Anti-proton, anti-neutron and anti-electron form anti-atom if they are combined like proton, neutron and electron. Matter composed of anti-atoms is antimatter. When you look in the mirror, what will happen if you really exist in the mirror and appear in front of you? Scientists have considered this problem. They call you "against you" in the mirror. Scientists imagine a world very similar to ours in a distant place. It will be an anti-world composed of all anti-stars, anti-houses, anti-food and so on. Antimatter is the opposite of general matter, which is the main part of the universe. [Edit this paragraph] The concept of antimatter in particle physics. As we know, all kinds of macroscopic objects in nature are attributed to their microscopic origins, and they are all composed of protons, neutrons and electrons. These particles are therefore called elementary particles, which means that they are the basic bricks for building everything in the world. In fact, the world of elementary particles is not that simple. In the early 1930s, positively charged electrons were discovered, which was the first step for people to know antimatter. In the 1950s, with the discovery of antiprotons and antineutrons, people began to clearly realize that any basic particle in nature has corresponding antiparticles.

Electrons and antielectrons have the same mass, but opposite charges. So do protons and antiprotons. So what's the difference between the properties of neutrons and antineutrons? In fact, particle experiments have confirmed that particles and antiparticles not only have opposite charges, but also all other properties can be reversed. Here we discuss the concept of baryon number.

Protons and neutrons are collectively called nucleons. From the study of nuclear phenomena, it is found that protons can be converted into neutrons, and neutrons can also be converted into protons, but the total number of nucleons in the system remains unchanged before and after the conversion. Particle experiments since 1950s show that there are many kinds of particles heavier than nucleons, which belong to the same category, so these particles are renamed baryons, and nucleons are only the lightest representatives. The general rule is that when particles are transformed through interaction, the baryon number in the system will not change.

Because of the conservation of baryon number, two protons will not produce a system containing three baryons when they collide, so how should the antinucleon be produced? Experiments show that antinucleons are always paired with nucleons in collisions. take for example

P+P→N+N+N '+ Some π mesons

Where n represents proton or neutron and n' represents antiproton or antiproton. Once antinucleons are produced, they often collide with nearby nucleons and annihilate in pairs. take for example

N+N'→ Some π mesons

According to this statement, there must be an antimatter world somewhere in the universe. If the antimatter world really exists, then it can only exist if it doesn't meet matter. can

How can matter and antimatter not be combined? Where is antimatter in the universe? This is still a mystery to be solved.

For baryons heavier than nucleons, the situation is exactly the same. Antibaryons are always produced and annihilated in pairs with baryons. These experiences make people realize that the law of conservation of baryon number needs to be re-recognized.

Now people regard baryon number b as a kind of charge that describes the properties of particles. The positive and negative baryons are not only opposite in charge, but also opposite in baryon number b. Let any baryon baryon number B=+ 1, then any anti-baryon B=- 1. Non-baryons, such as mesons, leptons and gauge, have no baryons, that is, B=0. The law of conservation of baryon number can be expressed as: no particle reaction will change the total baryon number b of the system. This expression not only reflects the constant baryon number when antiparticles are not involved, but also summarizes the pair generation and annihilation of antiparticles and particles. Now we can easily understand the difference between neutrons and antineutrons. They have the opposite baryon number b, so antineutrons can collide with nucleons and cause annihilation, while neutrons cannot.

In addition, people also found the conservation of lepton number. Although neutrinos are uncharged and have no baryon number, they have the opposite lepton number as antineutrinos. According to the conservation of lepton number, the physical behaviors of neutrinos and antineutrinos are also very different. Experiments also show that the number of mesons and gauge particles is not conserved. In this way, we can see that charge is only one property of particles, and there are other properties described by physical quantities such as baryon number and lepton number. These properties of positive and negative particles are also opposite.

1928, the young British physicist Dirac proved the existence of positrons for the first time in theory. This positron has the same properties as an electron, but its electrical property is opposite to that of an electron. 1932, American physicist Anderson discovered the positron predicted by Dirac in the laboratory. 1955, American physicist Sigri and others obtained antiprotons artificially. Since then, people have gradually realized that not only protons and electrons, but also all microscopic particles have their own antiparticles.

This series of scientific achievements has brought people closer to the antimatter world. However, the problem is not that simple. First of all, it is difficult to find antimatter on earth. Because particles meet antiparticles, just like ice meets fireballs, they either disappear together or become other particles. So on earth, antimatter will be swallowed up once it meets other substances. Secondly, it is very difficult and expensive to make antimatter, which requires high-tech instruments such as SSC or LHC, and even if antimatter is made, it is difficult to preserve it, because everything on earth is made of matter.

The macroscopic substances around us are mainly composed of protons and neutrons with positive baryon numbers. Therefore, such substances are called positive substances, and substances composed of their antiparticles are correspondingly called antimatter. From the point of view of particle physics, the properties of positive particles and antiparticles are almost completely symmetrical, so why is there a lot of positive matter in nature, but almost no antimatter? This is exactly what we are going to discuss now.

Fundamentally speaking, antimatter is the inverted form of matter. Einstein predicted the existence of antimatter according to the theory of relativity: "For a substance with mass m and charge e, there must be a substance with mass m and charge -e (that is, antimatter)".

Is there an antimatter universe in the positron spacecraft (under development) envisioned by NASA? From a philosophical point of view, this question is easy to answer. The ancient Taiji diagram in China seems to imply its existence, and some astronomers think it is possible, but modern astronomy has not produced convincing evidence. Many people deny antimatter. Shi Lamu, an American cosmologist, said: "Most theorists intuitively think that there is no antimatter. This means that if you find it, it will be a great discovery and prove these theorists wrong. But most likely, it means you can't find it. "

At present, scientists from 16 countries have participated in this research in Ding Zhaozhong, with an investment as high as10 billion US dollars. Many scientists say that as long as antimatter can be found in the universe, it will be a well-deserved Nobel Prize. The probe will be launched in 2005 and will remain in space forever. Southeast University will also establish a data reception and analysis center and a training center as supporting projects. Ding Zhaozhong believes that if antimatter does exist, it can generate enormous energy when positive matter collides with antimatter. The research on "looking for dark matter and antimatter in the universe" which he is now presiding over has been going on for many years, and some important achievements have been made so far. "However, judging from the development history of this field, people should be psychologically prepared. Maybe we will find unexpected things that have nothing to do with what we originally wanted to study. " Ding Zhaozhong said cautiously.

From Laplace's Great Prophecy

Celestial bodies have great gravity, and under the action of great gravity, various reactions will occur, and light and heat will be emitted. The extremes meet, Laplace (P? s？ Laplace boldly predicted that the largest celestial body in the universe might be invisible. When gravity increases with mass, the celestial body will become a region with nothing, neither heating nor emitting light. Now we call it a "black hole". So the universe is mostly made up of invisible dark matter or antimatter. What our naked eyes and astronomical instruments can "see" is only the cosmic structure in the form of stars or galaxies, which only account for 65,438+00% of the universe, and 90% of the matter exists in the form of dark matter or other structures. Obviously, for visible matter, the existence of gravity indicates the existence of dark matter or antimatter. But we can't detect them by light, and we can't find their footprints by infrared, ultraviolet and X-ray.

Similarly, corresponding to the existing galaxy structure system, is there an anti-cosmic structure system? In fact, as early as 1898, a British physicist suggested that, like the existence of matter, there is a mirror image of antimatter. Limited by the scientific level and experimental conditions at that time, this concept of antimatter has no factual basis, so the existence of a cosmic nebula composed of antimatter in the depths of the universe can only be a pure hypothesis.

1997, scientists announced the discovery of the "Silver Fountain of Antimatter", which greatly shocked the whole physics field and made scientists' enthusiasm for finding antimatter suddenly rise.

1On June 3rd, 998, Professor Ding Zhaozhong initiated the event of searching for cosmic antimatter with global significance, which made this field once become the focus of global scientists.