forces

The four fundamental interactions

a boson, how does it work?

About the importance of movement

We have seen the world of material particles.
This world is only one of the two facets of the Universe around us. In effect, all of the physical, chemical and biological phenomena such as the fall of an apple from a tree, the light emitted from an electric lamp, a good soufflé which inflates in the oven or even the beating of your heart; all of these phenomena could not exist if it wasn't for the fact that movement is possible. This appears obvious but all the same, of all things, how is movement possible? There is always at the origin of movement, an impulse, a force which is the cause... Now physicists, in their search for a unified vision of the world, have succeeded in reducing to exactly four (or maybe even less, we will see at the end of this chapter) the number of forces loose in the vast Universe.

These forces are:

Gravitation which makes apples fall on certain peoples heads. It is also this which pulls together the Earth and the Moon.

electromagnetic interaction which assures the cohesion of our bodies and governs all chemistry. It is this which pulls together the electron and the atomic nucleus.

the strong interaction which unites quarks together and thus the nuclei of atoms.

weak interaction which is responsible for beta radioactivity, thanks to which, matter defeated antimatter.

These forces are called interactions. Why?

Before quantum theory, forces were transmitted by virtue of a mysterious force field emitted by particles.
According to quantum theory, forces are not exherted between two fermions unless there is an exchange of a mediator particle, called a boson. Now the heavier the boson, the shorter will be the range of the interaction. This theory bears the indigestible name of quantum electrodynamics.

How to represent this curious concept?

Replace two fermions with two small boats, each with a quiet fisherman aboard. Imagine then that the two fishermen drift on a lake and come inexorably closer to each other... Now these absent minded fishermen have neither oars nor a motor to deviate their trajectories. What to do? By chance, one of the fishermen has in his boat a boson, excuse me a big bowling ball (and why not? he is very absent minded).

As the two boats approach, the amateur bowling fisherman has a flash of genius and forcefully throws the heavy ball towards his colleague.
What will happen?
He and his boat will be deviated by the reaction, by as much as the fisherman has pushed against the ball during the throw. The ball departs in one direction and the fisherman-boat ensemble departs in the opposite direction (of course more gently because the boat and its occupant are much heavier to move).

The scenario is the inverse for fisherman N°2 who receives the ball: The kinetic energy of the ball will, at the moment when fisherman 2 catches it, divert boat 2 under the force of the impact.
Final result: The two boats will be deviated and distance themselves from each other...

Sure, this analogy has its limits because it only works for repulsive forces, unless we replace the bowling ball by a heavy boomerang which could emulate attractive forces...
Anyway, it's clear that the heavier the ball, the greater the force of repulsion, but also that the force is greater the shorter the distance between the boats, because to throw a bowling ball a long way requires lots of energy!

Next you see each of these interactions in order of notoriety: