The atomic nucleus, the dense center of every atom, is built from protons and neutrons, collectively known as nucleons. Scientific exploration into the subatomic world revealed that protons and neutrons are not elementary particles. Instead, they are composites made up of smaller, fundamental constituents bound together by a powerful force. Understanding what lies within the proton and neutron is key to understanding the mass and stability of all matter in the universe.
Defining the Fundamental Components: Quarks
The fundamental components of protons and neutrons are particles called quarks. The quarks that make up everyday matter come in two “flavors”: the up quark (‘u’) and the down quark (‘d’). These quarks possess fractional electrical charges. The up quark carries a positive charge of \(+2/3\) of the elementary charge, while the down quark carries a negative charge of \(-1/3\).
Protons and neutrons are defined by their combination of three quarks, making them members of a particle class called baryons. A proton has a composition of two up quarks and one down quark (uud), resulting in a net electrical charge of \(+1\). The neutron is composed of one up quark and two down quarks (udd), giving it a neutral net charge of zero. This configuration differentiates the two types of nucleons and determines their electrical properties.
The Binding Mechanism: Gluons and the Strong Nuclear Force
The force that holds quarks together within a proton or neutron is the strong nuclear force. This interaction is mediated by force-carrying particles called gluons, which are a type of boson. Quarks carry a property called “color charge,” which acts as the source of the strong force, analogous to how electric charge is the source of the electromagnetic force.
The strong force is described by the theory of Quantum Chromodynamics (QCD), which dictates that all observable particles must be “colorless” overall, meaning the three quarks inside a nucleon must combine their color charges to cancel out. Unlike the photon in electromagnetism, gluons themselves carry color charge, allowing them to interact not only with quarks but also with other gluons.
The force between quarks does not diminish with distance; instead, it dramatically increases, similar to a stretching elastic band. This phenomenon, known as color confinement, means that it would take an infinite amount of energy to separate a single quark from a nucleon. If enough energy is supplied, that energy actually converts into new quark-antiquark pairs, preventing any single quark from being observed in isolation.
The Origin of Mass in Protons and Neutrons
A common misconception is that the mass of a proton or neutron is simply the sum of the masses of its three constituent quarks. However, the mass of the up and down quarks themselves is remarkably small, accounting for only about 1% of the total mass of the nucleon. For instance, a proton’s total mass is approximately 938 mega-electron volts (MeV), while its three quarks contribute only about 9 to 12 MeV.
The overwhelming majority of the proton and neutron’s mass, roughly 95% to 98%, comes from the energy associated with the motion and interaction of the quarks and gluons. According to Einstein’s equation, \(E=mc^2\), energy and mass are interchangeable; the immense kinetic energy of the quarks moving near the speed of light within the tiny confines of the nucleon contributes significantly to its total mass. This energy is trapped within the strong force field, giving rise to the particle’s mass.
The binding energy of the strong force and the kinetic energy of the gluons are what provide the bulk of the particle’s mass. This explains why the neutron, made of two down and one up quark, is only slightly heavier than the proton, made of two up and one down quark, despite the down quark being heavier than the up quark. The strong force’s binding energy is so dominant that it overrides the small difference in the constituent quark masses, ensuring the near-equal mass of the two nucleons.