While most people are familiar with three states, the scientific understanding includes four main states, along with several other exotic states that exist under extreme conditions of temperature, pressure, or energy. The state of matter is determined by the collective behavior of its constituent particles, such as atoms, molecules, ions, and electrons. Exploring these various conditions reveals a universe far more diverse than the common forms of matter we encounter daily.
The Three Familiar States
The states of matter most common in human experience are solid, liquid, and gas. These forms are primarily distinguished by the kinetic energy of their particles and the resulting molecular spacing. A solid maintains a definite shape and volume because its particles are tightly packed in fixed positions, with energy only allowing for vibration.
A liquid has a fixed volume but takes the shape of its container because its particles possess enough energy to move past one another while still remaining close together. In a gas, the particles have enough kinetic energy to overcome all intermolecular forces, causing them to move randomly and rapidly, expanding to fill any container and having neither a fixed shape nor volume.
Plasma The Most Abundant State in the Universe
Beyond the gaseous state, adding significant energy can lead to the formation of plasma, which is often called the fourth fundamental state of matter. Plasma is essentially an ionized gas, created when the temperature becomes high enough to strip electrons from their atoms, resulting in a superheated mix of free electrons and positively charged ions. This state is electrically conductive and its behavior is governed by electromagnetic forces rather than simple particle collisions.
This highly energetic state is the most common form of ordinary matter in the visible universe, making up an estimated 99.9% of it. Stars, including our Sun, are immense balls of plasma, and the vast expanse of the interstellar and intergalactic medium is also filled with this ionized matter. On Earth, plasma is less prevalent but can be observed in natural phenomena like lightning and the auroras.
States Created Under Extreme Cold
Moving to the opposite extreme of temperature, near absolute zero (0 Kelvin), matter enters highly exotic quantum states. One such state is the Bose-Einstein Condensate (BEC), formed when a gas of bosons is cooled to extremely low temperatures. At this point, the individual atoms lose their separate identities and collapse into a single quantum mechanical state, behaving as one unified wave of matter.
Another ultra-cold state is the Fermionic Condensate, created using fermions, the particles that obey the Pauli Exclusion Principle and cannot occupy the same quantum state. To form a condensate, fermions must first be paired up, typically using magnetic fields, to behave like bosons, which can then condense. Both the BEC and the Fermionic Condensate are purely laboratory creations, requiring highly specialized equipment.
Matter Under Extreme Astrophysical Pressure
A different class of exotic matter is created not by heat or extreme cold, but by immense gravitational pressure, typically found in the remnants of massive stars. When a star exhausts its fuel, gravity can compress the core into degenerate matter, which is supported by a quantum mechanical pressure independent of temperature. Electron degenerate matter occurs in white dwarf stars, where the pressure exerted by tightly packed electrons prevents further collapse.
If the star’s core is massive enough, gravity overwhelms the electron degeneracy pressure, crushing protons and electrons into neutrons. This creates neutron degenerate matter, the substance of neutron stars, which are essentially giant atomic nuclei held together by the pressure of the closely packed neutrons.
At the highest pressures, potentially existing in the core of the largest neutron stars or recreated briefly in particle accelerators, matter may transition into a Quark-Gluon Plasma (QGP). This is a state where the fundamental particles that make up protons and neutrons—quarks and gluons—are deconfined and flow freely, a condition thought to have existed just microseconds after the Big Bang.