What is the strong force?

Protons, made of three quarks, colliding
Protons, made of three quarks, colliding. The quarks are held together by the nuclear strong force carried by gluons. (Image credit: MARK GARLICK/SCIENCE PHOTO LIBRARY via Getty Images)

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. 

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

Illustration of quarks

Quarks come in different flavors. (Image credit: Shutterstock)

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.


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

Jim Lucas
Live Science Contributor
Jim Lucas is a contributing writer for Live Science. He covers physics, astronomy and engineering. Jim graduated from Missouri State University, where he earned a bachelor of science degree in physics with minors in astronomy and technical writing. After graduation he worked at Los Alamos National Laboratory as a network systems administrator, a technical writer-editor and a nuclear security specialist. In addition to writing, he edits scientific journal articles in a variety of topical areas.
With contributions from
  • Thomas Thompson
    Admittedly, being largely ignorant of physics, I got stuck on the concept of science being able to recognize the presence of something that lasts a trillionth of a billionth of a second. Or, for that matter, measuring that length of time! It boggles the untrained mind.
  • Hartmann352
    Thomas Thompson

    Despite the apparent complexity within the universe, there remain just four basic forces. These forces are responsible for all interactions known to science: from the very small to the very large to those that we experience in our day-to-day lives. These forces describe the movement of galaxies, the chemical reactions in our laboratories, the structure within atomic nuclei, and the cause of radioactive decay. They describe the true cause behind familiar terms like friction and the normal force. These four basic forces are known as fundamental because they alone are responsible for all observations of forces in nature. The four fundamental forces are gravity, electromagnetism, weak nuclear force, and strong nuclear force.

    The gravitational force is most familiar to us because it describes so many of our common observations. It explains why a dropped ball falls to the ground and why our planet orbits the Sun. It gives us the property of weight and determines much about the motion of objects in our daily lives. Because gravitational force acts between all objects of mass and has the ability to act over large distances, the gravitational force can be used to explain much of what we observe and can even describe the motion of objects on astronomical scales! That said, gravity is incredibly weak compared to the other fundamental forces and is the weakest of all of the fundamental forces. Consider this: The entire mass of Earth is needed to hold an iron nail to the ground. Yet with a simple magnet, the force of gravity can be overcome, allowing the nail to accelerate upward through space.

    The electromagnetic force is responsible for both electrostatic interactions and the magnetic force seen between bar magnets. When focusing on the electrostatic relationship between two charged particles, the electromagnetic force is known as the coulomb force. The electromagnetic force is an important force in the chemical and biological sciences, as it is responsible for molecular connections like ionic bonding and hydrogen bonding. Additionally, the electromagnetic force is behind the common physics forces of friction and the normal force. Like the gravitational force, the electromagnetic force is an inverse square law. However, the electromagnetic force does not exist between any two objects of mass, only those that are charged.

    When considering the structure of an atom, the electromagnetic force is somewhat apparent. After all, the electrons are held in place by an attractive force from the nucleus. But what causes the nucleus to remain intact? After all, if all protons are positive, it makes sense that the coulomb force between the protons would repel the nucleus apart immediately. Scientists theorized that another force must exist within the nucleus to keep it together. They further theorized that this nuclear force must be significantly stronger than gravity, which has been observed and measured for centuries, and also stronger than the electromagnetic force, which would cause the protons to want to accelerate away from each other.

    The strong nuclear force is an attractive force that exists between all nucleons. This force, which acts equally between proton-proton connections, proton-neutron connections, and neutron-neutron connections, is the strongest of all forces at short ranges. However, at a distance of 10–13 cm, or the diameter of a single proton, the force dissipates to zero. If the nucleus is large (it has many nucleons), then the distance between each nucleon could be much larger than the diameter of a single proton.

    The weak nuclear force is responsible for beta decay, as seen in the equation ZAXN → Z+1AYN–1 + e + v. Enrico Fermi was the first to envision this type of force. While this force is appropriately labeled, it remains stronger than the gravitational force. However, its range is even smaller than that of the strong force. The weak nuclear force is more important than it may currently appear, which can be explained when quarks are discussed..

    The strong force is not the only force with a carrier particle. Nuclear decay from the weak force also requires a particle transfer. In the weak force are the following three: the weak negative carrier, W–; the weak positive carrier, W+; and the zero charge carrier, Z0. As we will see, Fermi inferred that these particles must carry mass, as the total mass of the products of nuclear decay is slightly larger than the total mass of all reactants after nuclear decay.

    The carrier particle for the electromagnetic force is, not surprisingly, the photon. After all, just as a lightbulb can emit photons from a charged tungsten filament, the photon can be used to transfer information from one electrically charged particle to another.

    Finally, the graviton is the proposed carrier particle for gravity. While it has not yet been found, scientists are currently looking for evidence of its existence.

    So how does a carrier particle transmit a fundamental force? The transmitted photon is referred to as a virtual particle because it cannot be directly observed while transmitting the force. The graph of time versus position is called a Feynman diagram, after the brilliant American physicist Richard Feynman (1918–1988), who developed it.

    The Feynman diagram should be read from the bottom up to show the movement of particles over time. In it, you can see that the left proton is propelled leftward from the photon emission, while the right proton feels an impulse to the right when the photon is received. In addition to the Feynman diagram, Richard Feynman was one of the theorists who developed the field of quantum electrodynamics (QED), which further describes electromagnetic interactions and the strong force on the submicroscopic scale. For this work, he shared the 1965 Nobel Prize with Julian Schwinger and S.I. Tomonaga. A Feynman diagram explaining the strong force interaction hypothesized by Yukawa can

    The strong force, a fundamental interaction of nature that acts between subatomic particles of matter. The strong force binds quarks together in clusters to make more-familiar subatomic particles, such as protons and neutrons. It also holds together the atomic nucleus and underlies interactions between all particles containing quarks.

    The strong force originates in a property known as colour, from Richard Feynman's quantum chromo- dynamics (QCD). This property, which has no connection with colour in the visual sense of the word, is somewhat analogous to electric charge. Just as electric chargeis the source of electromagnetism, or the electromagnetic force, so colour is the source of the strong force.

    Particles without colour, such as electrons and other leptons, do not “feel” the strong force; particles with colour, principally the quarks, do “feel” the strong force. Quantum chromodynamics, the quantum field theory describing strong interactions, takes its name from this central property of colour.

    Protons and neutrons are examples of baryons, a class of particles that contain three quarks, each with one of three possible values of colour (red, blue, and green). Quarks may also combine with antiquarks (their antiparticles, which have opposite colour) to form mesons, such as pi mesons and K mesons. Baryons and mesons all have a net colour of zero, and it seems that the strong force allows only combinations with zero colour to exist. Attempts to knock out individual quarks, in high-energy particle collisions, for example, result only in the creation of new “colourless” particles, mainly mesons.

    In strong interactions the quarks exchange gluons, the carriers of the strong force. Gluons, like photons (the messenger particles of the electromagnetic force), are massless particles with a whole unit of intrinsic spin. However, unlike photons, which are not electrically charged and therefore do not feel the electromagnetic force, gluons carry colour, which means that they do feel the strong force and can interact among themselves. One result of this difference is that, within its short range (about 10−15 metre, roughly the diameter of a proton or a neutron), the strong force appears to become stronger with distance, unlike the other forces.

    As the distance between two quarks increases, the force between them increases rather, the inverse of the gravitational force, just as the tension does in a piece of elastic as its two ends are pulled apart. Eventually the elastic will break, yielding two pieces. Something similar happens with quarks, with sufficient energy it is not one quark but a quark-antiquark pair that is “pulled” from a cluster. Thus, quarks appear always to be locked inside the observable mesons and baryons, a phenomenon known as confinement.

    At distances comparable to the diameter of a proton, the strong interaction between quarks is about 100 times greater than the electromagnetic interaction. At smaller distances, however, the strong force between quarks becomes weaker, and the quarks begin to behave like independent particles, an effect known as asymptotic freedom.

    See: https://www.britannica.com/science/dispersion-physics
    If a particle you know has the property called color, then you get to examine the strong nuclear force. Your color can be one of red, green, or blue (confusingly there is also anti-red, anti-green and anti-blue, because of course life isn't that simple). To build a particle like a proton, all the colors of the quarks have to add up to white. Thus one quark gets assigned to be red, the other assigned to be green, and the last assigned to be blue. The particular assignment of color doesn't actually matter (and, in fact, the individual quarks constantly change color), what matters is that they all add up to white and that the strong force can do its work. This property of color is what allows the quarks to share a state inside a proton. With color, no two quarks are exactly the same — they now have different colors.