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Why do we exist? This is undoubtedly the biggest existential problem in the universe. Because our theories and observations show that whenever matter is produced, it produces the same amount of antimatter. But when matter and antimatter meet, they annihilate each other in huge bursts of energy. So the Big Bang should produce the same amount of matter and antimatter, leading to bursts of energy, leaving almost nothing left.

Although a universe made up of nothingness may have a simpler theory, it is obviously not our universe because the universe we observe is made up of matter. The question is why almost everything we can see is made of matter, and where is the antimatter? This is one of the biggest puzzles in physics, and in order to find possible clues, we must turn to quantum mechanics.

The best theory of matter in the universe is the standard model of particle physics, which describes not only matter, but also how matter interacts with three fundamental forces. We have another theory: Einstein's general theory of relativity, which describes how matter interacts on a large scale through the curvature of space-time. These two theories describe almost everything we know about how the universe works. But neither the standard model nor general relativity can explain why the universe is matter.

The first thing we have to solve from scratch is the problem of how things grow out of nothing. This problem was raised before quantum mechanics, but now that we have quantum mechanics, it provides us with a feasible solution.

If there is a box and you take out all the particles and radiation in the box, you might say that there is nothing in the box. But in quantum mechanics, that box is not really nothing, and the laws of quantum mechanics are still valid in that box, which means that it still has quantum fluctuations. This is the idea that particles can be created and destroyed in such a short time that we don't really notice them.

These virtual particles that cannot be measured directly, we know where they are, because we can detect their overall effects. Empty space has energy, and it can exert a force, as can be seen in the Casimir effect. This effect is caused by the pressure difference between the interior and exterior of a group of parallel metal plates, and the pressure difference is caused by the difference of internal and external quantum fluctuations. Therefore, when we regard the vacuum of space as nothingness, we must remember that it does have something. And this kind of thing is due to the laws of quantum mechanics, especially the Heisenberg uncertainty principle.

But when matter is created from a vacuum, it also produces the same amount of antimatter. Antimatter, like matter, has the opposite electric charge. For example, the antimatter equivalent of an electron is a positron, which, like an electron, carries a positive charge instead of a negative charge. There are also antineutrinos and antquarks. When matter and antimatter collide, they annihilate each other and produce huge amounts of energy. A simple Feynman diagram can be used to represent the process, or we can turn it upside down: energy becomes matter and antimatter.

Therefore, as long as energy is conserved, the same amount of matter and antimatter can be produced in principle from the vacuum of space. Assuming that the laws of physics today are the same as the laws of physics at the beginning of the universe, we would expect to see as much antimatter as matter. The problem, however, is that all the universes we can observe seem to be matter, and antimatter seems to account for only a tiny fraction. So we need to figure out what causes this matter-antimatter asymmetry, which is now called the baryon generation problem.

Sakharov's three conditions 13.8 billion years ago, when the universe was born, it was thought to be matter-antimatter symmetry, which meant that the two were equal in number. But within the first second of its existence, almost all antimatter in the universe disappeared. So the universe either gives priority to the creation of matter or the destruction of antimatter, but what causes it?

In 1967, Russian nuclear physicist Andre Sakharov proposed that the universe must meet three conditions to produce matter and antimatter at different rates. These conditions are as follows: first, the universe must be unbalanced, second, the universe must show a violation of C and CP symmetries, and third, the universe must have interactions that violate the conservation of baryon numbers. Basically, Sakharov mathematically proved that if these conditions were met, the universe could start with the same amount of matter-antimatter, but end up producing more matter than antimatter.

Let's look at what these conditions mean one by one. The first condition is easy for us because we live in an expanding universe that is not in equilibrium.

C symmetry represents charge conjugation symmetry, which simply means that the laws of physics should also apply to antimatter. But if the C symmetry is broken, the laws of physics will be opposite to each other in particles and antiparticles. It turns out that a weak force destroys C symmetry, and if you apply a left-handed neutrino to C transformation to make it an antineutrino, it doesn't work. Because neutrinos are always left-handed, while antineutrinos are always right-handed. In order to turn a neutrino into an antineutrino, it is also necessary to change its spin direction, which requires the so-called CP transformation. CP symmetry is a combination of C symmetry and P symmetry. P represents spatial parity, and the same weak force can also violate CP symmetry. Unfortunately, there is not enough violation of the CP interaction to explain the difference in the amount of matter and antimatter we see in the universe.

The last point is also a problem. Baryons are particles composed of three quarks. For example, protons are made up of two upper quarks and one lower quark, and neutrons are made up of two lower quarks and one upper quark. Generally speaking, we can't destroy baryons, we can only change them. For example, in beta decay, neutrons can be turned into protons and vice versa. Start with one baryon and end up with another baryon, which is the conservation of baryon numbers. In order to meet the third condition, we need a process, for example, one proton can become two protons, but we have never seen this happen in nature.

Therefore, Sakharov's first condition is satisfied, the second condition is partially met, and the third condition is not met, and the problem has not been solved. The question remains what causes matter-antimatter asymmetry.

Other ideas have a lot of theories trying to solve this problem, and one of the most interesting ideas comes from Richard Feynman who points out that antimatter is mathematically equivalent to ordinary matter moving backward in time. So some cosmologists suggest that perhaps during the Big Bang, antimatter began to go backwards in time and never encountered matter. The idea is, what happens if you rewind the cosmic clock back to the Big Bang and then continue to rewind? Maybe we'll see a universe made up of antimatter.

In our universe, the arrow of time is good for matter, while in another universe, the opposite arrow of time is good for antimatter. The biggest problem with this hypothesis, of course, is that whenever we create antimatter in the lab, they actually move forward rather than backward in time, so the above assumption is not true.

Another idea is that matter and antimatter may be separated too fast to annihilate each other, and that there may be mirrored antimatter galaxies in the distant universe. But if this is the case, we expect to see some huge fireworks in the form of high-energy gamma rays in the boundary area between matter and antimatter, but we have not observed this phenomenon.

This article comes from the official account of Wechat: Vientiane experience (ID:UR4351), author: Eugene Wang

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