# The Four Interactions

There are four forces in the universe, some you will have come across already and some will be new:

The electromagnetic interaction causes an attractive or repulsive force between charges.

The gravitational interaction causes an attractive force between masses.

The strong nuclear interaction causes an attractive (or repulsive) force between quarks (and so hadrons).

The weak nuclear interaction does not cause a physical force, it makes particles decay. ‘Weak’ means there is a low probability that it will happen.

# Exchange Particles

In 1935 Japanese physicist Hideki Yukawa put forward the idea that the interactions/forces between two particles were caused by ‘virtual particles’ being exchanged between the two particles.

He was working on the strong nuclear force which keeps protons and neutrons together and theorised that they were exchanging a particle back and forth that ‘carried’ the force and kept them together. This is true of all the fundamental interactions.

The general term for exchange particles is bosons and they are fundamental particles like quarks and leptons.

# Ice Skating Analogy

Imagine two people on ice skates that will represent the two bodies experiencing a force.

If A throws a bowling ball to B, A slides back when they release it and B moves back when they catch it. Repeatedly throwing the ball back and forth moves A and B away from each other, the force causes repulsion.

The analogy falls a little short when thinking of attraction, but bear with it.

Now imagine that A and B are exchanging a boomerang (bear with it), throwing it behind them pushes A towards B, B catches it from behind and moves towards A. The force causes attraction.

# Which Particle for What Force

Each of the interactions/forces has its own exchange particles.

In 1947 the exchange particle of the strong nuclear interaction were observed in a cloud chamber.

# Lending Money Analogy

Think of making exchange particles in terms of lending somebody some money.

If you lend somebody £50 you would want it paid back fairly soon.

If you lend somebody 50p you would let them have it for longer before paying you back.