Many people are curious about the physical world, and in order to give them a simple but accurate answer to the workings of the world, The Nobel Laureate in Physics and a professor at the Massachusetts Institute of Technology, Wilczek, wrote the book Fundamentals: Ten Keys to Reality, which explains how physicists understand the world from ten broad principles. If we know a lot of rules about the workings of this universe, but there are still many problems, so among these ten principles, there is also how to look at what we don't understand. Especially with regard to time inversion, dark matter and dark energy.
This article is authorized from Chapter 9 of "The Mystery Still Exists" of The Principle of All Things: 10 Answers to the Objective World (CITIC Publishing House, January 2022 edition), with deletions and deletions, and the title is added by the editor. Go to the "Back to Park" public account and click "Read the original text" at the end of the article to purchase this book. Click "Watching" and post your thoughts to the message area, as of January 30, 2022 at 12:00 noon, we will select 2 messages, each person will give away 1 book.
Written by Frank Wilczek
Translated 丨 Bai Jiang Zhu, Gao Ping
Mystery is the most beautiful thing we can experience, it is the source of all true art and science. If a person cannot experience mystery, no longer seeks out curiosity, no longer stops by amazement, then he is no different from a dead man - he cannot see anything.
—Albert Einstein
Although we already know a lot about how the world works, there are still many mysteries.
Here, we will focus our exploration on two more compelling mysteries. They are at the forefront of today's research efforts and aim to deepen our fundamental understanding of the physical world. The first mystery revolves around a singular feature of the fundamental laws. If you invert time, then the basic laws are almost exactly the same as the positive situation of time, but not the same. The second mystery comes from a confusing discovery. Astronomers have encountered gravitational pulls that appear to have no visible source in various situations. On the surface, their observations seem to reveal the "dark side" of the universe, which consists of two new forms of matter — "dark matter" and "dark energy," respectively. Although they provide most of the mass in the universe, somehow we have never observed them.
A promising idea may help solve these mysteries. The problem of time inversion has led many physicists to suspect the existence of a new particle, the axion. The axle afterglow left over from the Big Bang has the properties of dark matter. A series of studies around the idea sparked a fierce competition involving hundreds of scientists around the world.
Frank Wilczek 丨Education: Jordi Pay
Time Reversal (T)
Mirroring of time
In reality we've experienced, the most obvious asymmetry is between the past and the future. We remember the past, but we can only guess the future. If you play a movie backwards, like Charlie Chaplin's City Lights, you feel like a series of events that are reversed don't look like they would happen in reality at all. You definitely don't confuse an upside-down movie with a normal movie.
However, from the dawn of modern science, from Newton's classical mechanics to the present, one of the common properties of the fundamental laws is that you can turn them back in time. That is, you can invert past states based on the present state, and the same laws are used to predict future states based on the present state. For example, if you now want to make a movie of planets orbiting the sun according to Newton's laws, you'll turn the movie upside down and you'll see that the motion in the movie still follows Newton's laws. This feature of the law is called time inversion symmetry, abbreviated as T symmetry.
The scope of the law has been expanding, and the symmetry of time inversion still exists. For example, the Maxwell equations of the electromagnetic field and Einstein's modified gravitational equations have this property, as do the quantum versions of these equations. Observations of fundamental interactions all seem to confirm T symmetry.
This contrast between everyday experience and fundamental laws raises two questions. One of the questions is how the real-world universe chooses the direction for the flow of time. We find the answer in chapters 6 and 7 (especially chapter 7), where we see gravity upsetting the balance [1]. Another question is a simple "why." Since the feature of time-inverse symmetry has not appeared much in the world we experience, why does it appear in our basic description of nature?
Why? Step 1: Bottom line
Parents are often annoyed by their children's 100,000 "whys" . Why do I have to sleep? Because people need to rest. Why? Because the human body will be tired. Why? Because our muscles work for a while and then they can't continue to be in good shape. Why? Because they run out of energy from the food we eat and leave a pile of garbage to clean up. Why? Because according to the second law of thermodynamics, the energy within a system will become increasingly unavailable. Why? Because during the Big Bang, gravity created an imbalance... Eventually, you'll always come across questions that you can't answer. [2] There will always be answers that are the most basic, and we cannot explain them further: it is what it is, there is no why.
T symmetry seems to be an exact feature of the fundamental law, and I don't know to ask it "Why?" "Can you get any valid information?" It seems to be a concise nature of the law, though somewhat peculiar. T symmetry may be the answer that lies at the bottom line. Most physicists think so.
Why? Step 2: Sacred Principles
In 1964, things changed. James Cronin, Val Fitch, and their collaborators found a tiny, vague effect in the decay of the Κ meson[3] that violated T symmetry. Since T symmetry is no longer a perfectly correct proposition, it cannot be regarded as an unspoken basic fact. Clearly, there is a question that requires further in-depth study: Why is it that nature is so close to T symmetry, but not exactly? Follow-up studies have shown that this problem can indeed bring us a lot of valid information.
In 1973, Makoto Kobayashi and Toshiei Maskawa made a theoretical breakthrough on this issue. This breakthrough was built on a strong framework of quantum field theory and core theories about force that were not yet solid at the time. As I mentioned earlier, the framework is very strict – you can't easily change it without breaking its consistency. No one knows how to change its structure without violating the sacred principles of relativity, quantum mechanics, and locality.[4] But you can add something to it. Kobayashi and Masogawa found that by adding third-generation quarks and leptons to the known quarks and leptons,[5] it was possible to introduce an interaction that violated T symmetry and produce the effects observed by Cronin and Fitch. Only two generations of quarks and leptons are not enough.
Shortly after Kobayashi and Masochima's research work was completed, the third generation of particles they predicted began to appear in particle accelerators operating at higher energies. After that, there were many experiments that also confirmed the interactions they proposed.
However, this is not the grand finale of the story. In addition to the interaction utilized by Kobayashi and Masonari, there is another interaction that may also violate T symmetry, but it is fully in line with the strict framework of core theory and quantum field theory. Explaining the phenomena that Cronin and Fitch saw or any other observation does not require reference to this interaction. Nature doesn't seem to have chosen it. Why?
Why? Step 3: Evolution
In 1977, Roberto Peccei and Helen Quinn answered the third and perhaps final "why" about T symmetry. This is an evolutionary theory that expands on core theories. They propose that the strength of the excess extra interaction is not a simple number, but a quantum field that changes with time and space. They proved that if a new field has some appropriate, relatively simple properties, then the forces acting on it will make it tend to zero. The implicit assumption of Pechey and Quinn is that the value of this field tendency is zero. Big Bang cosmology holds that this field will gradually evolve toward this value.
Eventually, on this basis, we will arrive at a satisfactory answer: T symmetry can almost be seen as a feature of the fundamental law, which is close, but not entirely. It is an indirect result of deeper principles (relativity, quantum mechanics, and locality) acting on the fundamental constituents of the world.
These theoretical ideas have dramatic consequences, as we will soon say. Before we do that, let's look at the dark side of the universe.
The dark side of the universe
Dark matter and dark energy have similar properties, so it makes sense to discuss them together. They all point to those movements that we observe without an apparent source. In fact, if we express it as "there is an unexplainable acceleration", it may be more accurate than "the existence of dark matter and dark energy", but then the problem will not be so compelling. These extra motions all exhibit a pattern, which suggests that they are all caused by gravity, but the source of gravity is not visible. To explain all the observations, we need two different new sources, dark matter and dark energy. I have to emphasize that neither dark matter nor dark energy means the word "dark" in their names to refer to their color. So far, they have all proven invisible. Where dark matter and dark energy should be present, we detect neither the emission of light nor the absorption of light.
Dark matter may have been formed by a new particle produced by the Big Bang that interacts very weakly with ordinary matter. Dark energy may be the density of space that is ubiquitous in itself. These are the most popular ideas in the field of study, and they do quite convincingly explain a range of observations. Other views also have fans, but there is little evidence to support these views.
The "Standard Model" of cosmology
Knowing that dark matter and dark energy (still hypothetical) make up most of the universe's mass, it is to be expected that they must have also played an important role in the history of the universe. In order to be able to "play the film backwards" to validate this intuition, we need to understand more specifically the nature of dark matter and dark energy. Revisiting the Big Bang gives us the opportunity to learn more about the nature of the dark side of the universe. If our guesses about them are wrong, then the Big Bang model cannot produce the universe we observe.
Given how little we know about the dark side, guessing how dark matter and dark energy will behave in the early stages of the Big Bang seems like an impossible task. Fortunately, it turns out that we don't need to know much, and some simple guesses are already very effective.
For dark matter, we assume that it is made up of particles that interact weakly with ordinary matter and itself. We also have to assume that it was in equilibrium with the rest of the fireball in the early universe, but soon after that it was disconnected from other matter and became the kind of afterglow we mentioned in chapter 6. A delicate problem is that when particles flee, they must be moving much slower than the speed of light [6] (some previous early theories about dark matter failed for this reason). Because (according to the hypothesis) gravity is the only force associated with it, and gravity does not distinguish between different forms of matter, that's all we need to know. Once dark matter is disconnected, we can calculate how dark matter moves and how it affects other parts of the universe. This is known as the cold dark matter model.
For dark energy, we take Einstein's view that dark energy represents a pervasive density of space itself, which is related to the universal negative pressure.
Based on these assumptions, we can compare the density of the cosmic microwave background radiation, which contains information that reflects what the universe has been like 380,000 years after the Big Bang to the present. The addition of dark matter makes instability take effect faster. After the introduction of dark matter, the universe in the model evolved much like our own; without dark matter, it would have become different. In this way, the "dark forces" began with small initial density differences, and through the instability of gravity, allowed our Big Bang cosmology to produce the structures we observe in the universe today.
Axion: "Cleaner" quantum
As a teenager, I would sometimes go to the supermarket with my mom. During a visit to the supermarket, I noticed a washing powder called Axion. It occurred to me that this was really a good name for elementary particles. It is short, catchy, and does not contradict protons, neutrons, electrons, and π (pion). At that moment I thought, if I had a chance to name a particle, I would call it axion.
In 1978, I really got that opportunity. I realized at the time that Pechey and Quinn's idea of introducing a new quantum field had an important consequence that they themselves hadn't noticed. We discussed earlier that a quantum field produces particles, which are the quanta of that quantum field. This particular field produces a very interesting particle, and this new particle has an interesting technical feature that "clears" a problem with an axial current. Tatsuju Islet, and the Axis (in the physics literature) appears!
(Incidentally, if I had disclosed my true motivations before the paper was published, the editors of Physical Review Letters, and possibly the manufacturer of Drip Detergent, would never have agreed to my use of the name.) I mentioned "axial current" in the article at the time, and I took the opportunity to name this particle axion. ）
Look for their afterglow
The properties of the axions make it a candidate for the dark matter component of the universe: they interact very weakly with ordinary matter and with themselves. They are produced at high temperatures and then break free from cosmic fireballs. Their afterglow (i.e., the axion background) pervades the entire universe. The background density of the axions we calculated is consistent with the observed density of dark matter, and the axions are produced in an almost stationary state. Thus, the axion background satisfies the cosmological assumptions of "cold dark matter".
The story is wonderful, but is it true? As we said, the interaction between axions and matter is extremely weak, but we can know from theory that they do interact, and in what way. In order to detect axion backgrounds, we need to design new detectors that are sensitive according to their characteristics. Now, hundreds of physicists, both theoretical and experimental physicists, are trying to overcome this problem. If justice and luck are not absent, we may soon see a saga as important as the discoveries of Neptune, cosmic microwave backgrounds, Higgs particles, gravitational waves, and exoplanets. Solutions to the mysterious stories of science are often of great value.
Future outlook on the mystery
How the mystery ends
Val Fitch was a representative figure in the field of early T symmetry-breaking studies, and he was a smart man with a good sense of humor. He was previously the head of the physics department at Princeton University, and I was a professor there at the time, early in my research career. When I told him about new ideas about axions and dark matter,[8] I naturally mentioned the T symmetry flaw, as if it were a fact that had been unbreakable since ancient times. After all, when I first started learning, it was already widely accepted by the academic community. At some point in our conversation, he smiled kindly and said to me, "Yesterday's sensation is the yardstick for today." ”
This is the fate of successful scientific discoveries. I've felt a similar process at the end of the story of Asymptotic Freedom and QCD (Quantum Chromodynamics). In the years since our breakthrough, people have been excited and questioned whether it has actually solved the powerful mystery. At several major international conferences, the reports on the theme "Testing QCD" are highlights, introducing predictions made using the theory and progress in testing them in experiments. However, as doubts fade away, so does the excitement. Today, the same work continues behind the scenes, but it's much more complicated now than it was then, and we call it "background computing." Yesterday's sensation is the yardstick for today, and the background for tomorrow.
Understand and wonder
In addition to the future outlook for a particular mystery, there are many interesting questions about the future development of the mystery itself that are worth exploring.
The Clay Foundation provided $1 million to reward people who proved that QCD predicted quark confinement. Physicists have a lower standard (I think it would be more appropriate to say that physicists have different standards), and as far as I know, we have gone far beyond quark confinement. With the help of a computer, we can calculate which particles the QCD will produce within a small margin of error, but isolated quarks do not appear in the calculations. In fact, the masses and properties of the particles in these calculations are exactly the same as those we observe in nature.
Should supercomputers win? What about the programmers who write the code?
In 2017, a computer program called AlphaZero innovatively used artificial neural networks, and after mastering the rules of chess, it played against itself for several hours, learning from them, and finally achieved a performance that surpassed humans. Does Alpha Zero know chess? If you want to answer "I don't understand", then I suggest you look at what Emanuel Lasker said, who won the world championship for 27 consecutive years between 1894 and 1921.
On the chess board, lies and hypocrisy never last. Creative combinations will fully expose lies, and hypocrites will be killed in a harsh reality.
Cases like this show that there are methods that are not available to human consciousness. But to be honest, this is nothing new. There are many things that human consciousness also can't provide, but humans instinctively know what to do, such as how to process visual information at breakneck speed and how to keep the body upright, walking, and running.
The genomes of humans and other creatures on Earth are another vast and unconscious repository of knowledge. They have solved many of the complex problems that organisms encounter in order to thrive, and these feats are far beyond the reach of human beings. They do not gradually "learn" how to do this through any logical reasoning process, but through a long and inefficient process of biological evolution, and they cannot consciously know that they have mastered these things.
Our machines are able to perform lengthy and precise calculations, store large amounts of information, and learn at an extremely fast pace, all of which bring about a qualitative leap in the way problems are understood. Computers will push the boundaries of knowledge in all directions, eventually reaching places that the human brain cannot reach. Of course, having a computer-assisted brain can help with these explorations.
Humans have a special trait that evolution and machines don't have, that is, they can recognize gaps in their own understanding and derive pleasure from the process of filling in them. Experiencing mystery and power couldn't be better.
exegesis
[1] Of course, why this happens obviously requires further explanation. We discussed some related concepts in Chapters 6 and 7, especially inflation and complexity in simplicity.
[2] Alternatively, your answer may make the child sleepy.
[3] The Κ meson is a strongly interacting particle (hadron) that is extremely unstable and can be carefully studied using high-energy accelerators. The Κ meson is the lightest of all the hadrons that contain odd quarks.
[4] Of course, no scientific principle can be as sacred as religious dogma in a theological sense. But if relativity, quantum mechanics, or locality are wrong, we need to reclaim the frontier of knowledge, because these principles work well and explain a lot of things. In other words, they may be more fundamental than T symmetry.
[5] For more information on these "extra" particles, see the Appendix, the details of which are not important for what follows.
[6] If particles move too fast, they affect the growth of gravitational instability, so that you get a different model of the universe than ours.
[7] Steven Weinberg also discovered this independently.
[8] We also talk about the asymmetry between matter and antimatter in the universe.
[9] Lasker also made outstanding contributions to the field of pure mathematics.
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