Antimatter
Antimatter is composed of antiparticles, each of which has the same mass as its corresponding matter particle but equal and opposite quantum numbers, such as electric charge. When a particle meets its antiparticle they annihilate each other, converting their combined rest mass entirely into energy in the form of high-energy photons (gamma rays), governed by E = mc². Predicted theoretically by Paul Dirac in 1928 and first observed by Carl Anderson in 1932 (the positron), antimatter is produced in particle accelerators, certain radioactive decays (β⁺), and natural cosmic ray interactions. It has applications in PET medical imaging and is the subject of research into the matter-antimatter asymmetry of the universe.
Key Formula
E = 2 × m_e × c² ≈ 1.022 MeV (for electron-positron pair annihilation)
LaTeX: E = 2 m_e c^2 \approx 1.022 \text{ MeV} \quad \text{(electron-positron annihilation)}
| Symbol | Meaning | Unit |
|---|---|---|
| E | Total energy released as two gamma photons | MeV |
| m_e | Electron rest mass (9.109 × 10⁻³¹ kg = 0.511 MeV/c²) | kg |
| c | Speed of light (3 × 10⁸ m/s) | m/s |
Worked Example
Problem
A positron and an electron annihilate at rest. What is the energy and frequency of each gamma photon produced?
Solution
Step 1: Total energy released = 2 × m_e × c² = 2 × 0.511 MeV = 1.022 MeV. Step 2: By conservation of momentum, two equal photons are emitted in opposite directions, each with E_γ = 0.511 MeV. Step 3: Convert to joules: 0.511 × 10⁶ × 1.602 × 10⁻¹⁹ = 8.186 × 10⁻¹⁴ J. Step 4: Frequency: f = E/h = 8.186 × 10⁻¹⁴ / 6.626 × 10⁻³⁴ = 1.236 × 10²⁰ Hz.
Answer
Each gamma photon has energy ≈ 0.511 MeV, frequency ≈ 1.24 × 10²⁰ Hz.
Particle-Antiparticle Pairs and Their Properties
| Particle | Antiparticle | Charge (particle) | Charge (antiparticle) | Annihilation Product |
|---|---|---|---|---|
| Electron (e⁻) | Positron (e⁺) | −1e | +1e | Two 0.511 MeV γ photons |
| Proton (p) | Antiproton (p̄) | +1e | −1e | Pions and gamma rays |
| Neutron (n) | Antineutron (n̄) | 0 | 0 | Pions and gamma rays |
| Neutrino (νₑ) | Antineutrino (ν̄ₑ) | 0 | 0 | No annihilation (lepton number) |
| Up quark (u) | Anti-up quark (ū) | +2/3 e | −2/3 e | Gluons and mesons |
Interactive Tools
Khan Academy: Antimatter
Introduction to antimatter, annihilation, and particle-antiparticle pairs
Open ToolWolfram Alpha Annihilation Energy
Calculate annihilation energies for particle-antiparticle pairs
Open ToolBrilliant.org: Particle Physics
Deep dives into antimatter, the Standard Model, and particle interactions
Open Tool
Wikimedia Commons, CC BY-SA
Related Terms
Quark
Quarks are elementary subatomic particles that are the fundamental constituents of hadrons such as protons and neutrons, and are held together by the strong nuclear force via exchange of gluons in a theory called Quantum Chromodynamics (QCD). There are six types (flavours) of quarks — up, down, charm, strange, top, and bottom — each carrying a fractional electric charge of +2/3e or −1/3e and a colour charge (red, green, or blue). Quarks are permanently confined within hadrons and are never observed in isolation; a proton consists of two up quarks and one down quark (uud), while a neutron consists of one up quark and two down quarks (udd).
Standard Model of Particle Physics
The Standard Model of Particle Physics is the theoretical framework that describes all known fundamental particles and three of the four fundamental forces — the electromagnetic force, the weak nuclear force, and the strong nuclear force (excluding gravity). It classifies all elementary particles into fermions (quarks and leptons, which make up matter) and bosons (force-carrying particles), and predicts interactions with remarkable precision, including the existence of the Higgs boson — experimentally confirmed at CERN's Large Hadron Collider in 2012. Despite its extraordinary success, the Standard Model does not incorporate gravity, dark matter, or dark energy, and remains an incomplete description of the universe.
Gamma Radiation
Gamma radiation consists of high-energy electromagnetic photons emitted by an excited atomic nucleus as it transitions from a higher energy state to a lower one, typically following alpha or beta decay. Unlike alpha and beta radiation, gamma rays carry no charge and no mass, so gamma emission does not change the atomic number or mass number of the nucleus. Due to their extremely high energy (typically 10 keV to 10 MeV) and penetrating power, gamma rays are used in cancer radiotherapy, sterilisation of medical equipment, and industrial non-destructive testing.
The prefix "anti-" comes from Greek "ἀντί" (antí), meaning "against" or "opposite." The concept was first formulated by Paul Dirac in 1928 when his relativistic quantum equation predicted the existence of a positive electron, confirmed experimentally by Carl Anderson in 1932, who named it the "positron."