A neutron is a subatomic particle found within the atomic nucleus, alongside protons. While the interaction between two electrically charged particles is straightforward, the forces governing two neutrons—which possess no net electrical charge—are a delicate balance of attraction and repulsion. Understanding this balance requires moving beyond the familiar forces of daily life and delving into the physics of extremely short distances. The complete answer is not a simple yes or no, but rather a distance-dependent phenomenon involving two distinct mechanisms.
The Neutron’s Identity and Electrical Neutrality
The neutron’s defining characteristic is its electrical neutrality, meaning it carries a net charge of zero. This contrasts sharply with the proton, which possesses a single positive charge and readily repels other protons via the electromagnetic force. Because neutrons lack this net charge, the powerful electrostatic repulsion that threatens to tear atomic nuclei apart does not directly apply to neutron-neutron interactions.
The neutron is classified as a hadron, a composite particle made up of three smaller particles called quarks. Specifically, a neutron contains two down quarks (charge \(-1/3\)) and one up quark (charge \(+2/3\)), resulting in zero net charge.
The internal structure of these charged quarks is why the neutron, despite its overall neutrality, still exhibits a magnetic moment. This internal charge distribution allows the neutron to interact with magnetic fields. The absence of net charge removes the electromagnetic force from the primary discussion, setting the stage for the short-range nuclear forces to take over.
The Strong Nuclear Force: The Mechanism of Attraction
The primary force governing the binding of neutrons and all nucleons within the nucleus is the nuclear force, which is the residual effect of the strong interaction. This force overcomes the electromagnetic repulsion between protons to hold the nucleus together. The nuclear force is mediated by the exchange of particles called mesons, such as pions, between the nucleons.
Unlike the infinite range of electromagnetism and gravity, the nuclear force is extremely short-ranged, dropping to zero significance beyond about \(2.5\) femtometers. At typical nuclear separation distances, specifically around \(0.8\) to \(1.0\) femtometers, this force is overwhelmingly attractive. This attraction creates a low-energy configuration, often described as a “potential well,” where the nucleons are most stable and tightly bound together.
The attractive nature of this force is what makes stable atomic nuclei possible. Neutrons act as nuclear “glue” to increase the overall strong force attraction without adding to the disruptive electromagnetic repulsion. This intense, short-range attraction is confined strictly to the tiny volume of the atomic nucleus.
Repulsion at Extreme Proximity
The attraction provided by the nuclear force does not continue indefinitely as neutrons move closer together; the force reverses to become repulsive at very short distances. When the separation between the centers of two neutrons drops below a critical threshold, typically around \(0.7\) femtometers, the strong nuclear force rapidly transitions into a powerful repulsive interaction. This phenomenon is often termed the “hard core” repulsion.
This switch prevents the nucleus from collapsing under the pressure of the attractive force. The repulsive core acts as a quantum mechanical barrier, dictating the minimum physical size of a neutron and the overall volume of the atomic nucleus. This distance-dependent behavior is a consequence of the underlying quark-gluon dynamics within the composite neutrons.
The second source of repulsion is the Pauli Exclusion Principle (PEP), which applies because neutrons are fermions (particles with half-integer spin). The PEP dictates that no two identical fermions can occupy the exact same quantum state simultaneously. If two neutrons attempt to occupy the same small region of space, the principle forces them into different momentum states. This required difference in momentum translates into a degeneracy pressure that resists compression.
Implications for Nuclear Stability and Neutron Stars
The interplay between the nuclear force’s attraction at moderate distances and its repulsion at extreme proximity determines the stability and size of every atomic nucleus. The attractive component provides the binding energy necessary to hold the atom together, while the repulsive core establishes the nuclear radius and prevents catastrophic collapse. Stable nuclei exist only because the strong force has this dual nature.
This precise balance of forces is tested in the core of a neutron star. A neutron star is essentially a giant nucleus, composed almost entirely of neutrons packed to high density following the collapse of a massive star. Gravity attempts to crush the star into a black hole, but this is resisted by the repulsion mechanisms.
The Pauli Exclusion Principle creates a neutron degeneracy pressure that provides some initial support. However, this pressure alone is insufficient to support most observed neutron stars, which often exceed \(1.4\) solar masses. It is the short-range repulsive core of the strong nuclear force, which activates at femtometer scales, that provides the necessary outward pressure to counteract the crushing force of gravity and stabilize the star. The neutron-neutron repulsion is the primary structural feature that prevents a neutron star from collapsing further into a black hole.