What is the strong force?

The strong force or strong nuclear force is one of the four fundamental forces of nature, along with gravity, electromagnetism and the weak force. As the name suggests, the strong force is the strongest force of the four. It binds fundamental particles of matter, known as quarks, to form larger particles.

But in August 2023, a new discovery called the strong force into question. By smashing an isotope of oxygen with a beam of fluorine atoms, physicists have finally created oxygen-28 — a rare form of oxygen long-predicted to be ultrastable. The only problem is that it isn’t. Oxygen-28 decays within a zeptosecond, or a trillionth of a billionth of a second. This has left physicists baffled, and the Standard Model (the five-decade-old theory of how particles should behave) open to doubt.

The strong force in the Standard Model

The reigning theory of particle physics is the Standard Model, which describes the basic building blocks of matter and how they interact. The theory was developed in the early 1970s and, over time and through many experiments, has become established as a well-tested physics theory, according to CERN, the European Organization for Nuclear Research. 

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Under the Standard Model, one of the smallest, most fundamental elementary particles, or those that cannot be split up into smaller parts, is the quark. These particles are the building blocks of a class of massive particles known as hadrons, which include protons and neutrons. Scientists haven't seen any indication that there is anything smaller than a quark, but they're still looking.

The strong force was first proposed to explain why atomic nuclei do not fly apart. It seemed that they would do so due to the repulsive electromagnetic force between the positively charged protons located in the nucleus. Physicists later found that the strong force not only holds nuclei together but is also responsible for binding the quarks that make up hadrons. 

"Strong force interactions are important in … holding hadrons together," according to "The Four Forces," physics course material from Duke University. "The fundamental strong interaction holds the constituent quarks of a hadron together, and the residual force holds hadrons together with each other, such as the proton and neutrons in a nucleus."

Quarks and hadrons

Quarks were theorized in 1964, independently by physicists Murray Gell-Mann and George Zweig, and physicist first observed the particles at the Stanford Linear Accelerator National Laboratory in 1968. According to The Nobel Foundation, Gell-Mann chose the name, which is said to have come from a poem in the novel "Finnegans Wake," by James Joyce: 

"Three quarks for Muster Mark! Sure he has not got much of a bark, And sure any he has it's all beside the mark."

"Experiments at particle accelerators in the '50s and '60s showed that protons and neutrons are merely representatives of a large family of particles now called hadrons. More than 100 [now more than 200] hadrons, sometimes called the 'hadronic zoo,' have thus far been detected," according to the book "Particles and Nuclei: An Introduction to the Physical Concepts" (Springer, 2008). 

Scientists have detailed the ways quarks constitute these hadron particles. "There are two types of hadrons: baryons and mesons," Lena Hansen wrote in "The Color Force," a paper published online by Duke University. "Every baryon is made up of three quarks, and every meson is made of a quark and an antiquark," where an antiquark is the antimatter counterpart of a quark having the opposite electric charge. Baryons are the class of particles that comprises protons and neutrons. Mesons are short-lived particles produced in large particle accelerators and in interactions with high-energy cosmic rays. 

Quark flavors and colors

Quarks come in six varieties that physicists call "flavors." In order of increasing mass, they are referred to as up, down, strange, charm, bottom and top. The up and down quarks are stable and make up protons and neutrons, Live Science previously reported. For example, the proton is composed of two up quarks and a down quark, and is denoted as (uud).

The other, more massive flavors are produced only in high-energy interactions and decay extremely quickly. They are typically observed in mesons, which can contain different combinations of flavors as quark-antiquark pairs. The last of these, the top quark, was theorized in 1973 by Makoto Kobayashi and Toshihide Maskawa, but it was not observed until 1995, in an accelerator experiment at the Fermi National Accelerator Laboratory (Fermilab). Kobayashi and Maskawa were awarded the 2008 Nobel Prize in physics for their prediction. 

Quarks have another property, also with six manifestations. This property was labeled "color," but it should not be confused with the common understanding of color. The six manifestations are termed red, blue, green, antired, antiblue and antigreen. The anticolors belong, appropriately, to the antiquarks. The color properties explain how the quarks can obey the Pauli exclusion principle, which states that no two identical objects can occupy the same quantum state, Hansen said. That is, quarks making up the same hadron must have different colors. Thus, all three quarks in a baryon are of different colors, and a meson must contain a colored quark and an antiquark of the corresponding anticolor.

Gluons and the strong force

Particles of matter transfer energy by exchanging force-carrying particles, known as bosons, with one another. The strong force is carried by a type of boson called a "gluon," so named because these particles function as the "glue" that holds the nucleus and its constituent baryons together. A strange thing happens in the attraction between two quarks: The strong force does not decrease with the distance between the two particles, as the electromagnetic force does; in fact, it increases, more akin to the stretching of a mechanical spring. 

As with a mechanical spring, there is a limit to the distance that two quarks can be separated from each other, which is about the diameter of a proton. When this limit is reached, the tremendous energy required to achieve the separation is suddenly converted to mass in the form of a quark-antiquark pair. This energy-to-mass conversion happens in accordance with Einstein's famous equation E = mc2 — or, in this case, m = E/c2 — where E is energy, m is mass, and c is the speed of light. Because this conversion occurs every time we try to separate quarks from each other, free quarks have not been observed and physicists don’t believe they exist as individual particles. In his book "Gauge Theories of the Strong, Weak and Electromagnetic Interactions: Second Edition" (Princeton University Press, 2013), Chris Quigg of Fermilab states, "The definitive observation of free quarks would be revolutionary."

Residual strong force

When three quarks are bound together in a proton or a neutron, the strong force produced by the gluons is mostly neutralized, because nearly all of it goes toward binding the quarks together. As a result, the force is confined mostly within the particle. However, a tiny fraction of the force does act outside the proton or neutron. This fraction of the force can operate between protons and neutrons, collectively known as nucleons. 

According to Constantinos G. Vayenas and Stamatios N.-A. Souentie in their book "Gravity, Special Relativity and the Strong Force" (Springer, 2012), "it became evident that the force between nucleons is the result, or side effect, of a stronger and more fundamental force which binds together quarks in protons and neutrons." This "side effect" is called the "residual strong force" or the "nuclear force," and it is what holds atomic nuclei together in spite of the repulsive electromagnetic force between the positively charged protons that acts to push them apart. 

Unlike the strong force, though, the residual strong force drops off quickly at short distances and is significant only between adjacent particles within the nucleus. The repulsive electromagnetic force, however, drops off more slowly, so it acts across the entire nucleus. Therefore, in heavy nuclei, particularly those with atomic numbers greater than 82 (lead), while the nuclear force on a particle remains nearly constant, the total electromagnetic force on that particle increases with atomic number to the point that, eventually, it can push the nucleus apart. "Fission can be seen as a 'tug-of-war' between the strong attractive nuclear force and the repulsive electrostatic force," according to the Lawrence-Berkeley National Laboratory's ABC's of Nuclear Science. "In fission reactions, electrostatic repulsion wins." 

The energy released by the breaking of the residual strong force bond takes the form of high-speed particles and gamma-rays, producing what we call radioactivity. Collisions with particles from the decay of nearby nuclei can precipitate this process, causing a nuclear chain reaction. Energy from the fission of heavy nuclei, such as uranium-235 and plutonium-239, is what powers nuclear reactors and atomic bombs.

Limitations of the Standard Model

In addition to all the known and predicted subatomic particles, the Standard Model includes the strong and weak forces and electromagnetism, and explains how these forces act on particles of matter. However, the theory does not include gravity. Fitting the gravitational force into the framework of the model has stumped scientists for decades. But, according to CERN, at the scale of these particles, the effect of gravity is so minuscule that the model works well despite the exclusion of that fundamental force.

The Standard Model also predicts that the isotope oxygen-28 should be stable. As fermions, protons and neutrons cannot overlap with each other. Intead, they stack into discrete shells inside the atomic nucleus. 

When these shells are filled, atoms become ultra-stable or "magic" and have no need to decay into more stable forms. Yet oxygen-28 decays incredibly quickly in the tiniest fraction of a second. 

What this means for our understanding of subatomic forces is unclear but it could suggest that deeper, unknown physics is dictating the behavior of the bizarre isotope. Because the strong force is what holds an atom together, as well as ruling their actions at these short timescales, it is this force that the new findings call into question. 

Additional resources

CERN created a rich website describing all the intricacies of our efforts to understand the strong force, which you can see here. You can also check out interactive demos either on the web or via an app courtesy of The Particle Adventure. If you're in more of a listening mood, check out this podcast episode digging into the strong force.

Bibliography

Constantinos, G. et al. Gravity, Special Relativity, and the Strong Force (Springer Science & Business Media, 2012)

Quigg, C. Gauge Theories of the Strong, Weak, and Electromagnetic Interactions (Princeton University Press, 2013)

Povh, B. et al. Particles and Nuclei: An Introduction to the Physical Concepts (Springer Science & Business Media, 2008)

Thacker, T. (1995, Jan 29) The Four Forces https://webhome.phy.duke.edu/~kolena/modern/forces.html#005

Hansen, L. (1997, Feb 27) The Color Force https://webhome.phy.duke.edu/~kolena/modern/hansen.html