Particles and Antiparticles

In 1928 Dirac developed a quantum theory which included relativity and predicted the existence of a new particle. This new particle was the positron which is the antiparticle of the electron. The positron was predicted to have the same mass as the electron but carry a positive charge.

Positrons are created simultaneously with electrons to conserve charge in a process termed pair production. This process has a threshold energy of 1.022 MeV, because the mass of the positron and the electron is 0.511 MeV.

If a positron and an electron come near each other, they will annihilate each other, producing two photons each with energy equal to the mass of an electron. Thus it is not unusual to observe photons with exactly the energy of 0.511 MeV that result from the annihilation of positron/electron pairs at rest. Since photons carry momentum, the annihilation must produce two photons of equal energy going in opposite directions to conserve momentum.

In 1954 the antiproton was discovered by Segre and Chamberlain. The antiproton has the same mass as a proton but has a negative charge. An antiproton must be produced together with a proton in order to conserve charge.

In today's modern particle accelerators, beams of antiprotons are produced and made to collide with counter-rotating beams of protons in the same accelerator ring. These collisions can be used to study the fundamental building blocks of nature. Virtually any particle can have an antiparticle, but the more complex antiparticles are difficult to create and observe in the laboratory.

A remark on physicist-speak: Above, we give the mass of the electron as 0.511 MeV. What does this mean? Well, this is a shortcut that needs explanation:

According to Einstein, energy and mass are proportional to each other: E = m c²
So, if you multiply the mass of the electron, m = 9.11$\cdot$ 10^-31 kg, with the square of the speed of light, c = 3.00$\cdot$ 10⁸ m/s, then you get: E = (9.11$\cdot$ 10^-31 kg)$\cdot$ (3.00$\cdot$ 10⁸ m/s)²= 8.12$\cdot$ 10^-14 J
(1 J = 1 kg m²/s²)
Now, we can convert the J to eV. Both are energy units, and the conversion is: 1 eV = 1.60$\cdot$ 10^-19 J
Thus we have finally E = (8.12$\cdot$ 10^-14 J) (1 eV/ 1.60$\cdot$ 10^-19 J) = 0.511$\cdot$ 10⁶ eV = 0.511 MeV
(The "M" stands for "Mega" = 1 Million)
So this explains why physicists refer to the electron mass (or "rest mass") as 0.511 MeV or 511 keV.