In order to understand supersymmetry, we have to talk about another clue in the analysis of the basic structure of matter: force. No matter how complex the particle zoo is, it seems that there are only four basic forces: gravity, electromagnetic force (which is widely known because it is closely related to daily life), weak force and strong force. Of course, the strength between neutrons and protons can't be the basic force, because neutrons and protons are compounds themselves, not elementary particles. When two protons attract each other, what we actually see is the resultant force of six quarks. There are two reasons for this. First, quarks have three colors but only one charge, so one photon corresponds to eight different gluons. Second, gluons are also colored, so there is a strong interaction between them, while photons have no charge, so they are irrelevant to each other.
More than 20 years ago, some far-sighted theoretical physicists suddenly thought that there were four basic forces in nature, which seemed too many. It is quite possible that these four basic forces are not really independent. Maxwell put forward a mathematical formula in the 65438+60s, which unified electric power and magnetic force into a single electromagnetic field theory. It is likely that there will be further synthesis.
A lingering mathematical problem has prompted some theoretical physicists to think so. But the same mathematical stunt doesn't work on the other three forces. It is hoped that electromagnetic force and other three basic forces will be combined into a single description in some way, and the mathematical docility of this single description will eliminate the other three forces except electromagnetic force, so that people can draw an understandable formula.
The first step to achieve this grand goal was taken by Stephen Weinberg and abdul sallam in 1967. They successfully transformed the mathematical expressions of electromagnetic force and weak force, so that the two forces were combined into a unified mathematical expression. Their theory shows that we usually regard electromagnetic force and weak force as different forces (indeed, they are significantly different in nature) because the energy used in our current experiments is extremely low. Of course, the "low" here is relative: the current accelerator can provide enough energy for a collision. If this energy is added to a billiard ball instead of a proton, the released energy can provide the needs of an ordinary family for millions of years! However, Weinberg-Salam theory has an embedded energy unit, and the energy of this unit can only be achieved by the existing technology. The "low" energy used in the current experiment mentioned above is also relative to this unit.
In 1970s, experimental evidence gradually accumulated, and the situation became favorable to Weinberg-Salam theory. 1980, they won the nobel prize for their work in unified research. 197 1 year proves that the infinity of headache can be swept away in the unified formula as expected, and physicists begin to talk about three basic forces of nature instead of four.
The main reason why the headache infinity can be swept away is that more abstract symmetry groups appear in the unified force theory. It has long been known that Maxwell's beautiful electromagnetic theory is powerful and beautiful because of its balance and symmetry in its mathematical description. There is a kind of balance in the unified force theory, called gauge symmetry, which is an abstract balance. But this balance can remind people of things in daily life.
The example of climbing a cliff can be used to illustrate normative symmetry. It takes energy to climb from the bottom of the cliff to the top. But there are two ways to climb from the bottom up. One is relatively short and climbs vertically to the top of the cliff; The other one is longer and climbs to the top of the cliff along a slower ramp. Which of these two methods is more efficient? (See Figure 24) The answer is: the energy consumed by the two methods is the same (here, we ignore the unrelated complex situations such as friction). In fact, it is easy to prove that the energy required to climb the cliff top has nothing to do with the chosen path. This is gauge symmetry.
The above example is a normal gravity field symmetry, because if you want to climb to the top of the cliff, you must overcome gravity. Gauge symmetry is applicable to both electric fields and more complex magnetic fields.
It has been proved that the gauge symmetry of electromagnetic field is closely related to the massless characteristics of photons, and it is also a key factor to avoid the catastrophic infinity of unified force theory. Weinberg and Salam finally tamed the weak force and combined it with electromagnetic force.
Inspired by the success of the unified gauge theory, physicists turn their attention to another nuclear force-the chromatic force between quarks. Soon after, the color gauge theory was put forward. Later, some people tried to use a larger gauge symmetry to include all other symmetries in a gauge symmetry, and unified the weak force and the color force into a "great unified theory" (GUT). At present, it is too early to evaluate the achievements of GUT, but at least one of its predictions-protons may be unstable and spontaneously decay after an infinite time-is now being verified. However, gravity is still disobedient. Infinite problems in revenge depend on gravity. Now, physicists are more and more inclined to think that this problem will be solved only in the hyperunified theory which contains some hypersymmetry. A large group of mathematicians and physicists are busy creating such a theory. The goal of this theory is to unify the irresistible dream of field theory-a single force field, which covers all the forces in nature: gravity, electromagnetic force, weak force and strong force. However, this is not enough. Quantum particles and the forces between them show that any theory of force is also a theory of particles. Then, the hyperunified theory should also be able to describe all quarks and leptons completely, and explain why there are particles with three energy levels in the table 1
Some people say that if we can really achieve this dazzling goal, we will reach the apex of basic physics, because theories such as hyperunification theory can explain the behavior and structure of all substances-of course, in a reductionist way. With the super-unified theory, we can write down all the secrets of nature with an equation and a universal formula. Such an achievement will confirm the long-held belief that the universe operates according to a single, simple and amazingly beautiful mathematical principle. John wheeler's following words express people's urgency to achieve this ultimate goal: "One day, a door will definitely open, revealing the shining central mechanism of this world, which is simple and beautiful."
How far are we from this paradise of wisdom? Theoretical physicists now pin their hopes on a series of theories. The name of this theory is supergravity. The key to this theory is a strange supersymmetry, which is described as the square root of space-time. This means that if two supersymmetric expressions are multiplied, you will get an ordinary geometric symmetry operation, such as motion in space.
At first glance, this abstraction seems to be of little use, but after careful analysis, it will be found that supersymmetry is closely related to one of the most basic properties that a particle may have-rotation. It is found that all quarks and leptons rotate in a rather mysterious way. We don't care how it rotates now. What we should care about is that those "messenger" particles-gluons, photons, and particles corresponding to gravity and weak force-either don't rotate or rotate in a normal rather than mysterious way. The significance of supersymmetry is that it connects particles that rotate in mysterious ways with other particles, just as isospin symmetry connects protons and neutrons. So supersymmetric operation can turn a rotating particle into a non-rotating particle. Of course, the "operation" mentioned here refers to mathematical steps. In fact, it is impossible to turn a rotating particle into a non-rotating particle, just as you can't turn your left hand into your right hand.
By putting the theory of gravity into a supersymmetric framework, the messenger particles of gravity (called gravitons) get companion particles (called gravitons) that rotate in an interesting way, as well as other particles. So many kinds of particles have entered the theory of supergravity, which strongly shows that the terrible infinite problem has been suppressed, and all the concrete operations carried out so far in this theory have poor results.
In the most popular theory of supergravity, the total number of members of the whole particle family does not exceed 70. Many particles contained in this theory can be identified as known particles in the real world. Some people have raised objections to this view, arguing that the structure of matter has no lower level than quarks, and the quark world is already a big and vague thing in the nucleus. On this scale, it becomes meaningless to talk about what exists within what. So the research on whether there are more basic material units continues.
I hope that my brief introduction to the work that physicists are doing to reveal the final structure of matter will at least give you an understanding of modern physics research. Physicists treat their research objects with awe, because they are always dominated by a belief that nature is ruled by the beauty and simplicity of mathematics; By exploring the structure of matter, the unity of nature will be revealed. All the experience so far shows that the smaller the system, the more general the principle. According to this experience, the complexity of the world we stumbled upon is largely the result of the relatively low energy of our material sampling system. It is believed that with the increasing energy of sampling system, the unity and simplicity of nature will become more and more obvious. This is also the reason why so much manpower and material resources have been invested to build the ultra-high energy particle accelerator. People want to break into that simple state through the ultra-high energy particle accelerator to find out.
However, there was a time when this simple state was explored by nature. At that time, the universe had not been born in the big bang for a second. At that time, the temperature was as high as 1027 degrees, which was just the energy needed to explore the original state. Physicists in this period called it the era of great unity, because physics at that time was dominated by the process of great unity of fundamental forces. The crucial non-equilibrium we mentioned in the third chapter was established at that time. With that kind of non-equilibrium, matter is a little more than antimatter. Later, with the cooling of the universe, the original unified force was divided into three different forces-electromagnetic force, weak force and strong force. These forces are what we see in the relatively cool universe.
Today's complex physics is formed by the simple physical cooling of the original big bang flame. The scenery is wonderful and attractive. The ultimate principle of nature, that is, Wheeler's "central mechanism of flash", is difficult for us to see because of the lack of energy. If people trace back to those periods before the Great Unity era and catch up with places closer to the beginning of time and higher temperature, they can find supergravity. Supergravity represents the beginning of existence, where time and space are integrated with basic forces. Most physicists believe that the concept of space-time cannot be used in the era of supergravity. In fact, there are signs that time and space should also be regarded as two kinds of fields, which are themselves "cooled" by the original soup composed of geometric elements. Therefore, in this super-gravity era, the four forces of nature are chaotic and integrated, and time and space have not yet become an image. At that time, the universe was just a pile of ultra-simple components, which were used by some gods to create time, space and matter.
The latest progress in the study of fundamental forces in physics is described. These advances make people look at nature from a new angle. The influence of this view is rapidly expanding among physicists and astronomers. Now, people have begun to regard the universe as a complex thing produced by the cooling of simple things, rather than an invisible ocean frozen into ice floes with different postures. Scientists have a feeling that the research theme of cosmology and people's study of fundamental forces in matter are providing a unified description of the universe. In this description, the microstructure of matter is closely related to the overall structure of the universe, and both structures affect each other's development in subtle and complicated ways.
A series of successes described in Physics undoubtedly represent a victory of modern physics thought based on reductionism. Physicists try to simplify matter into its final components-leptons, quarks, messenger particles-so that they can understand this basic law. It is that basic law that controls the forces that form material structure and behavior and can explain many basic characteristics of the universe.
However, it is not enough to pursue some ultimate truth felt in this way. As we have seen in the previous chapters, reductionism cannot explain many obvious phenomena with holistic characteristics. For example, we can't use quarks to understand inanimate systems such as consciousness, living cells and even tornadoes. Otherwise there will be jokes.
When a physicist says that protons are made up of quarks, he doesn't mean that. For example, when we say that an animal is made up of cells, or a library is made up of books, we mean that we can take out a cell or a book, or anything from that larger system for isolated research. But quarks are not like this. As far as we know, it is impossible to really take apart protons and take out quarks.
However, disassembly has a glorious history. Disassembling atoms is now commonplace; It is difficult to crack the nucleus, but it will also split under the impact of high energy. This may mean that bombarding protons or neutrons with high-speed particles will crush them into quarks. However, this is not the case. A tiny high-speed electron will pass through the proton and violently rebound one of the quarks, thus making us sure that there is a quark somewhere inside the proton. However, if it is not a small electron, but a sledgehammer, that is, another proton, then we will not see quarks in the fragments of protons, but only more hadrons (protons, mesons, etc.). ). In other words, quarks never appear in isolation. Nature seems to only allow quarks to appear as a group, always two or three together.
So when a physicist says that protons are made up of quarks, he doesn't mean that these mysterious quarks can appear alone. He only refers to a descriptive level, which is more basic than the proton level. In a sense, protons are synthetic, not basic; But synthesizing protons from quarks is different from synthesizing books in the library.
This is because no subatomic particle (whether quarks or other elementary particles) is a real particle. In fact, subatomic particles may not even be "things". This makes us realize once again that the so-called description of matter as a collection of particles must actually be regarded as a description level determined by mathematics. Physicists can accurately describe the structure of matter only through abstract advanced mathematics, and only when people realize this background can they understand the true meaning of reductionism.
One aspect of Heisenberg's uncertainty principle well illustrates the difficulties brought by quantum factors to the study of "what is made of what". But this duality is not between waves and particles, nor between motion and position, but between energy and time. The two concepts of energy and time are mysterious and antagonistic: you know one but you don't know the other. Therefore, even if a system is observed in a short time, its energy may fluctuate greatly. In daily life, energy is always conserved. Conservation of energy is the cornerstone of classical physics. But in the quantum micro-world, energy may come out of nowhere or disappear in a spontaneous and unpredictable way.
When Einstein's famous formula E=mc2 is considered, the fluctuation of quantum energy becomes a complex structure. Einstein's formula says that energy and mass are equal, or that energy can create matter. This has been discussed in previous chapters. However, the energy mentioned in those chapters comes from the outside. Here, we want to discuss how to create matter particles from the fluctuation of quantum energy without external energy input. Heisenberg's principle is much like an energy bank. Energy can be borrowed for a short time, as long as it is returned quickly. The shorter the loan period, the larger the loanable amount.
For example, in the microscopic world, a sudden energy fluctuation may make a pair of positive and negative electrons appear and disappear in a short time. The short-lived existence of this pair of positive and negative electrons is the cumulative effect maintained by Heisenberg-style lending, which makes the empty space have a certain transformed texture, although it is a vague and unrealistic texture. Subatomic particles must swim in this constantly moving ocean. Not only electrons and positrons, but also protons and antiprotons, neutrons and antiparticles, mesons and antiparticles. In short, all the particles in nature are so turbulent.
From a quantum point of view, electrons are not just electrons. The pattern of energy change flashes around it, and I don't know when it suddenly leads to the appearance of photons, protons, mesons and even other electrons. In short, everything in the subatomic world is attached to electrons, for example, electrons are covered with an invisible coat, or a group of bees buzz around the middle hive like ghosts, forming the cover of the hive. When two electrons are close to each other, their covering layers are also entangled, so interaction occurs. The so-called cover is just a quantized expression that was previously considered as a force field.
We can never separate electrons from the ghost particles they carry. When someone asks "what is an electron", we can't say that an electron is such a small particle; We must say that electrons are an inseparable whole string of things, including ghost particles that generate force with them. There is also a strange circle here: force is generated by particles, and the generated force generates force.
For particles like photons, this strange circle means that photons can show many different faces. By borrowing energy, it can temporarily become an electron-positron pair or a proton-proton pair. Experiments have been carried out to observe how photons become positive-negative electron pairs or positive-negative proton pairs. However, people once again found that it is impossible to separate "pure" photons from this complex change.
As far as most unstable particles are concerned, it is difficult to distinguish which pairs of positive and negative electrons are caused by the "real Heisenberg principle", and their lifetimes are similar to those of ψ particles. Who can say that the former is true and the latter is just a ghost?
A few years ago, Jeffrey Qiu, an American physicist, compared this flickering change in the subatomic world to democracy. We can't catch a particle and say it is an entity of one kind or another. We must regard each particle as an endless cycle composed of all other particles. No particle is more basic than other particles. (This is what we briefly mentioned in Chapter 4. ).)
We will see that the essence of matter has a strong holistic flavor in its quantum theory: the descriptions of different levels of matter are interrelated, and everything is composed of everything else, but at the same time everything shows the hierarchical order of structure. It is in this all-encompassing integrity that physicists pursue the ultimate composition of matter and the ultimate unified force.