Antimatter is matter that is made of antiparticles. These are particles with the same characteristics as matter but with the opposite charge. Every component of matter at an atomic level has an antiparticle, except for photons. If the symbol of a proton is p, its antiparticle symbol will be \(\bar p\), and this is true for neutrons and neutrinos but not for electrons whose symbol is e+.
The discovery of antimatter
Paul Dirac first theorised antiparticles, but the first to discover an antiparticle was Carl Anderson when he found the antiparticles of an electron, also named positrons.
What is an antiparticle?
Antiparticles are the blocks that make antimatter. There is an antiparticle for every subatomic particle in the nucleus and orbit of an atom. Protons, neutrons, neutrinos, and electrons all have antiparticles. The characteristics of antiparticles are similar to particles; they just differ in their charge. Antiparticles can be created by radioactive decay processes in the atom. They can also interact with matter, leading to an annihilation process.
See the following table of particles and antiparticles:
Particle | Proton | Neutron | Electron | Neutrino |
symbol | p | n | e- | v |
Antiparticle | Antiproton | Antineutron | positron | Antineutrino |
symbol | \(\bar p\) | \(\bar{n}\) | e+ | \(\bar{v}\) |
Characteristics of antiparticles
Antiparticles and particles have the same mass and energy at rest; the only difference is their charge. The charge of a positron is \(1.6022 \cdot 10 ^ {-19}C\), which is the opposite of an electron charge, which has a value of \(-1.6022 \cdot 10 ^ {-19}C\). The same occurs with protons, which have a positive charge for normal matter and a negative charge for the antiproton.
Particle and antiparticle annihilation
When matter and antimatter interact, they destroy each other. This destruction has three main characteristics:
- The particles’ masses are destroyed.
- The destruction converts all the mass of the particles into energy.
- The energy is released in the form of high energy photons.
Figure 2. When matter and antimatter collide, this produces an annihilation event that results in photons being released.
Antimatter and matter pair creation
In some cases, a photon can interact with a particle, which creates a pair of particles. The event is accordingly named pair creation, and the pair consists of a particle and its antiparticle.
Energy must be conserved during pair creation. A photon has a certain amount of energy (X), and the energy of the two particles created (Y and Z) must be equal to the total energy of the photon that created them. The conservation of energy is expressed more simply below:
\(\text{Photon energy = Energy particle created + Energy antiparticle created}\)
Calculating the energy released by matter and antimatter
To calculate the energy of the impact between matter and antimatter, we need to obtain the total energy of the photon released during the collision. This energy is a relationship between the light velocity ‘c’, the Planck constant ‘h’, and the photon’s wavelength ‘λ’. The formula to calculate this is as follows:
\(\text{Energy} = \frac{c \cdot h}{\lambda}\)
Here is a simple example:
Calculate the energy released by a collision between a particle and an antiparticle.
The photon released by the collision has a wavelength of 0.005 nanometres. To calculate the released energy, you need to convert the number of nanometres to metres. One nanometre is equal to \(1 \cdot 10 ^ {-9}\) metres, so you need to multiply 0.005 by \(1 \cdot 10 ^ {-9}\) metres:
\(\text{Wavelenght} = (1 \cdot 10^{-9}) \cdot (5 \cdot 10^{-3}) = 5 \cdot 10^{-12} [m]\)
Now you need to multiply the approximate velocity of light in the vacuum by the Planck constant ‘h’, which has a value of \(6.63 \cdot 10 ^ {-34}J / s\):
\(c \cdot h = (3 \cdot 10^8 [m/s]) \cdot (6.63 \cdot 10^{-34}[Js]) = 1.989 \cdot 10^{-25} [Jm]\)
Then you need to divide this by the wavelength of the released photon. The results are expressed in the energy units joules (J):
\(\text{Energy} = \frac{c \cdot h}{\lambda} = \frac{1.989 \cdot 10^{-25}}{5 \cdot 10^{-12}} = 3.978 \cdot 10^{-14} [J]\)
Antiparticles - key takeaways
- Antiparticles are particles that have the opposite charge to normal particles.
- Antimatter is matter made of antiparticles.
- The mass of particles and antiparticles is the same, but their electrical charge is the opposite.
- When matter and antimatter collide, they destroy each other and create energy in the form of photons.
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