In the long journey of unifying general relativity and quantum theory, physicists initially adopted a more moderate method. They tried to adopt ancient tactics, that is, effective means to conquer strong and weak forces and electromagnetic forces, and apply them to gravity. In the theory of relativity, gravity is described as a geometric effect caused by curvature of spacetime. As we have seen, quantum field theory regards fundamental force as a function of exchanging particles, such as electromagnetic force exchanging photons, strong interaction force exchanging gluons, and so on. So, is gravity also the result of some kind of particle exchange? Before seeing this kind of particle, people have named it "graviton". According to the prediction, it should be a massless boson with spin 2 (a boson is a particle with an integer spin, such as a photon). The spin of fermions is a semi-integer, such as electrons. Roughly speaking, fermions are particles that make up "matter" and bosons are particles that carry "force". .
However, if the so-called gravitons and photons are treated equally, people will soon find that they are doomed to failure. In quantum field theory, no matter how clever we are, we can't make gravitons obey: the calculation result will inevitably lead to infinite divergence term, infinity! We still remember how physicists were tortured by this infinite ghost in the early days of the creation of quantum field theory, but now the situation is even worse: even if there is renormalization, we can't drive it out of the theory. In this war, we lost the first battle, and now all the moderate roads to reunification have been cut off. Quantum theory and general relativity glared at each other and finally broke up. We finally realized that they are incompatible with each other and cannot be properly combined! The future of physics is suddenly shrouded in shadow. Supporters of relativity are angry, and those who support quantum theory hesitate: if we persist in storming, the outcome may be even worse than Einstein's, but if we retreat strategically, won't physics be torn apart and unable to extricate itself?
New hope appeared in 1968, but it started with a very accidental clue: it had nothing to do with gravity. That year, Winicki Ciano, an Italian physicist at CERN (European Institute of Nuclear Physics, I believe that friends who have read dan brown's Angels and Demons will not be unfamiliar), leafed through a math book and found something called "Euler β function" on it. Winicki Yano conveniently applied it to the so-called "Regge trajectory" and made some calculations. As a result, he was surprised to find that this Euler function, which was studied as early as 177 1 for purely mathematical reasons, can well describe the actions of many strong relative forces in nucleons!
Winicki Yano didn't foresee the changes that happened later, and he didn't know what kind of door he opened. In fact, he probably inadvertently did something that made us surpass the times. Edward witten often said later that superstring originally belonged to the science of 2 1 century, and it was actually a very lucky accident in history that we were able to invent and study it in the 20th century.
Winicki's Arno model was noticed by three people almost at the same time: Yoichiro Nambu of the University of Chicago, Suskin of the Jewish University and Nelson of the Bohr Institute. Three people proved that this model is equivalent to describing a one-dimensional "string" when describing particles! This is a very strange result. In quantum field theory, any elementary particle has always been regarded as a small point without length and width. How can it become a string?
Although this result was unexpected, John Schwarz of California Institute of Technology, in cooperation with Calker, a French physicist who was visiting there at that time, studied some properties of this theory. They regard this string as a bond that binds quarks, that is, quarks are bound at both ends of the string, which makes them never separate from the nucleus. That sounds good, but they found something strange at the end of the calculation. For example, the theory needs a zero-mass particle with spin 2, but this particle can't find its place in the nuclear genealogy (can you imagine how crazy a chemist would be if he found an element that can't be inserted into the periodic table? )。 In addition, the theory also predicts a particle whose speed exceeds the speed of light, the so-called "superluminal particle". You may think that this violates the theory of relativity at first, but strictly speaking, superluminal particles can exist in the theory of relativity, as long as their speed never drops below the speed of light! The real trouble is that if this superluminal particle is introduced into the quantum field theory, then the vacuum is no longer the lowest energy state of the field, that is to say, even the vacuum will become unstable, and it will definitely decay into something else! This is obviously nonsense.
What is even more incomprehensible is that if string theory wants to justify itself, it must require our time and space to be 26 dimensions! Ordinary space-time is easy to understand: it has three-dimensional space, plus 1 dimensional time. What is the extra 22 dimensions? This theory of introducing multidimensional space has appeared before. If you remember Bohr's assistant in Copenhagen (Oscar Klein), you may remember that he once introduced the concept of "the fifth dimension" into the Schrodinger equation. Klein started from the quantum point of view. Before him, Theodor Kaluza, a loyal follower of Einstein, also made the same attempt from the perspective of relativity. Later, people collectively referred to this theory as Kaluza-Klein theory (or KK theory). But these theories are stillborn in the end. It's really hard to imagine how we can convince the public that we actually live in a space of more than 4 dimensions.
Finally, the rise of Quantum Chromodynamics (QCD) makes string theory lose its final appeal. As we mentioned earlier, QCD has successfully captured the strong interaction force, and the mountain is king, which has been recognized by most physicists. In such internal and external troubles, the original string theory was soon abandoned and left out in the corner.
In the darkest days of string theory, only Schwartz and Calker persisted along this road. 197 1 year, Schwartz and Pierre Raymond cooperated to simplify the string theory that originally needed 26 dimensions to only need 10 dimension. At first, the idea of "supersymmetry" was introduced, and each boson corresponds to a corresponding fermion. The alliance with supersymmetry gives string theory unprecedented power, which enables it to deal with fermions at the same time. More importantly, it makes some theoretical problems (such as superluminal particles) disappear, and its bright future in gravity gradually emerges. Unfortunately, when the first light of string theory appeared, Karl died before he could conquer it. He suffered from severe diabetes and died in 1980. Schwartz had to ask Michael of Queen Mary College in London for help. Michelle Green finally completed the supersymmetric string theory combining the two. They were surprised to find that this theory was completely transformed and a powerful upgrade was completed. Now, the old "string theory" is dead, and the new one is the powerful "superstring" theory. This new title of "supersymmetry" is the highest glory given to it.
Schwartz and Green were ecstatic. When they applied their model to gravity, they could hear their own heartbeat. Although the spin predicted by the old string theory is 2 and the particle with mass of 0 can't find its place in hadron, it conforms to the theory of relativity! In fact, it is the legendary "graviton"! After forming an alliance with supersymmetry, the new superstring absorbed another promising army, the so-called "supergravity theory". Now, thankfully, there is no longer infinity when calculating gravity! The calculation results are limited and meaningful! Gravitational defense forces guard against the attack of particles all day, but when we no longer regard particles as a point, but as a string, we can sneak in and bypass the infinite defense line painstakingly arranged, thus going deep into the kingdom of gravity for the first time. The original intention of superstring is to deal with strength, and now its attention has completely turned to gravity: My God, if we can conquer gravity, what else?
The calculation of gravity is completed around 1982. At 1984, Schwartz and Green won a key victory, which shocked the whole physics world: they solved the so-called "abnormal" problem. Originally, there were infinite kinds of symmetries in superstrings to choose from, but after careful investigation, Schwartz and Green found that only in extremely limited symmetrical forms can the theory eliminate these anomalies and be self-consistent. This enables us to carefully examine those specific superstring theories without having to deal with infinite possibilities at the same time. Even better, the selected group can just contain the existing gauge field theory and the standard model of particles! Great victory!
The "first superstring revolution" broke out. Not long ago, superstring was dismissed, and the extremely cold physics community was suddenly possessed, pouring out rare enthusiasm and attention. Hundreds of people rushed into this field, so much so that Grouse later said, "In my experience, I have never seen such enthusiasm for a theory." In just three years, Superstring completed an extremely beautiful imperial counterattack, which quickly spit out his pent-up anger in those years. Meanwhile, like Edward? Wilton and the "Princeton Superstring Quartet" group headed by grouse have made extremely important contributions, but we can't describe them in detail. There is a lot of information about superstring on the Internet. If interested readers can refer to this detailed index:
arxiv.org/abs/hep-th/03 1 1044
After the first revolution, we got such an image: any particle is actually not a point in the traditional sense, but an open or closed string (the head and tail are connected into a ring). When they vibrate in different ways, they correspond to different particles in nature (electrons, photons ... including gravitons! )。 We still live in a 10 dimensional space, but the six dimensions are tightly curled up, so we usually don't notice it. Imagine a water pipe. If viewed from a distance, it is as thin as a line, and its structure is only 1 dimension. But if you really enlarge it, you will find that it has a cross section! This second element is so curly that it can't be seen at a glance. In the image of superstring, so is our world. For some reason, the six dimensions shrink so tightly that at first glance, the universe has only four dimensions (three-dimensional space plus 1 dimensional time). But if we enlarge the space-time to the scale of the so-called Planck space (about 10-33 cm), we will find that what was originally regarded as a "point" in space-time is actually a 6-dimensional "ball"! These six curled dimensions are constantly disturbed, leading to all quantum uncertainties (see, this theory can explain the uncertainty principle from the root)!
This revolution made superstring famous and became a universal theoretical candidate. Of course, there are a few physicists who are still skeptical about this, such as glashow and Feynman. Hawking (a professor who spoke to 6000 people in a wheelchair in the Great Hall of the People the day before yesterday) is not very enthusiastic about this either. You may remember what we described before. After the Spector experiment in 1982, Brown of BBC and Davis of Newcastle University interviewed several experts in quantum theory. Now the superstring craze in physics is in the ascendant, and these two guys are not idle. They came out again and invited nine most outstanding experts in string theory and quantum field theory to the BBC for an interview. These records are also listed in 1988 as "superstring: theory of everything?" ? Published by Cambridge Press. Reading these records, we can find that although experts are not as noisy as quantum theory, the differences are still obvious. Feynman even said with a vicissitudes of life that when he was young, he noticed that many old people pedantically resisted new ideas (such as Einstein's resistance to quantum theory), but when he became an old man himself, he actually did the same thing involuntarily, because some new ideas were really weird-such as string theory!
People naturally ask, why are there six dimensions curled up? What's the difference between these six dimensions? Why not curl up in five or eight dimensions? What is the topological property of this curl? Is there any way to prove it? Because the scale of string theory is so small (Planck space), people lack the necessary technical means to understand it directly through experiments, and the calculation of string theory is so complicated and difficult. Needless to say, we can't determine the equation itself, so we can only use approximation! To make matters worse, after the first revolution, although people washed away many possible symmetries, there are still five superstring theories, each of which adopts 10 dimensional space-time and can be proved to be justified. Which of these five theories is correct? People arrived here with great enthusiasm, only to find themselves trapped. The upsurge of string theory soon subsided, many people returned to their own fields, and the dust of the first revolution settled.
It was not until the mid-1990s that Superstring woke up from a deep sleep again and completed the Jedi counterattack. Edward woke it up this time? Wilton. Wilton surprised everyone at 1995 superstring annual meeting held by the University of Southern California. He proved that string theories with different coupling constants are essentially the same! We can only deal with the weak coupling theory by perturbation method, that is to say, the coupling constant is very small. In this case, the five string theories look very different. However, if we enlarge the coupling constants gradually, they should be five different variants of a big theory! Especially when the coupling constant is enlarged, a new dimension-11dimension appears! It's like a piece of paper has only two dimensions, but when you stack many pieces of paper together, a new dimension appears-height!
In other words, there is a more basic theory, and the existing five superstring theories are its limits in different situations, and they are mutually inclusive! Just like the famous fable-the blind touch the elephant. Some people touch their noses, some touch their ears and some touch their tails. Although these people feel very different, they touch the same elephant-only everyone touches a part of it! Brian Greene gave a rather funny example in 1999' s Elegant Universe. Let's play: imagine a local in a tropical rain forest. He likes water, but he has never seen ice. On the contrary, an Eskimo likes ice, but because the place where he lives is too cold, he has never seen liquid water (there is no doubt that the real Eskimo has seen water, but something wonderful happened: on a hot day in the desert, the Eskimo's ice melted into water! And on a cold night, the water freezes into ice again! They finally realized that they actually like the same thing, but in different forms under different conditions.
In this way, all five superstrings are contained in a unified image, and physicists can finally breathe a sigh of relief. This unified theory is called "M theory". Just as no one knows why the doctor inventor in the 007 film is called "Q" (the old actor who played him died in a car accident in 1999, so I'm here to commemorate it), no one knows what this "M" really means. Perhaps the original intention of the inventor is "mother", which means that it is the mother theory of the five superstrings, but some people think it is "mysterious" (. Some people in China like to call it "touch theory", which means "the blind touch the elephant"!
In M theory, space-time becomes 1 1 dimension, from which all five superstring theories of 10 dimension can be deduced. In fact, because there is one more dimension, we have another variant of supergravity, so one * * * is six derivatives! At this time, if we look at the basic structure of space-time, we will find that it is not only a chord of 1 dimension, but also a point of 0 dimension, a membrane of 2 dimension, a bubble of 3 dimension, or a title of 4 dimension … I can't think of it. In fact, this basic structure may have any dimension-from 0 to 9! M theory is more bizarre than superstring.
Both superstring and M-theory have just started, and there is still a long way to go. Although it is extremely complicated, the superstring /M theory has achieved some success, and can even explain the problem of black hole entropy-1996, and the papers of Strominger and Vafa have opened the way for this. In a speech not long before that, Hawking also sarcastically said: "String theory has been quite miserable so far: it can't even describe the structure of the sun, let alone a black hole." But he finally changed his mind and joined the trend of string theory. M theory is a part of the "second superstring revolution", and now the smoke of this revolution has been exhausted, and superstring has entered a dormant period. PBS later produced a TV program about superstring based on Green's book, which caused quite a stir among the public. Perhaps there will be the third and fourth superstring revolutions soon, so as to finally complete the unification of physics, which none of us can foresee.
It is worth noting that starting from string theory, we began to notice that it seems that the structure of quantum theory is more basic. In the past, people liked to determine the general framework of the theory by classical means, and then modify the quantum theory in detail, which can be called the "arrogant and small" method. But in string theory, quantum theory must be introduced first, and then the space-time structure on a large scale can be derived! People are beginning to realize that maybe "growing up" is the fundamental way to explain the universe. At present, most string theorists believe that quantum theory plays a key role in it, and the quantum structure does not need to be revised. The way of general relativity is probably wrong. Although its geometric structure is wonderful, it can only be wronged to retreat to the position of inference-not the basic assumption! Many people believe that superstring will have a brighter future only by further relying on the power of quantum. Although our quantum is so strange, God endows it with unparalleled power and controls the whole universe under its brilliance.
Finally, in 2006, the World Congress of String Theory was held in China, but in fact, there are no available scientists in China, the frontier territory of theoretical physics.