By Ryan Ridden 16/01/2017

Colours are everywhere. The colours we see come from light with different wavelengths, but light isn’t the only thing physicists say have colour. In the quantum world the subatomic particles known as quarks have colour. As with everything quantum mechanical the colour of quarks isn’t what it may seem.

Quarks can come in a variety of six colours, from red, green and blue to antired, antigreen and antiblue. Straight away something seems fishy, why are there only three colours and what is an anticolour? To answer these questions we will need to explore some ideas of quantum mechanics, and try to understand the mad mind of a physicist at the dawn of particle physics.

The Colourful Nature of Quantum Mechanics

The colour of quarks has nothing at all to do with colours of light we are familiar with. You see physicists went a bit mad in the 1960s and started naming the newly discovered world of particles and particle properties quite bizarre things: for example we have particles called quarks of course, but it doesn’t stop there, we also have gluons, leptons and bosons, (what are these words even!) with equally peculiar properties such as strangeness, beauty and of course colour.

All these weird names fit well with the counterintuitive weirdness of quantum mechanics. An advantage of being bizarre makes particle physics so fun to learn and talk about. But to understand what colour means for a quark we will need to dive into the weird and wonderful terms of quantum mechanics, but don’t worry we will stay at the shallow end of the quantum pool!

Quarks are known as fundamental building blocks. Combinations of different types of quarks produce larger particles like protons, neutrons and a plethora of other particles. The zoo of particles that are made from quarks are known as hadrons.

The colossal CMS instrument of the LHC under construction. The CMS is used for tracking particle pathways.
The colossal CMS instrument of the LHC under construction. The CMS is used for tracking particle pathways.

To investigate the properties of quarks, physicists need to be destructive. Quarks always stick together, you will never find one by itself, so if we want to understand how they work we need to smash hadrons together and analyse the shrapnel. Physicists smash particles together in gigantic particle colliders, like the well-known Large Hadron Collider.

Particle colliders do exactly what you might expect; they accelerate protons to incredible speeds, close to the speed of light, and smash them together. When protons collide at high energy the protons break apart releasing the quarks along with enormous amounts of energy. A cascade of particles is produced in the aftermath of the collision as quarks interact with one another and find new groups of quarks to join, or even make a buddy quark from the energy of the collision.

Detecting and analysing the particle cascades push science and technology to their limits. Instruments must perform measurements with incredible accuracy, to trace swarms of subatomic particles as they race out from the collision. As challenging as it may be to detect the cascades, they are invaluable to understanding the quantum world. From these destructive experiments we discovered the different types of quarks and their properties.

Piecing Together Properties

The internal quark structure of neutrons and protons. Left: proton, Right: neutron.

Collisions aside we can begin to infer some quark properties from the particles they make. One property which proved to be a bit puzzling was electric charge. We know that protons have a positive charge and we also know that it takes 3 quarks to build a proton, so some quarks must have positive charges.  Likewise we know that the neutrons have a neutral electric charge and again it takes three quarks to make a neutron. From these two particles it seems that at least some quarks have positive charge.

By looking at the properties of other hadrons we find that all quarks have a charge associated with them, which can be positive or negative and the charges are either a third or two thirds the charge of a proton.  All this charge deduction seems fine until you think carefully about the implications.

There is a property of charge which we are all familiar with; like charges repel and opposite charges attract. If this is the case then how can a hadron like a proton stick together? A proton has two “up” quarks which have a charge of +2/3 each and a ‘down’ quark which has a charge of -1/3. Quite clearly the charge doesn’t balance, so surely the proton should rip itself apart from the two positively charge up quarks pushing away from each other? Well nature seems to disagree.

Uncovering the New Physics of Colours

What’s going on? Do we simply not understand how electromagnetism works? Or has nature hidden new physics inside of quarks?

It turns out it was the latter, there was something about nature that hadn’t been discovered yet. Through experiments to understand quarks physicists discovered a new fundamental force in nature—the strong nuclear force or simply, the strong force. The strong force is what sticks quarks that have the same charge together and stops protons from tearing apart. When quarks are close together the attraction through the strong force is greater than the electromagnetic repulsion. If you have ever tried to force two ends of a magnet together you know the strong force must really be strong!

You may be wondering when colours come into all of this, well the answer is now. It turns out its colour that makes quarks stick together! But what does that mean? Well we can think of colour as a kind of charge for the strong nuclear force. Unlike electrical charge the strong “charge” is a bit more complicated, being broken up into six different colours, instead of two charges like in electromagnetism.

A combination of the three colour charges give white, or neutral colour charge. Combinations of colours and their anticolours can also give white.
A combination of the three colour charges give white, or neutral colour charge. Combinations of colours and their anticolours can also give white.

Like any physical force there are rules associated with it that are expressed through the colours. In order for the quarks to stick together they want to add their colours up to ‘white’, which is neutral. This can happen two ways; either quarks with corresponding colours and anticolours pair up to cancel out to white, like blue and antiblue; or all three colours (blue, red and green) come together to make white, or neutral.


The Quarky Nature of the Universe

We owe our existence to the colourful little quarks. If quarks didn’t come in colours, then the strong force wouldn’t exist leading to an entirely different reality. Protons, neutron and all hadrons would never form and all visible matter that makes up you, me and all of the galaxies would not exist. Our existence is owed to the colours we will never see that hold everything together.

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