Most particles disintegrate, and some do not.
Although most particles disintegrate, or decay, into other particles, some of them do not behave this way. But why?
There are many types of particles in the world, some of them look elementary, others can be built from elementary — for example, protons, neutrons, and the atomic nucleus — but most of them decay in a small fraction of a second. In a previous article I explained why they break up; in fact, it is a form of dispersion, about which we have an intuitive notion derived from our experience with waves and vibrations. But why don't several types of particles decay at all, or at least live much longer than 13.7 billion years, longer than the age of the Universe?
The only stable particles known in nature are the electron (and the anti-electron), the lightest of the three types of neutrinos (and its antiparticle), the photon, and the supposed graviton (both of the latter are anti-particles themselves). Other neutrinos, protons, and many atomic nuclei (and their antiparticles — here I stop mentioning antiparticles, it will be implied) are probably unstable, but they live very, very, very long. Protons, for example, live so long that a very small number of them decayed from the Big Bang, so that from all practical points of view they are stable. Another long-lived particle is a neutron, which by itself, outside the atomic nucleus, lives only about 15 minutes. But inside atomic nuclei, neutrons can live longer than the age of the Universe. Finally, it is worth adding that if dark matter consists of particles, then these particles must also be stable or very, very long-lived.
Why are these particles stable? It turns out that in the microcosm there are rules for the behavior of particles unknown to us from everyday life filled with waves and vibrations. These laws prevent the disintegration of particles, both fast and slow. The fundamental rules are the laws of conservation, which state that certain values ​​of the Universe do not change in any physical processes. Among them - energy, impulse, electric charge and several others. There are also some approximate laws of conservation, suggesting that some quantities change very rarely. These laws did not appear out of nowhere and were not invented by theorists from scratch. They are associated with other properties of the world. For example, if the laws of nature do not change with time, it follows from this (thanks to the theorem of the mathematician Emmy Noether ) that energy is saved. We will see that the stability of the matter of which we are composed allows us to verify these laws quite well.
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The combination of these laws with the properties of particles gives us a few simple rules that determine when particles simply cannot decay, or can very rarely decay. And these rules (almost) are enough to explain the stability of the particles of which we are composed, and of the particles with which we most often interact.
In the world in which Einstein's theory of relativity works, space has three dimensions, and quantum mechanics works, all particles must be either fermions (named after Italian physicist Enrico Fermi ) or bosons (after Indian physicist Satyendra Nath Bose ). This statement is a mathematical theorem, not the result of observations. But data from the last 100 years of observations supports it - all particles known in the Standard Model are either fermions or bosons.
An example of a boson is a photon. Two or more bosons (of the same type of particle) are allowed to do the same. For example, a laser is a machine for creating a large number of photons that do absolutely the same things, and produces a very bright light with a very precisely defined color and propagating in a certain direction. All photons in the beam are synchronized.
You cannot make a laser out of fermions. An example of a fermion is an electron. Two fermions (of the same type of particle) cannot do the same at the same time. Since the electron is a fermion, two electrons cannot be on the orbit of an atom in the same way. This is due to the principle of the prohibition of Pauli , which we teach in chemistry lessons, which has enormous implications for the periodic table of elements and for chemistry. The electrons in an atom occupy different orbits, in different shells around the atomic nucleus, since they cannot all fall into one orbit at the same time - fermions are forbidden to do this. More precisely, only two electrons can occupy one orbit, and only if they rotate in different directions around the axis, i.e. have a different spin. If electrons were bosons, chemistry would not know!
Among the elementary particles known in our world there are many fermions: charged leptons, neutrinos, quarks, and many bosons: all carriers of interactions and the Higgs particle.
Also, boson fields can, on average, significantly differ from zero. Fermion fields cannot do this. The Higgs field, non-zero in our Universe, giving a mass to all elementary particles is a boson field (and its particle is a boson, therefore it is called the Higgs boson).
In addition, Bose-Einstein condensate, predicted by Einstein in the 1920s, but obtained only in the 1990s, in an experiment that won the Nobel Prize, can be formed from boson particles. In such experiments, condensate is obtained by forcing a large number of boson atoms to remain in the most “quiet” state accessible to a quantum object, i.e. atoms are in the lowest possible quantum states, and then quantum effects begin to manifest themselves at the macroscopic level.
All this relates to quantum mechanics. Although Einstein did not like the consequences of quantum mechanics, you should not get the impression that he did not understand it. On the contrary, his work was critical for the development of some aspects of quantum theory.
Here are the main rules. In bold are their main consequences for our Universe.
Therefore, with each decay of particles in nature, two or more particles appear from a single particle. This follows from the law of nature, according to which the total energy and total momentum must remain constant in any physical process (physicists say that “energy and momentum are conserved”). And that is why the 1st rule follows from them:
Suppose a particle of type 1 can decay only into a particle of type 2. Let us prove that there is a contradiction. Take particle 1 and place it in front of us in a stillness. All her energy will be enclosed in its mass. Now, let's say it broke up into particle 2. The law of conservation of energy states that
Since the energy of motion is positive, the rest energy of particle 2 can be less than or equal to the rest energy of particle 1. But the energy of motion of particle 2 is positive, therefore if the rest energy of particle 2 is less than the rest energy of particle 1, then particle 2 should move. But particle 1 rested, so it had no momentum. Particle 2 moves, so it has momentum. But this is impossible - the momentum must be maintained. Therefore, such a decay is impossible, unless the masses of these particles are equal to each other. But in this case, if particle 1 can decay into particle 2, the reverse is also true - particle 2 can decay into particle 1. But this is not decay - it is just a confusion between the two types of particles.
The total energy and the total momentum during decay are saved, but the total mass is always reduced. A “parent” particle with a mass m1 can decay only into “daughter” particles 2 and 3, if the sum of their masses is less than the mass of the parent: m2 + m3 <m1. This is a simple consequence of the law of nature - the total energy must remain constant in any physical process. Evidence:
Imagine that you are watching particle 1 alone. All her energy is rest energy, m 1 c 2 . Then it breaks up into particles 2 and 3. Each of them has a rest energy and a motion energy. As energy is conserved,
But the energy of motion is always greater than zero, therefore the initial rest energy exceeds the final rest energy, therefore m 1 c 2 > m 2 c 2 + m 3 c 2 , hence m 1 > m 2 + m 3 .
Since the photon, as all experiments show, does not have mass, it cannot decay . Therefore, the waves of light can pass through the whole room, all the space from the Sun to us, and the entire Universe, without disintegrating on the way at all. It is assumed that the graviton has the same properties.
Another conserved property is the electric charge. A particle W-, very heavy and negatively charged, with a charge of –e, can decay into an electron with a negative charge –e and an antineutrino without charge. But W- cannot decay into a positron with a positive charge + e and a neutrino without charge, since the total charge would change from –e to + e. Also, W- cannot decay into an electron with a negative charge and a positron (anti-electron) with a positive charge, since their combination would give a zero charge.
Since the electron is the lightest of particles with an electric charge, it cannot decay into anything else . Only neutrinos, photons, gluons and gravitons are lighter, but they are electrically neutral, so any combination of them will have a zero charge. Any unknown particle lighter than an electron must be electrically neutral, or we would easily find it in experiments. Therefore, the electron is stable .
The rule follows from the fact that the angular momentum, as well as energy and momentum, is conserved (which explains the tendency of rotating things, for example, the Earth, to preserve rotation). The rule forbids a neutron to decay into a proton and an electron. Such a decay would fall under laws 1, 2 and 3, but not under 4, since all these particles are fermions. The neutron decays into a proton, an electron and an antineutrino. Then we will initially have one fermion and three in the end, 3 - 1 = 2.
There are three types of neutrino, and it is now believed that all of them have a mass (two of them most likely, and the third probably). The lightest neutrino is the lightest of the known fermions , but the only particles that are lighter than it can break into are bosons (photon and graviton). Therefore, it does not decay : you cannot start with a fermion and end up with bosons. It can in principle be unstable if there are even lighter fermions, which we have not yet met - they would interact with ordinary matter even weaker than the neutrino. And we know that neutrinos live long enough, because we have seen how they travel great distances from remote explosions of supernova stars.
A proton contains three quarks, a set of gluons and a pair of quarks-antiquarks; therefore, in a proton the number of quarks minus the number of antiquarks is three. There is also a surplus of three quarks in the neutron. Therefore, a neutron, as a heavier particle, can decay into a proton without breaking rule 5 - and it does so (generating an electron and an antineutrino).
But the proton is the lightest of particles containing more quarks than antiquarks, and this rule follows, together with rule 2, that it is stable . It is clear that a proton cannot decay into any combination of electrons, photons and neutrinos, since they do not contain quarks. There are several hadrons (particles consisting of quarks, antiquarks and gluons), in particular, pions, but they differ from protons and neutrons in that they contain an equal number of quarks and antiquarks. Therefore, a heavy proton cannot decay into any combination of pions and non-hadrons (photons, electrons, neutrinos), since the daughter particles will have an equal number of quarks and antiquarks, and the parent particle will not. But peonies can decay without breaking the rules; for example, an electrically neutral pion (being a boson) can decay into two photons, and a positively charged pion can decay into neutrinos and an antimuon — which is very useful for creating neutrino rays.
Many theorists believe (although this was not confirmed by experiment) that this rule is a little broken , and the proton is very, very little unstable, while possessing an extremely long lifetime. For more than ten years, watching the huge number of protons in a huge tank of water in the Super-Kamiokande experiment , and not having received any decay, we know that a proton lives at least 10,000,000,000,000,000,000,000,000,000,000 years old. I hope I did not miss a single zero. The age of the current phase of the Universe is approximately 13,700,000,000, so there will be quite a lot of protons in the future.
There are other laws, but most of the effects observed by us follow only from those listed.
Now we have rules to explain:
• why photons are stable,
• why are electrons stable,
• why protons are stable, or live for very long,
• why at least one type of neutrino is stable or has a very long life.
What is quite enough to explain the usual matter, chemistry, sunlight, many other processes in life - except for one. What about an unstable neutron?
Neutron is a very amazing thing. Nothing prevents it from disintegrating, and it disintegrates after about 15 minutes into a proton, electron and antineutrino. Why does he live so long? Partly due to the fact that the proton and neutron masses are very close. Although the rest mass of the neutron is approaching GeV, it is only 0.0007 GeV more than the sum of the rest mass of the proton, electron and antineutrino. And the frequency of decays becomes very small, when the total mass of daughter decay particles is obtained very close to the mass of the parent particle. This is not surprising, since rule 2 postulates that the decay must completely stop if the mass of daughter particles exceeds the mass of the parent.
But what is strange is that if you place a neutron in the atomic nucleus, it becomes stable! For example, in helium there are two protons and two neutrons. And although the neutron itself lives a quarter of an hour, the helium nucleus can live as long as the Universe exists, and even longer. This is true in general for all stable elements of the periodic table to them. Mendeleev and their neutrons. This fact is an extremely important consequence of Einstein's theory of relativity and some features of strong nuclear interaction, and without it our chemical world would not have any diversity. This feature deserves a separate article.
And, by the way, if dark matter consists of unknown particles - why are they stable? No one knows for sure, but probably the laws I have described will not be enough for this. Most likely, there is another conservation law, exact or approximate, which is yet to be discovered.
There are many types of particles in the world, some of them look elementary, others can be built from elementary — for example, protons, neutrons, and the atomic nucleus — but most of them decay in a small fraction of a second. In a previous article I explained why they break up; in fact, it is a form of dispersion, about which we have an intuitive notion derived from our experience with waves and vibrations. But why don't several types of particles decay at all, or at least live much longer than 13.7 billion years, longer than the age of the Universe?
The only stable particles known in nature are the electron (and the anti-electron), the lightest of the three types of neutrinos (and its antiparticle), the photon, and the supposed graviton (both of the latter are anti-particles themselves). Other neutrinos, protons, and many atomic nuclei (and their antiparticles — here I stop mentioning antiparticles, it will be implied) are probably unstable, but they live very, very, very long. Protons, for example, live so long that a very small number of them decayed from the Big Bang, so that from all practical points of view they are stable. Another long-lived particle is a neutron, which by itself, outside the atomic nucleus, lives only about 15 minutes. But inside atomic nuclei, neutrons can live longer than the age of the Universe. Finally, it is worth adding that if dark matter consists of particles, then these particles must also be stable or very, very long-lived.
Why are these particles stable? It turns out that in the microcosm there are rules for the behavior of particles unknown to us from everyday life filled with waves and vibrations. These laws prevent the disintegration of particles, both fast and slow. The fundamental rules are the laws of conservation, which state that certain values ​​of the Universe do not change in any physical processes. Among them - energy, impulse, electric charge and several others. There are also some approximate laws of conservation, suggesting that some quantities change very rarely. These laws did not appear out of nowhere and were not invented by theorists from scratch. They are associated with other properties of the world. For example, if the laws of nature do not change with time, it follows from this (thanks to the theorem of the mathematician Emmy Noether ) that energy is saved. We will see that the stability of the matter of which we are composed allows us to verify these laws quite well.
')
The combination of these laws with the properties of particles gives us a few simple rules that determine when particles simply cannot decay, or can very rarely decay. And these rules (almost) are enough to explain the stability of the particles of which we are composed, and of the particles with which we most often interact.
Fermions and bosons
In the world in which Einstein's theory of relativity works, space has three dimensions, and quantum mechanics works, all particles must be either fermions (named after Italian physicist Enrico Fermi ) or bosons (after Indian physicist Satyendra Nath Bose ). This statement is a mathematical theorem, not the result of observations. But data from the last 100 years of observations supports it - all particles known in the Standard Model are either fermions or bosons.
An example of a boson is a photon. Two or more bosons (of the same type of particle) are allowed to do the same. For example, a laser is a machine for creating a large number of photons that do absolutely the same things, and produces a very bright light with a very precisely defined color and propagating in a certain direction. All photons in the beam are synchronized.
You cannot make a laser out of fermions. An example of a fermion is an electron. Two fermions (of the same type of particle) cannot do the same at the same time. Since the electron is a fermion, two electrons cannot be on the orbit of an atom in the same way. This is due to the principle of the prohibition of Pauli , which we teach in chemistry lessons, which has enormous implications for the periodic table of elements and for chemistry. The electrons in an atom occupy different orbits, in different shells around the atomic nucleus, since they cannot all fall into one orbit at the same time - fermions are forbidden to do this. More precisely, only two electrons can occupy one orbit, and only if they rotate in different directions around the axis, i.e. have a different spin. If electrons were bosons, chemistry would not know!
Among the elementary particles known in our world there are many fermions: charged leptons, neutrinos, quarks, and many bosons: all carriers of interactions and the Higgs particle.
Also, boson fields can, on average, significantly differ from zero. Fermion fields cannot do this. The Higgs field, non-zero in our Universe, giving a mass to all elementary particles is a boson field (and its particle is a boson, therefore it is called the Higgs boson).
In addition, Bose-Einstein condensate, predicted by Einstein in the 1920s, but obtained only in the 1990s, in an experiment that won the Nobel Prize, can be formed from boson particles. In such experiments, condensate is obtained by forcing a large number of boson atoms to remain in the most “quiet” state accessible to a quantum object, i.e. atoms are in the lowest possible quantum states, and then quantum effects begin to manifest themselves at the macroscopic level.
All this relates to quantum mechanics. Although Einstein did not like the consequences of quantum mechanics, you should not get the impression that he did not understand it. On the contrary, his work was critical for the development of some aspects of quantum theory.
Laws of nature for particles
Here are the main rules. In bold are their main consequences for our Universe.
The laws of nature, which are considered for good reasons, must be followed exactly
1) A particle must break up into two or more particles.
Therefore, with each decay of particles in nature, two or more particles appear from a single particle. This follows from the law of nature, according to which the total energy and total momentum must remain constant in any physical process (physicists say that “energy and momentum are conserved”). And that is why the 1st rule follows from them:
Suppose a particle of type 1 can decay only into a particle of type 2. Let us prove that there is a contradiction. Take particle 1 and place it in front of us in a stillness. All her energy will be enclosed in its mass. Now, let's say it broke up into particle 2. The law of conservation of energy states that
particle rest energy 1 = particle rest energy 2 + particle motion energy 2
Since the energy of motion is positive, the rest energy of particle 2 can be less than or equal to the rest energy of particle 1. But the energy of motion of particle 2 is positive, therefore if the rest energy of particle 2 is less than the rest energy of particle 1, then particle 2 should move. But particle 1 rested, so it had no momentum. Particle 2 moves, so it has momentum. But this is impossible - the momentum must be maintained. Therefore, such a decay is impossible, unless the masses of these particles are equal to each other. But in this case, if particle 1 can decay into particle 2, the reverse is also true - particle 2 can decay into particle 1. But this is not decay - it is just a confusion between the two types of particles.
2) The mass of the decaying particle must exceed the sum of the masses obtained during the decay of particles
The total energy and the total momentum during decay are saved, but the total mass is always reduced. A “parent” particle with a mass m1 can decay only into “daughter” particles 2 and 3, if the sum of their masses is less than the mass of the parent: m2 + m3 <m1. This is a simple consequence of the law of nature - the total energy must remain constant in any physical process. Evidence:
Imagine that you are watching particle 1 alone. All her energy is rest energy, m 1 c 2 . Then it breaks up into particles 2 and 3. Each of them has a rest energy and a motion energy. As energy is conserved,
Particle rest energy 1 = Particle rest energy 2 + Particle rest energy 3 + Particle motion energy 2 + Particle motion energy 3
But the energy of motion is always greater than zero, therefore the initial rest energy exceeds the final rest energy, therefore m 1 c 2 > m 2 c 2 + m 3 c 2 , hence m 1 > m 2 + m 3 .
Since the photon, as all experiments show, does not have mass, it cannot decay . Therefore, the waves of light can pass through the whole room, all the space from the Sun to us, and the entire Universe, without disintegrating on the way at all. It is assumed that the graviton has the same properties.
3) The total charge before and after the decay is preserved
Another conserved property is the electric charge. A particle W-, very heavy and negatively charged, with a charge of –e, can decay into an electron with a negative charge –e and an antineutrino without charge. But W- cannot decay into a positron with a positive charge + e and a neutrino without charge, since the total charge would change from –e to + e. Also, W- cannot decay into an electron with a negative charge and a positron (anti-electron) with a positive charge, since their combination would give a zero charge.
Since the electron is the lightest of particles with an electric charge, it cannot decay into anything else . Only neutrinos, photons, gluons and gravitons are lighter, but they are electrically neutral, so any combination of them will have a zero charge. Any unknown particle lighter than an electron must be electrically neutral, or we would easily find it in experiments. Therefore, the electron is stable .
4) The total number of fermions before and after the decay can only change by an even number.
The rule follows from the fact that the angular momentum, as well as energy and momentum, is conserved (which explains the tendency of rotating things, for example, the Earth, to preserve rotation). The rule forbids a neutron to decay into a proton and an electron. Such a decay would fall under laws 1, 2 and 3, but not under 4, since all these particles are fermions. The neutron decays into a proton, an electron and an antineutrino. Then we will initially have one fermion and three in the end, 3 - 1 = 2.
There are three types of neutrino, and it is now believed that all of them have a mass (two of them most likely, and the third probably). The lightest neutrino is the lightest of the known fermions , but the only particles that are lighter than it can break into are bosons (photon and graviton). Therefore, it does not decay : you cannot start with a fermion and end up with bosons. It can in principle be unstable if there are even lighter fermions, which we have not yet met - they would interact with ordinary matter even weaker than the neutrino. And we know that neutrinos live long enough, because we have seen how they travel great distances from remote explosions of supernova stars.
The laws of nature, which are thought to be for slightly less solid reasons, should be almost exactly fulfilled.
5) The difference between the total number of quarks and the total number of antiquarks does not change during decay
A proton contains three quarks, a set of gluons and a pair of quarks-antiquarks; therefore, in a proton the number of quarks minus the number of antiquarks is three. There is also a surplus of three quarks in the neutron. Therefore, a neutron, as a heavier particle, can decay into a proton without breaking rule 5 - and it does so (generating an electron and an antineutrino).
But the proton is the lightest of particles containing more quarks than antiquarks, and this rule follows, together with rule 2, that it is stable . It is clear that a proton cannot decay into any combination of electrons, photons and neutrinos, since they do not contain quarks. There are several hadrons (particles consisting of quarks, antiquarks and gluons), in particular, pions, but they differ from protons and neutrons in that they contain an equal number of quarks and antiquarks. Therefore, a heavy proton cannot decay into any combination of pions and non-hadrons (photons, electrons, neutrinos), since the daughter particles will have an equal number of quarks and antiquarks, and the parent particle will not. But peonies can decay without breaking the rules; for example, an electrically neutral pion (being a boson) can decay into two photons, and a positively charged pion can decay into neutrinos and an antimuon — which is very useful for creating neutrino rays.
Many theorists believe (although this was not confirmed by experiment) that this rule is a little broken , and the proton is very, very little unstable, while possessing an extremely long lifetime. For more than ten years, watching the huge number of protons in a huge tank of water in the Super-Kamiokande experiment , and not having received any decay, we know that a proton lives at least 10,000,000,000,000,000,000,000,000,000,000 years old. I hope I did not miss a single zero. The age of the current phase of the Universe is approximately 13,700,000,000, so there will be quite a lot of protons in the future.
There are other laws, but most of the effects observed by us follow only from those listed.
Conclusion
Now we have rules to explain:
• why photons are stable,
• why are electrons stable,
• why protons are stable, or live for very long,
• why at least one type of neutrino is stable or has a very long life.
What is quite enough to explain the usual matter, chemistry, sunlight, many other processes in life - except for one. What about an unstable neutron?
Neutron is a very amazing thing. Nothing prevents it from disintegrating, and it disintegrates after about 15 minutes into a proton, electron and antineutrino. Why does he live so long? Partly due to the fact that the proton and neutron masses are very close. Although the rest mass of the neutron is approaching GeV, it is only 0.0007 GeV more than the sum of the rest mass of the proton, electron and antineutrino. And the frequency of decays becomes very small, when the total mass of daughter decay particles is obtained very close to the mass of the parent particle. This is not surprising, since rule 2 postulates that the decay must completely stop if the mass of daughter particles exceeds the mass of the parent.
But what is strange is that if you place a neutron in the atomic nucleus, it becomes stable! For example, in helium there are two protons and two neutrons. And although the neutron itself lives a quarter of an hour, the helium nucleus can live as long as the Universe exists, and even longer. This is true in general for all stable elements of the periodic table to them. Mendeleev and their neutrons. This fact is an extremely important consequence of Einstein's theory of relativity and some features of strong nuclear interaction, and without it our chemical world would not have any diversity. This feature deserves a separate article.
And, by the way, if dark matter consists of unknown particles - why are they stable? No one knows for sure, but probably the laws I have described will not be enough for this. Most likely, there is another conservation law, exact or approximate, which is yet to be discovered.
Source: https://habr.com/ru/post/370493/
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