dark substances), which is enough to overcome their attractiveness to expand the universe. After the expansion, the density of material is smaller, and the density of dark energy is still the same, so the expansion will become faster and faster.

Although the vacuum energy is not completely 0. From the perspective of particle physics, it is too small. The contribution of any dark energy is dozens of magnitude larger than this value! Logically speaking, the original vacuum energy of the universe and the contribution of various quantum fields can be completely offset, which can be said, but it is unreasonable. If there is no higher mechanism to restrict, how can the original vacuum energy of the universe and various quantum fields be offset so clean? This huge question is still the unsolved mystery of modern physics.

About the Author

Dai Jin graduated from the Department of Physics of Peking University in 1985. Through the Li Zhengdao Cupspea project to study in the United States, he obtained a doctorate degree in physics from the University of Texas University of Austin. During the work of the Nobel Prize winner Winberg's research team, he studied the D film theory in the String theory from Bolcinski, the winner of the Basic Physics Breakthrough Award. It is an Intel executive.

This article is reproduced from WeChat public account: Return to Park Author: Dai Jin

Reprinted content only represents the author's point of view

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It does not represent the high energy office of the Chinese Academy of Sciences

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