On February 15, 2022, Zhang Yuanzhong, a famous theoretical physicist in mainland China and a researcher at the Institute of Theoretical Physics of the Chinese Academy of Sciences, died of illness at the age of 81. Mr. Zhang Yuanzhong has long been engaged in theoretical physics research and teaching, especially in the fields of special and general relativity theory, gravitational theory and cosmology, and has authored monographs such as "Experimental Basis of Special Relativity", which has made important contributions to the research and education of gravity and relativity in mainland China.
This article was written by Mr. Zhang Yuanzhong to clarify that "special relativity" is a major pillar of modern physics and not general relativity.
Written by | Zhang Yuanzhong
(This article is a special contributor to Physics and Engineering, No. 2, 2017.) The picture is written by Zhang Yuanzhong, a researcher at the Institute of Theoretical Physics, Chinese Academy of Sciences)
(Institute of Theoretical Physics, Chinese Academy of Sciences)
01
background
As early as 1970, we saw in the literature the accepted comment of the physics community: "Special relativity and quantum mechanics are the two pillars of modern physics"; the third film "Chapter 15 Fundamentals of Special Relativity Mechanics", which was recently signed DHBD219 and uploaded to the Internet on May 5, 2012, also shows this statement.
Figure 1 is a screenshot of the network, and the download address is:
http://wenku.baidu.com/link?url=wGmEmuuPlIB-jaCrYUBwDIDQTeG lWGZAVVu1X8R0JOPzP3iZVm4vHgKIoDimvW NQI8u0xNgdhp31AZ_65U_8Z-kqp1Nun3as5D 5oGMms00q
Figure 1 Network screenshot
It should be noted that in the early years, some literature omitted the word "narrow sense" in order to save trouble, and the word "narrow sense" was also omitted from other physics names, such as relativistic mechanics, relativistic quantum mechanics, etc., in which "relativity" refers to "special relativity". Einstein made a special statement in his 1916 book Foundations of General Relativity[1]: "The theory to be discussed below is the most detailed generalization imaginable of what is commonly called 'relativity' today. For the sake of distinction, I will henceforth call the above 'theory of relativity' 'special relativity', and assume that it is known to all. ”
It is now impossible to verify which experts or doctoral students in previous years took the "theory of relativity" in "the theory of relativity is a pillar of modern physics" as "general relativity"! As a result, in individual doctoral dissertations of universities and research institutes, there has been a completely erroneous comment: "General relativity is the backbone of modern physical theory"; this erroneous comment has also appeared in the proposals and applications of some expert professors in recent years. In order to avoid such mistakes from continuing to mislead people, the following explains why it is "special relativity" rather than "general relativity", which is a major pillar of modern physics.
02
Two Basic Principles (or Assumptions) of Special Relativity
(1) The principle of special relativity: All laws of physics are valid in all inertial frames.
(2) The principle of invariant speed of light: Light always travels at an invariant speed c in a vacuum and has nothing to do with the state of motion of the light source.
These two basic principles are explained as necessary below.
03
Lorentz transform
The three-dimensional Euclidean spatial coordinates (x, y, z, z) in the inertial system K (x, y, t) to which special relativity applies are Cartesian coordinates (indistinguishable from the spatial coordinates in the Galileo transformation); however, the definition of time coordinate t is completely different from that of classical mechanics, where the time coordinates are defined by the principle of invariance of the speed of light: there is a standard clock everywhere in space to measure local time, but the time order can only be compared with each other after the clocks in each place are aligned with each other (that is, the definition of simultaneity). At this time, the time indicated by the clocks in various places is the coordinate time of the K system t; the process of clock alignment is as follows:
Now explain what would happen if the principle of relativity were left unchanged and the one-way assumption of the invariance of the speed of light was replaced. For example:
(1) By replacing the one-way invariance assumption of the speed of light in both directions (one-way variable speed of light), the coordinate time thus defined differs from equations (1) and equation (2), and the coordinate transformation derived together with the principle of relativity is not the Lorentz transform but the Edwartz transform.[2] The corresponding theory is called special relativity with an invariant speed of light in a loop.
(2) Replace the one-way invariance assumption of the speed of light with the assumption of instantaneous signals (i.e., the propagation velocity is infinite), so that the time coordinates defined are Galilean time coordinates, derived together with the principle of relativity are the Galileo transformation; that is, in the Lorentz transformation, c is equal to infinity.
The above analysis shows that 3 different definitions of simultaneity, together with the principle of relativity, derive 3 different coordinate transformations, so the (unidirectional) principle of invariance of the speed of light and the principle of relativity of special relativity are basic assumptions that are independent of each other.
04
Special relativity is a major pillar of modern physical theory
With the Lorentz transform (4) and equation (5) above, the principle of special relativity can be expressed as follows: the equations of all laws of physics remain invariant (or covariance) under the Lorentz transform.
Modern physical theory is constructed from this formulation of the (narrow) principle of relativity. The method of construction is usually the action method, that is, the kinetic variables of the physical system are used to construct the amount of action that is unchanged under the Lorentz transform, and then the variation of the action on the kinetic variable is equal to zero (the principle of the minimum amount of action) to obtain the kinetic equation of the physical system (Euler-Lagrange equation), so that the resulting equation remains unchanged under the Lorentz transform (that is, to meet the requirements of the principle of special relativity). For example, the macroscopic theories of flat space-time include: (special) relativistic mechanics, (special) relativistic electromagnetics of moving media, etc.; microscopic theories include: (special) relativistic quantum mechanics, (special) relativistic quantum electrodynamics, (special) relativistic particle physics theory, etc. All of these (macroscopic and microscopic) theories have their dynamical equations kept formally unchanged under the Lorentz transform (i.e., to satisfy the requirements of the principle of special relativity); moreover, the invariance of the amounts of action of these physical systems under the non-homozygos Lorentz transform gives the law of conservation (the invariance of the homology lorentz gives the law of conservation of angular momentum; the translational invariance of time coordinates gives the law of conservation of energy; the translational invariance of spatial coordinates gives the law of conservation of momentum). So special relativity is a major pillar of all these modern physical theories (that is, without special relativity, there would be no these modern physical theories). Of course, quantum mechanics is another pillar of microphysical theory).
After the birth of special relativity in 1905, Newton's law of gravitation must also be generalized to the covariant form of the Lorentz transformation; this cannot be done in flat space-time, so Einstein established the gravitational theory of curved space-time, general relativity, in 1915.
General relativity also has two basic assumptions: (1) the principle of general relativity (or general covariance) ;(2) Einstein's equivalence principle (or strong equivalence principle). The principle of strong equivalence can be expressed as [3]: a local inertial system can be established near each point in curved space-time , in which the physical laws obtained from non-gravitational physical experiments are in the form of special relativity ( that is , these laws of physics remain unchanged under the Lorentz transformation , for example , macroelectromagnetic experiments give electrodynamics , microscopic electromagnetics experiments give quantum electrodynamics ; mechanical experiments give special relativistic mechanics , and so on ). Therefore, special relativity is also a pillar of general relativity (the principle of special relativity that satisfies partially in curved space-time). General relativity is the (local) special relativistic theory of gravitational attraction, it is only a theory describing gravitational interaction, and electromagnetic theory, weak action theory, strong action theory, etc. belong to the same level, and it is impossible to be the pillar (or foundation) of who is who. Only special relativity is the backbone of modern physical theory of all 4 fundamental interactions (gravity, electromagnetism, weak force, strong force). Thus, the statement that general relativity is the backbone of modern physical theory is a confusion of the concept of physics.
bibliography
[1] Einstein. Fundamentals of General Relativity, Chronicles of Physics, Germany, Series 4[M].1916,49:769-822.
(For the Chinese translation, see page 36 of selected works by Einstein published by the Shanghai People's Publishing House in 1973.)
[2] Edwards W F. Special relativity in anisotropic space[J]. Am.J.Phys.,1963(31): 482.
(Or see Zhang Yuanzhong. Fundamentals of Experiments on Special Relativity, Section 1.2, Beijing: Science Press, 1979)
Weinberg. Gravity theory and cosmology (principles and applications of general relativity)[M].Beijing:Science Press,1980:75-76.
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