The strong nuclear force is the glue that holds reality as we know it together

Of the four fundamental forces in physics, the strong nuclear force, or the strong force for short, is the glue that holds the universe as we know it together.
John Loeffler
A wireframe model of the atom with a tightly bound nucleus
A model of the atom with a tightly bound nucleus held together by the strong nuclear force.

DepositPhotos 

The strong nuclear force is easily the most powerful of the four fundamental forces in the standard model of physics, but it's also one that none of us will ever 'feel', despite it being foundational to the universe as we know it.

Like the other three fundamental forces in nature (the weak nuclear force, the electromagnetic force, and gravity), the strong nuclear force is an essential component of how the universe is shaped and composed, and it's been a part of the universe longer than matter itself.

Suppose you've ever looked at an atomic model with a nucleus of more than a single proton. In that case, you might have asked yourself how more than one positively charged proton in a nucleus could clump together when two positive electromagnetic charges should repel each other. You definitely aren't alone in asking.

If we want a universe with elements other than hydrogen, this quintessential problem of a positively charged nucleus needs to be resolved, and the strong nuclear force is how the universe does so, though it's only been in the past century that we've come to recognize its existence and its importance.

What is the strong nuclear force?

The strong nuclear force is one of the four fundamental forces in the Standard Model of particle physics, and it is responsible for the stability of the atomic nucleus. It holds together the quarks that make up protons and neutrons and bind together the atomic nucleus, allowing elements more complex than primordial hydrogen to form.

As everyone knows, electromagnetism is responsible for opposite electric charges attracting one another but it is also what drives like charges apart, often with considerable energy. To overcome the force of electromagnetism pushing protons apart in the atomic nucleus, there needs to be an even stronger force binding them together, which is what the strong nuclear force does.

At its most basic level, matter is composed of elementary particles, such as quarks and leptons. Quarks are the building blocks of protons and neutrons, which are the building blocks of atomic nuclei. The strong nuclear force is responsible for holding these particles together, allowing them to form the structures that make up the matter around us.

The strong nuclear force is carried by particles called gluons, which interact with the quarks within the nucleons. The interaction between gluons and quarks is what gives protons and neutrons their mass, and allows them to bind together to form atomic nuclei.

Without the strong nuclear force, the protons and neutrons in atomic nuclei would repel each other due to their positive charges, and the nuclei would break apart. This would prevent the formation of atoms, and so the strong force is essential to the formation of all matter as we know it.

What is the residual strong force?

While the strong nuclear force is mainly a function of the interactions between quarks inside a nucleon (a proton or neutron), the extent of its range is somewhat greater than the radius of a proton or neutron. Again, this can't be emphasized enough, it is a very strong force — about 1039 times stronger than the force of gravity — and it's this residual force that is responsible for holding the nucleus of an atom together despite the repulsive forces between the positively charged protons.

The strong residual force has several important characteristics. Firstly, it is a charge-independent force, which means that it is not affected by the electric charge of the nucleons. This is in contrast to the electromagnetic force, which is responsible for the interaction between charged particles and is affected by the electric charge of the particles.

Second, the strong residual force has a spin-dependent component, meaning that it depends on the orientation of the spins of the nucleons. Nucleons with aligned spins experience a stronger attractive force than those with anti-aligned spins. This is known as the spin-spin interaction, and it is responsible for the magnetic properties of atomic nuclei.

Finally, the strong residual force is a complex force that involves the exchange of mesons, which are particles made up of quarks and anti-quarks. The exchange of mesons creates a field that attracts nucleons toward each other. The exchange of different types of mesons gives rise to different types of nuclear forces, which are characterized by their range and strength.

The strong residual force is responsible for the stability of atomic nuclei, as it holds the protons and neutrons together despite the repulsive electromagnetic forces between the protons. It also plays a crucial role in nuclear reactions, such as fusion and fission, and in the behavior of matter in extreme conditions, such as in the core of stars and in nuclear explosions.

What did scientists think held matter together before the strong nuclear force was proposed?

One of the earliest theories was the "plum pudding" model, proposed by J.J. Thomson in 1904. This model suggested that the atom was made up of a positive "pudding" of matter with negatively charged electrons scattered throughout, like plums in a pudding. However, this model was quickly disproven by Ernest Rutherford's gold foil experiment in 1911, which showed that atoms had a dense, positively charged nucleus at the center.

Following this discovery, Niels Bohr proposed the first widely-accepted model of the atom in 1913. His model, known as the Bohr model, suggested that electrons orbited the nucleus at fixed distances, like planets orbiting the sun. This model was successful in explaining many properties of atoms, but it did not explain what held the nucleus together.

Who discovered the strong nuclear force?

Once Bohr's model of the atom gained widespread support, it became obvious that there had to be some countervailing force within the nucleus strong enough to keep the positively charged protons stable. In the 1930s, scientists proposed the idea of a new fundamental force that could explain the stability of atomic nuclei, calling it the "strong nuclear force" because it was thought to be stronger than the electromagnetic force, which was the only other known fundamental nuclear force at the time.

Though the idea of the strong nuclear force was in the scientific zeitgeist of the 1930s and 1040s, the first real theory of the strong nuclear force was proposed by Hideki Yukawa in 1935 to explain the force that held protons and neutrons together in the nucleus. He proposed that the force was transmitted by a new particle, which he called the "meson." The meson was later found to be a type of particle called a "pion."

Yukawa's theory was successful in explaining the stability of atomic nuclei, but it was later found that the pion did not have the properties required to transmit the strong nuclear force. In the 1970s, scientists developed a new theory of the strong nuclear force based on the principles of quantum chromodynamics, which described the interactions between quarks and gluons. This theory is the one currently accepted by the scientific community.

Fundamental particles and the strong force

The strong nuclear force is the glue that holds reality as we know it together
A first of its kind Tetraquark, discovered at CERN in 2020

Another key responsibility of the strong nuclear force is binding together quarks into protons and neutrons. The strong force is actually a mediated exchange of particles called gluons between the quarks that make up the proton and neutron.

Quarks are never found in isolation, as the strong nuclear force is so strong that the quarks are always bound together in combinations to form particles such as protons and neutrons. The gluons responsible for producing the strong nuclear force carry a type of charge called color charge, which comes in three values or types: red, green, and blue. Quarks also have color charge, and they can have one of these three color values.

Quarks come in different color values: red, blue, and green. In order to form a bound state, such as a proton or neutron, quarks must combine in such a way that the overall color charge is "colorless". This means that if you add up the colors of all the quarks in the particle, the result is 'white' (which is equivalent to no color).

The way this works is that quarks of different colors attract each other while quarks of the same color repel each other. For example, a red quark might be attracted to a blue quark, but it would be repelled by another red quark. This attraction and repulsion is mediated by the exchange of gluons between the quarks.

The strong force is very powerful, but it only acts over very short distances. This is why quarks can only exist within particles that are very small, such as protons and neutrons. If you try to separate quarks by pulling them apart, the energy you'd need to overcome the strong force becomes so great that new quarks are created from the energy, forming new particles instead of individual quarks (a phenomenon known as "confinement").

How did the strong nuclear force form?

The strong nuclear force is thought to have emerged during the first few moments after the Big Bang, when the universe was still in a hot, dense, high-energy state.

During these first few moments of the early universe, all four fundamental forces (gravity, electromagnetism, weak force, and strong force) were thought to be unified into a single force, which is thought to have existed at extremely high temperatures and energies. As the universe expanded and cooled, this force gradually devolved into the different forces we see today.

Once gravity devolved into a distinct force, the strong nuclear force became distinct shortly after once the temperature had cooled to around 1028 Kelvin. At this temperature, it is believed that the universe was filled with a primordial soup of quarks and gluons, which would quickly go on to become the building blocks of protons and neutrons.

What was the role of the strong nuclear force in the early universe?

The strong nuclear force is the glue that holds reality as we know it together
The strong nuclear force might have had a profound impact on the evolution of cosmic expansion

In the early universe, the strong force was much stronger than the other forces, which is why it was able to act over such short distances and bind quarks together to form protons and neutrons. As the universe continued to expand and cool, the strong force weakened, and the other forces began to play a larger role in the behavior of particles, but many of these early interactions had lasting repercussions on the universe we see today long after those interactions stopped.

During the quark soup stage of the universe, a process known as nucleosynthesis formed the lighter elements like hydrogen, helium, and lithium.

Later, during the period of cosmic inflation, the universe underwent a rapid expansion, during which the strong force played a crucial role in determining the distribution of matter in the universe. The fluctuations in the density of matter, caused by the random variations in the strength of the strong force, were the seeds that, after billions of years of evolution and expansions, gave rise to the large-scale structure of the universe, including galaxies, clusters of galaxies, and superclusters.

Which is stronger, the strong nuclear force or electromagnetism?

The strong nuclear force is stronger than electromagnetism. In fact, the strong nuclear force is the strongest of the four fundamental forces of nature, hence the name.

To give you an idea of just how much stronger the strong nuclear force is than electromagnetism, consider this: the strong nuclear force is estimated to be about 1038 times stronger than electromagnetism. This means that the forces that hold atomic nuclei together are billions of times stronger than the forces that hold atoms together through their electrons and electric charges.

The reason for this vast difference in strength is due to the properties of the particles that are involved. Electromagnetism is carried by photons, which have no mass and only interact with charged particles. In contrast, the strong nuclear force is carried by gluons, which are themselves charged and interact with other charged particles, as well as with quarks, the building blocks of protons and neutrons.

What evidence do we have for the strong nuclear force?

The strong nuclear force is so powerful that it keeps the quarks and nucleons affected by it packed tight enough that pretty much only advanced particle accelerators can break these bonds. So how do we even know that a strong nuclear force exists? Well, there are several major lines of evidence for the existence of the strong nuclear force, so even though we'll never be able to glean a gluon on its own, there's a lot that we can deduce from observation and experimentation.

First, one of the strongest pieces of evidence for the strong nuclear force is the binding energy of atomic nuclei. The energy required to break apart the nucleus of an atom into its constituent protons and neutrons is much larger than the energy required to break apart the atoms themselves.

Scientists have also conducted scattering experiments using high-energy particles to study the structure of atomic nuclei. By analyzing the way these particles scatter off of atomic nuclei, physicists can infer the presence and strength of the strong force.

The original theory of the strong force proposed by Yukawa in the 1930s involved the exchange of particles called mesons between nucleons (protons and neutrons) in generating the attractive force that holds the nucleus together. While this theory was later found to be incomplete, it provided a foundation for the development of the modern understanding of the strong force.

The strong force is also responsible for quark confinement, has been observed in high-energy experiments, and is a fundamental prediction of the theory of the strong force.

Can we use the strong nuclear force?

The strong nuclear force is the glue that holds reality as we know it together
The nuclear fusion reaction, which generates energy from the strong nuclear force

John Loeffler, for IE

Like all the fundamental forces, we've been able to put the strong nuclear force to use in technology and scientific research.

Since the strong nuclear force is what binds together the atomic nucleus and the hadrons within it, it is likewise responsible for the energy produced in nuclear reactions, such as nuclear fusion and fission.

Nuclear fusion, the energy source for stars like our sun, is the process where atomic nuclei combine to form elements heavier than hydrogen, usually releasing energy in the process if the resulting element is lighter than iron. While we often talk about nuclear fusion fusing two hydrogen atoms into a single helium atom, what is actually happening is the fusion of two hydrogen isotopes containing at least one or two neutrons. In the fusion process, one of these neutrons gets pushed out of the resulting nucleus, and the energy from the strong nuclear force that kept it bound to the nucleus is released as the neutron breaks free.

Nuclear fission, meanwhile, is the process of splitting a heavy atomic nucleus into two or more lighter nuclei. This releases a large amount of energy in the process through a similar means. Free neutrons are shot at an atomic nucleus which then disrupts the nucleus to form two or more smaller but more stable nuclei, releasing the energy from the strong nuclear force that kept the two or more nuclei together as a single unit. This is the process used in nuclear power plants to generate electricity and in nuclear weapons like those dropped on Hiroshima and Nagasaki, Japan, in 1945.

The Strong Nuclear Force also plays a role in the behavior of particles in particle accelerators, where particles are accelerated to near the speed of light and collide with other particles. These collisions can produce new particles, allowing physicists to study the fundamental properties of matter and energy.

The mystery of the strong nuclear force

While our understanding of the strong nuclear force has advanced significantly over the years, there are still many open questions that remain unresolved and areas of ongoing research.

One of the most intriguing aspects of the strong nuclear force is its ability to confine quarks and gluons within hadrons, such as protons and neutrons. However, the mechanism behind this confinement is not well understood and remains an active area of research.

While scientists have made significant progress in studying quark-gluon plasma, the free-floating quark soup present after the Big Bang, there is still much we don't know about this state of matter. For example, the precise mechanisms behind its formation, its properties at different energy densities, and its role in the early universe are all still being actively studied.

While the strong nuclear force is well understood at low energies, its behavior at very high energies is still not well understood. This is particularly important for understanding the behavior of protons and other hadrons in high-energy collisions and is an active area of research at facilities such as the Large Hadron Collider.

What's more, while the standard model of particle physics provides a framework for understanding the strong nuclear force, it is known to be incomplete, and there may be additional particles or forces that are not yet understood. Understanding the behavior of the strong nuclear force in the context of a more complete theory of physics is an important goal for many physicists.

Finally, while the strong nuclear force is not directly involved in the behavior of dark matter, its properties could have important implications for our understanding of this mysterious substance. For example, the behavior of dark matter particles in the early universe could be influenced by the strong nuclear force, and understanding this could shed light on the nature of dark matter.

Overall, while we have made significant progress in understanding the strong nuclear force, there is still much we don't know, and ongoing research in this area is crucial for advancing our understanding of the universe at its most fundamental level.

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