Elementary particles have the lifetime and the mass. The electron is believed to have an infinite lifetime. The mass of the electron is 0.5 MeV. On the other hand, proton is believed to have a finite lifetime. However, the length of the lifetime is not yet known. The mass of the proton is approximately 940 MeV. If we suppose the mass of the electron is 0.5g, the mass of the proton becomes about 940g.
In the outer space, protons are flying at high speed. They are known as cosmic rays. The protons collide with atoms in the atmosphere and various elementary particles are generated. Some of them are the tau particle and the mu particle.
The property of the tau particle is similar to the electron. The mass of the tau particle is about 1800 MeV. If we suppose the mass of the electron is 0.5g, the mass of the tau particle becomes about 1,800g. The average lifetime of the tau particle is 300 femtoseconds. 1 femtosecond is 1000 trillionth of a second. The tau particle decays into neutrinos and electrons.
The property of the mu particle is similar to the electron. The mass of the mu particle is about 100 MeV. If we suppose the mass of the electron is 0.5g, the mass of the mu particle becomes about 100g. The average lifetime of the mu particle is two microseconds. 1 microsecond is one millionth of a second. If we suppose the average lifetime of the tau particle is 1000 years, the average lifetime of the mu particle becomes 700 million years. The mu particle decays into neutrinos and electrons.
The photon goes 600m in two microseconds. The mu particle should not go 600m because the lifetime of the mu particle is two microseconds. However, it actually goes more than 6,000m and arrives at the surface of the earth. This is because an effect of the special theory of relativity extended the lifetime of the mu particle.
| Elementary particle | Mass | Lifetime |
|---|---|---|
| The electron | 0.5 MeV (0.5g) | Infinity |
| The proton | 940 MeV (940g) | Unknown |
| The tau particle | 1800 MeV (1800g) | 300 femtoseconds (1000 years) |
| The mu particle | 100 MeV (100g) | 2 microseconds (700 million years) |
It is very mysterious that the elementary particles have the lifetime. The life of the candle is determined by the length of the portion of the wax. However, The lifetime of each elementary particle is not decided from the beginning. The 1000-year-old elementary particle is identical to a 700 million-year-old elementary particle. In other words, they do not remember the age of their owner.
Each elementary particle collapses with a fixed probability per unit time. For example, the elementary particles throw the dice every second and act as follows.
The expression to throw the dice is a metaphor. However, how does the elementary particle throw the dice? How long is the time interval to throw the dice? I think that the time interval is very short. The time interval might be the smallest unit of the time.
In this way, there are many mysterious things about the decay of the elementary particle.
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