# Four Fundamental Forces

What holds the universe and everything in it together ## What are the four fundamental forces?

There are four fundamental forces of physics, each of which varies in terms of its strength and the range it works on.

The strong nuclear force is the strongest force but only works on the quantum (very small, subatomic) level. Gravity is the weakest force but has the largest range – the entire universe.

The four forces are:

1. Gravity (weakest force with an infinite range)
2. Weak nuclear force (next weakest force with short range)
3. Electromagnetic force (stronger force with infinite range)
4. Strong nuclear force (strongest force with short range)

Gravity
If two objects have mass, they are pulled together by the force of gravity. Gravity has an infinite (unending) range and is always attractive.

Newton’s universal law of gravity is

F = GM1M2/r2

Where F is the force due to gravity between the two masses M1 and M2. These masses are a distance r apart. G is the gravitational constant.

Because the r2 is the denominator (the bit below the line) in the equation and the r is squared, this is called an inverse square law.

Here, it means that if you double the distance between the two objects with mass M1 and M2 then the force of gravity is four times smaller.

Weak nuclear force
This force is responsible for the interactions that change particles.

For example, the weak force causes a type of radioactive decay called beta decay. In beta decay, there are either too many protons or neutrons in the nucleus.

So, a neutron may change to a proton, an electron and an antineutrino.

Or, a proton may change to a neutron, a positron and a neutrino.

In both cases, the electron and positron are called beta particles.

It is the only force to be felt by all fermions (particles that make up matter), including neutrinos.

Neutrinos do not feel the electromagnetic force as they’re electrically neutral and do not feel the strong force as they’re not quarks.

Electromagnetic force
The electromagnetic force is a combination of the electric and magnetic forces. The electromagnetic force is attractive and repulsive. It causes like electrical charges and magnetic poles to repel (push away). It causes opposite electrical charges and magnetic poles to attract (pull together).

Experiments by Michael Faraday in the early 19th century and some mathematical wizardry from James Clark Maxwell showed electricity and magnetism arise from the same force of electromagnetism.

The electromagnetic force keeps negatively charged electrons orbiting around the positively charged nucleus.

These electrons interact with other electrons in other atoms, forming electron bonds to produce molecules and, eventually, visible matter.

Strong nuclear force
The strong nuclear force holds the nuclei of atoms together. It is mainly attractive but can be repulsive in some circumstances.

It keeps the nucleus together by overcoming the electromagnetic repulsion felt between positively charged protons. It also glues quarks together to form particles, including protons and neutrons.

## So what?

The four fundamental forces of physics are responsible for keeping everything in us and around us working in the right way.

They maintain the Earth’s orbit around the Sun (gravity), govern all chemical processes (electromagnetic), power the Sun by helping nuclear fusion (weak) and hold nuclei together (strong).

## What else?

There are a lot of attempts to unify all four forces into one Grand Unified Theory (GUT).

There has been some progress towards a GUT. At extremely high temperatures and densities (the sort of conditions that existed in the early universe), the electromagnetic and weak nuclear forces are indistinguishable. These are referred to as the electroweak interaction.

It’s more difficult when we try to unify the other forces, particularly gravity because it affects the space and time in which the other forces exist.

## Want more?

This is a great synopsis of the four fundamental forces from Seeker:

#### The End of Discovery

Each chapter in this book by Russell Stannard looks at specific questions in science, including GUT.

#### The Forces by Fermilab

A more in-depth introduction to the four fundamental forces from the leading US accelerator lab.

#### All four one, one for all

An excellent article from Symmetry magazine investigating whether a GUT will ever be possible.

### Special relativity

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