Matter is defined as anything that has mass and takes up space. Its state depends primarily on the thermal energy contained within its particles, which dictates how they are arranged and how they move. Historically, science focused on the three states observable in everyday life. However, examining particle behavior across a vast temperature range, from super-hot stars to temperatures near absolute zero, has confirmed the existence of five distinct states of matter.
Defining the States: Solids, Liquids, and Gases
The most familiar forms of matter are solid, liquid, and gas, distinguished by their shape, volume, and the kinetic energy of their particles. In a solid, particles are tightly packed and held in fixed positions by strong intermolecular forces. They possess the lowest kinetic energy, causing them to vibrate only slightly in place. This gives the solid a definite shape and a fixed volume that resists compression.
When energy is added to a solid, particles gain enough kinetic energy to overcome some attractive forces, transitioning the substance into a liquid. Liquid particles remain close together, retaining a fixed volume, but they can move and slide past one another. This movement allows a liquid to take the shape of any container it occupies.
Further addition of energy causes the substance to reach its gaseous state, where particles possess a high degree of kinetic energy. Gas particles are far apart and move rapidly and randomly. A gas has neither a fixed shape nor a fixed volume and will expand indefinitely to fill the container, making it easily compressible.
A common example illustrating these transitions is water, which exists as solid ice, liquid water, and gaseous steam. These three states demonstrate a progression of increasing particle energy and decreasing particle organization. The transition from one state to the next results from energy transfer affecting the forces between atoms and molecules.
The Fourth State: Plasma and Extreme Energy
Applying intense energy to a gas, such as intense heat or a strong electromagnetic field, results in the formation of plasma, the fourth state of matter. Plasma is often described as an ionized gas, a condition where the energy input is so great that it strips electrons from the atoms. This ionization creates a swirling mixture of positively charged ions and negatively charged free electrons.
The presence of these charged particles makes plasma an excellent conductor of electricity, unlike a neutral gas. Although it contains both positive and negative charges, plasma is electrically neutral overall because the number of ions and electrons is roughly equal. Plasma behavior is dominated by collective electromagnetic forces, differing significantly from the random motion seen in ordinary gases.
Plasma is the most abundant form of matter in the visible universe, making up an estimated 99.9% of all ordinary matter. Natural examples include the material that composes stars, lightning, and the auroras on Earth. Scientists also generate plasma artificially for technological applications, including neon signs, plasma televisions, and fusion research.
The Fifth State: Bose-Einstein Condensate (BEC)
In contrast to the high-energy environment required for plasma, the fifth state of matter, the Bose-Einstein Condensate (BEC), exists at the opposite extreme of the temperature scale. The BEC is formed when a gas of bosons (particles with integer spin) is cooled to temperatures incredibly close to absolute zero (0 Kelvin, or approximately -273.15 degrees Celsius).
At these ultracold temperatures, individual atoms lose their separate identities and transition into the lowest possible quantum energy state. A large fraction of the atoms condense and coalesce, behaving as a single, unified entity often called a “superatom.” The entire group is described by a single wave function, making microscopic quantum phenomena visible on a near-macroscopic scale.
The BEC was first predicted in the 1920s by Albert Einstein, building on the work of Satyendra Nath Bose, but it was not successfully created in a laboratory until 1995. The difficulty lies in achieving and maintaining the necessary temperature, often within a few billionths of a degree above absolute zero. BECs have properties related to superfluidity, allowing scientists to study the fundamental laws of quantum physics in a controlled environment.