Democritus believes that the primitive or basic elements of everything are "atom" and "void". "Atom" means "inseparable" in Greek. Democritus used this concept to refer to the most basic material particles that make up concrete things. The fundamental feature of an atom is "full and solid", that is, there is no gap inside the atom, it is solid and impenetrable, so it is inseparable. Democritus believes that atoms are eternal, immortal and immortal; The number of atoms is infinite; The atom is in a state of constant motion, and its only form of motion is "vibration"; Atoms are too small to be seen by the naked eye, that is, they cannot be perceived by the senses and can only be understood through reason. 1897, Thomson discovered electrons in his experiments.1910/year, Rutherford conducted large-angle elastic scattering experiments of α particles, which confirmed the existence of positively charged nuclei. Thus, the existence of atoms and the theory that atoms are composed of electrons and nuclei are proved experimentally.
1932, chadwick discovered neutrons in the experiment of bombarding the nucleus with alpha particles. Then people realized that the nucleus is composed of protons and neutrons, thus obtaining a unified world image that all substances are composed of basic structural units-protons, neutrons and electrons. At this time, the modern concept of elementary particles began to form. 1905, Einstein proposed that the basic structural unit of electromagnetic field is photon, which was confirmed in Compton et al.' s experiment in 1922, so photon is considered as a kind of "elementary particle". 193 1 year, Pauli theoretically assumes the existence of a particle with no static mass-neutrino (strictly speaking, the existence of neutrinos was experimentally confirmed by Raines and Cohen in 1956).
Relativistic quantum mechanics predicts that electrons, protons, neutrons and neutrinos all have antiparticles with the same mass as them. The first antiparticle-positron was discovered in 1932 when Anderson recorded cosmic ray particles in a cloud room placed in a strong magnetic field. Since the mid-1950s, antiparticles of other particles have been discovered one after another. With the development of nuclear physics, it is found that there are two new kinds of interactions besides the known gravitational interaction and electromagnetic interaction-strong interaction and weak interaction.
In 1934, in order to explain the strong interaction short-range force between nucleons, Hideki Yukawa proposed that this force is caused by the exchange of a basic particle between protons and/or neutrons with mass mesons. 1936, Anderson and Niedermeyer experiment confirmed a new particle, its mass is 207 times that of electrons, and it was later called muon. Muons are unstable particles that decay into electrons, a neutrino and an anti-neutrino, with an average life span of two millionths of a second. The charge of mesons proposed by Yukawa is positive and negative. 1938, Kemer developed the concept of isospin, which appeared earlier. Based on the fact that the charge of nuclear force is independent, the symmetry theory of nuclear force was established.
In 1947, Convercy et al. found that muons have no strong effect by using counter statistics. 1947, Powell and others discovered mesons with strong interaction in cosmic rays by using nuclear latex, and later confirmed the existence of such mesons in accelerators. Since then, more and more elementary particles have been recognized by human beings. 1947, rochester and butler discovered the v particle (i.e. the k meson) in the cosmic ray experiment, which was the beginning of a series of new particle discoveries later called exotic particles. Because of their unique properties, a new concept of quantum number-singularity number is introduced into particle physics. Among these strange particles, there are strange mesons with lighter mass than protons and various hyperons with heavier mass than protons. Under the usual conditions of the earth, they don't exist. At that time, they could only be produced by high-energy cosmic rays flying from space.
These discovered elementary particles, together with the gravitational field quantum-graviton predicted in theory but not confirmed by experiments, can be divided into four categories according to the nature of interaction: graviton, photon, lepton and hadron. In order to overcome the limitation of weak cosmic ray current, particle accelerators with higher and higher energy and stronger current have been built since the early 1950s. In the experiment, new powerful detection methods appeared one after another, such as large bubble chamber, spark chamber and multi-wire proportional chamber. The period of great discovery of new particles began. In the first few years of the 1960s, the number of elementary particles observed in the experiment has increased to more than the number of chemical elements found when the periodic table appeared, and the discovery momentum is getting stronger and stronger. 196 1 year, put forward by Gail-Mann and Naiman's analogy to the periodic table of chemical elements, hadrons are classified according to the symmetry of strong interaction.
Octuple classification not only gives the position of hadrons discovered at that time, but also accurately predicts some new particles, such as ω particles discovered by bubble chamber experiment in 1964. Octuple method well explains the regularity of static properties such as spin, parity, charge, singularity and mass of particles.
At this stage, it is proved that not only electrons but also all particles have their antiparticles (the antiparticles of some particles are themselves). The first charged anti-hyperon was discovered by Wang Yu of China in 1959. In addition, it is also found that a large number of particles with very short life are decayed by strong interaction resonance States. The discovery of a large number of elementary particles makes people doubt the alkalinity of these elementary particles. The concept of elementary particles is facing a mutation.
From 1940s to 1960s, the greatest progress in understanding the rationality of the micro-world was the establishment of quantum mechanics. With the efforts of a generation of physicists, quantum mechanics can explain the atomic structure, the regularity of atomic spectrum, the properties of chemical elements, the absorption and radiation of light and so on. Especially when it is combined with special relativity to establish relativistic quantum mechanics, it becomes the basic theory of the micro-world at the atomic and molecular levels.
However, there are still several shortcomings in quantum mechanics: it cannot reflect the particle nature of the field; Can not describe the process of particle production and annihilation; It has a negative energy solution, which makes the physical concept difficult. Quantum field theory was developed by Dirac, Jordan, Wigner, Heisenberg and Pauli on the basis of relativistic quantum mechanics, which solved these three problems well.
The anomalous magnetic moment of electrons discovered by Kush and Foley in 1947, and the splitting of hydrogen atom level discovered by Lamb et al. can only be correctly explained by the renormalization theory of quantum electrodynamics. Today, quantum electrodynamics has been verified by many experiments and has become the basic theory of electromagnetic interaction.
Fermi and Yang Zhenning first proposed in 1949 that not all elementary particles are "elementary". They think that mesons are not basic, but basic nucleons, and mesons are only a combination of nucleons and antinucleons. 1955, Sakata Shyoichi extended Fermi model and Yang Zhenning model, and proposed that hadrons are composed of nucleons, hyperons and antiparticles. 196 1 year, many resonance states were found in the experiment. 1964, the number of elementary particles (including resonant states) increased to several hundred, which led Gail-Mann and zwick to propose that the basis of symmetry is the formation of all hadrons. There are three kinds of them, named quarks.
Since 1960s, quarks have been searched experimentally in cosmic rays, accelerators and rocks, but it has not been confirmed as a successful report. In 1960s and 1970s, more accelerators with higher energy and better performance were built. Although quarks were not found on these accelerators. However, indirect but more powerful evidence of quarks is obtained.
Contrary to the sharp increase in the number of hadrons, since 1962 confirmed two types of neutrinos in a large spark chamber, there were only four known leptons for a long time, but the situation changed in 1975. This year, Pell and others discovered a new lepton in the electron-positron collision experiment, which was positively or negatively charged, twice as much as the proton, so it was called heavy lepton. Accordingly, it is generally believed that there should be another neutrino, but it has not been confirmed by experiments.
Shortly after the quark theory was put forward, some people realized that the study of strong and weak interactions of hadrons should be based on quarks, and at the same time, the structural and kinematic characteristics of hadrons should be fully considered in order to correctly explain the dynamic properties of hadrons such as life, width, shape factor and cross section. 1965, the hadron structure model developed in China is one of the earliest studies in this direction. The name of the layer is to emphasize the infinite level of the material structure. Deeper than hadron is quark. In recent 20 years, the mainstream of particle physics experiments and theoretical development has been along this direction, with a breakthrough in weak interaction and significant progress in strong interaction.
The earliest weak interaction theory was put forward by Fermi in 1934 to explain neutron decay. The discovery of parity non-conservation of weak interaction has brought great impetus to the study of weak interaction theory. Soon after, the form of flow describing weak interaction under Lorentz transformation was established, which is suitable for all weak interaction processes and is called the universal Fermi-type weak interaction theory. Glashow put forward the unified theory of electromagnetic interaction and weak interaction in 196 1. This theory is based on the non-Abelian gauge field theory proposed by Yang Zhenning and Mills in 1954. However, in this theory, whether these particles have static mass and how to renormalize them in theory have not been answered.
From 1967 to 1968, Weinberg and Salam expounded that particles as gauge fields can have static masses, and also calculated the relationship between these static masses and weak coupling constants and electromagnetic coupling constants. A very important point in this theory is to predict the existence of weak neutral flow, but the phenomenon of weak neutral flow was not observed in the experiment at that time. Because there was no experimental support, this model did not attract people's attention at that time.
1973, Fermilab and CERN in the United States successively discovered weak neutral flow, and then people began to pay attention to this model. 1983, Lu Biya's experimental group and others found that the characteristics of high-energy proton-antiproton collisions were completely consistent with the theoretical expectations of gauge particles, which gave great support to the theory of electro-weak unity, thus making it possible to become the basic theory of weak interaction.