What are Quarks?
Quarks are a group of fundamental particles that are indivisible, meaning that they cannot be broken into smaller pieces; building blocks that combine in groups to make up a whole zoo of other (composite) particles. They were first thought up by physicists Murray Gell-Mann and George Zweig while attempting to mathematically explain the vast array of new particles popping up in experiments throughout the 1950’s and 1960’s.
Debris that results from smashing protons and protons into each other was seen in experiments to be a whole lot messier than debris from two electrons colliding headlong. Gell-Mann and others reasoned that this would happen if the proton were not a single entity like the electron but instead, like a bag of groceries, containing multiple particles within itself.
The menagerie of particles being discovered each week at particle accelerators could, in Gell-Manns model, all be explained as different composites of just a few types of truly fundamental particles. The multiple that seemed to fit the data in most cases was three and Gell-Mann got the spelling for his ‘kwork’ from a passage in James Joyce’s ‘Fineganns Wake’ – “Three quarks for Muster Mark”. Proof of Gell-Mann’s model came when a particle he predicted in 1962 to exist (which he called Ω–) was seen at an experiment at Brookhaven National Lab in the US in 1964. Gell-Mann received the Nobel Prize in Physics in 1969 for this work which was the birth of the quark.
Below are diagrams showing Murray Gell-Mann’s mathematical idea of explaining experimental data of the time, called the Eightfold way. These two patterns show all ways you can create Baryons made from up, down, and strange quark building blocks. The particle made of three strange quarks at the very bottom of the second diagram (Baryon Decuplet) is the Ω– particle that Gell-Mann predicted to exist and won him the nobel prize in 1968 after it was discovered.
This method can be extended further to include more of the three generations of quark. When considering the first two generations (up, down, strong, and charm quarks) we find ourselves at the Baryon multiplets:
Why groups of three?
It is all down to the way the strong force, responsible for binding the quarks together, works. The electromagnetic force has a possible two charges; which we label positive electric charge (like protons) and negative electric charge (like electrons). These different charges attract, which is the reason electrons remain orbiting the proton heavy nucleus of an atom. The strong force it seems has not two but three possible charges! As there is no clear way to describe this in terms of whole numbers like positive and negative another analogy had to be found. The best way to think of strong charge is as colours of light.
The three primary colours of light we see are red, green, and blue. The reason we have decided upon these colours is a selfish biological one; our eyes have evolved to be sensitive in particular to these three colours individually. When these three colours are combined, added together, they form what we perceive as white light. If we assigned the three primary colours of light to the three possible strong charges we could say that a quark can have a strong charge of red, green, or blue.
An atom is electrically neutral because it has a balance of positively charged protons in the nucleus and negatively charged electrons surrounding it; a helium nucleus contains two protons and has two electrons surrounding it which means the electric charge is +2 -2 = 0. In the same vein a proton has to be strong force neutral, it must have a balance of the three strong charges; composed from one green charged quark, one red charged quark, and one blue charged quark. Which of the two up or one down quarks is charged with each colour doesn’t matter – the fact is just that we need one of each to make a stable proton.
We can then say that the stable proton is white as green plus blue plus red light equals white. The same rules applies for all other particles made in a similar way, the group of particles known as Baryons. Almost any combinations of three quarks can create a Baryon as long as the Baryon is white in strong charge. I am in no way saying that quarks have colour in the traditional sense, because we cannot see quarks in the traditional sense – assigning them a colour is an analogy that fits the way in which the strong force behaves.
For more on this see my blog posts on quarks and rule of three.
LEGO® is a trademark of the LEGO Group of companies which does not sponsor, authorize or endorse this site.