The new trend of thought to explore what happened before the Big Bang is actually just the latest swing of the rational pendulum for thousands of years. In almost every civilization, the question of ultimate origin will keep philosophers and theologians busy. Its concern is overwhelming. The famous painting 1897, which appeared in paul gauguin, is: Where do we come from? What are we? Where are we going? This work depicts the cycle of birth, aging, disease and death: the origin, identity and fate of each person, and this concern for the individual is directly related to the fate of the universe. Humans can trace back to their ancestors, cross generations, return to our animal ancestors, trace back to the early form and initial life of life, then return to the synthetic elements in the primitive universe, and then return to the ethereal energy in the earlier space. Can our family tree go on like this endlessly? Or will it end somewhere? Isn't the universe, like human beings, eternal?
The ancient Greeks had a heated debate about the origin of time. Aristotle advocates that everything can't be born, but he stands in the camp where time has no starting point. If the universe can't be created out of nothing, it must have always existed in the past. According to these theories, time must extend infinitely to the past and the future. Christian theologians tend to take the opposite view. Augustine insisted that God exists outside time and space, and created time and space and the whole world. Someone asked: What was God doing before he created this world? Augustine replied: Time itself is one of the products created by God, so there is no precedent!
Einstein's general theory of relativity makes contemporary cosmologists come to almost the same conclusion. General relativity holds that space and time are soft and plastic entities. On a large scale, space is dynamic in nature and will expand or contract with time; It carries matter in the same way that waves carry floating objects. In the1920s, astronomers observed that distant galaxies were moving away from each other, thus confirming that the universe was expanding. Then, physicists Stephen Hawking and roger penrose proved in the1960s that time cannot go back all the time. If we go backwards in the history of the universe, all galaxies will eventually squeeze into an infinitesimal point (called a singularity), which is similar to what they mean by falling into a black hole. Each galaxy or its predecessor is compressed to zero size, while physical quantities such as density, temperature and curvature of spacetime become infinite. Singularity is the beginning of everything in the universe. Beyond this limit, our cosmic genealogy tree can no longer be extended.
The universe is homogeneous?
This inevitable singularity poses a serious and disturbing problem to cosmologists. In particular, the singularity seems to contradict the high uniformity and isotropy of the universe on a large scale. Because the universe is the same everywhere on a large scale, information must be transmitted between distant regions in some way to coordinate their properties. However, this contradicts the old cosmological norms.
Specifically, let's think about what happened in the past 65.438+037 billion years after the universe released microwave background radiation: due to the expansion of the universe, the distance between galaxies increased by 654.38+0000 times, while the radius of Hubble volume increased by 654.38+ million times (because the speed of light exceeded the expansion speed of the universe). A large part of the universe we see today is something we couldn't see in 654.38+037 billion years. Indeed, in the history of the universe, this is the first time that light from the most distant galaxy has reached the Milky Way.
Nevertheless, the nature of the Milky Way is basically the same as those of distant galaxies. It's like going to a party and finding yourself wearing the same clothes as a dozen friends. If only two people wear the same clothes, it can be explained by coincidence. But if a dozen people wear the same clothes, it is likely that they made an appointment in advance. In cosmology, this number is not a dozen, but tens of thousands-this is the number of sky regions under the microwave background all day, which are independent from each other, but statistically the same.
One possibility is that these spatial regions were endowed with the same attributes at the beginning of their birth, in other words, this consistency is just a coincidence. However, physicists have proposed two more natural ways to get rid of the deadlock: to make the early universe either much smaller than the standard universe or much older. Under any conditions (or both), it is possible to realize the interconnection between various spatial areas.
At present, the most popular way is the first way. Suppose the universe experienced a rapid expansion in the early days, called inflation. Before inflation, galaxies or their predecessors were tightly packed together, so their properties were easy to coordinate. In the inflation stage, they lost contact because the speed of light could not keep up with the inflation speed. After the inflation ended, the expansion rate began to slow down, so the galaxies gradually resumed contact.
Physicists attribute the energy released by expansion to the potential energy stored in a new quantum field inflator about 10*-35 seconds after the Big Bang. Potential energy is different from static energy and kinetic energy, which can produce gravitational repulsion effect. The usual gravity of matter will slow down the expansion of the universe, but inflation will accelerate the expansion of the universe. 198 1 year inflation theory came out, and many accurate observation results have been explained so far [see 1984 No.9 "Exploding Universe" written by Allen H. Gus and Paul J. steinhardt and the special report "Four Keys to the Universe" No.4 in 2004]. However, there are still a series of potential theoretical problems that have not been solved. The first thing is, what is an inflation field? And where does such a huge initial potential energy come from?
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It seems problematic to assume that relativity is always valid in the process of derivation. When approaching the universally recognized singularity, the quantum effect will inevitably become more and more important, even play a leading role. Orthodox relativity does not take this effect into account, so it is undoubtedly an excessive belief in relativity to draw the conclusion that singularity is inevitable. In order to find out what happened, physicists must incorporate relativity into the theory of quantum gravity. This task is a headache for physicists after Einstein. Until1mid-1980s, the progress was almost zero.
Revolution of string theory
Now, there are two good plans. The first is called loop quantum gravity, which completely retains the essence of Einstein's theory, but changes the program to meet the conditions of quantum mechanics [see the article Quantized Time and Space written by Lee smolin in the third issue of 2004]. In the past few years, researchers of loop quantum gravity have made great progress and gained a very profound understanding. However, perhaps the revolution of traditional theory has not gone deep enough to solve the fundamental problem of gravitational quantization. A similar problem occurred in 1934, when Enrico Fermi put forward his theory of weak nuclear force effectiveness, which made particle physicists nervous. All efforts to establish quantum Fermi theory have failed miserably. Therefore, what is really needed is not a new branch skill, but a fundamental innovation brought by the theory of weak electricity of glashow, Steven Weinberg and abdul sallam in the late 1960s.
The second is string theory, which I think is more promising. String theory completely changed Einstein's theory, and this paper will focus on it. Although supporters of loop quantum gravity claim that they have reached many of the same conclusions.
String theory was born in 1968, which is the model I used to describe nucleons (protons and neutrons) and their forces. Although it caused quite a stir at first, this model finally failed and gave way to quantum chromodynamics. The latter uses more basic quarks to describe nucleons, while the string theory is abandoned. Quarks are trapped in protons or neutrons as if they were tied together by rubber ropes. Looking back now, the original string theory has actually mastered the elements of strings in the nuclear world. After a period of silence, string theory made a comeback by combining general relativity and quantum theory.
The core concept of string theory is that the elementary particle is not a point, but an infinitely fine one-dimensional entity, that is, a string. In the huge family of elementary particles, each particle has its own characteristics, which are embodied in various possible vibration modes of a string. How can such a seemingly simple theory describe the complex world of particles and their forces? The answer can be found in what we call quantum string magic. Once quantum mechanics is applied to vibrating strings (just like violin strings, but the vibration on them propagates at the speed of light), new properties appear. All these properties are of far-reaching significance to particle physics and cosmology.
First of all, the scale of quantum strings is limited. If quantum effect is not considered, a violin string can be split in two, then split in two, and so on until it becomes some massless point particles. But if it is divided to a certain extent, Heisenberg's uncertainty principle will intervene to prevent the lightest string from being divided below 10*-34 meters. This inseparable length quantum, denoted by ls, is a brand-new natural constant introduced by string theory, which is juxtaposed with the speed of light C and Planck constant H. It plays a decisive role in almost all aspects of string theory, setting upper and lower limits for various physical quantities to prevent them from becoming zero or infinite.
Secondly, even a quantum string without mass can have angular momentum. In classical physics, angular momentum is a property that an object rotates around an axis. The formula for calculating angular momentum is the product of speed, mass and the distance from the object to the axis of rotation, so an object without mass cannot have angular momentum. But in the microscopic world, because of quantum fluctuations, the situation is different. Even if a tiny string has no mass, it can obtain an angular momentum of no more than 2h. This property surprises physicists because it coincides with the properties of all known basic force carriers (such as photons that propagate electromagnetic force or gravitons that propagate excitons). Looking back at history, it is angular momentum that makes physicists notice that string theory contains quantum gravity.
Third, quantum strings need to add extra spatial dimensions beyond the usual three dimensions. Classical violin strings can vibrate without the limitation of space-time properties, while quantum strings are much more critical. To make the equation describing the vibration of quantum strings self-consistent, spacetime must be highly curved (which contradicts the observed results), otherwise it should contain six additional spatial dimensions.
Fourth, physical constants (constants that appear in physical equations and determine natural properties, such as Newton's constant and Coulomb's constant) no longer have any given fixed values. They appear as fields in string theory, just like electromagnetic fields, and their values can be dynamically adjusted. These fields may take different values in different cosmic periods or distant space areas; Even today, these constants may change slightly. As long as any such changes are observed, it will be a great progress in string theory [related articles will be published in this journal soon].
The so-called expansion subfield is the key of the whole string theory, which determines the total strength of all forces. String theorists are particularly interested in dilators, because its magnitude can be reinterpreted as a scale of an extra spatial dimension, thus giving a space-time of 1 1 dimension.
Fasten the loose head
Quantum string makes physicists finally realize that there is a new important symmetry in nature, called duality, which changes our intuition about the microscopic world with very small scale. I mentioned a duality: usually, the shorter the string, the lighter the string, but if we want to shorten the length of the string below the basic length ls, the string will become heavier again.
Another symmetry, called t-duality, points out that all extra dimensions are equivalent, regardless of their scale. This symmetry occurs because the motion of a string may be more complicated than that of a point particle. Consider a closed chord (called a circle) in a cylindrical space. The circular cross section of this space represents a limited extra dimension. In addition to vibration, the string can rotate around the cylinder as a whole, or it can rotate around the cylinder once or several times, just like a rubber band wound around a paper tube [see page 40].
In these two States, the energy consumption of the string is related to the cylindrical scale. The winding energy is proportional to the radius of the cylinder. The larger the cylinder, the more the string is stretched, so the more energy it contains in winding. However, when the whole string moves around the cylinder, its energy is inversely proportional to the radius of the cylinder. The larger the cylinder, the greater the wavelength (equivalent to the lower the frequency), so the smaller the energy. If a large cylinder is used instead of a small cylinder, then the two motion states can exchange roles. The energy generated in the previous circle is now generated by winding, and the energy generated in the previous winding is generated by circular motion. External observers only see the size of energy, but can't see its source. To an external observer, the radius of a cylinder is physically equivalent, regardless of its size.
T- duality is usually described by a circular space (one dimension of this space, that is, the circumference is limited), but a variant of it is suitable for the usual three-dimensional space, and each dimension of this space can extend indefinitely. Be careful when talking about the expansion of infinite space. The total size of infinite space will not change; Forever is infinite. But this space contains galaxies and other celestial bodies, but the distance between them can be farther and farther. In this sense, infinite space can still be expanded. The key variable is not the size of the whole space, but its scale coefficient, that is, the value that measures the change of the distance between galaxies, which is manifested as the red shift of galaxies observed by astronomers. According to t-duality, the universe with smaller scale coefficient is equivalent to the universe with larger scale coefficient. There is no such symmetry in Einstein's equation; String theory realizes the unity of relativity and quantum theory, and this symmetry naturally stands out, and the expander plays a key role in it.
For many years, string theorists believe that T- duality is only suitable for closed strings, but not for open strings (the two ends of open strings are loose, so such strings cannot be wound. ) 1995, joseph Polchinski of the University of California, Santa Barbara, realized that if the conditions at both ends of the string change accordingly when the radius changes from large to small or from small to large, then the T-duality is suitable for opening the string. The boundary condition assumed by physicists before is that the end of the string is not affected by any force and can swing freely from side to side. T duality requires that these conditions become the so-called Dirichlet boundary conditions, that is, the endpoints are in a fixed state.
Any given string can have two boundary conditions. For example, the endpoint of a chord corresponding to an electron may move freely in three of the 10 spatial dimensions, but it is fixed in the other seven dimensions. These three dimensions constitute a subspace called Dirichlet membrane (D membrane). 1996, Peter Khorava of the University of California, Berkeley and edward witten of the Institute for Advanced Studies in Princeton, USA, proposed that our universe is located on such a membrane. Electrons and other particles can only move in certain dimensions, which explains why we can't enjoy the whole 10 dimensional space landscape.