The 4 fundamental forces of nature.
It is that time of year again where people dress up in stormtrooper visors and wave lightsabers around in celebration of international Star Wars Day. While you may contend that the ‘Force’, which Star Wars characters use to levitate objects and battle each other, has not a lot in common with what physicists mean by a ‘force’, I refuse to accept that this is not an excellent opportunity to shed some light on the 4 fundamental forces of nature.
From walking on the street to sticking a magnet on your fridge, physical forces are acting all around us. These can be whittled down to just four fundamental forces, or fundamental interactions, which themselves cannot be reduced to more basic interactions: Gravity; Electromagnetism; the Strong Interaction; and the Weak Interaction. For each of them, I shall give a short introduction addressing their significance, and then I shall delve a little deeper into how scientists attempt to explain them. So without further ado, let’s start by addressing gravity: the most large-scale, intuitive and familiar interaction, yet simultaneously one of the most challenging to explain.
As living organisms, we have a lot to thank gravity for. Not only is it the reason why year after year we remain locked at an ideal distance from our Sun, but it is also the reason why that very Sun doesn’t blow itself apart due to the nuclear fusion that occurs within it. (It is also the reason why Yoda is able to remain standing on the planet of Dagobah in the image.) Gravity can be described as the phenomenon by which all things with mass or energy (proved by Einstein to be equivalent) are brought towards one other. It was first proposed by Newton in the mid-1680s as a literal attraction between two objects, the strength of which is given by Newton’s equation for universal gravitation. Newton’s law of universal gravitation was accepted as the primary theory of gravitation until as recently as the early 20th century but was eventually superseded by Einstein’s ground-breaking General Theory of Relativity. Through this new theory, Einstein suggested that gravitation is a consequence of all objects with mass causing a curvature in spacetime (similar to the way in which a large ball placed in the middle of a sheet deforms it), which in turn causes what we observe as gravitational attraction. This interaction is specified by the Einstein field equations. General Relativity, as it became to be known, has been successful in describing a number of astronomical phenomena which Newton’s theory failed to describe fully – the most famous of these is perhaps the precession in the elliptical orbit of Mercury. However, while Einstein’s theory was revolutionary in how accurately it described gravitational interaction, the work of scientists into describing gravity is far from over. Indeed, our current theory of gravity seems to stick out like a sore thumb when compared to our theories of the other 3 fundamental forces. Electromagnetism, the Weak Interaction, and the Strong Interaction are all interactions which involve quantum effects and which are mediated by elementary particles. This has led to the creation of theories of ‘quantum gravity’, which rely on the existence of the ‘graviton’, a hypothetical elementary particle that mediates the force of gravity. These theories may seem far-fetched, but in order for there to be a Grand Unified Theory, we will first need to find a way to incorporate quantum mechanics into Einstein’s theory of relativity or vice versa.
If we zoom inwards on the image of Yoda, in fact quite a lot inwards, we can begin to see the workings of the next fundamental force: electromagnetism, which is responsible for all chemical processes (among many other things). Electromagnetism refers to the interaction between all particles with an electric charge and includes the electrostatic force acting between charged particles at rest, as well as the combined effect of electric and magnetic forces acting between charged particles moving relative to each other. (This is what enables phenomena such as the motor effect or induction). Though it acts on a much smaller scale than gravity, it is in fact 10 to the 36th power stronger. This statement can be quite a confusing one, so to better explain it, I shall describe a simple illustration of two bottles of water placed on a table-top. It is not difficult to calculate that in a 2 litre bottle of water, there is approximately 10 to the 8 coulombs of charge. Thus, if the bottles are placed a metre apart, the electrons in one bottle repel those in the other bottle with a force roughly equivalent to 10 to the 26 Newtons. However, these repulsive forces are cancelled by the attraction of the electrons in the first bottle to the nuclei in the second bottle, which clearly illustrates why electromagnetism is nowhere as influential as gravity on the scale of everything we can see around us. This is one of the reasons why explanations for electromagnetism only came about long after scientists were attempting to explain gravity. Originally, electricity and magnetism were considered to be two separate forces, but this changed in 1873 with the publication of Maxwell’s ‘A Treatise on Electricity and Magnetism’, in which he showed that the two phenomena were inextricably linked by a single fundamental interaction. Our understanding of electromagnetism has evolved since then and our current theory, called quantum electrodynamics, describes how charged particles interact by means of exchanging photons.
If we zoom in still further on the image, right down to the scale of nuclei, we can begin to see that there is a much stronger force at work. More than a hundred times stronger than electromagnetism, but operating on an even smaller scale, the strong force is capable of holding together the protons and neutrons that make up the nucleus. Before the 1970s, physicists were unsure as to how the atomic nucleus was held together. The existence of a ‘strong nuclear force’ was proposed to explain this. It was later discovered that protons and neutrons were not fundamental particles but were made up of quarks. Our current theory of quantum chromodynamics (the theory that explains the strong force) describes the attraction between protons and neutrons as a residual effect of the interaction between the quarks within them. You see, besides their electromagnetic charge, quarks have a property called strong nuclear charge (a.k.a colour charge – entirely unrelated to visible colour, but nonetheless a very helpful analogy which enables particle physicists to describe the strong interaction). In simple terms, quarks with unlike colour charge are attracted to one another. In order to explain how the attraction between quarks relates to the binding of protons and neutrons in the nucleus, the existence of massless particles called gluons was proposed. Gluons are described as mediators of the strong nuclear force, and this means that they travel between quarks and assist in holding them together. It is worth noting that the existence of gluons was experimentally ‘confirmed’ in 1978, further backing up the theory.
We have now had a look at the three main forces which govern how things are held together. We now move on to the weak nuclear force which is an exception: it primarily plays a role in the way things fall apart. It is highly probable that somewhere in the image of Yoda is an atom with an unstable nucleus that is about to decay. If we manage to zoom in on this atom with perfect timing, we will observe an event that is known as beta decay. What makes beta decay interesting is that it cannot be explained by the three previously-described interactions. Beta decay is when a neutron transforms into a proton and simultaneously emits a beta particle (a high-energy electron) and an antineutrino, resulting in a more stable ratio of protons to neutrons in the nucleus. The question of how beta decay works was the basis on which Enrico Fermi (who would later play a key role in the Manhattan Project to develop the first nuclear weapons during WW2) proposed the first theory of the weak interaction in 1933. Fermi’s theory was ground-breaking but he certainly wasn’t correct on all accounts. For example, he originally believed that the force was a zero-distance force; that spontaneous beta decay was a result of a proton coming into contact with another nucleon. We now know that the weak nuclear force is in fact an attractive interaction between two nucleons that occurs at a range of less than a thousandth the diameter of a proton. We also know that it is mediated by particles called W and Z bosons which are capable of acting as force-carriers and of changing the properties of quarks (which can ultimately result in a neutron changing into a proton or vice-versa).
All in all, what I have described above seems as fantastical and awe-inspiring as any Star Wars battle could ever be. On top of that, everything I have mentioned is happening around you and within you every millisecond of every day. So I encourage you now to look around you with wonder, no matter what setting you’re in, and marvel at how the four fundamental forces contribute to the existence of absolutely everything you can see. And with that said, May the fourth be with you.