Plasma is a state of matter created when a gas is heated to such extreme temperatures that its atoms lose their electrons, resulting in a superheated mixture of free electrons and positively charged ions. This electrically charged medium is often referred to as the “fourth state of matter,” distinct from solids, liquids, and gases. The recognition of this unique state was a gradual understanding that emerged from decades of scientific curiosity into electrical phenomena. Scientists in the 19th and 20th centuries probed the fundamental components of the universe, and their experiments with electrical currents in vacuum environments illuminated the existence of this ubiquitous charged substance.
Early Experiments with Electrical Discharges
The path to identifying plasma began with intense experimentation involving partially evacuated glass tubes and high-voltage electricity in the late 19th century. Early researchers, including Sir William Crookes, were fascinated by the glowing rays that appeared when a strong current was passed through a low-pressure gas inside these so-called Crookes tubes. Crookes conducted a series of ingenious experiments, which led him to believe that he had found a new form of matter.
Crookes observed that these “cathode rays,” which traveled from the negative electrode, could be deflected by a magnet, proving they possessed an electrical charge. He also designed a tube with a small paddlewheel, which the rays caused to spin, suggesting they were composed of particles with mass and momentum. Based on these observations, Crookes proposed in 1879 that this phenomenon was a fourth state of matter, which he termed “radiant matter.”
While Crookes was correct in identifying a unique state, his “radiant matter” was later more precisely characterized by J.J. Thomson. In 1897, Thomson used modified Crookes tubes to measure the properties of these cathode rays. He demonstrated that the rays were composed of tiny, negatively charged particles—the electron—which were far smaller than any known atom. This work confirmed that Crookes had been observing an ionized gas, though the collective behavior of this ionized medium was not yet formally named.
Defining the New State of Matter
The formal naming of this ionized gas as a collective entity occurred decades later in the 1920s through the work of American chemist and physicist Irving Langmuir. Langmuir was studying electric discharge phenomena at the General Electric Research and Development Center. His investigations focused on the internal structure and dynamics of the glowing, electrified gas created inside the discharge tubes.
Langmuir, with his colleague Lewi Tonks, observed that in the center of the discharge, the gas contained nearly equal numbers of positive ions and negative electrons, making the region electrically neutral overall. This core region exhibited a distinct collective behavior, unlike the less-ionized gases near the electrodes. Langmuir needed a term to describe this electrically active fluid.
In 1928, Langmuir settled on the word “plasma,” borrowing a term already in use in biology to describe the fluid part of blood. He chose this name because the electrically neutral core region acted as a fluid “substratum” that carried high-velocity electrons, ions, and gas impurities, much like blood plasma carries red and white corpuscles. This analogy cemented the term “plasma” in physics, giving the fourth state of matter its permanent name.
Recognizing Plasma’s Ubiquity
Following Langmuir’s definition, scientists began to recognize that plasma was not merely a laboratory curiosity confined to discharge tubes, but a state of matter with cosmic significance. The vast majority of the observable universe exists in this state. Stars, including our Sun, are dense balls of plasma, where the extreme heat has ionized the constituent atoms.
The space between stars and galaxies, once thought to be an empty vacuum, is also filled with a diffuse, low-density plasma. It is estimated that over 99% of the visible matter in the universe is in the plasma state, from the solar wind that streams off the Sun to the distant nebulae. This recognition spurred new fields of study, such as plasma astrophysics, which investigates cosmic plasmas, and fusion science, which attempts to replicate the Sun’s plasma processes on Earth to produce clean energy.