Einstein’s General Theory of Relativity: Gravity is the curvature of space-time

Albert Einstein (German/English: Albert Einstein, March 14, 1879 - April 18, 1955), was born in Ulm, Baden-Württemberg, Germany City, graduated from the Federal Institute of Technology in Zurich and is a modern physicist. ?[1-4]

Einstein was born in a Jewish family in the city of Ulm, Germany (both parents are Jewish). He graduated from the Swiss Federal Institute of Technology in Zurich in 1900 and became a Swiss national.[1-4]. In 1905, Einstein received a doctorate in physics from the University of Zurich, and proposed the photon hypothesis and successfully explained the photoelectric effect (thus winning the 1921 Nobel Prize in Physics)?[1-4]?; in the same year, he founded the special theory of relativity, and in 1915 he founded General theory of relativity. He immigrated to the United States in 1933 and worked at the Institute for Advanced Study in Princeton. In 1940, he became an American citizen while retaining his Swiss nationality?[1-4]?. On April 18, 1955, Einstein died in Princeton, New Jersey, USA at the age of 76?[1-4]?. In December 1999, Einstein was selected as the "Person of the Century" of the 20th century by Time Magazine.

Einstein’s theory laid the theoretical foundation for the development of nuclear energy. In order to help fight against the Nazis, he once wrote a letter to U.S. President Franklin Roosevelt with the assistance of Leo Szilard and others, directly promoting the The launch of the Manhattan Project, and after World War II, he actively advocated peace, opposed the use of nuclear weapons, and signed the "Russell-Einstein Declaration"[4-8]?. Einstein ushered in a new era of modern science and technology and is recognized as the greatest physicist after Galileo and Newton.

The photoelectric effect was discovered by German physicist Hertz in 1887, and the correct theoretical explanation was proposed by Einstein. Einstein argued that the energy of light is not uniformly distributed, but is carried in discrete light quanta, and the energy of photons is related to the frequency of the light it is composed of. This breakthrough theory not only explained the photoelectric effect, but also promoted the birth of quantum mechanics.

In 1905, Einstein proposed the photon hypothesis and successfully explained the photoelectric effect, thus winning the 1921 Nobel Prize in Physics. ?[2]?[16]? [14]

The photoelectric effect is divided into photoelectron emission, photoconductivity effect and photovoltaic effect. The former phenomenon occurs on the surface of an object and is also called the external photoelectric effect. The latter two phenomena occur inside the object and are called the internal photoelectric effect. ?[14]

Hertz discovered the photoelectric effect in 1887. Einstein was the first to successfully explain the photoelectric effect (the effect of a metal surface emitting electrons under the action of light irradiation. The emitted electrons are called Optoelectronics). Only when the wavelength of light is smaller than a certain critical value can electrons be emitted, that is, the limit wavelength, and the corresponding frequency of light is called the limit frequency. The critical value depends on the metallic material, while the energy of the emitted electron depends on the wavelength of the light and has nothing to do with the light intensity, which cannot be explained by the wave nature of light. There is another point that is inconsistent with the wave nature of light, that is, the instantaneous nature of the photoelectric effect. According to the wave theory, if the incident light is weak and the exposure time is longer, the electrons in the metal can accumulate enough energy and fly out of the metal. surface. But the fact is that as long as the frequency of light is higher than the limiting frequency of metal, regardless of the brightness of the light, the generation of photons is almost instantaneous, no more than ten to the negative ninth power of seconds. The correct explanation is that light must be composed of strictly specified energy units (ie photons or light quanta) related to wavelength. ?[2]

In the photoelectric effect, the ejection direction of electrons is not completely directional, but most of them are emitted perpendicular to the metal surface, regardless of the direction of light. Light is an electromagnetic wave, but light oscillates at high frequency. The amplitude of the orthogonal electromagnetic field is very small and will not affect the direction of electron emission. ?[14]

General Relativity is a theory that describes the gravitational interaction between matter. The foundation was completed by Einstein in 1915 and officially published in 1916. This theory explains the gravitational field as the curvature of space-time for the first time.

Einstein once tried to incorporate the law of universal gravitation into the framework of the theory of relativity. After several failures, he finally realized that the special theory of relativity could not accommodate the law of universal gravitation. Therefore, he extended the principle of special relativity to general relativity, and used the principle that gravitation and inertial force are equivalent in the local inertial system to establish the theory of general relativity that uses the Riemannian geometry of curved space-time to describe gravity.

In 1907, Einstein wrote a long article on the special theory of relativity, "On the Principle of Relativity and Conclusions Drawn from It". In this article, Einstein mentioned equivalence for the first time. Since then, Einstein's ideas about the equivalence principle have continued to develop. He used the natural law that inertial mass and gravitational mass are proportional as the basis of the equivalence principle, and proposed that a uniform gravitational field in an infinitely small volume can completely replace the reference frame of accelerated motion. Einstein also proposed the concept of a closed box: no matter what method is used, an observer in a closed box cannot determine whether he is at rest in a gravitational field or in a space that is accelerating without a gravitational field. , this is the most commonly used statement to explain the equivalence principle, and the equality of inertial mass and gravitational mass is a natural corollary of the equivalence principle. ?[14]

In November 1915, Einstein submitted four papers to the Prussian Academy of Sciences. In these four papers, he put forward new ideas and proved the precession of Mercury's perihelion. , and gives the correct gravitational field equation. At this point, the basic problems of general relativity have been solved, and general relativity was born.

In 1916, Einstein completed a long paper "The Foundation of General Relativity". In this article, Einstein first called the theory of relativity that was previously applicable to inertial systems the special theory of relativity, which only applies to inertial systems. The principle that the physical laws of the inertial frame are also established is called the principle of special relativity, and further expresses the principle of general relativity: the laws of physics must be true for any frame of reference that is moving in any way. ?[14]

The significance of the theory of relativity: A long time has passed since the establishment of the special theory of relativity and the general theory of relativity. It has withstood the test of practice and history and is a truth generally recognized by people. The theory of relativity has had a huge impact on the development of modern physics and the development of modern human thought. The theory of relativity unifies classical physics logically and makes classical physics a perfect scientific system. The special theory of relativity unifies the two systems of Newtonian mechanics and Maxwell's electrodynamics on the basis of the special principle of relativity, pointing out that they both obey the special principle of relativity and are covariant to the Lorentz transformation. Newtonian mechanics is nothing more than the movement of objects at low speeds. A good approximation for motion. On the basis of general covariance, the general theory of relativity established the relationship between the local inertial length and the universal reference coefficient through the equivalence principle, obtained the general covariant form of all physical laws, and established the general covariant gravity theory, and Newton's theory of gravity is only its first approximation. This fundamentally solves the previous problem of physics being limited to inertial systems, and provides a logically reasonable arrangement. The theory of relativity strictly examines the basic concepts of physics such as time, space, matter and motion, and provides a scientific and systematic view of time, space and matter, thus making physics a logically perfect scientific system. ?[14]

The special theory of relativity gives the motion rules of objects moving at high speeds, and suggests that mass and energy are equivalent, and gives the mass-energy relationship. These two results are not obvious for macroscopic objects moving at low speeds, but they show extreme importance when studying microscopic particles. Because the movement speed of microscopic particles is generally relatively fast, and some are close to or even reaching the speed of light, particle physics is inseparable from the theory of relativity. The mass-energy relationship not only creates necessary conditions for the establishment and development of quantum theory, but also provides a basis for the development and application of nuclear physics. ?[14]

For these brand-new concepts introduced by Einstein, most physicists on the earth at that time, including Lorentz, the founder of the relativistic transformation relationship, found it difficult to accept. Some people even said that "only two and a half people in the world understood the theory of relativity at that time." The obstacles of the old way of thinking meant that this new physical theory was not familiar to the majority of physicists until a generation later. Even when the Royal Swedish Academy of Sciences awarded the Nobel Prize in Physics to Einstein in 1922, it only said "Because of his contribution to theoretical physics, and even more because of his discovery of the law of the photoelectric effect.

"The Nobel Prize in Physics speech to Einstein did not even mention Einstein's theory of relativity. (Note: An important reason why the theory of relativity did not win the Nobel Prize is that it lacks a large number of factual verifications.

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