We take for granted that the world doesn’t fall apart, but what is it that holds the most basic of particles together?
If we looked at any everyday object on the earth, from a glass of water, to the air, or even the computer screen you are reading this on right now we would be looking at atoms and molecules. These are held together by the electromagnetic force, but if we looked in more detail at the atoms we would realise that they are also made of smaller pieces. An atom consists of a nucleus and orbiting electrons. The nucleus consists of even smaller pieces, called protons and neutrons. However, protons all have a charge of +1, and we know that the electromagnetic force dictates that like charges repel, so how can they stay together so compactly? Well what we are seeing here is a demonstration of the strong force in action. Later in the post we will have to get into some messy particle physics, so I recommend reading our other post on the particles of the standard model.

We can see the effects of the strong force with protons, but to see where the strong force is at its most influential we have to look to the components of these protons. Protons (and neutrons) are made of quarks. Quarks are fundamental particles, they are not made of anything and cannot be broken down any further:

There are different types of quarks, called flavours. One of the things that make quarks fundamentally different to other significant particles, for example the electron, is that they are the only particles we know of  that interact through the strong force, excluding the carrier of the force, the gluon. This property leads to a somewhat strange property of quarks, they can never exist alone. They can only form in groups called hadrons (hence that large hadron collider, LHC, at CERN). These hadrons come in two varieties, baryons and mesons. Baryons consist of three quarks, or three antiquarks, whilst a meson consists of one quark and one antiquark:

So what governs which quarks attract which? Well quarks hold a sort of charge. It is intrinsically different to the charge felt by particles that interact through electromagnetism. In electromagnetism there exists only positive and negative charge, though in different values. In the strong force there are three different types of positive and negative charge and these are expressed with colours. Quarks can have either Red, Green or Blue charge and just as anti particles have the opposite electromagnetic charge to their matter counter-particles, as do the antiquarks. These charges are called Antired, Antigreen and Antiblue. I have to stress that there are no actual colours in these charges, but they are used to clasify the different types. Any normal coloured charged quark will attract any anti coloured charged quark, as opposite charges attract. For example, a red charged up quark could attract a antired, antiblue or antigreen down antiquark to form a π+ meson. Not only do the normal and anti colour charges attract, so do the different colour charges. A red quark will repel another red quark, but will attract a blue and a green. All baryons have one quark of each charge to make a RGB combination. The same happens with antiquarks, which come together in Antired-Antigreen-Antiblue triplets. All these are shown in the diagram above, with the meson having a green quark and an antigreen antiquark. Once three quarks have come together as a baryon, another of any colour charge will be attracted by two, but repelled by one. This means that the quark cannot stick to the baryon, however if that quark is part of a baryon itself, each quark in each baryon would be attracted to two quark in the other. This leads to baryons, for example protons and neutrons, being able to hold together. This is what we see in the atomic nucleus. This force is called residual strong force, or the nuclear force.

As with all the other forces, with the unusual exception of gravity, there is a particle carrier, or boson, that moves the effects of the force over distance. The electromagnetic force has the photon, and the weak force has the W and Z bosons. The strong force has a particle called the gluon. The gluon has the ability to change the colour charge of a quark. For example, if a quark has a charge R then if it emits a gluon with a sort of ‘positive R, negative B’ charge, then the quark will become charge B. The theory of the strong force, called quantum chromodynamics (or QCD), allows for only eight different colours of gluons. Due to the fact that gluons have colour charge, they attract each other. This property is not seen in any other bosons so instead of filling a space like a photon, the gluons concentrate in a line between the two quarks they are acting upon. In theory, this attraction could lead to gluons clustering into ‘glueballs’.

This mutual attraction leads to other unusual properties and one that only the strong force has. Unlike with gravity or the electromagnetic force which weaken with distance (the hardest part of pulling two magnets from one another is the initial pull), the strong force gets stronger, like an unbreakable elastic band wrapped around the quarks. If two attracting quarks are pulled to a distance of around 10-15 meters from each other, the force required to pull them any further would be infinite. The strong force is called the strong force because, in comparison to the electromagnetic force, it is stronger, and it is also the strongest of all the forces; however, if we created energy levels similar to those soon after the big bang, we find that the weak, electromagnetic and strong force get similar strength and this point is known as the grand unification of the forces. It is a key component in forming a theory of everything.

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Ned Summers.

Check out our last two posts:

What is φ, the ‘Golden Ratio’ – This ratio appears in everything from art to nature, but what is it? And why is it so special?
What is thelifetime of a star? – Many of you will have learned about how a stars life unfolds, but what are the basic processes behind it all?
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