(Zhang Qinyuan, East China Normal University, June 65438+1October 65438+September 2005)
First, physical science outside the common sense space.
Go deep into the macro and micro extremes.
Great achievements have been made in science in the 20th century. As far as the understanding of material energy levels and their basic laws of motion is concerned, it involves the following five energy levels of material systems: first, atomic and molecular energy levels; Second, the nuclear level; Third, quark level; Fourth, the lepton level; Fifth, the cosmic level. The first four levels can be classified as microscopic systems. The fifth level can be classified as a macro system together with celestial bodies, planets and stars on earth.
Although the atomic and molecular level was initially understood in the19th century, it was not until the 20th century, especially after the 1920s when quantum mechanics was born, that the research on the atomic and molecular level involved many disciplines such as physics, chemistry and biology, such as quantum chemistry, molecular biology and condensed matter physics.
Scientists headed by Madame Curie studied the radioactivity of matter, which made people know another level of matter-nucleus. The nucleus is a system composed of neutrons and protons, which are closely combined through strong interaction to form the nucleus. The study of atomic nucleus has enabled people to obtain a new energy source-nuclear energy, and successfully built a nuclear reactor, realizing the peaceful use of nuclear energy.
Quark level was discovered and studied in 1960s. Scientists have found that quarks are more basic particles than neutrons, protons and other particles, which are all made of quarks. Since then, scientists have discovered six kinds of quarks by using extremely high-energy accelerators. Quark hierarchy follows the law described by quantum chromodynamics.
The study of lepton energy level is earlier than that of quark energy level. Initially, the celestial bodies were mainly photons and electrons. Later, muons and τ-neutrons and their corresponding three neutrinos were discovered, namely, electron neutrinos, muons and τ-neutrinos. 1956, Yang Zhenning and Li Zhengdao discovered that parity is not conserved at lepton level. 10 years later, Weinberg and other scientists established an electrically weak unified theory to describe the law of lepton motion.
The research on the cosmic level is based on general relativity and astronomical observations, such as the red shift of the spectral lines of galaxies outside the river and the background radiation of 3K microwaves. The redshift of spectral lines of extragalactic galaxies and Hubble's law indicate that the universe is expanding; K microwave background radiation shows that the early universe was in a state of high temperature and high density, and the universe was born in the Big Bang. Nowadays, the black hole predicted by general relativity has also been indirectly confirmed by observation.
The spatial span of the above five levels ranges from the minimum10-19m to the maximum1017m (about1017m), with a difference of 1036 times. However, the essential difference between these five material levels does not lie in their spatial scope. At the atomic and molecular level, the most important thing is electromagnetic interaction; At the level of nucleus and quark, the most important thing is the strong and weak interaction; At the lepton level, the most important thing is the weak electromagnetic interaction. It is through the study of these five material levels that people realize four basic forms of interaction in nature.
On the cosmic level, in 19 17, Einstein published a paper entitled "Cosmological Investigation Based on General Relativity", which initiated the study of the universe as a physical whole in theory and became a landmark document for the birth of modern cosmology. 1946, Gamov, a Russian-American physicist, first linked general relativity with nuclear theory. Explore the origin of the expansion of the universe and the abundance of elements in the universe, and put forward a cosmological model of the Big Bang. 1965, the 3K microwave background radiation discovered by penzias and Wilson became the most powerful support for the cosmological model of the Big Bang. Since then, BIGBANG's cosmological model has become the mainstream of cosmological theoretical research, so it is called the standard cosmological model. In 1980s, based on the cosmological model of the Big Bang, an explosion describing the very early universe was proposed.
1989, the United States launched the "cosmic background detector" COBE. 1990. The detection data of the first batch show that the matching degree between microwave background radiation and the theoretical curve of 2.730K is 99.75%. The background radiation inhomogeneity detected by COBE satellite in 1992 accords with the very early explosion required by the cosmological model of thermal explosion.
According to the cosmological model of the Big Bang, we have explained the structure, movement and evolution of the universe with confidence. The universe has a uniform and isotropic structure, and the universe as a whole is expanding. The universe originated from high temperature and high density, and every space point is experiencing a big bang (no center). From the moment of BIGBANG, the universe began to expand, and with the expansion, the density of the universe decreased. Temperature of the universe fell. In the hundredth second after the Big Bang, the temperature dropped to 1000 billion degrees Celsius. There are electrons, positrons, neutrinos, photons, a small number of protons, neutrons and other basic particles in the universe, and every 654,380 billion photons (or electrons, positrons or neutrinos) corresponds to a proton or neutron. After the Big Bang 180 seconds, the temperature dropped to10 billion degrees Celsius, and protons and neutrons combined to form helium nuclei. In 700,000 years after BIGBANG, the temperature dropped to 4,000 degrees Celsius, and electrons and nuclei combined to form atoms, which later formed galaxies and stars. Today is/KLOC-0.5 billion years after BIGBANG, and the temperature is below -270 degrees Celsius (3K).
So what will the future of the universe be like? It depends on the current density of matter in the universe. If the density of matter is equal to or less than a critical density, the universe will expand forever. If the density of matter is greater than this critical density, the expansion of the universe will gradually slow down, from expansion to contraction, and finally return to the high temperature and high density state like the Big Bang. 1998, scientists found that the expansion of the universe is accelerating, and it seems that the universe is likely to expand forever, and the evolution of the universe is endless.
The road to reunification
Along the road guided by relativity and quantum mechanics, in the first half of the 20th century, people began to look for a higher level of unity, that is, the unity of natural forces or the unity of interaction. So far, only four natural forces have been discovered, namely, strong force, electromagnetic force, weak force and gravity. These four forces or interactions are quite different in strength and force range. At first glance, there seems to be no unified basis. But is there really no unified mechanism behind it? Einstein didn't think so. He believes that pursuing the unity of nature is not only the sacred duty of scientists, but also the ability of scientists. He said, "Do we really have any hope of finding the right path? Not only that, is there room for this correct path besides our fantasy? Can we hope that experience will lead us to the right path? If there is a theory (such as classical mechanics) that can make a correct judgment on experience to a large extent, but it does not trace back to the source, I can answer without hesitation. In my opinion, there is indeed a correct path and we have the ability to find it. " In fact, Einstein has been committed to the unification of electromagnetic interaction and gravitational interaction since he founded general relativity. Although he did not succeed, he always believed that someone would continue his unfinished business in the future.
In the year of Einstein's death (1955), Yang Zhenning founded the theory of gauge symmetry. According to gauge symmetry theory, strong interaction, electromagnetic interaction, weak interaction and gravitational interaction are all governed by gauge symmetry principle. Because these four interactions correspond to four forces, Yang Zhenning calls them symmetrical advantages. Because all four interactions are dominated by gauge symmetry, there are four interactions.
1969, Weinberg and Salam first realized the unification of weak interaction and electromagnetic interaction on the basis of canonical symmetry theory. An important prediction of the unified theory of weak current is that there should be a neutral flow in the basic particle reaction of weak interaction and electromagnetic interaction, such as the scattering of electrons and neutrinos. Neutral flow is a particle flow formed by neutral particles such as Z. In addition to neutral flow, there are also charged currents transmitted by W and W. W, W and Z are gauge particles. 1982, an experimental team of the western European center used a computer to screen 140000 collision cases on a proton-antiproton collider. Finally, five examples are found to prove the generation of W particles. 1983, six more examples of Z particles were found in the center of Western Europe. The mass of these three particles measured in the experiment is completely consistent with the theory. The unified theory of weak current has been confirmed by experiments. Weinberg and Salam 1979 won the Nobel Prize in Physics.
The success of weak current unification increases the confidence of unifying all four interactions. In 1970s, a unified theory of strong interaction, weak interaction and electromagnetic interaction based on SU(5) group mathematics was established, which was called SU(5) grand unified theory.
Su (5)' s grand unified theory can explain the existing understanding of strong interaction, electromagnetic interaction and weak interaction, but its two important predictions of proton decay and the existence of magnetic monopole have not been confirmed by experiments. At present, the lifetime of protons given by experiments is almost infinite, because the half-life of protons is more than 654.38 billion years, but in the grand unified theory, the half-life of protons is less than 65438+/kloc-0.0 billion years. As for the magnetic monopole, no one has ever seen it, because the magnetic south pole is always accompanied by the magnetic north pole. Although Dirac, a famous physicist, put forward the hypothesis that there may be a magnetic monopole in 1937, people did not take it seriously. Now the grand unified theory puts forward the hypothesis of the existence of magnetic monopole, which leads many laboratories to try to find it, but so far.
Although there are still many difficulties in today's great unification theory, many scientists don't think we should give up the idea of unification. For example, Yang Zhenning thinks that unity is the direction, and the problem now is probably because the concept of symmetry has not been well expanded. When talking about the relationship between symmetry and the unified theory of interaction, he said: "The problem that has not been solved today is probably because we have not extended the concept of symmetry to the end. In recent 20 years, the popularization and re-popularization of the concept of symmetry has become a hot topic. 1973 someone has proposed a very clever mathematical structure called supersymmetry. 1976 someone put forward a new concept of symmetry called supergravity. Although it is different from supersymmetry, it is closely related. Actually, someone put forward superstring in 1984. It is precisely by expanding the concept of symmetry that we have expanded to a considerable extent today, which was unimaginable before ... we dare not talk about its future now. However, if you ask a person who studies basic physics, this general direction-I hope to expand the concept of symmetry a little, and then extend it a little, and try to solve some problems that we have not solved through these efforts. Is the direction right? I think 90% of people will think that this is the right direction, although some details of this correct direction are still being explored. " (Yang Zhenning, Journal of Nature, 1995, 10, p. 257)
Science will not stop developing, and the physical science in 2 1 century will face new breakthroughs. Although both relativity and quantum mechanics are successful and have been combined in the field of quantum field theory, in the recent 10 years, the development of relativity and quantum mechanics has been out of harmony. As Weinberg said, it is now found that "quantum mechanics and special relativity are almost incompatible, and their harmony in quantum field theory has brought strong restrictions to the interaction mode of particles." (S. Weinberg's Dream of the Ultimate Theory, Hunan Science and Technology Press, 2003, p. 1 14) The disharmony between quantum mechanics and relativity mainly shows that relativity is a localized theory. On the other hand, quantum mechanics is a nonlocality theory. The nonlocality of quantum mechanics is the most important progress in quantum mechanics experiments in recent ten years. The nonlocality of quantum mechanics is mainly manifested as a very strange phenomenon-quantum entanglement, that is, there is a correlation between two distant parts in a quantum system, but relativity believes that it cannot exist. Quantum entanglement is supported by Bell's theorem and Esbeckett's experiment. (About quantum entanglement, we are
Li Zhengdao said in a speech at Fudan University not long ago that he also mentioned the problems faced by physical science. He believes that there are two major problems in contemporary science, one is quasars and the other is dark matter. There are two problems in particle physics, one is symmetry breaking, and the other is invisible quark. If we can understand these problems, it will play an important role in the scientific development of 2 1 century.
Li Zhengdao pointed out that the quasar problem is actually an energy mechanism problem.
The brightness of a star can be 1-65438+ ten thousand times higher than that of the sun in an instant, and a supernova is1-200 thousand times brighter than a nova. However, quasars are more powerful, and their brightness is the sum of the brightness of 65,438+0,000 galaxies. Since the discovery of quasars in 65,438+0,966 and 5438+0 years, its energy mechanism is still unknown. All we know is that it is much more than nuclear energy. 1987 It was found that the brightness of quasars suddenly doubled, and the reason is not clear. Through the study of quasars, we expect to find a much stronger energy mechanism than nuclear energy. The problem of dark matter is about the existence of unknown forms of matter. Astronomical observation shows that 3/4 of the matter in the galaxy cluster of the Milky Way is invisible, and its energy cannot be measured by optical, infrared and radiation means. Looking for dark matter.
Symmetry breaking problem in particle physics is related to unknown force. Many theories in particle physics, such as quantum chromodynamics and Einstein's general theory of relativity, are derived from symmetry. However, our world is asymmetric. This shows that besides the known strong force, electromagnetic force, weak force and gravity, there may be unknown forces, which is the reason of symmetry breaking. If we understand this force, it is possible to know the mechanism of symmetry breaking. We can know the origin of asymmetry, and then it is possible to know the source of mass (including dark matter). Quark problem is not only a matter structure problem, but also an energy mechanism problem. According to the existing particle physics theory, all hadrons and nucleons are composed of quarks, but so far no free quarks have been observed in the experiment. Whether quarks exist or not, and whether the energy mechanism of quarks combining into hadrons is special, these are extremely fundamental problems.
In order to solve the coordination and unity of interaction between quantum mechanics and relativity, and the four major problems mentioned by Li Zhengdao, the physics community is making difficult explorations. Among them, it is known as "physics that unexpectedly fell into the 20th century from 2 1 century"-M theory has become the hottest.
M theory is developed from 10 dimensional superstring theory, and there is no definite name yet. M takes the first letter of the mother of magic, mystery, Marvel Comics, film, Martic and all theories.
M theory holds that the basic component of our world is a supersymmetric supermassive (membrane or
Nuggets), a superblock contains not only 1 dimensional chords and 2-dimensional films, but also 3-dimensional and higher physical entities. In M theory, space-time is 1 1 dimension, where 1 dimension time, 10 dimension space. The space in M theory is seven dimensions more than the three dimensions in real space. The compactness of space is Planck length, so we can't observe four-dimensional space-time, which is our real world. M theory has a special property-duality, which means that all kinds of superstring theories can give the same result within the framework of M theory.
At present, M-theory has given a reasonable explanation for the unification of four kinds of interactions, and put forward a new view on the origin of the universe, forming M-theory cosmology. In M-theory cosmology, the expanding three-dimensional space universe we live in is regarded as a piece or regional wall of air movement in higher dimensions, and the universe originated from a quantum vacuum.
Second, complexity science.
From self-organization to hypercycle
Prigogine's self-organization theory was founded in 1960s. Prigogine studied the open system far away from the equilibrium state, and pointed out that such a system would move from disorder to order under the condition of nonlinear interaction (such as fluctuation) without violating the second law of thermodynamics, and this process was called self-organization. Self-organization is a kind of phase change, which is a system with dissipative structure. In the process of developing towards entropy reduction, it will enter a new and more orderly steady state. This increase in order corresponds to a relative increase in the amount of information. Self-organization includes self-creation, self-growth, self-evolution and self-replication. An open system far from the equilibrium state (whether it is a mechanical, physical, chemical, biological or even socio-economic system) will experience fluctuations when the change of a certain parameter in the system reaches a certain threshold, by constantly exchanging materials and energy with the outside world.
According to the self-organization theory, how to keep the order of life and natural objects such as the earth can be well explained. A living organism is an open system, which obtains negative entropy from the outside world by constantly exchanging matter and energy with the outside world, and maintains self-organized order and vigorous vitality.
I.llyaPrigogine's dissipative structure theory not only solves the mystery that the second law of thermodynamics is incompatible with the theory of biological evolution, but also eliminates the suspense of self-organization phenomenon in nature, which opens a channel for us to understand the complexity of the whole world, including society.
Stuart Kaufman of the Santa Fe Institute believes that only by relying on the theory of self-organization can we truly understand the phenomenon of life. He wrote in the book "The Universe is Home": "I want to say that there is another source of life besides this, and that is self-organization; This is the root of the orderly state. I finally believe that the order in the biological world is not only established piecemeal. This is because the principle of self-organization is innate and spontaneous. Complex principles have just been revealed and understood. " (Stuart Kaufman, Hunan Science and Technology Press, Universe is Home, Preface, 2003)
German scientist Harken founded Synergetics in 1973 shortly after he established the theory of self-organized dissipative structure in Prigogine. Synergetics believes that when the subsystems in the biological and abiotic open systems are far away from the equilibrium state, they will produce synergistic effects through nonlinear interaction and reach a critical point through fluctuations in a certain range. We can make the system move from disorder to order through self-organization, and make the old structure develop into a new structure with fundamental changes in time, space, nature and function. Therefore, we can think that synergetics is a science about self-organized collective behavior. Synergetics regards a complex self-organizing system as a collective composed of many subsystems, and self-organization is the result of the synergy of these subsystems under dissipative structure conditions.
Hypercycle theory was put forward by German scientist Manfred Eigen in 1970. Egan thinks that hypercycle is a process of molecular self-organization, and he divides the cycle phenomenon in biochemistry into different levels: the first level is the transformation reaction cycle, which is a self-regeneration process as a whole; The second level is called catalytic reaction cycle, which is a self-replicating process as a whole; The third level is the so-called hypercycle, which refers to the cycle in which the catalytic cycle is functionally coupled, that is, the catalytic hypercycle. In fact, in the hypercycle organization, it is not required that all components play the role of self-catalyst. Generally speaking, as long as one link in this cycle is a self-replicating unit, this cycle can show the characteristics of hypercycle. Hypercycle is characterized by not only self-regeneration, self-replication, but also.
The internal factors of the evolution of hypercycle structure mainly come from two aspects: first, the mistakes of self-replicating units in the process of replication, similar to gene mutation; Secondly, the hypercycle structure is a multi-level system composed of many components, and there are complex nonlinear interactions inside. In this case, as the chaos theory points out, the inherent randomness will play a role to a great extent, exerting another internal disturbance on the hypercycle structure. Therefore, the evolution of hypercycle structure is usually related to three factors: replication error, inherent randomness and environmental interference.
Hypercycle structure can only exist in evolution. For hypercyclic structures to exist, evolution must meet three preconditions: (1) promoting the metabolism of the structure with a large enough negative entropy flow; (2) Make the system information accumulation and inheritance have strong enough replication ability; (3) Ensure the existence and development of structures with strong enough functional coupling between components. Only when these three conditions are met at the same time can the hypercycle structure exist stably and develop and evolve.
The above complexity theory mainly discusses the formation mechanism, structure and evolution of complex systems, and seldom studies the dynamics of complex systems. Chaos theory, which appeared in 1960s, is about the dynamics of nonlinear complex systems. Chaos in chaos theory is defined as the aperiodic behavior of deterministic nonlinear systems under bounded sensitive initial conditions.
Chaos theory originated from the study of weather system. Chaos theory holds that the weather system is very sensitive to initial conditions, and any slight change will cause unexpected changes in the subsequent development of the system. Lorenz, an American meteorologist and one of the founders of chaos theory, vividly compared the situation that a system is extremely sensitive to initial conditions to the butterfly effect. He said that a butterfly flapping its wings in Brazil may be the cause of a tornado in Texas. Lorenz also believes that the butterfly effect is not accidental, but inevitable.
Chaos theory is another challenge to determinism. Quantum mechanics abandons Laplacian determinism in the micro field, while chaos theory excludes Laplacian determinism in the familiar macro field. Now we know that chaos is everywhere. The rising smoke column broke into a dazzling whirlpool, the flag fluttered back and forth in the wind, and the water dripping from the faucet was a mess. In the flight of an airplane, chaos exists in the behavior of cars being blocked on the highway, and even in the financial process. Chaos is a science about the nature of the system, which breaks the boundaries of various disciplines and brings people from different disciplines together. Chaos is not only a theory, but also an effective method to deal with complex, nonlinear and aperiodic systems.
As Gleike said in his book Chaos: Creating a New Science, "Chaos is an idea that makes all scientists believe that we are all members of the same joint venture. Physicists, biologists or mathematicians believe that simple deterministic systems can breed complexity; People think that systems that are too complicated for traditional mathematics may still follow simple laws; Also, no matter what their field of expertise is, I believe everyone's task is to understand complexity itself. " (James Gleike, Chaos: Creating a New Science, Shanghai Translation Publishing House, 1990, page 32 1).
Third, life science.
Discussion on Molecular Biology and Nature of Life
What is life? Where is the boundary between life and death? This is a problem that has been puzzling mankind from the beginning. Today, although there is still a long way to go to the final answer, at least we can think that the emergence of molecular biology in the 20th century has completely brought this question into the orbit of modern science. As early as the19th century, Engels once said, according to the scientific understanding at that time, "life is the way of existence of protein Group, and the essence of this way of existence lies in the constant self-renewal of the chemical components of these protein Groups." As for what protein Group was, it was not clear at that time, and Engels himself was not satisfied with using the word "protein Group" to represent the primitive people of life. He believes that the development of science will give a more reasonable name.
In the first half of the 20th century, protein, one of the basic elements of life, was finally discovered, and then nucleic acid was discovered. Protein and nucleic acid are both high molecular weight molecules, so they are usually called biomacromolecules. 1953, Orson and Crick discovered that DNA (deoxyribonucleic acid) has a double helix structure. Since then, a new discipline-molecular biology was born.
With the development of molecular biology, people can not only grasp the internal mechanism of biological structure and its function at the molecular level, such as heredity and variation, material metabolism and energy metabolism, biological regulation, etc., but also some biological macromolecules have been artificially synthesized, some life processes can be reproduced under experimental conditions, and the gap between chemical reactions and life activities is getting smaller and smaller. So some people think that life is the movement of these biological macromolecules. The discovery of viruses also blurs the boundary between life and molecules to some extent, because some viruses, such as foot-and-mouth disease virus, are smaller than molecules (such as hemoglobin molecules); There is another virus, only nucleic acid, no protein. It is dozens of times smaller than ordinary viruses and carries little information. It can only encode dozens of amino acids, but it can also reproduce in the host. So is the virus life? These new situations in life science make some biologists feel that the boundary between life and death is blurred, and it is more difficult to answer the question of what life is. For example, biologist Sass in his book Evolutionary Biology. Biologist Cushing said in the book The Origin of Life: "It is impossible to strictly define all biological substances (past and present) and exclude all abiotic substances."
In addition to biologists, physicists and chemists have also paid great attention to the question of what life is. Xue, one of the founders of quantum mechanics, published a booklet entitled "What is life-the physical view of living cells" in 1944. In this book of less than 100 pages, he put forward some enlightening ideas and concepts. He said: "The only way for a living organism to survive is to constantly absorb negative entropy from the environment." Organisms "depend on negative entropy to make a living". Or more accurately, the essence of metabolism is to let organisms successfully eliminate all the entropy they have to produce when they are alive. "Here, Xue introduced the concept of negative entropy for the first time. According to the second law of thermodynamics, the entropy of an isolated system will always increase, and the system will become more and more disordered, but life will develop in an organized and orderly way. Xue grasped this important feature of life and linked negative entropy with life activities. Bertalanfi, the founder of system theory, said in his book The Problem of Life: "A living organism is an open order system, which keeps itself in the exchange of components by virtue of its own conditions."
It is of far-reaching significance to study life phenomena with modern physics and system thought, and it is a pioneer in exploring the unity of physics and biology. In fact, it was after reading What is Life that Orson and Crick decided to devote themselves to the study of molecular biology. Japanese geneticist Hirao Kondo believes that the publication of What is Life is a symbol of the biological revolution.
Of course, it is not advisable to completely attribute life phenomena and life movements to physical and chemical phenomena and physical and chemical movements.
In the understanding of the nature of life, the complexity science discussed in the last section will provide a new explanation. Stuart kauffman of the Santa Fe Institute wrote in his book "The Universe is Home": "In short, the naked RNA and naked ribosome polymer hypothesis does not give a profound description of the minimum complexity of all free cells we have observed. I insist that the principle of orderly origin described in chapter 3 still makes sense. This principle clarifies the reason why matter must reach a certain point. I insist that it is the proper meaning of the essence of life. " (page 55)
In a word, the complexity of life phenomena must be considered when using reduction method in life science, because life phenomena and life movements do have their own characteristics. First of all, life on earth is based on nucleic acids and protein macromolecules. Nucleic acids (DNA and RNA) play the role of information carrier in the continuation of life as a whole and individuals, while protein plays the role of information recognition and functional expression. Second, life is always exchanging material and energy with the environment, and the mechanism of this exchange is metabolism. Moreover, if the entropy value of life decreases in the process of material and energy exchange (that is, negative entropy is absorbed from the outside), life will develop positively, and vice versa. Third, life has the characteristics of heredity and variation. Fourth, life is a highly organized and open system. It can accept and process external information and generate information. Fifth, it can adjust itself and adapt to the environment. Finally, life is the unity of occurrence and death. Because of the existence and death of individual life, the whole life is eternal.
In this way, the current understanding of the nature of life can be summarized as follows: life is essentially a complex multi-molecular system composed of nucleic acid and protein, which has the ability of self-renewal and processes, processes and stores substances, energy and information in a highly orderly manner. It has the characteristics of self-regulation, self-replication and selective response to internal and external environment.
The essence of consciousness
The essence of consciousness and the study of brain is another important topic in life science. The brain is an organ that produces consciousness, and the complexity of consciousness is largely due to its high complexity. The human brain weighs only 3-4 pounds, but it contains about 654.38+00 billion neurons. This is an astronomical figure, and the total number of stars in the Milky Way is in an order of magnitude. So is the complexity of the brain just because there are too many neurons? Of course not. There are nearly 1000 billion cells in the liver, but no amount of liver together can replace the colorful life activities of the brain. The high complexity of the brain is partly due to the diversity of nerve cells that make up the brain. Nerve cells can be divided into neurons and glial cells. Among them, neurons can be divided into many kinds according to morphological differences, such as large pyramidal cells, small pyramidal cells, granular cells, short axonal neurons and general wild cells, and can be divided into afferent neurons according to different functions. Efferent neurons and intermediate neurons. There are many types of glial cells, such as astrocytes, protoplasmic astrocytes, microglia, oligodendrocytes and ependymal cells. For neurons, besides morphological differences, there are also molecular differences. In addition, although all cells have the same gene, cells in different tissues have different choices for gene expression. At present, there are acrocrine cells in the retina. This selectively expressed gene exists in Purkinje cells of cerebellum and motor neurons of spine. In addition to structural and molecular differences, neurons also differ in input and projection.
While studying the complexity of neurons, it is also found that neurons have identity. Identity shows that neurons with similar functions gather into columns or plates and pass through the cortex. All neurons transmit information in a very similar way. Information exists along the axon in the form of short electrical pulses of action potential.