Do electrons really spin and do quarks really have colour?

Do electrons really spin and are quarks really coloured? Well, the answers to both are no. Why then do scientists come up with such terms? To deliberately obfuscate people? I do not think so. The limitations of human languages in describing the quantum world have made it difficult for us to assign appropriate words to describe properties of subatomic particles. As such, analogies such as spin and colour are used to create a more intuitive understanding of reality. What do scientists mean by the spin of electrons and colours of quarks then?

Before I explain electron spin, imagine a classical spinning top. If you look at the spinning top from the top or bottom, you will see that it is either spinning clockwise or anti-clockwise. If you view it from the side, you will not observe any spin. (Note: Spin is defined to be the rotation of an object about an axis which passes through it.) In order to understand the spin of an electron, we have to move away from the classical notion of spin. The electron is actually not a point-like particle but rather, it is a probability cloud. The spin of an electron is just a name used to describe one of its properties. This electron spin is a result of its intrinsic angular momentum. If you view the electron about any axis in 3-dimensional space, you will always observe either a clockwise or anti-clockwise spin, and the value of the spin is defined to be 1/2. It is difficult to imagine how this can happen, for the quantum world is totally different and exists on different scales from the macroscopic world on which we live in. It is only natural then that we will have trouble visualizing the world of the subatomic, just as a bacterium would have much difficulty in comprehending the human world.

Now let's move on to quarks. Quarks are the constituent particles of hadrons (a class of subatomic particles which interact via the strong force) and come in six different flavours called, 'up', 'down', 'top', 'bottom', 'strange' and 'charm'. These are not names of some newfangled ice-cream flavours but are merely names of different types of quarks. Apart from flavours, quarks also posses another property called colour and come in red, blue and green. A hadron is colour neutral as it consists of three quarks from each of the colours, just like mixing the three primary colours of light produces white light. In a meson, there are two quarks of similar colour but each being the matter and anti-matter counterpart of each other.

Quarks themselves are not coloured in the classical sense in which we see macroscopic objects in different colours. We can see and perceive colour from macroscopic objects because incident light or photons on the objects are absorbed by electrons in the atoms and then re-emitted or transmitted to our eyes. In the simplified model of an atom, bound electrons 'orbit' the nuclei in shells of fixed distances from the nuclei. (Note that electrons are actually probability clouds which flow around the nuclei of atoms and this flow is called a probability current.) Whenever a photon with the right amount of energy (corresponding to the gap between two energy levels of an atom) is incident on a bound electron, it will be absorbed by the electron and cause the latter to become ‘excited’ and jump to a higher orbit. The excited electron subsequently re-emits a photon as it falls back to a lower state. (Note that the excited electron may return to the ground in a single or multi-step process.) Since different atoms and molecules, and thus different objects, have different gaps in energy levels, they reflect different frequencies of visible light, resulting in our perception of colour. Anything smaller than an atom will not possess colour in the classical sense. Hence, it makes no sense to ask how quarks look like.

In trying to understand and describe the quantum world, we are inevitably bogged down by our limited sensory perceptions, preconceived notions of the world and semantic limitations. Nevertheless, we have come very far in building a scientific model of the world since the dawn of civilization. Will we ever be able to fully comprehend the world? Some say that it is impossible for mere human minds which are constrained by the limitations imposed by our physical senses to comprehend the universe. I personally think that Science alone will not be able to fully explain every aspect of reality. However, many people have been wrong countless times in predicting the limits of Science. Thus, only time will tell.