Fundamental Forces of Nature

As promised, I give you my next article in the standard model series!! This article will be about the fundamental interactions that take place and is extremely important in order to understand particle physics.

Forces or interactions are necessary for any change that takes place. From preventing atoms from collapsing to stabilising a star through fusion reactions, everything is a consequence of interactions between particles. In order for two particles to interact with each other there must be some “driving” or “mediating” particle that carries out the interaction. Such mediating particles are known as force carriers or bosons. It is interesting to note that these particles are virtual particles they are transitory quantum fluctuations i.e. exist only for a very short duration of time (during which the interaction takes place). The closer their characteristics come to real particles, the longer they exist. Also they always conserve energy. And as far as quantum numbers go, different quantum numbers are conserved depending upon the interactions or the exchanged bosons. One important tool we use for representing interactions is the Feynman Diagram.

Certain conventions are followed while making these diagrams. A Feynman diagram consists of an incoming fermionic line, an outgoing fermionic line, two vertices and a propagator connecting these two vertices. The propagator may be bosonic or fermionic, if it is bosonic then it’s represented by a wiggly line and if it is fermionic then it’s represented by a straight line. Time can be considered to be moving either upwards or from left to right. These Feynman diagrams though look very simple, are representations of all possible ways that a particle can follow to undergo a specific interaction. Any one such Feynman diagram is one possible way and represents a term in the Wick’s Expansion.

Any interaction can be classified into 4 basic or fundamental interactions. Fundamental forces or interactions are the basic forces which cannot be broken down further into simpler forces. At present there are 4 such forces : The Strong Nuclear Force, The Electromagnetic Force, The Weak Nuclear Force and finally the Force of Gravity.

Since these topics are a bit long; therefore, I have decided to make the rest of this article about Quantum Chromodynamics and the next article about Quantum Electrodynamics.

The Strong Nuclear Force is the strongest of the four forces. The boson for this interaction is the Gluon (g). This interaction makes up all matter as we know it. Quarks interact with each other through the strong force and this force confines the quarks to their bound state which results in the formation of all hadrons which make up the atoms that are the building blocks of everything from carbon molecules to high rise buildings. Now you may wonder why only quarks interact with the strong force. Well for starters quarks and gluons are the only particles which do not have a vanishing colour charge. Colour charge is analogous to the electromagnetic charge we see on protons, electrons, etc. While we have + and – in electromagnetic charge, in colour charge we have ±red, ±green and ±blue. Gluons have two colour charges for example red and anti-green. Gluons can exist in a total of eight independent color states also know as the octet of gluons, the states are:

\(\)LATEX\(\)

Other than this gluons are massless and electrically neutral particles with a spin 1. Unlike gluons, quarks have only one color charge, a spin 1/2 and an electric charge of +2/3 or -1/3 depending on their flavor.

Now moving on to the interactions, as the name suggests the strong nuclear force is the strongest of the 4 forces of nature. This force acts on the scale of 0.8 fm. One interesting fact about this is that it is independent of the distance. Whereas the rest of the three forces decrease their magnitude with increasing distance. Now what will happen if u try to bombard hadrons with high energy particles? This extra energy instead of separating the quarks and releasing free moving radiation (gluons) creates quark-antiquark pairs. This leads us to the phenomenon of Hadron Jets, wherein, as the name suggests, as we provide energy jets of a large number of hadrons are created in a chain reaction so to say. This property by which the energy added to pull two quarks apart creates a pair of new quarks that pair up with the original ones is called Color Confinement.

Now moving on to, a force that is extremely popular inside the nucleus of an atom, the Residual Strong Nuclear Force. Although some assume the force binding the nucleons to be the weak force, however, that is incorrect. It is actually the same force binding the quarks but slightly weaker and distance dependent. Instead of the gluons mediating the residual force this force is actually carried by the mesons. Now if u remember mesons are bounded quark-antiquark pairs and these 2 quark mesons are basically bosons (carriers of the residual force in this case). Let’s look at the following gif :

In this we see that a quark interacts with another quark and changes the color charge of the other quark. Consider a case in which a blue quark changes to a green quark. Look at the Feynman diagram below, a blue quark interacts with a green quark through a blue-antigreen gluon and this interaction causes the blue quark to gain a green color charge and loose the blue color charge at the same time causing the green quark to do the exact opposite.

This is just one example of the quark interactions that take place in hadrons. In baryons like protons or neutrons, quarks keep on changing their colors through gluons and this can sometimes lead to the formation of a meson (quark-antiquark pair) which gets removed from the said baryon and goes to be absorbed by the second baryon. In this case if a d quark changes its color from blue to green in a proton then this will lead to a formation of a pion (a meson having d quark and d antiquark). This pion now goes on to the neutron and gets absorbed by another quark in the neutron and this process continues. This my friends is the Residual Nuclear Force. This force is the reason why our phones can exist without blowing up. This force keeps the nucleus from flying out, it overcomes the repulsive electromagnetic force and keeps the protons inside the nucleus. Its transitions lead to the production of Gamma Rays.

Thus, in conclusion, the Strong interactions bind quarks together, allow quarks to change their color charge, bind the nucleons together and give us marvelous phenomena like hadron jets arising due to color confinement.

I bid you farewell reader and as always reach out to me if you have any queries and….Stay Tuned!