Strong interaction

• 1. Introduction

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  The strong interaction is the reason a proton does not fly apart. Inside it sit quarks that carry only about 1 percent of the proton's mass. The rest is pure binding energy, the residue of a force so fierce that physicists named it for its raw power. At a distance of 10 to the minus 15 meters, roughly the width of a nucleon, this force is about 100 times stronger than electromagnetism. It is around a million times stronger than the weak interaction. And it is some 10 to the 38th power stronger than gravity. Yet before 1971 nobody could explain why the nucleus held together at all. Positive charges should repel. Protons crammed together should scatter. Something stronger had to be pulling the opposite way. What follows is the story of that something: how it confines quarks so completely that none has ever been seen alone, how it leaks out to bind whole nuclei, and why pulling two quarks apart only makes more quarks.

• 2. The Nucleus That Should Not Hold

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  Protons carry positive electric charge and neutrons carry none. From the rules of electromagnetism, that arrangement is a problem. Like charges repel, so a cluster of protons should drive itself apart, and the nucleus should never survive. But it does survive, and that contradiction sat unresolved for decades. Physicists postulated a stronger attractive force to account for it. They called it the strong force and treated it as fundamental, acting on the protons and neutrons that make up the nucleus. The idea was a placeholder for a mechanism nobody had yet found. In 1964 the placeholder began to dissolve. Murray Gell-Mann, and independently George Zweig, proposed that baryons and mesons were built from smaller elementary particles. Zweig called them aces; Gell-Mann called them quarks, and that name stuck along with the quark model. The attraction between nucleons turned out to be a side effect of something deeper, a force binding quarks inside the protons and neutrons themselves.

• 3. Color And The Gluon

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  Quantum chromodynamics, or QCD, is the theory that describes the strong force, and it forms part of the Standard Model of particle physics. Mathematically it is a non-abelian gauge theory built on a local symmetry group called SU(3). At its center is a property called color charge, which has no relation to visible color. Color charge comes in three types rather than one: red, green, and blue, each with a positive and negative form. That threefold structure gives the strong force rules unlike anything in electromagnetism. The gluon is the force carrier, a massless gauge boson that ferries this interaction between particles. Unlike the photon, which is electrically neutral, the gluon itself carries color charge. Quarks and gluons are the only fundamental particles with non-vanishing color charge, so they participate in strong interactions only with each other. The strength of their coupling is set by the strong coupling constant. Because gluons carry the very charge they respond to, they interact not only with quarks but with other gluons, a feedback that shapes everything the force does.

• 4. Why Quarks Cannot Escape

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  The strong force breaks the rule that every other force obeys. Electromagnetic, weak, and gravitational forces all weaken as objects move apart. The strong force between two quarks does not. Past a limiting distance, about the size of a hadron, it holds steady at a strength of around 10000, no matter how far the quarks are pulled. Pulling them apart adds energy to the system. That energy does not stretch a bond thinner; it manufactures new matter. The work done against a force of 10000 is enough to create particle and antiparticle pairs across a very short distance. So the energy from separating two quarks conjures a fresh pair of quarks that promptly bond with the originals. This is why no one can isolate a single quark. The phenomenon is called color confinement, and it means only hadrons, never free quarks, can be observed. Every experiment that has hunted for a free quark has failed, and that long record of failure is taken as evidence the confinement is real. When a quark in one proton is struck by a fast quark from another inside a particle accelerator, the elementary particles themselves stay hidden. What emerges instead are jets of newly created hadrons, born from mass-energy equivalence. Quark-gluon plasmas, however, have been observed.

• 5. The Force That Leaks Out

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  At distances near or beyond the radius of a proton, a leftover of the strong force survives between hadrons. This is the nuclear force, also called the residual strong force, and historically the strong nuclear force. It acts between the mesons and baryons that count as hadrons, even though those hadrons are colorless overall. The mechanism is indirect: gluons inside virtual pi and rho mesons carry the interaction from one nucleon to the next, holding together every nucleus beyond the lone proton of hydrogen-1. This residue is faint compared with the force inside a nucleon. The strong force is mostly neutralized within each proton and neutron, leaving only a minor remainder to reach across. The analogy is van der Waals forces, the weak electromagnetic attraction between neutral atoms, set against the powerful electromagnetic grip that holds electrons to a nucleus. Unlike the strong force inside hadrons, this residual force fades with distance, and it fades fast, dropping roughly as a negative exponential power of distance with no simple formula, a behavior captured by the Yukawa potential. That rapid fade has consequences. The attractive residual force drops off quickly while the repulsive electromagnetic force between protons drops more slowly. Beyond atomic number 82, the element lead, that imbalance makes large nuclei unstable. Still, the nuclear force remains highly energetic, and its transitions throw off gamma rays.

• 6. Mass Defect And Stellar Power

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  The mass of a nucleus is not the sum of its parts. Add up the individual nucleons and the total comes out different from the nucleus they form. That gap is the mass defect, and it traces back to the potential energy stored in the nuclear force. Differences between mass defects are what power nuclear fusion and nuclear fission. Fusion accounts for most of the energy produced in the Sun and other stars. Fission drives the decay of radioactive elements and isotopes, though that decay is often mediated by the weak interaction rather than the strong force directly. Humanity has tapped both. The energy bound up in the nuclear force is partly released in nuclear power and in nuclear weapons, from uranium and plutonium fission devices to fusion weapons such as the hydrogen bomb. The same force that quietly stabilizes a nucleus can, when its balance is broken, light a star or level a city.

• 7. The Search For One Force

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  Grand Unified Theories, known as GUTs, aim to fold the strong interaction and the electroweak interaction into a single force. The model is the earlier success of Glashow, Weinberg, and Salam, who unified electromagnetism and the weak interaction into the electroweak interaction. What makes unification plausible is a property called asymptotic freedom: the strong force grows weaker at higher energies and temperatures. There is a theorized energy, the grand unification energy, where the strong force would match the electroweak force in strength. No Grand Unified Theory has yet been successfully formulated, and grand unification remains an unsolved problem in physics. The idea reaches back to the universe's first moments. If a GUT is correct, then during the electroweak epoch after the Big Bang the electroweak force split away from the strong force. Before that, physicists hypothesize, came a grand unification epoch when the two had not yet parted, the briefest window in which the universe may have been governed by a single law.