Look at yourself in the mirror. If you jump up and your image in the mirror moves down, you will be dumbfounded.
There is even a creepy feeling! Fortunately, this kind of thing has never happened to anyone in the world. But such a strange thing really appears in the particle world, specifically, in a basic particle.
The problem of mirror image asymmetry is not a simple problem, but an important topic in physics research in the 20th century.
Symmetry is always perfect.
When you look in the mirror, you form a symmetrical relationship with the image in the mirror. Symmetry appears not only in the mirror, but also in the nature around us. Honeycomb is a building that is symmetrically arranged by regular hexagons. Each regular hexagon is even in size and the distance from top to bottom to left and right is equal. This structure is the most compact and orderly, and it also saves the most materials. The structure of the butterfly's left and right wings is symmetrical, even the patterns and colors on the wings are symmetrical, so it can become the most beautiful insect in nature; All conchs have wonderful left-right rotational symmetry; People are symmetrical, not only in appearance, but also in the shapes of eyes, ears and left and right brains. Imagine a person without an eye or with his mouth tilted to one side, which will definitely be regarded as unsightly.
Humans have advocated the beauty of symmetry since ancient times, and the concept of symmetry has penetrated into almost all disciplines. In architecture, architects are always inseparable from symmetry when planning, designing and building various buildings. Most famous buildings handed down from ancient times are extremely symmetrical, such as the Forbidden City in China, the Temple of Heaven, the promenade in the Summer Palace, the Great Pyramid in Egypt and the Colosseum in Rome. Geometrically, there are various symmetrical shapes such as circle, ellipse, square, regular triangle, cone and cylinder. In algebra, there is a symmetry of two roots of a quadratic equation, a symmetric function of the equation, and even a mathematical theory of symmetric group theory.
In crystallography, symmetry is particularly prominent. In fact, there are few things in nature that are completely symmetrical with 100%, but crystals are an exception. No matter from the macroscopic or microscopic point of view, crystals are strictly symmetrical. There are many atoms in the crystal, and there is a strict spatial arrangement. If you draw a part of the atomic arrangement diagram at will, whether it is translation, rotation or left-right exchange, the obtained image can not be distinguished from the original image, that is to say, most crystals have the properties of translation symmetry, rotation symmetry and mirror symmetry. For example, the snowflake has a six-fold rotational symmetry, that is, after the snowflake crystal rotates 60 degrees, 120 degrees, 180 degrees, 240 degrees, 300 degrees or 360 degrees along a fixed axis, the spatial arrangement of its atoms is exactly the same as the original arrangement.
Symmetry in physics
In fact, in physics, the concept of symmetry is definitely not just "left-right identity", it is much broader than what we usually understand, and it is applicable to almost all natural phenomena-from the birth of the universe to every microscopic subnuclear reaction process. In physics, changing an object from one state to another is called transformation. If a transformation does not change the situation of the object, it can be said that the object is symmetrical to this transformation, which is called the symmetrical transformation of the object.
Exchanging things on the left with things on the right without any change is called mirror symmetry, which means that things in the mirror are the same as those outside the mirror. Most human and animal bodies are mirror images, so are Tiananmen Square and Temple of Heaven in China.
In space, a unit can translate in any direction. If the translated image is indistinguishable from the original image (that is, completely coincident) and this operation can continue, it is translational symmetry. Regular meshes have translational symmetry. In nature, honeycombs, slubs or beads all have translational symmetry.
Rotating a uniform ball around the center of the ball at any angle, its shape, size, mass, density distribution and so on, all properties remain unchanged, which is rotational symmetry. A flower with five identical petals (such as plum blossom and bauhinia) rotates by an angle of 2π/5 or an integer multiple of 2π/5 around an axis perpendicular to the surface of the flower. It is exactly the same before and after rotation, and there is no change. We say it has a rotational symmetry of 2π/5. On the other hand, if there is a point or some defects on the edge of a ball, this point or defect can distinguish the situation before and after rotation, and it does not have rotational symmetry-or its rotational symmetry is broken.
All the above are the symmetry of the external form of the object. There is a more important symmetry in physics: the symmetry of physical laws. Take Newton's law as an example. No matter how an object rotates, its motion follows Newton's law. So Newton's law has rotational symmetry. The movement of objects entering and leaving the mirror obeys Newton's law and has mirror symmetry; Newton's law still holds after the object moves freely in space, and Newton's law also has spatial translation symmetry; At different times, yesterday, today or tomorrow, the motion of objects also obeys Newton's law, Newton's law also has time translation symmetry, and other known physical laws also have similar properties.
Physicists have always had a special interest in symmetry. Symmetry often enables us to obtain some knowledge without accurate solution, which simplifies the problem. For example, a simple pendulum swings without resistance, which is symmetrical left and right. So we can know that the height of swinging to the left and the height of swinging to the right must be equal, the time from the middle to the highest point on the left must be equal to the time to the highest point on the right, and the speed and acceleration of the pendulum at the corresponding position on the left and right sides must be the same.
Relationship between symmetry and conservation
These symmetries of physical laws actually mean the invariance of physical laws under various transformation conditions. From the invariance of physical laws, we can get an invariant physical quantity, which is called a conserved quantity or an invariant. For example, the most important parameter of space rotation is angular momentum. If an object is rotationally symmetric in space, its angular momentum must be conserved. Therefore, the rotational symmetry of space corresponds to the law of conservation of angular momentum. For another example, 500,000 tons of water fell from the height of 1000 meters, forming a waterfall. If all the water flow power of the waterfall is converted into electric energy, the power generated by the same water flow will be the same at any time, and this energy will not change with time. So the symmetry of time translation corresponds to the conservation of energy. Also, space translation symmetry corresponds to momentum conservation, charge yoke symmetry corresponds to electric quantity conservation, and so on.
The conservation of physical laws is of great significance. With these conservation laws, the change of nature presents a simple, harmonious and symmetrical relationship and becomes easy to understand. Therefore, scientists have a special enthusiasm and sensitivity to the law of conservation in scientific research. Once the law of conservation is recognized, people are extremely reluctant to overthrow it.
Therefore, when we understand the corresponding relationship between various symmetries and the law of conservation of physical quantities, we also understand the significance of symmetry principle. We can't imagine what a chaotic and confusing world it would be without symmetry and changes in physical laws!
The correspondence between the symmetry of physical laws and the law of conservation of physical quantities was first discovered by the German mathematician emmy noether in 19 18, so it is called "Nott Theorem". Since then, physicists have formed such a mindset: as long as a new symmetry is discovered, we must look for the corresponding conservation law; On the contrary, as long as a conservation law is found, the corresponding symmetry must always be found.
Nott's theorem pushes the importance of "symmetry" in physics to an unprecedented height. However, physicists don't seem satisfied. 1926 someone put forward the law of parity conservation, which further extended the relationship between symmetry and conservation law to the micro world.
What is parity conservation?
Let's first understand the meaning of "parity conservation" "Parity" means that a fundamental particle is completely symmetrical to its "mirror image" particle. When people look in the mirror, the image in the mirror and the real self always have exactly the same attributes-including appearance, dress, expression and action. Similarly, all the properties of a basic particle and its mirror image particle are exactly the same, and their motion laws are also exactly the same, which is the "parity conservation". If a particle rotates clockwise, its mirror particle looks counterclockwise from the mirror, but all the motion laws of the two rotating particles are the same, so the particles inside and outside the mirror are parity-conserved.
In a sense, we can understand the same particle as a mirror image of each other. Suppose one electron rotates clockwise and the other electron rotates counterclockwise, then one electron can take the other electron as his own in the mirror, just like a person looks at himself through the mirror. Therefore, according to the parity conservation theory, all electrons should follow the same physical laws in their own environment and mirror environment, and so should other particles.
The so-called "parity conservation" sounds nothing special. At least before 1926, Newton's law has been proposed to have mirror symmetry. However, most of the physical laws of mirror symmetry put forward by scientists before are macroscopic, and parity conservation is aimed at the most basic particles that make up all substances in the universe. If the symmetry of the most basic level of this matter can be established, then symmetry becomes the fundamental attribute of the cosmic matter.
In fact, the theory of parity conservation has indeed been verified in almost all fields-except weak force. As we know, modern physics divides the interaction between substances into four types: gravity, electromagnetic force, strong force and weak force. In the environment of strong force, electromagnetic force and gravity, the theory of parity conservation has been well verified: as we usually think, particles show absolute and unconditional symmetry in these three environments.
In the eyes of ordinary people, symmetry is the guarantee of a perfect world; In the eyes of physicists, parity conservation is so in line with scientific ideals. Therefore, although the parity conservation in the weak force environment has not been verified, it naturally follows the parity conservation law.
Li and Yang's insights.
However, after all, truth must speak for itself. 1956, two Chinese-American physicists, Li Zhengdao and Yang Zhenning, boldly challenged the "perfect symmetrical world" and directed at the law of parity conservation, which became one of the most shocking events in physics in the last century. The most direct cause of this shocking event is the "θ-τ mystery" that has long puzzled scholars, which is a hurdle that the law of parity conservation cannot bypass. In the early 1950s, scientists observed two new mesons (i.e. particles with a mass between protons and electrons) from cosmic rays: θ and τ. The spin, mass, lifetime and charge of these two mesons are exactly the same, and many people think they are the same particle. But they decay in different ways (decay refers to the transformation of unstable particles with high energy into stable particles with low energy). When θ decays, two π mesons will be produced, while τ decays into three π mesons, which shows that they follow different motion laws.
If τ and θ are different particles, how can they have exactly the same mass and lifetime? And if we admit that they are the same kind of particles, how can they have completely different laws of motion?
In order to solve this problem, physics put forward various ideas, but none of them succeeded. Physicists carefully bypass the possibility of "parity non-conservation"
1956, Li Zhengdao and Yang Zhenning boldly asserted that τ and θ are exactly the same particles (later called k mesons), but their motion laws are not necessarily exactly the same in the weak interaction environment. Generally speaking, if these two same particles look at each other in the mirror, their decay patterns in the mirror and outside the mirror are actually different! In scientific language, the "θ-τ" particle is parity-nonconservative under weak interaction. The views of Li Zhengdao and Yang Zhenning shocked the physics circle at that time, and they tore a gap in the perfectly symmetrical world of physics!
Wu Jianxiong's excellent experiment.
At first, "θ-τ" particles were only considered as a special exception. Not long after, Wu Jianxiong, an experimental physicist in China, verified the "parity non-conservation" with a clever experiment. Since then, "parity non-conservation" has really been recognized as a basic scientific principle with universal significance. Wu Jianxiong used two sets of experimental devices to observe the decay of cobalt 60. At a very low temperature (0.0 1K), she used a strong magnetic field to make the spin direction of cobalt 60 in one set of devices to the left, and the spin direction of cobalt 60 in the other set of devices to the right. Cobalt 60 in these two devices is a mirror image of each other. The experimental results show that the number of electrons emitted by cobalt 60 in these two devices is very different, and the directions of electron radiation cannot be symmetrical with each other. The experimental results show that parity is not conserved in weak interaction.
We can use a similar example to illustrate this problem. Suppose two cars are mirror images of each other. The driver of car A sits in the front left seat with the accelerator pedal close to his right foot. The driver of car B sits in the right front seat with the accelerator pedal close to his left foot. Now, the driver of car A turns on the ignition key clockwise, starts the car, and presses the accelerator pedal with his right foot to make the car move forward at a certain speed. The driver of car B did exactly the same thing, but switched left and right-he turned on the ignition key counterclockwise and stepped on the accelerator with his left foot. Now, how will car B move?
Perhaps most people will think that two cars should move at exactly the same speed. Unfortunately, Wu Jianxiong's experiment proves that in the particle world, car B will travel at a completely different speed and in a different direction! The particle world is an incredible proof that parity is not conserved. Obviously, people have made a mistake in applying the symmetry observed in the macro world to the micro world.
Three Chinese-American physicists won a great reputation for their wisdom. 1957, Li Zhengdao and Yang Zhenning won the Nobel Prize in Physics. This is a scientific theory, which is unprecedented in the second year after its publication. Unfortunately, Wu Jianxiong, who proved that parity was not conserved through exquisite experiments, never won the prize.
Why on earth do particles show parity non-conservation under weak interaction? The root cause is still a mystery.
The universe originated from asymmetry.
The discovery of parity non-conservation is not isolated. In the microscopic world, there are three basic symmetry ways for elementary particles: one is that particles and antiparticles are symmetrical to each other, that is, for particles and antiparticles, the law is the same, which is called charge (C) symmetry; One is spatial reflection symmetry, that is, for a pair of particles that are mirror images of each other, their motion laws are the same, which is called parity (P); One is time inversion symmetry, that is, if we reverse the motion direction of particles, the motion law of particles is the same, which is called time (t) symmetry.
That is to say, if antiparticles are used instead of particles, and time to go back is allowed, then the transformed physical process still follows the same physical laws.
However, since the law of parity conservation was broken by Li Zhengdao and Yang Zhenning, scientists soon found that the behavior of particles and antiparticles is not exactly the same! Some scientists have further suggested that it may be because of the slight asymmetry of the laws of physics and the asymmetry of the charge (C) of particles, which led to a little more matter produced at the beginning of the Big Bang than antimatter. Most of the matter and antimatter were annihilated, and the rest of the matter formed the world we know today. If the laws of physics are strictly symmetrical, the same amount of matter and antimatter will be born after BIGBANG, but the positive and negative matter will be annihilated immediately after they meet. In this case, galaxies, the earth and even humans will have no chance to form.
Next, scientists found that even time itself is no longer symmetrical!
Perhaps most people thought that time could not be turned back. In daily life, the arrow head of time always has only one direction, "the deceased is like this", the old man becomes young, the broken vase cannot be restored, and the boundary between the past and the future is clear. However, in the eyes of physicists, time is always considered reversible. For example, a pair of photons collide to produce an electron and a positron, and a pair of photons are also produced when the positive and negative electrons meet. These two processes conform to the basic laws of physics and are symmetrical in time. If one of the processes is filmed with a video camera and then played, the viewer will not be able to judge whether the video tape is played forward or backward-in this sense, time has no direction.
However, at the end of 1998, physicists first discovered the events that violated the symmetry of time in the micro-world. Researchers at CERN found that there is time asymmetry in the transformation process of positive and negative K mesons: the rate of anti-K mesons transforming into K mesons is faster than its inversion process, that is, K mesons transforming into anti-K mesons.
At this point, the symmetry of the physical laws of the particle world has been completely broken, and the world has been proved to be imperfect and flawed in essence.
God is left-handed?
When "parity non-conservation" was put forward in 1950s, most people did not agree that the law of parity conservation of "complete harmony" was challenged. Before Wu Jianxiong's experiment, Professor Pauli, a famous authority on theoretical physics at that time, even said, "I don't believe that God is a weak left-hander. I'm ready to make a big bet. I bet the experiment will come to a symmetrical conclusion. " However, rigorous experiments proved that Professor Pauli lost the bet this time.
Pasteur, the father of modern microbiology, once said: "Life shows us the asymmetric function of the universe. The universe is asymmetric and life is dominated by asymmetry. " Nature may really not be so symmetrical and perfect. In addition to favoring matter and rejecting antimatter, nature also has a preference for left and right.
Among the 20 amino acids in nature, 19 has two configurations, namely, left-handed and right-handed. In the experiment of producing amino acids by abiotic reaction, the probability of left-handed and right-handed is equal, but in life, 19 amino acids are surprisingly consistent with left-handed-except for a few low-level viruses containing right-handed amino acids. There is no doubt that life has a strong preference for lefties.
It has also been suggested that in the origin of life, amino acids are actually left-handed, which coincides with the direction of the magnetic field around the sun. However, most scientists believe that the asymmetry between the left hand and the right hand means that there is a difference between the two energies. It is generally believed that levorotatory energy is low and stable, and stability is easy to form life.
What is even more puzzling is that although protein amino acid molecules that make up life are all left-handed, ribose and deoxyribose molecules that make up nucleic acid are all right-handed, although the probability of left-handed and right-handed in natural sugar is almost the same.
It seems that God really leans to the left and to the right. If everything is absolutely balanced and symmetrical, the life of "the spirit of all things" will not be produced.
Asymmetry has a world.
In a sense, asymmetry created the world. The reason is actually very simple. Although symmetry reflects the * * * nature of different material forms in motion, only when symmetry is destroyed can they show their respective characteristics. It's like architecture, with only symmetry and no symmetrical destruction. Although the building looks neat, it will certainly look very monotonous and boring at the same time. Only when it is basically symmetrical but not completely symmetrical can beautiful buildings be formed.
Nature is such an architect. When constructing macromolecules like DNA, nature always follows the principle of replication. The molecules are connected by symmetrical spiral structures, and the spatial arrangement of spiral structures is basically the same. However, in the process of replication, a slight deviation from the exact symmetry will change. So the destruction of symmetry is the reason why things continue to evolve and become colorful.
As the famous German philosopher Leibniz said, there are no two identical leaves in the world. If you carefully observe the fine structure of midrib (the main vein in the middle of a leaf), you will find that even the number and distribution of midrib on both sides of a leaf are different. Most people's facial development is asymmetric. 66% people's left ear is slightly larger than their right ear, 56% people's left eye is slightly larger, and 59% people's right face is bigger. The trunk and limbs are not completely symmetrical, and the left shoulder is often higher. 75% people's right upper limb is longer than their left upper limb.
It can be said that the asymmetry in the biological world is absolute, and symmetry is only relative. Experimental studies have proved that this is caused by the asymmetry of protoplasm in cells. It can be clearly seen from the molecular structure of protein and other substances in organisms that they are generally asymmetric in structure. Scientific research has also found that the metabolic activity of asymmetric protoplasm is at least three times faster than that of symmetric chemicals. It can be seen that asymmetry is of great significance to the evolution of life. The development of nature is a process of decreasing symmetry.
In fact, not only in nature, but also in perfect human civilization, absolute symmetry is not pleasing. A seemingly symmetrical landscape painting can give people a beautiful enjoyment. However, a completely symmetrical landscape painting will appear bland and lifeless, which has nothing in common with the vibrant natural landscape and has no aesthetic feeling at all.
Sometimes, some kind of damage to symmetry or balance, even a small damage, will bring incredible and wonderful results. In this sense, perhaps perfection does not mean absolute symmetry, but it is the breaking of symmetry that brings perfection.