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7. On the last day of the 19th century, famous European scientists gathered together. At the meeting, W? Thomson (Baron Kelvin) made a new year's speech. Looking back on the great achievements of physics, he said that the physics building has been completed, and only some decoration work remains. At the same time, while looking forward to the future of physics in the 20th century, he said thoughtfully: "the dynamic theory affirms that heat and light are two ways of motion. Now, its beautiful and clear sky is covered by two dark clouds." the first dark cloud appears in the wave theory of light, ”"The second dark cloud appears in the Maxwell Boltzmann theory of equipartition of energy." W. In April 1900, Thomson published an article entitled "dark clouds over the dynamic theory of heat and light in the 19th century". The first dark cloud he said mainly refers to the contradiction between the results of Michelson Morey experiment and the theory of ether drift; What he said about the second dark cloud mainly refers to the results of the theoretical interpretation of the specific heat of gas and the potential radiation spectrum by the energy equipartition rule in thermodynamics, which are different from the experimental results, especially the "ultraviolet disaster" in the blackbody radiation theory. Kelvin is an outstanding theoretical physics and experimental physicist in 19th century England. He is an influential authority in physics. His statement tells the basic situation of physics development to the end of 19th century and reflects the main trend of thought in the field of physics at that time
physics developed to the end of the 19th century, which can be said to be quite perfect and mature. All physical phenomena seem to be able to get satisfactory answers from the corresponding theories. For example, all mechanical phenomena can be explained from classical mechanics in principle. Newtonian mechanics and analytical mechanics have become effective tools to solve mechanical problems. For the analysis of electromagnetic phenomena, Maxwell's electromagnetic field theory has been formed, which is the unified theory of electromagnetic field. This theory can also be used to explain the basic problems of wave optics. As for the thermal phenomenon, there are phenomenological thermodynamics and statistical mechanics theories, which can make a reasonable explanation for the macroscopic law of the thermal motion of matter and the microscopic statistical law of the thermal motion of molecules. In a word, the classical physics building with classical mechanics, classical electromagnetic theory and classical statistical mechanics as the three pillars has been built, and the foundation is solid and magnificent! Under such circumstances, it is no wonder that physicists will feel intoxicated and feel that physics has achieved great success, thus asserting that they will not be able to do anything in the future. At that time, this kind of thought not only existed universally in physics, but also had a long history
Planck made a speech in 1924. In his speech, he recalled that when he was studying physics at the University of Munich in 1875, his physics teacher P. Jolie (1809-1884) advised him not to study pure theory, because physics "is a highly developed and almost perfect science". Now this science "seems very close to taking the most stable form.". Perhaps, there is a st or a small bubble in a corner, which can be studied and classified. However, as a complete system, it is established firmly enough. And theoretical physics is clearly approaching the level of perfection that geometry has had for hundreds of years. " Another famous teacher of Planck, G? Kirchhoff (1824-1887) also said a similar thing, he said that "physics has done nothing but add a few numbers after the decimal point of the known law." Although Kelvin's evaluation of the achievements of physics is too extreme, he can find "two dark clouds" in the clear sky and is worried about them, which shows that he is far sighted. The history of the development of physics shows that it is these two small dark clouds that finally led to a big storm
the first dark cloud -- Michelson Morey experiment and the collapse of "Ether" theory. Sunlight passes through a vacuum to the earth, and light from galaxies billions of light-years away also passes through space to the earth. Why can light wave propagate in vacuum? What is its medium of transmission? Physicists have found a medium for light transmission - "Ether"
Aristotle, the ancient Greek philosopher, first proposed "Ether". Aristotle thought that the lower boundary was composed of fire, water, earth and air; Add the fifth element to the upper bound, "Ether". After Newton discovered the universal gravitation, he met a difficult problem: in the vacuum of the universe, what medium does gravitation propagate? In order to find a complete solution, Newton revived Aristotle's "Ether" theory, and believed that "Ether" was the propagation medium of gravity in the cosmic vacuum. Later, physicists developed the theory of "Ether", believing that "Ether" is also the medium of light wave propagation. Light, like gravity, is transmitted by the ether. They also assume that the whole universe is full of "Ether", which is a thin, imperceptible medium composed of very small elastic spheres. In the 19th century, Maxwell's electromagnetic theory also described the medium that transmits light and electromagnetic waves as a kind of "Ether" that has no weight and can absolutely penetrate“ "Ether" not only has the nature of electromagnetism, but also is the transmitter of electromagnetic effect, and also has the nature of mechanical mechanics. It is an absolutely static reference frame, and all movements are relative to it. In this way, the electromagnetic theory is consistent with Newtonian mechanics“ The concept that "Ether" is the common carrier of light, electricity and magnetism is generally accepted by people, forming a "etherology"
however, after confirming the existence of "Ether", a new problem arises: if the earth moves around the sun at the speed of 30 kilometers per second, it must encounter the wind of 30 kilometers per second. At the same time, it must also have an impact on the propagation of light. The emergence of this problem causes people to explore whether "yitaifeng" exists or not< In 1887, Michelson (1852-1931) cooperated with American chemist and physicist Murray (1838-1923) to carry out a famous experiment in Cleveland: "Michelson Murray experiment", namely "ether drift experiment". The experimental results show that no matter the direction of the earth's motion is the same as or opposite to the direction of light, the measured speed of light is the same, and there is no relative motion between the earth and the "Ether". Therefore, there is no "Ether" or "absolutely static space". Because this experiment is easy to understand in theory and accurate and reliable in method, there is no doubt that the experimental result denies the existence of "Ether"
the Michelson Morey experiment puts scientists in a dilemma. They may have to abandon the etheric theory that has explained many phenomena of electromagnetism and light. If they dare not give up ether, then they must give up Copernican's theory of earth movement, which is older than etherology. Classical physics is at a loss in the face of this famous experiment
the second dark cloud -- blackbody radiation and "ultraviolet disaster"
at the same temperature, the brightness and color (wavelength) of different objects are different. Dark objects have strong ability to absorb radiation. For example, the absorption rate of coal to electromagnetic wave can reach about 80%. The so-called "blackbody" refers to the ideal object that can absorb all external radiation without any reflection and transmission, and the absorptivity is 100%. There is no real blackbody, but a cavity with a small hole on its surface can be regarded as an approximate blackbody. Because the radiation entering the cavity through the small hole will not leak out from the small hole after many times of reflection and absorption in the cavity
at the end of the 19th century, lummer (1860-1925) and others' famous experiment blackbody radiation experiment found that the energy of blackbody radiation is not continuous, and its distribution according to wavelength is only related to the temperature of blackbody. From the perspective of classical physics, the results of this experiment are incredible
how to explain the results of blackbody radiation experiment? At that time, people started from classical physics to find the laws of experiments. Incorrect premise and starting point lead to failure in the end. For example, Wien, a German physicist, established a formula for the distribution of blackbody radiation energy according to wavelength, but this formula is only consistent with the experimental facts when the wavelength is relatively short and the temperature is relatively low. British physicist Rayleigh and physicist, astronomer King think that energy is a continuous change of physical quantity, establish the blackbody radiation formula which is more consistent with the experimental facts when the wavelength is longer and the temperature is higher. However, according to the Rayleigh gins formula, in the short wavelength region (ultraviolet region), the radiation intensity can increase endlessly with the shortening of the wavelength, which is far from the experimental data. So this failure was called "ultraviolet disaster" by Ellen fester. Its failure undoubtedly indicates the failure of classical physics theory in the problem of blackbody radiation, so it is also the "disaster" of the whole classical physics.