The concept of matter is anything that has mass and takes up space. However, matter can exist in a multitude of physical forms. The form matter takes depends entirely on environmental conditions, particularly the amount of energy and the external pressure applied to the particles. A single substance can dramatically alter its characteristics in response to these changes, leading to different physical behaviors. These distinct forms are known as states of matter, and their number is far greater than the three most commonly taught forms.
Addressing the Count: Why the Number Varies
The idea that there are twenty-two, or any other large, specific number of states of matter, is a common source of confusion stemming from definition. In physics, a true state of matter is separated from another by a fundamental change in the organization and collective behavior of its particles, known as a phase transition. This transition involves an abrupt change in a material’s properties, such as density or conductivity.
The varying count arises because many subcategories are often mistakenly included as full states. For instance, the solid state has many phases, such as crystalline solids, amorphous solids (like glass), and quasicrystals, which possess unique structures but are fundamentally solids. Different magnetic arrangements, such as ferromagnetism or antiferromagnetism, are sometimes counted separately because they are delineated by their own phase transitions. However, they do not represent a wholly new form of matter in the same way that a gas differs from a liquid. The scientific consensus focuses on a smaller set of fundamental states, with many exotic forms existing only under extreme conditions.
The Four Fundamental States of Matter
The most familiar forms of matter are defined by the kinetic energy of their particles and the strength of the intermolecular forces holding them together. At the lowest energy level, a solid is characterized by particles tightly locked into fixed positions, only vibrating in place. This strong internal cohesion gives a solid a definite shape and a fixed volume, regardless of its container.
Adding energy weakens the bonds, allowing particles to move past one another while remaining close, resulting in the liquid state. A liquid maintains a fixed volume because attractive forces are still significant. Its lack of a fixed shape allows it to conform to the boundaries of its container, permitting the characteristic property of flow.
When enough energy is added to overcome nearly all attractive forces, the particles gain sufficient kinetic energy to separate widely and move rapidly, forming a gas. A gas neither possesses a fixed volume nor a fixed shape, expanding indefinitely to fill any container it occupies. The large spaces between particles also make the gas state easily compressible.
Heating a gas to extremely high temperatures, such as those found on the sun, strips electrons from their atoms, creating the fourth fundamental state, plasma. This highly energetic state consists of a neutral mixture of positively charged ions and free-moving electrons. Plasma is the most common state of matter in the visible universe, making up the bulk of stars and lightning, and its charged nature makes it highly responsive to electric and magnetic fields.
Quantum and Low-Temperature Condensates
Moving away from the kinetic energy of classical states, some forms of matter are defined by quantum mechanics near absolute zero. The Bose-Einstein Condensate (BEC) is achieved by cooling a dilute gas of bosonic atoms to a few billionths of a degree above absolute zero. At this temperature, the atoms’ matter waves overlap so significantly that particles coalesce into a single quantum state. This collective behavior means the entire BEC acts like one giant, coherent matter wave rather than individual particles.
A closely related state is the Fermionic Condensate (FC), which is more difficult to create because fermions, like electrons, are restricted by the Pauli Exclusion Principle from occupying the same quantum state. To circumvent this, scientists force fermionic atoms to pair up into composite particles, often called Cooper pairs, which collectively behave as bosons. Once paired, these composite bosons can undergo the same condensation as a BEC.
These low-temperature quantum effects also lead to superfluids and superconductors. A superfluid, such as liquid helium-4 cooled below 2.17 Kelvin, flows with zero viscosity, meaning it experiences no internal friction and can flow up and over the sides of a container. Similarly, a superconductor exhibits zero electrical resistance when cooled below a critical temperature. This zero resistance occurs because the Cooper pairs are bound together by an energy greater than the energy lost to scattering off the crystal lattice, allowing current to flow indefinitely.
High-Energy and Astrophysical States
At the opposite end of the energy spectrum, matter exists in forms generated by immense gravitational pressure, typically found in the remnants of massive stars. Degenerate Matter is a state where gravity has compressed a star’s core so tightly that its density is governed by quantum mechanics, not thermal pressure. In a white dwarf star, electron degeneracy pressure prevents further collapse. The Pauli Exclusion Principle forces electrons into higher energy levels, creating an outward pressure that counteracts gravity. This pressure can only support the star up to the Chandrasekhar limit of about 1.4 solar masses.
If a star’s core exceeds this limit, gravity overcomes the electron degeneracy pressure, forcing electrons and protons to combine into neutrons. The resulting neutron star is supported by neutron degeneracy pressure, a stronger quantum force exerted by the densely packed neutrons. Neutron stars are among the densest objects in the universe, with a teaspoon of their material weighing approximately one billion tons.
The most extreme high-energy state is the Quark-Gluon Plasma (QGP), a state that last existed just microseconds after the Big Bang. This plasma is formed when matter is heated to temperatures exceeding four trillion degrees Celsius, a condition replicated in particle accelerators like the Large Hadron Collider. Under these conditions, the fundamental constituents of protons and neutrons—quarks and gluons—are deconfined from their normal bounds and flow freely in a dense, liquid-like state.