History of classical physics

Archimedes in ancient Greece had brilliant achievements in hydrostatics and solid balance, but at that time he classified these as applied mathematics, but did not classify his achievements, especially his precise experiments and strict mathematical demonstration methods, into physics. From Greece and Rome to the long Middle Ages, natural philosophy has always been Aristotle's world. During the Renaissance, Copernicus, Bruno, Kepler and Galileo challenged the old traditions regardless of religious persecution. Among them, Galileo established the theory and laws of physics on the basis of strict experiments and scientific argumentation, so he was honored as the father of physics or science.

Galileo's achievements are manifold. As far as mechanics is concerned, his conclusion is that an object sliding down a smooth slope will rise to the same height on another slope. For example, if the inclination of the other slope is very small, in order to reach the same height, the object will move to infinity at a uniform speed, thus drawing the conclusion that if there is no external force, the object will move endlessly. He accurately determined that objects with different weights slide down a smooth slope with the same acceleration, and deduced the acceleration and motion equation of objects falling freely, refuting Aristotle's conclusion that heavy objects fall faster than light objects. By integrating the uniform motion in the horizontal direction and the uniform acceleration motion in the vertical direction, the parabolic trajectory and the maximum range angle of 45 were obtained. Galileo also analyzed that "the earth always moves without being known" and put forward the famous "Galileo relativity principle" (written by China 18). But his analysis of the relationship between force and motion is still wrong. Newton's three laws of motion comprehensively and correctly summarize the relationship between force and motion. Newton also extrapolated the gravity of the ground to the moon and the whole solar system, and established the law of universal gravitation. Newton solved the two-body problem in the solar system by using the above four laws and the "flow number method" (that is, preliminary calculus), deduced Kepler's three laws, and solved the tidal problem on the earth theoretically. Historically speaking, Newton was the first physicist who integrated the mechanical movements of heaven and earth and made great achievements. At the same time, geometric optics has also made great progress. At the end of 16 or the beginning of 17, microscopes and telescopes were invented one after another, and Kepler, Galileo and Newton all made great improvements to telescopes.

Before and after the Great Revolution, France was full of talented people. French scientists, led by P.S.M Laplace (known as Laplace School in history), developed Newton's mechanics theory, applied partial differential equations to celestial mechanics, found approximate solutions to the three-body and many-body problems of the solar system, preliminarily discussed and solved the origin and stability of the solar system, and made celestial mechanics reach a quite perfect state. In Newton's and Laplace's solar systems, gravity, not the creator, dominates the motion of celestial bodies. No wonder Napoleon asked after hearing Laplace's introduction to the solar system: Where did you put God? Laplace, an atheist, replied bluntly: I don't need this hypothesis.

Laplace School also widely applied the laws of mechanics to rigid bodies, fluids and solids, and with the joint efforts of W.R. Hamilton and G.G. Stokes, it perfected analytical mechanics and pushed classical mechanics to a higher stage. The school also attributed various physical phenomena, such as heat, light, electricity, magnetism and even chemical interaction, to the attraction and repulsion between particles. For example, the reflection was explained by the repulsion of matter to photons, the refraction and diffraction were explained by the attraction of matter to light particles, the polarization was explained by photons of different shapes, and the thermal expansion and evaporation were explained by the mutual repulsion of hot particles and mass particles, all of which were successful, thus making the world view of mechanical materialism dominate for decades. Just when this school was in full swing, it was challenged by the British physicist T. Young, the backyard of this school, the French Academy of Sciences and the scientific community. J.B.V Fourier talked about heat conduction, T. Yang, D.F.J arago and A.-J Fresnel talked about optics, especially the wave theory and particle theory of light (see the duality of light). In order to refute the particle theory, Fresnel, a young civil engineer, with the support of arago, made a variety of interference and diffraction devices named after him, introduced the interference of light waves into Huygens' theory of wavefront propagation in media, and formed the huygens-fresnel principle. He also boldly put forward the hypothesis that light is shear wave, and used it to study the polarization of all kinds of light and the interference of polarized light. He created the "Fresnel zone" method, which fully explained the diffraction of spherical waves. Assuming that light is an etheric shear wave, the intensity and polarization problems of light reflection and refraction at the interface of different media are solved, thus completing the classical wave optics theory. Fresnel also put forward the hypothesis that the rotation of the earth makes some ether drift on the surface, and gave the traction coefficient. With the support of arago, J.B.L Foucault and A.H.L Fizzo confirmed that the speed of light in water is indeed less than that in air, thus confirming the victory of wave theory. History calls this experiment the decisive experiment of light. Since then, it is said that the fluctuation of light dominated the second half of the19th century, with famous physicists such as Faraday, Maxwell and Kelvin. Everyone believes in ether theory. On the other hand, tiny changes in length, velocity and curvature can be accurately measured by using the movement of interference fringes in the interferometer. Using the spectrum produced by prism and diffraction grating, we can determine the composition of matter on the ground and in the sky and the change of atoms. Therefore, these optical instruments have become important experimental means in physics, analytical chemistry, physical chemistry and astrophysics.

The invention of the steam engine promoted the development of heat energy. 18 in the 1960s, while J. Watt improved the steam engine, his close friend J. Blake distinguished temperature from heat, established the concepts of specific heat and latent heat, and developed calorimetry and calorimetry. The concepts of heat theory, heat and mass conservation formed by him have ruled for more than 80 years. During this period, although the gas law was discovered and the specific heat capacity and latent heat of different substances were measured, it did not help much to improve the steam engine, and the steam engine always operated inefficiently. 1755 French academy of sciences firmly rejects perpetual motion machines. 1807, T. Yang replaced Leibniz's "vitality" with "energy"; 1826, J.V. Poncelet coined the word "work". In 1798 and 1799, Langford and H. David analyzed the friction heat generation and challenged the heat theory. J.P. Joule spent nearly 40 years from 65438+1940s to 1878, and accurately determined the mechanical equivalent of heat by electric heating, mechanical work and other methods. Physiologists J.R. Mayer and H.von Helmholtz comprehensively expounded that energy can neither be produced nor disappeared from the transformation of mechanical energy, electrical energy, chemical energy, biological energy and heat, and established the first law of thermodynamics, namely the law of conservation of energy. 1824 or so, S. Cano deduced the law that the ideal heat engine efficiency is determined by the temperature of heat source and cold source according to his investigation of steam engine efficiency and heat theory. After the article was published, it did not attract attention. After Clausius and Kelvin put forward two expressions respectively, it was confirmed as the second law of thermodynamics. Clausius also introduced a new state entropy function; Later, enthalpy, Helmholtz function, Gibbs function and other state functions were introduced one after another, creating an important branch of physical chemistry-thermochemistry. Thermodynamics pointed out the direction for inventing new heat engines and improving their efficiency, and initiated thermal engineering. But also in physics, chemistry, mechanical engineering, chemical engineering, metallurgy and other aspects have a wide range of pointing and promoting role. All these make W. ostwald, one of the founders of physical chemistry, once denied the existence of atoms and molecules, but advocated the "energy theory" and regarded energy as the ultimate existence of the world. On the other hand, J.C. Maxwell's molecular velocity distribution rate (see Maxwell distribution) and L. Boltzmann's energy equipartition theorem combine heat and mechanics, introduce the law of probability into physics to study the movement of a large number of molecules, and establish the gas molecular dynamics theory (now called gas dynamics theory), establish the statistical properties of gas such as pressure, internal energy and specific heat capacity, and get the conclusion consistent with thermodynamics. Boltzmann further thinks that the second law of thermodynamics is a statistical law, and establishes statistical thermodynamics by linking entropy with state probability. Any actual physical phenomenon inevitably involves energy conversion and heat transfer, and the law of thermodynamics has become the basic law to synthesize all physical phenomena. After the physics revolution in the 20th century, these laws still hold. Moreover, the concepts of balance and imbalance, reversibility and irreversibility, order and disorder, and even fluctuation and chaos are transplanted from the relevant branches of natural science to social science.

Before the 1920s of 19, electricity and magnetism were always considered as two different substances. So although W. Gilbert published an article on magnetism in 1600, and analyzed the phenomena of magnetism and geomagnetism in depth, B. Franklin put forward the single fluid theory of electricity in 1747, and defined the positive and negative charges, but the development of electricity and magnetism is that 2000 photovoltaic cells need to be connected to use arc lamps, so the application of electricity is not popular. 1920, H.C. Oster's current magnetic effect experiment started the synthesis of electricity and magnetism, and electromagnetism developed rapidly. Within a few months, through the experiment of A.-M. Ampere, the ampere law between parallel currents was established and the theory of magnetic molecules was put forward. J.-B. Biot and F. Savart established the acting force of current-carrying wire on magnetic pole (hereinafter referred to as Bi-Sa-) 183 1 year, Faraday discovered the electromagnetic induction phenomenon, and the change of magnetism produced current in the closed loop, which completed the integration of electricity and magnetism, and made mankind obtain a new power supply. 1867, W.von Siemens invented the self-excited motor and used the transformer to complete the long-distance transmission. These devices based on electromagnetic induction have changed the face of the world and created new disciplines-electrical technology and electrical engineering. Faraday also introduced the concept of field into electromagnetism; 1864, Maxwell further mathematicized the concept of field, put forward the hypothesis of displacement current and rotating electric field, established Maxwell equations, perfected the electromagnetic theory, and predicted the existence of electromagnetic waves propagating at the speed of light. However, his achievements were not immediately understood until H.R. Hertz completed the differential form of this set of equations, and proved through experiments that Maxwell's predicted electromagnetic wave has all the properties of light wave propagation speed, reflection, refractive interference, diffraction, polarization and so on, thus completing the synthesis of electromagnetism and optics, enabling mankind to master the fastest tool for transmitting all kinds of information and creating a new discipline of electronics.

Until the second half of19th century, the nature of charge was not clear. The popular etherism holds that charge is only a vortex element in the etheric ocean. H.A. Lorenz first combined the electromagnetic theory of light with the molecular theory of matter, and thought that molecules were charged harmonic oscillators. Since 1892, articles on "electronic theory" have been published one after another, and it is considered that the cathode ray discovered by J. Pluckel in 1859 is an electron beam; Lorentz force formula was put forward in 1895, which combined with Maxwell equation to form the basis of classical electrodynamics. The normal dispersion, abnormal dispersion (see the dispersion of light) and Zeeman effect are explained by electronic theory. 1897 J. J. Tang Musun applied electric and magnetic fields to cathode-ray tubes made of electrodes of different rare gases and different materials, and accurately determined that the particles constituting cathode-ray had the same charge-mass ratio, which provided an exact experimental basis for electronic theory. The electron became the first subatomic particle to be discovered. 1895, w.k. roentgen discovered x-rays and expanded the electromagnetic spectrum. Its strong penetration into matter makes it a tool for diagnosing diseases and finding internal defects in metals. 1896, A.-H. Bekkerel discovered the radioactivity of uranium, and 1898, the Curies discovered new elements with stronger radioactivity-polonium and radium, but these discoveries have not attracted wide attention in the field of physics for the time being.

Physics in the 20th century reached the end of19th century, and classical physics has developed to a perfect stage. Many physicists believe that physics is drawing to a close, and the future work is only to increase the number of significant figures. Kelvin said in his New Year message on the last New Year's Eve of the19th century: "The physical building has been built ... the dynamics theory has determined that heat and light are two ways of movement. Now there are two dark clouds in its beautiful and clear sky, one in the wave theory of light and the other in the energy sharing theory of Maxwell and Boltzmann. " The former refers to the etheric drift and Michelson-Morey experiment to measure the earth's speed to ether (absolute rest), while the latter refers to the blackbody radiation spectrum and the specific heat of solids at low temperature, which cannot be explained by the principle of energy sharing. It is these two basic problems and radioactivity neglected by Kelvin that gave birth to the physics revolution in the 20th century.

1905A。 In order to solve the asymmetry of electrodynamics applied to moving objects (hereinafter referred to as the disharmony between electrodynamics and Galileo's relativity principle), Einstein established the special theory of relativity, that is, the theory of relativity applicable to all inertial reference systems. Starting from the invariance of the speed of light in vacuum, that is, the speed of light emitted by a moving light source is the same in all inertial frames, he deduced the conclusions of simultaneous relativity, scale contraction and clock slowness in a moving system, and perfectly explained the Lorentz transformation formula proposed by Lorenz to explain Michelson-Morey experiment, thus completing the synthesis of mechanics and electrodynamics. On the other hand, special relativity also denies absolute space and time, combines time and space, and puts forward a unified and relative view of time and space, which constitutes four-dimensional time and space; And completely deny the existence of ether, fundamentally shake the philosophical foundation of classical mechanics and classical electromagnetism, and promote Galileo's theory of relativity to a new stage, which is applicable to all dynamic mechanics and electromagnetic phenomena. But when the speed of a moving object or system is much less than the speed of light, relativistic mechanics is consistent with classical mechanics. Mass, energy and momentum in classical mechanics also have new definitions in relativity, and the derived mass-energy relationship provides theoretical preparation for the release and utilization of nuclear energy. 19 15 years, Einstein established the general theory of relativity and extended it to the non-inertial system. He believes that the gravitational field is physically equivalent to a non-inertial system with considerable acceleration, and the space-time in the gravitational field is curved, and its curvature depends on the strength of the gravitational field, which innovates the old concept that the universe is a straight Euclidean space. However, for gravitational fields with small scope and intensity, such as the earth's gravitational field, the curvature of space can be completely ignored, while for spaces with strong gravitational fields, such as the sun and other stars, and large-scale spaces, such as the entire observable universe, the curvature of space must be considered. So the general theory of relativity explains some astronomical phenomena that Newton's gravity theory can't explain, such as the abnormal precession of Mercury's perihelion and the gravitational segregation of light. Cosmology based on general relativity has become the fastest growing branch of astronomy.

On the other hand, in 1900, M Planck put forward the formula of blackbody radiation in the whole wavelength range, and deduced it theoretically on the assumption of energy quantization, and put forward the discontinuity of physical quantities for the first time. 1905, Einstein published the light quantum hypothesis, and explained the photoelectric effect with the wave-particle duality of light. 1906 published the quantum theory of solid heat capacity; 19 13 N Bohr (see Bohr and his son) published Bohr's theory of hydrogen atom, accurately calculated the balmer formula of the spectrum of hydrogen atom with quantum concept, and predicted the existence of other spectral lines of hydrogen atom, which was later confirmed. 19 18, Bohr put forward the correspondence principle and established a bridge from classical theory to quantum theory. 1924, L.V. de Broglie put forward the hypothesis that microscopic particles have wave-particle duality and predicted the diffraction of electron beams. 1925, W. Pauli published Pauli's incompatibility principle, W. K. Heisenberg founded matrix mechanics with the help of M. Born and mathematician E. P. Jordan, and P.A.M Dirac put forward noncommutative algebra theory; 1926

E. Schrodinger published a series of papers on wave mechanics based on wave-particle duality, established wave functions, and proved that wave mechanics and matrix mechanics are equivalent, so they are collectively called quantum mechanics. In June of the same year, Born put forward the statistical explanation of wave function, indicating that a single particle follows statistical laws rather than classical deterministic laws. Heisenberg published the uncertain relation in 1927; The relativistic electron wave equation published in 1928 laid the foundation of relativistic quantum theory. Because all microscopic particles follow the laws of quantum mechanics, they become the theoretical basis for studying particle physics, nuclear physics, atomic physics, molecular physics and solid state physics, and also an important means for studying molecular structure, thus developing a new branch of quantum chemistry.

Almost at the same time, quantum statistical methods have been developed to study particle systems composed of a large number of particles, including Bose-Einstein distribution established by 1924 and fermi-dirac distribution established by 1926, which are suitable for particle systems with integer and semi-integer spins respectively. Later, quantum field theory also gradually developed. 1927, Dirac first proposed the scheme of quantizing the electromagnetic field into a system with infinite degrees of freedom to deal with the spontaneous radiation and absorption of light in atoms. 1929 Heisenberg and Pauli established the general form of quantum field theory and laid the foundation of quantum electrodynamics. The divergence difficulty is solved by renormalization method, and the radiation correction of each order is calculated. The difference between the obtained electron magnetic moment and the experimental value is only 2.5× 10- 10, and its accuracy is unprecedented in physics. Quantum field theory is also developing in the direction of unified field theory, that is, electromagnetic interaction, weak interaction, strong interaction and gravitational interaction are unified in a normative theory, and some achievements have been made in weak unified theory of electricity, quantum chromodynamics and grand unified theory.

"Practice is the only criterion of truth", and physics also follows this criterion. All hypotheses must be based on experiments and must stand the test of experiments. However, physics is also a highly speculative science, which has formed an indissoluble bond with philosophy since its birth. Whether it is Galileo's principle of relativity, Newton's law of motion, the law of conservation of momentum and energy, Maxwell's equation, relativity and quantum mechanics, it has strong scientific speculation. Some scientists, such as J.C. Poggendorff, editor-in-chief of Physics and Chemistry in19th century, once tried to expel speculation from physics, twice refused to publish Mayer and Helmholtz's articles on energy conservation on the grounds of speculative content, and were finally criticized by later generations. To discover the laws hidden behind the experimental facts requires profound insight and rich imagination. How many physicists pay attention to the mystery of θ-τ? Only Chinese-American physicists Li Zhengdao and Yang Zhenning, after careful speculation and examination of a large number of documents, found that there was a hypothesis of parity conservation of weak interaction behind this mystery, which had not been verified by experiments. From the development history of physics, every synthesis has promoted the great development of physics itself and related disciplines, and every synthesis is based on a large number of accurate observation and experimental facts, and also has profound speculative content. Therefore, in order to better apply and impart physical knowledge, ordinary physicists and physics teachers should also understand important concepts and laws from the whole system of physics.

Applied physics is a science widely used in various production departments. Someone once said that an excellent engineer should be an excellent physicist. The development of some aspects of physics is really driven by the needs of production and life. In previous centuries, Cano discovered the second law of thermodynamics to improve the efficiency of steam engines, Abbe established the theory of optical system to improve microscopes, and Kelvin invented many sensitive electrical instruments to make more effective use of Atlantic cables. In the 20th century, the rapid development of nuclear physics, electronics and semiconductor physics, plasma physics and even ultrasonic, hydroacoustics, architectural acoustics and noise research is obviously related to the needs of production and life. Therefore, it is very necessary to vigorously carry out the research of applied physics. On the other hand, many physical achievements that promote social progress and production greatly begin with the exploration of basic theories. For example, Faraday was inspired by the magnetic effect of electricity, studied the electric effect of magnetism, and promoted the birth of the electric age; Maxwell predicted electromagnetic waves in order to improve the electromagnetic field theory, which brought the electronics century; X-rays, radioactivity and even the discovery of electrons and neutrons all come from the study of the basic structure of matter. Considering the importance of knowledge and talents, we should pay special attention to the study of basic theories. Therefore, in order to make science and technology reach the forefront of the world, basic theoretical research can never be ignored.

Looking forward to the eve of 2 1 century, scientists will consider the prospect of a century from their own disciplines. Whether physics will be in the leading position like the first two or three centuries is controversial, but no scientist will assert that physics is close to the end of development like Kelvin. With the shortage of energy and mineral resources and the deterioration of the environment, the physical principles and technologies to solve new energy, new material processing and new testing methods are put forward to physics. The in-depth exploration of particles and the solution of the most basic structure and interaction of matter will provide new means for human beings to understand and transform the world, which requires new particle acceleration principles, higher energy accelerators and more sensitive and reliable detectors. To realize controlled thermonuclear fusion, it is necessary to integrate the knowledge of plasma physics, laser physics, superconducting physics, surface physics and neutron physics to solve a series of theoretical and technical problems. In short, with the in-depth development of the new technological revolution, physics will also be infinitely extended.