Matter can exist in several distinct forms called states, or phases. A state of matter is defined by the energy and temperature of its particles, which dictate how they behave and interact. While the three most common forms are familiar from everyday experience, modern physics recognizes many more states that reveal how matter behaves under extreme conditions of heat or cold. These states have been observed in laboratories or exist naturally in space.
States Defined by Familiarity
The three classical states of matter are distinguished by their structure and volume. A solid maintains a fixed shape and a fixed volume because its particles are closely packed and held in rigid positions. A liquid, in contrast, has a fixed volume but no fixed shape, allowing it to flow and conform to the container it occupies. This occurs because the particles are still close but can move past one another.
A gas possesses neither a fixed shape nor a fixed volume, expanding completely to fill any container it occupies. Gas particles are widely separated and move randomly at high speeds, with minimal interaction between them. The phase transitions between these states—like melting, freezing, boiling, and condensation—are everyday occurrences triggered by changes in temperature or pressure. Establishing these three fundamental states provides the basis for understanding the more exotic states that exist beyond normal terrestrial conditions.
The High Energy State
The fourth state of matter, plasma, is considered a high-energy form of gas. It is created when a gas is heated to extremely high temperatures or subjected to a strong electromagnetic field, causing the atoms to become ionized. Ionization strips the electrons from the atoms, resulting in a soup of negatively charged electrons and positively charged ions.
Plasma differs fundamentally from a neutral gas because it is an excellent conductor of electricity and is strongly influenced by magnetic fields. This unique behavior allows plasma to be manipulated and confined by electromagnetic forces. Plasma is the most common state of ordinary matter in the universe, making up stars, nebulas, and the space between galaxies. On Earth, it is found in lightning, the polar aurora, and technological applications like neon signs and plasma torches.
States Defined by Quantum Cold
The fifth and sixth states of matter are defined by the extreme opposite of high energy: near-absolute zero temperatures. These states reveal macroscopic quantum phenomena that are usually confined to the subatomic world. The nature of these states depends on the type of particle involved, specifically whether they are bosons or fermions.
The Bose-Einstein Condensate (BEC) is formed when a gas of bosons—particles with integer spin—is cooled to mere billionths of a degree above absolute zero (0 Kelvin). At this temperature, the atoms’ matter waves overlap so significantly that the individual particles lose their separate identities and coalesce into a single quantum mechanical entity occupying the lowest possible energy state. This collective behavior allows scientists to observe quantum effects, such as superfluidity and interference, on a scale large enough to be seen.
The Fermionic Condensate (FC) is the counterpart to the BEC, formed from fermions—particles with half-integer spin. Fermions are governed by the Pauli Exclusion Principle, which forbids two identical fermions from occupying the exact same quantum state. To bypass this rule and condense, the fermions must first pair up, often into composite particles called Cooper pairs, which collectively behave like bosons.
This pairing allows the collective to form a superfluid phase, meaning it can flow with zero viscosity. The first FC was created in 2003 using potassium-40 atoms. This state is closely related to the phenomenon of superconductivity, where electrons flow through a material without resistance.
The Most Extreme State
The seventh state, Quark-Gluon Plasma (QGP), exists at temperatures and pressures vastly exceeding those of regular plasma. It is a state where the building blocks of protons and neutrons—quarks and gluons—are no longer confined within those larger particles. This deconfined state is achieved at temperatures estimated to be over 4 trillion degrees Celsius, a condition thought to have existed only microseconds after the Big Bang.
This state is created in laboratories by colliding heavy ions at nearly the speed of light in specialized particle accelerators. Under these extreme collision conditions, the protons and neutrons “melt,” releasing their constituent quarks and gluons to move freely. Although named a plasma, QGP behaves less like an ideal gas and more like a nearly perfect liquid with extremely low viscosity, meaning it flows almost frictionlessly. Studying the properties of QGP provides scientists with unique insights into the fundamental nature of the strong nuclear force and the earliest moments of the universe.