The four fundamental interactions

In a nutshell

The four fundamental forces of the Universe act on particles through exchange particles. Feynman diagrams are a pictorial way to represent particle interactions.

The four fundamental forces

The four fundamental forces govern how particles interact and decay. In the standard model, the forces act on particles through exchange particles, which are unique for each one of them. The forces and their exchange particles are shown below:

force	What does it do?	Exchange particle
Strong nuclear	Acts on quarks.	Gluon
Electromagnetic	Acts on charged particles.	Virtual photon
Weak nuclear	Is responsible for radioactive decay.	$W^+$ , $W^-$ , $Z^0$ bosons
Gravitational	Acts on all particles with mass.	Graviton

Note: the standard model describes all forces except gravity. The graviton is the exchange particle predicted by the model but it has not been observed yet.

The exchange particles mediate the interactions between particles. A popular model illustrates how they work in the case of a repulsion between two particles with the same charge.

Imagine two skaters standing in front of each other. One skater passes a ball to the other and the exchange of momentum will make both of them move in opposite directions. This can be represented as two particles with the same charge exchanging a virtual photon, the exchange particle for the electromagnetic force, which causes a repulsion.

Physics; Particle physics; KS5 Year 12; The four fundamental interactions

1.	A skater passes a ball to another
2.	The exchange of momentum makes them both move in opposite directions
3.	Two particles exchange an exchange particle which makes them move in opposite directions
4.	The exchange particle is a virtual photon

Strong nuclear force

The strong nuclear force is the force that holds the nucleus of an atom together. It acts between quarks and its exchange particle is the gluon, often denoted by $g$ . It is the strongest of the fundamental forces and has a very short range.

Electromagnetic force

The electromagnetic force acts on all static or moving charged particles. Its exchange particle is a virtual photon, virtual because they exist only during the interaction. It has an infinite range and its strength follows an inverse square law, that is, it is inversely proportional to the square of the distance.

Weak nuclear force

The weak nuclear force is responsible for beta plus and minus decay as well as interactions such as electron capture and proton-electron collisions. It has three exchange particles, known as $W^+$ , $W^-$ and $Z^0$ bosons. It is weaker than the strong nuclear force and has an even shorter range.

Gravitational force

The gravitational force acts on all particles with mass and it is the weakest of the forces. The standard model predicts its exchange particle to be the gluon, but it has not been observed yet.

Feynman diagrams

To represent particle interactions in a simple and pictorial way, physicist Richard Feynman devised the Feynman diagrams. These are generally read from left to right and follow the following rules:

Straight lines with arrows represent particles while wavy lines represent exchange particles.
Particles are created and annihilated at the vertices.
Time generally moves from left to right.

Below are some examples of Feynman diagrams:

Electron capture

Physics; Particle physics; KS5 Year 12; The four fundamental interactions

$^1_1p+^{\enspace 0}_{-1}e\rightarrow^1_0n+\nu_e$

A proton in a nucleus absorbs an electron and decays into a neutron and a $W^+$ boson which interacts with the electron and forms an electron neutrino.

Electron-proton collision

Physics; Particle physics; KS5 Year 12; The four fundamental interactions

$^1_1p+^{\enspace 0}_{-1}e\rightarrow^1_0n+\nu_e$

A proton and an electron collide exchanging a $W^-$ particle and decaying into a neutron and an electron neutrino.

$\beta^+$ decay

Physics; Particle physics; KS5 Year 12; The four fundamental interactions

$^1_1p\rightarrow^1_0n+^{\enspace 0}_{+1}e+\nu_e$

A proton decays into a neutron and a $W^+$ boson which decays into a positron and an electron neutrino.