Matter constitutes the physical substance of the universe that we can observe and measure. All physical objects, from the air we breathe to distant stars, are composed of this substance. While matter exists in wildly different forms—from solids to gases—it shares fundamental, quantifiable characteristics. These universal properties allow scientists to categorize and study everything from subatomic particles to galaxies. Any substance that qualifies as matter must exhibit a specific pair of defining traits.
The Universal Property of Mass
The first property shared by all matter is mass, an intrinsic measure of the quantity of matter a substance contains. Mass is a fixed property of a body, unlike weight, which measures the gravitational force exerted on that mass. This fundamental attribute is tied directly to inertia, which is the resistance an object offers to any change in its state of motion. For instance, an object with greater mass requires a stronger force to start or stop moving.
This resistance to acceleration is defined by Newton’s second law of motion (force equals mass times acceleration). The inertial mass of an object determines how much it resists an applied force, making it a universal characteristic of all matter. Furthermore, mass creates a gravitational field, meaning all matter attracts all other matter. This gravitational mass is equivalent to inertial mass, providing a unified physical property that affects every particle and object, including subatomic particles like quarks and leptons.
The Requirement to Occupy Space
The second defining trait of all matter is that it occupies space, a property known as volume. Volume is an extensive physical property, meaning its measurement varies with the amount of the substance present. This requirement to take up space means that two distinct pieces of matter cannot exist in the exact same location simultaneously. This principle, sometimes called impenetrability, is a direct consequence of fundamental quantum mechanics.
The reason atoms and subatomic particles take up space is linked to the Pauli exclusion principle, which applies to fermions like electrons. This principle dictates that no two electrons can occupy the same quantum state, forcing them to maintain distance. This repulsion prevents matter from collapsing and gives objects their apparent volume and structure. For example, water spills from a full cup because new molecules cannot displace the existing ones and share the same volume. This spatial requirement also allows for the calculation of density, defined as the ratio of an object’s mass to its volume.
Distinguishing Matter from Pure Energy
The dual requirements of having mass and occupying space serve to clearly differentiate matter from pure energy. Forms of energy, such as light, heat, and sound, are not considered matter because they lack these defining properties. Light, which is composed of photons, is a form of electromagnetic radiation that possesses no rest mass and does not occupy a fixed volume. Photons travel at the speed of light and are pure energy, making them fundamentally different from the particles that compose atoms.
Heat is the transfer of thermal energy, and sound is a wave of mechanical energy transmitted through a medium. Neither phenomenon has intrinsic mass or occupies measurable space like a physical object. While mass and energy are interconvertible (as described by E = mc²), only matter exhibits both mass and the requirement to occupy space. This distinction establishes mass and volume as the two universal properties shared by every substance categorized as matter.