2.1 Quantum and Nuclear Physics


Quantum and nuclear physics describe objects and actions that elude our imagination.


The material world is made up of two forms of matter:

  1. Substance – composed of particles with rest mass, atoms, molecules, and finally structures with a more complex organisation
  2. Fields

The particles of a substance are hold together by four types of fundamental interaction

  • Weak Forces – Are responsible for the radioactive decay of subatomic particles, and it plays an essential role in nuclear fission. The weak interactions are caused by the emission or absorption of W and Z bosons. An example of weak interactions is the Beta Decay.
  • Strong Forces – Also known as nuclear strong force, maintain the compactness of the atomic nucleus by overcoming the repulsive electrostatic forces between protons.
  • Electromagnetic interactions – A type of forces that occur between electrically charged particles. Stationary charges are described by Coulomb’s Law, while moving charges are described by the laws of electromagnetism.
  • Gravitational Forces – Are interactions between all things with a mass. The gravitate towards one another, i.e. they are brought together. For most applications, gravity is well approximated by Newton’s law of universal gravitation, however gravity is most accurately described by the general theory of relativity.

Bosons are elementary particles, characterised as quanta of energy,  that also behave as waves. Their spin number may be either 0 or 1. Particles that have a non-integer spin number are called fermions. Bosons come into existence due to excitations of the fields of basic interactions.

  • Pions are excitations of a strong interaction field
  • Photons are excitations of electromagnetic fields
  • Gravitons are excitations of gravitational fields

Elementary particles (known as quarks) cannot be decomposed into simpler objects, but can, however, undergo mutual conversions. They have a certain structure and therefore should not be considered mass points (Mass originates from one point only).

Elementary particles can classified into groups according to:

  1. rest mass (m0),
  2. spin (s) given as a multiple of angular momentum,
  3. electric charge (q), and
  4. magnetic spin (μS)

The most frequent classification of elementary particles goes as follows:

  • Photons (hypothetical gravitons) – rest mass is zero, and spin number is one(1).
  • Leptons (for instance electrons) – rest mass is small, almost zero, and spin number equals ½
  • Mesons (for instance pions) – rest mass is slightly smaller that in protons, spin number equals 0
  • Baryons (nucleons – protons and neutrons) – relatively high mass, spin number equals ½

Hadrons are combinations of two or more quarks. Mesons and Baryons are sometimes collectively referred to as hadrons. There are many types of quarks (u, c, t, d, s, b) and it pays off to know that the protons is a combination of uud, while the neutron is a combination of ddu. The quark d has a slightly higher mass than quark u and therefore neutrons are heavier than protons.

Strong forces between quarks are facilitated by gluons.

All elementary particles also have anti-particles. Antiparticles (or antimatter) have the opposite charge, spin number, and possibly other quantum parameters.

The interaction between particles and their antiparticles leads to their collective conversion into photons or other particles.

  • The electron and its antiparticle produce two photons of Gamma Radiation
  • The proton and its antiparticle produce Pi Mesons.

Among the elementary particles, the more stable ones are photons, electrons, neutrons, and from a practical viewpoint the photon. The average lifetime of a neutron is about 650 seconds.