← Particle Physics

# The Four Basic Forces

Friday, April 8, 2022

## The Gravitational Interaction

Gravity is the weakest of the fundamental forces, having essentially no substantial effect on subatomic particles. For context, the strong interaction is $10^{38}$ times stronger than gravity at the scale of protons. Instead, gravity is important due to its influence on the macroscopic world. Gravity is cumulative and infinite in range, so it has a noticeable effect on large-scale objects like planets and stars.

## The Weak Interaction

The weak nuclear force is responsible for nuclear beta decay, but does not play a substantial role in the bonding of nuclei (the strong force is about $10^7$ times stronger). Additionally, the weak force only acts on the scale of about $0.001~\text{fm}$.

## The Electromagnetic Interaction

Electromagnetism is important in the structure and interactions of fundamental particles. While the electromagnetic interaction has infinite range, it can be shielded, which diminishes its effects for ordinary objects. Many common macroscopic forces (like friction, air resistance, drag, and tension) are caused by the electromagnetic interaction at the atomic level. Although it is only 1% as strong as the strong force, it can affect the bonding of nuclei due to its cumulative nature.

## The Strong Force

The strong nuclear force is responsible for the bonding of nuclei, and is one of the dominant forces in the reactions and decays of fundamental particles. However, it has a relatively short range of about $1~\text{fm}$.

## Comparisons Between the Forces

The strength of a force can determine the timescale at which it acts. For instance, since the strong force acts so strongly, decays involving it usually last the shortest, down to $10^{-23}~\text{s}$.

Particles interact with each other through any of the basic forces, depending on what type of particles are involved. All particles interact with gravity and the weak interaction, but only some use electromagnetic interaction, and even fewer use the strong interaction.

Type Range Strength Timescale Particles
Strong $1~\text{fm}$ $1$ $\lt~10^{-22}~\text{s}$ $\pi,\text{K},\text{n},\text{p}$
Electromagnetic $\infty$ $10^{-2}$ $10^{-14}-10^{-20}~\text{s}$ $\text{e},\mu,\pi,\text{K},\text{n},\text{p}$
Weak $10^{-3}~\text{fm}$ $10^{-7}$ $10^{-8}-10^{-13}~\text{s}$ $\text{All}$
Gravitational $\infty$ $10^{-38}$ $\text{Years}$ $\text{All}$

## Field Particles

Each of the four fundamental forces can be represented by the emission or absorption of particles that "carry" the interaction. With each force is a force-carrying particle called its field particle.

Force Field Particle Symbol Charge ($e$) Spin ($\hbar$) Rest Energy ($\text{GeV}$)
Strong Gluon $\text{g}$ $0$ $1$ $0$
Electromagnetic Photon $\gamma$ $0$ $1$ $0$
Weak Weak boson $\text{W}^+,\text{W}^-$ $\pm~1$ $1$ $80.4$
$\text{Z}^0$ $0$ $1$ $91.2$
Gravitational Graviton $0$ $2$ $0$