Modern physics, what does modern physics include?

Modern physics is based on relativity and quantum mechanics, and its research scope extends to all fields from elementary particles to cosmic celestial bodies, forming many branches and marginal disciplines.

1. Relativity

Albert Einstein (1879- 1955) founded the theory of relativity, abandoned Newton's absolute time and absolute space, established the space-time view of relativity, and fundamentally changed the physical concept. In relativity, the theory limited to inertial reference frame is called special relativity, and the theory extended to generalized reference frame and gravitational field is called general relativity.

(1) Special theory of relativity.

1905, Einstein established the special theory of relativity. Special relativity has two basic assumptions:

① Principle of relativity: All inertial reference frames are equivalent, and physical laws can be expressed in the same form for all inertial reference frames;

(2) Principle of invariability of light speed: The light speed in vacuum is invariable at C in any direction relative to any inertial system, regardless of the movement of the light source.

Based on these two assumptions, Einstein deduced the space-time transformation relationship between the two inertial systems, namely Lorentz transformation. Thus, the existence of "ether" is completely denied, and the "length contraction" of the moving rigid body, the "time delay" of the moving clock, the relativity of simultaneity and the new law of speed synthesis are derived. The space-time view of special relativity shows that: firstly, time, space and the motion of matter are closely related, and the characteristics of time and space are relative, and the measurement of time interval and space interval is not constant, but changes with the change of the motion state of matter; Second, time and space are inextricably linked. They are inseparable and exist independently. All physical phenomena and processes exist in the unified four-dimensional continuous area of X, Y, Z and T. ..

Einstein applied special relativity to electrodynamics, proved that Maxwell's equations conformed to the principle of relativity, and established relativistic electrodynamics. Here, the electric field and magnetic field are no longer vectors, but anti-symmetric four-dimensional tensors, which change according to certain laws in different inertial systems. Electric field and magnetic field are different components of this unified tensor, and their effects on different inertial systems are different. What is shown in a certain inertial system is pure electric field or magnetic field; In another inertial system, both electric and magnetic fields will be displayed. In other words, the electromagnetic field is divided into electric field part and magnetic field part, which has only relative significance and is related to the inertial system where the observer is located.

Einstein also applied relativity to mechanics and established relativistic mechanics. Relativistic mechanics can correctly describe the law of high-speed motion, and when the speed v

(2) General relativity.

From 1907 to 19 15, Einstein put forward and established general relativity. The starting point of this theory is the fact that the gravitational mass and inertial mass are equal, from which the hypothesis of equivalence principle can be put forward: the gravitational field is physically equivalent to the equivalent acceleration of the reference system. According to the general theory of relativity, the gravitational effect is a manifestation of space-time bending. The bending structure of space-time depends on the energy density and momentum density distribution of matter in space-time; The curved structure of space and time in turn determines the orbit of the object. Einstein's predictions about the red shift of spectral lines, the bending of light and the perihelion motion of planetary orbits have been confirmed by some experiments.

2. Quantum mechanics

Quantum mechanics is a theory to study the basic laws of microscopic particle motion. 1923, Louis de Broglie (1892-) put forward the theory of matter wave, which initiated the era of quantum mechanics. De Broglie believes that not only light has wave-particle duality, but also physical particles have wave-particle duality. He also linked the physical quantity describing the properties of matter particles with the physical quantity describing the fluctuation of matter, and wrote a relationship named after him. 1926, Schrodinger (1887- 196 1) introduced the wave function according to de Broglie's thought of matter wave, and obtained the Schrodinger equation (wave equation), the basic equation of quantum mechanics, and further established the perturbation theory.

Almost at the same time, Werner Karl Heisenberg (190 1- 1976) and others established matrix mechanics from the quantization conditions, and successfully solved the problems of hydrogen atomic energy level, Stark effect, energy level movement of hydrogen atoms in electric and magnetic fields and so on. Wave mechanics and matrix mechanics study the same problem from two different aspects, which have the same function and can be transformed from one theory to another through mathematical transformation. People put wave mechanics and matrix mechanics together, collectively called quantum mechanics. 1925- 1930, dirac (Paul Adrien Maurice Dirac, 1902- 1984) comprehensively summarized the theory of quantum mechanics and established relativistic quantum mechanics.

3. Various fields of modern physics

(1) quantum optics and modern optics.

1900, Max Planck (1858- 1947) put forward the energy quantum hypothesis when explaining blackbody radiation. He thought that electromagnetic waves of various frequencies could only be emitted from the oscillator in a certain energy quantum way, and the energy quantum was discontinuous, and its size could only be an integer multiple of the product of the frequency of electromagnetic waves (or light) and Planck's constant. 1905, Einstein developed Planck's energy quantum hypothesis, put quantum theory into the whole process of radiation and absorption, and put forward the light quantum (photon) theory, which explained the photoelectric effect satisfactorily. The subsequent Compton effect further proved the light quantum theory.

The theory of quantum mechanics shows that light has the properties of both wave and particle, that is, wave-particle duality. But photons are different from particles in the particle theory of17th century, which is related to the frequency of light.

Around the 1960s, the advent of laser, the application of holographic technology, the development of optical fiber communication, and the appearance of infrared technology and remote sensing technology made optics enter a new era of modern optics and formed some new branches or marginal disciplines, such as Fourier optics, nonlinear optics, laser spectroscopy and integrated optics.

(2) Atomic physics.

19 1 1 year, Ernst Rutherford (1871-1937) put forward the nuclear model of atoms through experiments, but this model contradicts the stability of atoms in classical physics. 19 13 years, niels bohr (1885- 1962) introduced the quantum concept into the atomic system, established his atomic structure theory through two hypotheses: the steady-state hypothesis and the frequency hypothesis, and successfully explained the spectral laws of hydrogen atoms. Later, people put forward the concept of spatial quantization, studied the shell structure of atoms, discovered the spin of electrons, and constantly revised the theory of atomic structure.

This atomism formed before quantum mechanics has great limitations. The key point is that the atomic problem is not considered by wave-particle duality. In this theory, every step to expand the scope of research is generally accompanied by some new assumptions or some empirical formulas, so it is not a complete theory. Only by studying the atomic structure based on quantum mechanics can we get an accurate description of the atomic structure.

(3) Nuclear physics.

Nuclear physics studies the characteristics, structure and changes of atomic nuclei. Before 1920, Rutherford and others discovered protons, 1932, and chadwick (1891-kloc-0/974) discovered neutrons. Since then, people have realized that the nucleus is composed of protons and neutrons. Since then, people have put forward various nuclear model hypotheses to explain some laws and phenomena of nuclear motion. These models include droplet model, α particle model, Fermi gas model, shell model, single particle shell model, multi-particle shell model, collective motion model, unified model and so on. But until now, no model can explain all the experimental facts, and nuclear structure is still an important topic that people are exploring.

As early as 1896, people discovered the phenomenon of natural radioactivity, which greatly impacted the traditional concept of unchangeable elements. Since 19 19, people have realized the artificial transformation of the nuclear, which is a major breakthrough in realizing the artificial nuclear reaction. 1938, neutron bombardment of uranium led to the discovery of nuclear fission. According to the mass-energy relationship of relativity, the mass defect of nuclear fission will produce huge energy. 1942, the first atomic reactor was built and put into operation at the university of Chicago, which opened a new era of human utilization of atomic energy. After 1952, people realized light nuclear fusion, which produced much more energy than fission.

(4) particle physics.

At present, the deepest research on the structure of matter that can be detected by experiments is called particle physics, also called high energy physics. 1932, Karl Dudley-Sen (1905-) discovered positrons in cosmic rays, which marked the birth of particle physics. Then a series of new particles were gradually discovered. All the particles discovered in the early days came from cosmic rays. Since 1950s, due to the appearance of various accelerators, a large number of particles have been discovered continuously. So far, hundreds of particles have been found, and it seems that new discoveries will continue.

① Four kinds of interactions between particles.

There are complex interactions between particles, which can be generated or destroyed. There are four kinds of interactions between particles: gravitational interaction, weak interaction, electromagnetic interaction and strong interaction. The four interactions all weaken with the increase of the distance between particles. Gravity and electromagnetic force change with distance according to inverse square law, belonging to long-distance force. With the increase of distance, the speed of strong and weak forces is much faster than the inverse square, which belongs to short-range forces. According to the different interactions involved, the discovered particles can be divided into three categories: gauge particles, leptons and hadrons.

② Symmetry and its corresponding conservation laws.

The study of symmetry provides clues for the establishment of particle physics theory. A certain symmetry of physical laws corresponds to the corresponding conservation laws. Conservation of mass and energy, conservation of angular momentum, conservation of momentum and conservation of charge established in macroscopic physics are still valid in particle physics. In addition, the particle motion also follows the conservation laws of baryon number, electric lepton number and μ lepton number. In particle physics, some conservation laws are broken under certain interaction, such as parity conservation law, which is not valid under weak interaction.

③ The internal structure of hadron.

Since 1950s, people have realized that hadrons have internal structure, which has been confirmed by experiments. 1964, gherman (1929-) proposed the quark model of hadron structure. 1974, Ding Zhaozhong (1936-) and richter (193 1-) discovered the J/ψ particle at the same time, which provided strong evidence for the authenticity of the quark model. Theoretically, it was predicted that there were six kinds of quarks, and now five kinds have been found. The experimental discovery of the sixth quark needs further confirmation. Although quarks can move quite freely inside hadrons, they can't be knocked down even with the largest accelerator at present. Many people think this is caused by quark confinement. Because the interaction between quarks is realized by exchanging gluons, gluons play the role of "glue" inside hadrons. There are eight gluons with different color charges that bind quarks together in different forms and transfer the interaction between quarks. From 65438 to 0979, Ding Zhaozhong and others confirmed the existence of gluons in experiments, which provided strong support for the study of quantum chromodynamics with strong interaction.

④ Quantum field theory.

Wave-particle duality, as well as the generation and elimination of particles, are common phenomena in microscopic and high-speed physics. In the case of high energy, it is impossible to distinguish particles from fields as in the case of non-relativity. The basic theory that can reflect particle transformation by treating particles and fields in a unified way is called quantum field theory. Starting from 1927, the quantum electrodynamics established by Dirac and others after more than 20 years is the earliest quantum field theory. In quantum electrodynamics, all kinds of particles are described by corresponding quantum fields. The quantum field at every point in space-time is represented by an operator, which is called the field operator. The field operator satisfies regular exchange relation and formal Hamiltonian equation. On the basis of Schrodinger equation, adding production and annihilation operators is called secondary quantization. With the introduction of renormalization method, quantum electrodynamics has become a complete and accurate theory to describe the micro-electromagnetic interaction, and the degree of agreement between theory and experiment has reached an amazing level. However, quantum electrodynamics itself is not logically self-consistent, and its research method has encountered insurmountable difficulties when it is extended to weak interaction and strong interaction.

⑤ gauge field theory.

The quantum field theory that is most likely to unify the four kinds of interactions is the gauge field theory that has emerged in recent years. In the local transformation of supersymmetry, this theory tries to introduce a gauge field into every symmetry involved in the equation, so that the four interactions, including gravity, are contained in a theoretical framework with the same * * *, and a comprehensive unity is realized. Glashow (196 1) proposed a unified theoretical model of weak interaction and electromagnetic interaction. In 1967 and 1968, Weinberg (1933-) and abdul sallam (1926-) realized the unification of weak interaction and electromagnetic interaction on the basis of gauge field theory, which was proved by a series of experiments.

(5) Quantum statistical physics.

Planck put forward the energy quantum hypothesis in 1900, which also marked the beginning of the original quantum statistics. By adding the hypothesis of energy quantization to the classical statistical method, Planck's formula consistent with the blackbody radiation experiment can be successfully derived, and the specific heat formulas of solid and polyatomic gases can also be derived in good agreement with the experiment. The establishment of quantum mechanics changed the statistical methods of classical statistical mechanics and formed quantum statistical physics.

The difference between quantum statistics and classical statistics is mainly reflected in the following four points:

(1) Because the change of energy is discontinuous, the representative points of energy in the phase space are not everywhere, but only in a certain area, so the phase space integral in classical statistics should be changed to directly calculate the sum of the distribution numbers of each energy level;

(2) Due to the indecipherability of identical particles, the exchange with particles cannot be regarded as a new microscopic state;

(3) Due to the limitation of uncertainty relation, the small volume of phase space cannot be arbitrarily small;

(4) Fermions are limited by the Pauli exclusion principle, and only one particle is allowed in each phase lattice, while for bosons, there is no limit to the number of particles allowed in each phase lattice, so different methods should be used to count fermions and bosons.

Using quantum statistics, we can accurately explain blackbody radiation and specific heat of free electrons in metals, and deduce the third law of thermodynamics.

(6) condensed matter physics.

Condensed matter physics studies the microstructure, physical properties and internal motion law of condensed matter (solid and liquid). It is developed from solid state physics and is the largest branch of modern physics. Including solid physics, crystallography, metal physics, semiconductor physics, superconductor physics, surface physics, amorphous physics and so on. Here is a brief introduction to solid-state physics, semiconductor physics and superconductor physics.

① Solid state physics.

The main research object of solid state physics is crystalline solid. In the19th century, people accumulated a lot of knowledge about crystal geometry. At the beginning of the 20th century, both experiments and theories provided a solid foundation for the establishment of solid state physics. 19 12 years, Max von Lane (1879—1960) first pointed out that crystals can be used as X-ray diffraction gratings, which made people have a deeper understanding of crystal structure through experimental observation. The discovery of quantum theory enables people to describe the movement process of microscopic particles in crystals more deeply and accurately. On this basis, 1928 Bloch (F.BLoch, 1905 ——) proposed that the periodic arrangement of atoms in the crystal forms a periodic potential field, which has an influence on the motion of free electrons. In this potential field, the possible energy levels occupied by electrons and closely spaced from each other form energy bands, and there is a certain gap between energy bands, which is called forbidden band. This energy band theory provides a universally applicable microscopic model for solids. Solid-state energy band theory and lattice dynamics make solid-state physics a systematic basic subject, and have made great success in dealing with crystal properties. For example, these theories have obtained microscopic criteria to distinguish conductors, semiconductors and insulators, and formed a systematic theory about dislocations and crystal defects.

② Semiconductor physics.

The energy band theory lays the foundation for the development of semiconductor physics. Semiconductors conduct electricity through electrons in the conduction band or holes in the valence band, and their conductivity can be controlled by doping impurity atoms instead of original atoms. In recent years, the research of semiconductor physics has been deepened and extended to the study of ultra-fine structure of semiconductor energy band, the study of semiconductor luminescence mechanism and the properties of semiconductor light guide, and surface physics has also become an important research content of semiconductor physics. The research of semiconductor physics led to the invention of 1947 transistor and 1959 integrated circuit. The combination of contemporary integrated circuit technology and computer technology has fundamentally changed the face of the whole industry and even the whole society, and promoted the arrival of a new world technological revolution.

③ Superconducting physics.

Superconducting physics studies superconducting phenomena and the properties of superconducting materials. When the temperature drops to the critical temperature, the metal suddenly loses its resistance, which is called superconductivity. It was first discovered by Agnes in 19 1 1 year. 1933 discovered the complete diamagnetism of superconductors, namely the Mesner effect. 1958, Jhon Bardeen (1908-) and others put forward the microscopic theory of superconductivity, which generally explained the origin of superconductivity. 1962, people discovered the superconducting tunneling effect and put forward the strong coupling superconducting theory of electron-phonon interaction. At present, all countries in the world are stepping up the research on high-temperature superconducting materials, and have developed high-temperature superconducting materials with superconducting temperatures of tens of degrees Celsius below zero.

(7) Astrophysics.

Astrophysics studies the material structure, formation and evolution of celestial bodies. From 1930s to 1960s, a relatively unified theory about stars was gradually formed. The predecessor of a star (embryo) is a dense gas and dust cloud condensed by scattered and thin interstellar matter through gravitational collapse. In the process of collapse, the density and temperature of the star embryo center increase, and gradually heat and glow, forming a pre-stellar object. Gravitational contraction is the energy source of celestial bodies before stars. When the embryo's core temperature rises to10 million degrees, hydrogen nuclear fusion becomes the main energy source, and then enters the main sequence stage, and real stars are formed. According to calculation, it only takes millions or even hundreds of thousands of years for a star to complete the pre-stellar stage, while the main sequence is as long as 65.438+0 billion years to 65.438+0 billion years. At the end of the evolution of stars, there will be three kinds of celestial bodies: white dwarfs, neutron stars and black holes. At present, a large number of white dwarfs and neutron stars have been discovered, and the discovery of black holes needs further confirmation. In the study of the universe as a whole, people put forward the expansion theory of the universe and the big bang theory, and found some experimental evidence.

(8) Non-equilibrium statistical physics.

Non-equilibrium statistical physics studies material systems in non-equilibrium state. Classical statistical mechanics holds that the evolution of material system is an irreversible process from order to disorder. However, some phenomena in the biological world are contrary to this, such as the evolution from low level to high level, from disorder to order or even highly orderly development. In this way, the two viewpoints of evolution, physics and biology, show sharp opposition. This tells us that the physical system should also have an evolutionary process from disorder to order. 1969, n.g. pri-gogine (1917-) put forward the theory of dissipative structure, which provided a new idea for searching from disorder to order. According to Prigozin, an open system which is in an unstable state far from the equilibrium state, if there is nonlinear interaction between internal elements, it may move towards a new stable state after the stability is destroyed, and in this process, an ordered structure (dissipative structure) may appear. 1973, Haken Hermann (1927-) put forward the theory of studying from disorder to order from another angle-synergetics, which is a theory that produces self-organized ordered structure and functional behavior.

(9) Biophysics.

Biophysics uses physical theories and experimental techniques to study life phenomena. From 1930s to 1950s, a group of physicists gradually figured out the basic structure of protein on the basis of crystal analysis technology. 1944, Schrodinger discussed the problem of heredity from the perspective of quantum mechanics. He imagined that a gene is an aperiodic crystal composed of a homogeneous continuum, and in the arrangement and combination of a huge number of atoms or atomic groups, there is a micro-code to form genetic information. In the early 1950s, some physicists began to study the crystal structure of DNA, the material basis of heredity. 1953, physicist F.H.C.Crick (1916-) and virus geneticist J.D.Watson (1928-) put forward the molecular model of DNA double helix structure and proposed DNA. They believe that the double helix structure of DNA is a gene carrying genetic code, and one DNA molecule can copy two identical DNA molecules. In the further exploration of how DNA controls protein synthesis, physicist G. Gamov (1904-1968) put forward the hypothesis of "triplet codon" according to permutation and combination, and proposed that * * has 64 genetic codes. By 1969, all 64 genetic codes have been detected and listed in the code table. Deciphering the mystery of gene information is one of the greatest achievements of natural science in the 20th century.