We have grown up accepting the fact that like charges repel each other while the opposite ones attract. But, if this is the case, then shouldn’t the components of a nucleus, that are merely positively charged protons and neutral neutrons should just fly away from each other. What is that mysterious entity which is holding these nucleons closely bound together, despite them being likely charged! The answer is the “Strong Nuclear Force”.

Strong Force: A Fundamental Interaction

Our universe consists of a variety of bodies; small and large, charged and uncharged, massless and extremely massive, all interacting via different interactions known as fundamental interactions. The fundamental force of gravity leads to the attraction between bodies that carry a non zero mass. The weak nuclear force explains radioactive decay. Then comes the electromagnetic force which defines the interaction between charged particles, paving the way to the “opposite charges attract and the same ones repel” rule. 

An overview of four fundamental interactions: Strong force, Electromagnetic force, weak force and the gravitational force (Image: Researchgate _ E.Dudas)
An overview of four fundamental interactions: Strong force, Electromagnetic force, weak force and the gravitational force (Image: Researchgate _ E.Dudas)

Before the 1970s, we only knew sufficiently well about the gravitational and electromagnetic interactions. We didn’t know much about the processes occurring at the nuclear level. We didn’t know what was keeping the atomic nucleus bound together,  despite the fact that its constituents were either neutral or likely charged.

So, a completely new force,  rather a new physics was needed to explain this phenomenon. This is when the concept of an attractive force, stronger than the electromagnetic force was postulated to explain how the atomic nucleus was bound together. This hypothesized force was then called the strong force, because of its greater strength over other known fundamental forces.

Watch the first video of our ‘Introduction to Quantum Mechanics’ Series

A New Perspective:

Initially, the strong force was considered to be a fundamental force that acted on the protons and neutrons to keep the nucleus stable. However, later it was found that the story had another chapter that was yet to be read. In the following years,  it was discovered that protons and neutrons were not fundamental particles like electrons and other leptons, rather they were made up of smaller constituent particles called quarks.

Also Read: An Introduction to the Standard Model of Particle Physics

Soon the mystery unfolded that the strong attraction between nucleons (protons and neutrons) was just a side-effect or a residual force arising as a result of a more fundamental force that bound the quarks together into protons and neutrons.

Mechanism of Strong Force:

To begin at the very basic level, just as electric charge is the source of electromagnetism or the electromagnetic force, color is the source of the strong force. No, I’m talking about the regular colors that you see around yourselves every day,  rather I’m talking about the term “color”, which is an intrinsic characteristic associated with quarks. The field of quantum chromodynamics, which basically deals with the quantum field theory describing strong interactions, takes its name from this central property of color itself.

As we know,  there are six kinds of quarks: up, down, charm, strange, top, and bottom. Each quark has six color manifestations termed as red, blue, green, antired, antiblue, and antigreen. The anti-colors belong to the antiquarks. So, in all, all three quarks in a baryon are of different colors, and a meson contains a colored quark and antiquark of the corresponding anti-color.

Baryons are a family of particles that contain three quarks and mesons are the ones that contain a quark and an antiquark. Now, in order to bind these quarks together, glue is required. So, according to the concept of the strong force, these quarks are glued together by something called gluons. In other words,  just like photons are exchange carriers of the electromagnetic force, gluons are the initiators of the strong force.

Ever Wondered What Binds The Protons Together? It's The Strongest Force In The Entire Universe. 2
Composition of nucleons and pi-mesons (Image: lumenlearning.com)

How? Suppose you are playing with a bouncy ball with your little sibling. What you are actually doing is that you are exchanging that ball continuously among yourselves and this continuous exchange of ball is hindering both of you from drifting apart from each other physically. Eventually, this process of exchanging ball is keeping you both bound together. The same process keeps the quarks together, the difference is that they are playing with gluons instead of a ball in your case.

A gluon has the ability to change the color of a quark in order to keep the overall color charge of the baryon neutral, that is; red + blue + green = white (neutral). Now,  when the three quarks are bound together in a proton or a neutron, the strong force produced is mostly neutralized and most of it goes toward binding the quarks together. Eventually, the force is confined mostly within the particle.

However, there exists a tiny fraction of the force that does act outside of the proton or neutron. This fraction of the force, known as the residual force can now operate between protons and neutrons, and it is this residual force which further holds the atomic nuclei together via exchange of mesons, in spite of the repulsive electromagnetic force existing between the positively charged protons.

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Ever Wondered What Binds The Protons Together? It's The Strongest Force In The Entire Universe. 3
Nucleons interact via exchange of pi-mesons (image: lbl.gov)

A Short Range Force:

Now, a question might have arisen in your mind that if this attractive force is so strong, then why isn’t it applicable everywhere! Why do we still take into account the concept of electromagnetic force if it is so feeble in front of this attractive strong nuclear force? Well, the answer lies in their range. The strong force is a short-range force while the electromagnetic force is the one that possesses long-range. So, the strong force dominates only at the distances of the order of the diameter of a proton, but at larger distances, it is the electromagnetic force that dominates.

Ever Wondered What Binds The Protons Together? It's The Strongest Force In The Entire Universe. 4
A comparison of fundamental interactions (image:slideshare.net)

Still, the strong force is undoubtedly one of the most important entities required for the existence of this gigantic universe. Without the presence of the strong force, the matter would cease to exist. Without it, the gluons won’t be able to hold together quarks, due to which quarks would fall out of the proton. Eventually, the universe would contain nothing but only the up and down quarks, lurking chaotically all around!

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John Z Dill
John Z Dill

How much SPACE is contained within the average atom? What is SPACE?

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