1.1.2: Fundamental Forces

The fundamental particles may interact through four forces. These four forces interact via force carrier particles, which are fundamentally different to the particles that form matter. The four forces, and their corresponding particles, are:

  • The strong force or nuclear force, mediated by gluons
  • The electromagnetic force, mediated by photons
  • The weak force, mediated by the W-bosons and the Z-boson.
  • Gravity, for which the force carrier particle still eludes detection

For the purposes of radiation oncology, the strong force and the electromagnetic force are the most important. The electromagnetic force is responsible for the behaviour of charged particles and photons. The strong force explains why the nucleus of atoms does not explode despite being packed full of positively charged protons. The weak force is involved in radioactive decay.

The electromagnetic force

The electromagnetic force binds particles of opposite charge together, and repels objects of like charge apart. The force carrier particle is the photon, a massless particle that travels at the speed of light.
In the nucleus, the electromagnetic force is responsible for binding the negatively charged orbital electrons to the positively charged nucleus.
The residual electromagnetic force occurs through interactions of electrons/protons of one atom with the electrons/protons of another. These interactions form the basis of molecules, where two atoms are tightly bound together through the residual electromagnetic force.

The strong force

The electromagnetic force repulses objects of same charge. This would cause difficulties for the nucleus, which consists of a number of positively charged protons that should repel each other.
The strong force is responsible for the attraction of quarks to one another, through the force carrier particle known as a gluon. The strong force is responsible for the binding of quarks within a hadron. The gluons that are exchanged between the quarks in a hadron cancel each other out in most cases.
The residual strong force, in a similar way to the residual electromagnetic force, is the attraction, mediated through gluons, of quarks within one hadron to the quarks of another hadron. This attraction is responsible for binding the protons and neutrons within the nucleus to each other.
Gluons have a very small range - not much outside of the nucleus. Therefore the strong force only exists over a small distance. Beyond this distance, the electromagnetic force is dominant.

This an important concept when discussing neutrons and protons. Both are hadrons; however protons carry an electromagnetic charge whereas neutrons do not. If a proton approaches a nucleus, it is typically repelled by the electromagnetic force radiating out from it. This prevents it from reaching the region that would cause it to be affected by the residual strong force. Only protons with very high kinetic energy can penetrate the electromagnetic force.
Neutrons, with no electromagnetic charge, may easily approach the nucleus and be attracted by the residual strong force. This allows them to more easily cause atomic transformations.

The weak force

The weak force is responsible for the decay of particles into less massive states. It is the only force that can change a quark or lepton into another type. The name comes from the relative strength of the interaction - about 10-11 less powerful than the electromagnetic force.
The force carrier particles are unusual, as they have a large amount of energy and can also carry electromagnetic charge (unlike photons). The three force carrier particles are the W+, W- and Z boson.

The best example of the weak force occurs during β decay, where a proton turns into a neutron or vice versa. This involves one of the quarks making up the hadron changing from an up to a down quark (for proton->neutron) or from down to up (neutron -> proton). When this occurs, a W boson carrying the charge is released. The W boson rapidly decays, creating two leptons, one of which retains the charge (an electron or positron) and the other with no charge (an electron neutrino / antineutrino).


Gravity is the force which causes objects with mass to attract one another. The force carrier particle for gravity remains elusive, and is known as a graviton. Gravity is the weakest of the forces but also has the longest range.