The Nuclear Force & Strong Force

When we think of atomic nuclei we think of protons and neutrons in the center and electrons on the outside. Have you ever wondered why the protons stay together in the center? If they’re all positivity charged, shouldn’t they repulse each other and fly away?

Instead of spraying everywhere, they remain together through the strong force, but not to each other. Protons and neutrons bind together via the strong force and that is what stabilizes the nucleus. The electromagnetic force which should repel them is a weakling compared to the strong force. The strong force is one of the fundamental forces in nature (along with the weak force, electromagnetic force and gravity) which underpin physics by being a basic type of interaction which can’t be reduced to anything else. It can be broken down into two types; the strong interaction and the residual strong force or nuclear force.

How does it work?

To describe the overall interaction, there is a short answer and a long answer. Protons and neutrons are made of quarks which you may have heard somewhere before. Essentially, particles called gluons bind them together to form larger particles. This is the strong force because of its strength. At the proton/neutron level there are particles called pions which are exchanged between the protons and neutrons and this is the nuclear force. It isn’t as strong as the strong force but actually a consequence of it which is why it is also referred to as the residual strong force. The strength of this force is more powerful than electromagnetism and can force sub-atomic particles like protons together when they’d normally speed away from each other at relativistic speeds. If you want you can stop here but from this point on we’re going to get technical (insofar as I can manage).

Nuclear Force

When we look at the nuclear force we have to examine different sub-atomic particles other than protons and neutrons but at the same time we need to classify them. One class of sub-atomic particles is the hadron which contains two or more quarks. One type of hadron is the meson (made of 1 quark and 1 anti-quark), which are highly unstable and degrade quickly. Another is the baryon which has an odd number of quarks, usually 3. Protons and neutrons are baryons while the pions are mesons which are similar to force carriers and bind the baryons together which will form atoms. The protons and neutrons are also collectively called nucleons to group the particles in the nucleus together. Using these terms, we can say that mesons bind baryons together and in the case of atoms we would say mesons bind the nucleons together in the nucleus. They do this by swapping quarks between nucleons in the form of mesons and they do this constantly which exerts the necessary force on the nucleons required to overcome electromagnetism.

This force has a limited range which is why we don’t experience it. It mainly works at the atomic level and has different zones of effect. The size we’re using is femtometers (fm) which are 1 x 10^−15 m and usually applies to the size of a proton or neutron. At 0.7 fm the nuclear force is repulsive as the nucleons move too close to one another. The force maximizes in power at about 0.9 femtometers but then drops off after this point. This keeps the nucleons in a state of always moving short distances but never separating as the force will always drag them back from moving to far away. It may diminish after 0.9 fm but due to the strength of the force this also means that the nucleons have difficulty moving much further due to the restraint on them. If we get more specific the nuclear force also involves vector mesons which aid pions but for our purposes we’ll stop here and look at something harder. The next point which needs to be addressed is the strong interaction between quarks.

Strong Force

Protons and neutrons are composed of quarks but each quark has their own charge which adds up to a positive charge (protons) or a neutral charge (neutrons) in the case of the atomic nuclei. Each particle even has different quarks. Protons have two up quarks and one down quark and neutrons have two down quarks and one up quark. These quarks are bound together with gluons which carry the binding energy between quarks and relates to the color charge. The color charge is a specific quantum state that every quark has and it comes in red, blue and green. It also has nothing to do with color but the name stuck and we just use it. Quarks with different color charges are attracted to each other and gluons are massless particles which bind quarks together.

The energy binding the quarks together is called the quantum chromodynamics binding energy and it results in the phenomenon called color confinement because the binding energy is extremely strong. The bond keeping quarks together doesn’t break and is the reason there are few free-floating quarks. It is also why matter can exist in the way it does. Inside of nucleons quarks are still moving, almost at light speeds, while always exchanging gluons and shifting their color. They cannot escape because the strong force is repulsive at short distances but stronger at larger one’s which will always pull them back. This is the same principle as the nuclear force and where it derives from. Just as gravity isn’t strong enough that you can still jump up in the air, gravity is strong enough to keep you on Earth. All of this also collectively falls under the discipline of quantum chromodynamics which studies the strong force at the quantum level.