The question of whether all matter possesses mass is central to modern physics, and its answer shifts depending on the definition used. Classically, the two concepts were inseparable, but 20th-century discoveries introduced exceptions that challenge this traditional understanding. While the familiar world is built on massive particles, the universe is also governed by entities that do not possess mass. Understanding this relationship requires moving beyond everyday experience to the quantum realm and the fundamental forces that shape reality.
Defining the Core Concepts: Matter and Mass
In classical physics and chemistry, matter is defined as any substance that has rest mass and occupies space, giving it volume. This definition describes everyday objects, from a grain of sand to a star, which are ultimately composed of atoms and subatomic particles.
Mass is an intrinsic physical property of a body. In modern terms, mass is best described as a measure of inertia, which is the resistance an object offers to a change in its state of motion when a force is applied. The greater an object’s mass, the harder it is to accelerate.
The Fundamental Link Between Matter and Mass
The strong link between matter and mass is evident in the fundamental components of ordinary matter: protons, neutrons, and electrons. All three particles possess an inherent, non-zero rest mass. Since all atoms and ordinary objects are constructed from these massive particles, the macroscopic world is inherently massive.
This connection is solidified by Albert Einstein’s mass-energy equivalence, \(E=mc^2\). This equation shows that mass and energy are interchangeable forms of the same fundamental entity. In composite particles, such as protons or atomic nuclei, the total mass is not merely the sum of the rest masses of its constituent parts.
A significant portion of an atom’s mass comes from the immense binding energy that holds the quarks together within the protons and neutrons. When energy is contained within a system, that energy contributes to the total rest mass of the composite system. For instance, the mass of a nucleus is less than the sum of the individual masses of the separate protons and neutrons, because some mass is released as energy when they bind together.
Particles That Lack Mass
The concept that all matter must have mass breaks down when examining certain elementary particles in the Standard Model of particle physics. The primary exception is the photon, the quantum of light and the carrier of the electromagnetic force. Photons have zero rest mass, yet they carry energy and momentum.
Because they are massless, photons must always travel at the speed of light in a vacuum and can never be at rest. Photons are not considered “matter” under the strict classical definition, which requires rest mass. Instead, they are classified as force-carrying particles, or bosons, which mediate interactions.
Other particles expected to be massless include the gluon, the carrier of the strong nuclear force, and the theoretical graviton, the proposed carrier of the gravitational force. Although gluons are confined within composite particles, theoretical models suggest they have zero rest mass. These massless particles demonstrate that not all fundamental entities that exist and interact in the universe possess the property of mass.
The Origin of Mass in Modern Physics
Modern physics, specifically the Standard Model, explains how fundamental particles acquire their mass through the Higgs mechanism. This mechanism involves the Higgs field, an energy field that permeates all of space. Unlike other quantum fields, the Higgs field has a non-zero value everywhere, even in a vacuum.
Fundamental particles acquire mass by interacting with this pervasive Higgs field. The strength of a particle’s interaction, or coupling, with the field determines the particle’s mass. For example, the electron interacts weakly with the Higgs field, giving it a small mass, while the top quark interacts very strongly, resulting in a significantly larger mass.
The Higgs boson is an excitation, or quantum, of the Higgs field, confirmed by its discovery in 2012. Particles that do not interact with the Higgs field, such as the photon and gluon, remain massless and therefore must travel at the speed of light. This mechanism explains mass not as an inherent, unchangeable property, but as a result of a particle’s relationship with a universal field.