The wonder of the world is far beyond human imagination, and the Creator's means of creating the world are so clever that they are far beyond our human understanding.
Sometimes we feel as if we have touched the truth, but in the end we find that nature always brings us some unexpected surprises, and every amazing discovery makes us have to admire the superb skill of the Creator.
But our human ability to explore and create is also amazing, sometimes something is secretly hidden in a dark corner by the Creator, but in the end it is discovered by us humans.
That's our theme today, we've found something that the Creator has hidden, and it's antimatter. Let's start with what is antimatter?
This is well understood, antimatter is the negative version of positive matter, positive matter is all the things that we can see around us, including you, your pet, your wife are all positive matter, because we are all made of protons, neutrons and electrons.
Antimatter, on the other hand, is made up of antiprotons, anti-neutrons, and antielectrons, so the question is, what's the difference between these antiparticles and positive particles?
In the case of electrons and antielectrons, since none of us have ever seen electrons and antielectrons, we can distinguish between them not by their appearance, but by some of the properties they carry.
For example, the electron is negatively charged, the anti-electron is positively charged, the number of leptons of the electron is 1, the number of leptons of the anti-electron is -1, the reason why they are their own anti-versions is because they also have the same place, such as the same mass, the same spin size are 1/2.
Here I will say what is the number of leptons, leptons are a classification of electrons according to quality, so electrons are members of the lepton family, because we found that in any reaction in the laboratory the number of leptons will not increase, nor will it decrease, so we think that the number of leptons is a conservation quantity, to put it bluntly, you create an electron at the same time, you must create a corresponding anti-electron, the number of electron leptons is 1, the number of leptons of the anti-electron -1, the structure is that the number of leptons does not increase or decrease, understand.
Then you want to destroy an electron at the same time, you must also destroy an anti-electron, so that you can ensure that the number of leptons before and after the reaction is preserved.
Then the above law means that there will be no net leptons in our universe, to put it bluntly, there will be no electrons, then there will be no matter without electrons, and there will be no everything we can see now, including various planets in the universe.
In fact, there will be no protons and neutrons, because protons and neutrons are called baryons, and it can be seen from the name that because these things are heavier than barnitons, they are called baryons, and we found that the number of baryons is also conserved, and it always does not increase or decrease in the reaction in the laboratory.
So according to current physics, the conservation of the number of leptons and baryons means that there should be no matter in the universe, and there is nothing, because the number of positive and negative matter in the universe is symmetrical at the beginning, and their number of leptons and baryons is opposite, and after the annihilation of positive and negative matter, the number of leptons and baryons in the universe is 0, leaving nothing behind.
But the truth is clear, there is a lot of matter left in the universe, and antimatter has disappeared, why is this? This question is one of the biggest mysteries in cosmology and particle physics today. But that's not our topic today, so I won't go into detail.
Then I say the above content, just to let you know two key points, the number of barons and the number of baryons, these two conserved quantities, in the following article will be a lot of mention of these terms, as well as the isotopic rotation we said yesterday, are some basic concepts.
In fact, after you look at the entire content of particle physics, you will find that most of the content of particle physics is based on conservation quantities and symmetry, so some conservation quantum numbers are still more important.
Well, let's move on to antimatter, so you might now be thinking, since there is no antimatter in the universe, where did we find antimatter? After a lot of preparation above, we can say today's content.
At that time, around 1925, Schrödinger wrote a wave equation describing the function of electron waves, because it did not know the concept of electron spin at that time, so its equations were non-relativistic.
That is, its equations are not based on special relativity, and this work was finally completed by Dirac in 1927, through Dirac's relativistic wave equations, it is possible to perfectly explain the spectral problem of hydrogen atoms, and it is also possible to deduce that the spin value of the electron is 1/2, then the spin of the electron before that is completely a guess, without any basis.
But it takes only two components to describe the spin of an electron, but Dirac's equation derives four, and a negative energy solution is present, which describes the motion of an electron with negative energy.
Negative energy What the hell is this? The best solution is to ignore the solution that has no physical significance, but Dirac is different, and its physical intuition tells itself that the solution may really have physical significance.
In fact, the problem of negative energy is not the first encountered by Dirac, in Einstein's formula for describing the energy carried by matter, there is a square for the energy item, which also means that energy may have positive and negative. The lowest value of positive energy can be mc², the maximum value of negative energy is -mc², and there is a gap of 2mc² width between the positive energy world and the negative energy world.
Since Einstein's equation is entirely an equation that describes the classical world. The macroscopic matter in the classical world is in continuous motion, and it is impossible to cross such a wide energy gap in the middle through continuous motion, so people at that time ignored this negative energy solution.
But the quantum world is different, it is discontinuous, with the spread of the wave function, particles like mysterious transitions, beyond the energy gap is also possible, so Dirac even if it is, the electrons in this atom have a certain chance of jumping to the negative energy level in 10^-8 seconds.
If such a thing really happens, the consequences are very serious, and the electrons of the hydrogen atom will disappear in an instant, running to a state of negative energy, and the entire material world will collapse.
So Dirac felt that this negative energy level could not be avoided, so it had to explain why the electron did not run to the negative energy level. To solve this difficulty, Dirac thought of Pauli's principle of incompatibility.
This principle says that two identical fermions cannot stay in the same space. Since the electron is a fermion, because its spin is 1/2, there are no two electrons with the same quantum state in atomic space.
This explains why electrons have not jumped to the negative energy level, because the orbits of the negative energy level have been filled with electrons with negative energy, so the positive energy of the electrons cannot continue.
This explanation means that if we supply energy to this negative energy electron, they will be excited into a positive energy state, this is because an electron runs, then there will be a hole in the vacuum, this hole is equivalent to a positive energy particle, but its charge is opposite to the electron, the mass and spin and all properties are the same as the electron, it is the positron.
This is Dirac's prediction of positrons, or antimatter. Around the same time, Zhao Zhongyao, the originator of nuclear physics in China, worked with Mirrigan at The California Institute of Technology, worked with Rutherford at cavendish laboratories, and returned to China in 1931 to serve his country.
Just in 1930, Zhao Zhongyao was bombarding the material with high-energy γ rays, observing the absorption law of the material on γ rays, he found an abnormal phenomenon, the material after the gamma ray impact, will radiate a γ photon with an energy of 0.5 mev in different directions, not only is the reverse random, the energy of the γ photons radiated out is also determined.
This phenomenon cannot be explained by the scattering of gamma photons and electrons outside the nucleus, but more like an interaction with the nucleus of an atom, which was then called the anomalous nuclear absorption phenomenon.
This was first discovered by Zhao Zhongyao, so it is also called Zhao Zhongyao characteristic radiation, and later many colleagues have discovered this phenomenon. In 1932, when the American physicist Carl Anderson was studying cosmic rays on the mountain, he discovered the positron predicted by Dirac in the cloud chamber.
Cloud chamber we said before, it can show the traces of particles, we are putting the cloud chamber in the magnetic field, according to the deflection of the charged particles in the magnetic field, we can see whether the particle is positively charged or negatively charged, but also calculate the particle's charge, mass and other information.
So it took Anderson only one night to confirm that this was positron, the antimatter prophesied by Dirac. It was because of this work that Anderson won the Nobel Prize in Physicist in 1936.
After the discovery of positrons, people quickly made an explanation for Zhao Zhongyao's characteristic radiation, which is what Dirac said, giving energy to the vacuum will excite an electron from the vacuum, generating a hole (positron), and then the electron falls back to the hole, that is, the electron and positron annihilation become two photons, the reason why it becomes two photons, not a photon, is that two photons can shoot out in the opposite direction, so that not only the energy is conserved, but the momentum is also conserved. The reason why the energy of each photon is 0.51 mev is actually the mass of an electron.
So what are the nuclei doing in this process? The nucleus is to ensure that the entire energy conversion process is conserved, because a γ photon cannot directly become two positive and negative electron pairs, so that momentum and energy cannot be guaranteed to be conserved at the same time, so the participation of a third party is required, that is, the nucleus. Same as the situation just now.
Now this effect is also called the positive and negative electron pair effect, which is a phenomenon in which a high-energy γ photons are absorbed by the nucleus to produce a positive and negative electron pair after the photon is incidental.
Here I would like to add that when photons are incident matter, there are three situations that occur from low energy to high energy, the photoelectric effect, Compton scattering, and the positive and negative electron pair effect.
Then let's talk about the last question, why Zhao Zhongyao did not win the Nobel Prize, but he discovered for the first time the generation of positive and negative electrons, and the process of annihilation.
The reason is actually very simple, he only described this phenomenon, and did not specifically explain the phenomenon, not to mention that γ light produced a positive and negative electron pair, if he can infer from the radiant photon is always 0.51 mev energy, this is the positive and negative electrons released after annihilation, it may have the opportunity to share the Nobel Prize with Anderson, which can also be regarded as indirectly discovering the positron. Therefore, Zhao Zhongyao and antimatter seem to be a step away, in fact, there is still a long distance.
In general, Dirac's hollow theory is significant, he allows us to discover a different world, in fact, not only has anti-electrons, all the particles we know now have their own version of antimatter, which is only because when God created the world, the lowest logic was symmetry.
This is why many of our current unexplained physical phenomena need to be answered in the spontaneous breaking of symmetry. Finally, I would like to add that Dirac's hole theory is actually just to give us a better understanding of the origin of antielectronics.
In the end we found that all particles have their own anti-version, so this hole theory is useless and should be deleted from the book, but this example is more vivid, so it has been retained. Therefore, the vacuum is not filled with electrons, protons, and neutrons, and the positive and negative matter is produced because of the conversion of energy into matter, which follows the requirements of symmetry.
Well, today's content will end here, and in the next lesson we will talk about the discovery process of neutrinos.