State of matter

• 1. Introduction

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  State of matter is the answer to a question most people stop asking after childhood: why does ice feel different from steam, when both are water? In physics, a state of matter is one of the distinct forms in which matter can exist. Four are observable in everyday life: solid, liquid, gas, and plasma. The difference comes down to how the component particles are arranged and how they behave together. But the four familiar states are only the opening. Cool certain particles close to absolute zero and they pile into a single quantum state. Crush atoms inside a dead star and electrons fuse with protons into a sea of neutrons. Push a nucleus near the speed of light and it appears as a wall of gluons. This is a story about ordering, and about how far that ordering can be pushed. It also raises a quieter question. If ice has fifteen known crystal structures, what does it even mean to call something simply solid?

• 2. The Familiar Four

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  In a solid, the particles are tightly packed and held in fixed positions, so the material keeps a definite shape and a definite volume. The forces between them are strong enough that the particles can only vibrate. A solid changes shape only when an outside force breaks or cuts it. In a liquid, the particles stay close but can move past one another, holding a nearly constant volume while taking the shape of the container. Heat a solid above its melting point and it becomes liquid, provided the pressure stays above the substance's triple point. A gas is a compressible fluid that conforms to its container and then expands to fill it. In a gas, the molecules carry enough kinetic energy that intermolecular forces matter little, and neighbors sit much farther apart than their own size. Plasma resembles a gas but carries charged particles, ions and free electrons, that move independently and answer to electric and magnetic fields. A gas turns into plasma either through a huge voltage difference between two points or through extremely high temperatures. Plasma is the most abundant of the four: 99% of all ordinary matter in the universe is plasma, since it composes all stars.

• 3. One Substance, Many Phases

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  Iron has a body-centred cubic structure below 912 C and a face-centred cubic structure between 912 and 1394 C, two solid phases of a single metal. The term phase is sometimes used as a synonym for state of matter, but a single compound can form different phases that share the same state. Ice is the solid state of water, yet it has multiple phases with different crystal structures, formed at different pressures and temperatures. A phase transition marks a change in structure, recognizable as an abrupt change in properties. Any set of states separated from another set by a phase transition counts as a distinct state of matter. The appearance of superconductivity is tied to a phase transition, so there are superconductive states; ferromagnetic states are demarcated the same way. When the change happens in stages, the intermediate steps are called mesophases, a behavior put to work in liquid crystal technology. A supercritical fluid lives above both the critical temperature and the critical pressure, where the line between liquid and gas disappears. Supercritical carbon dioxide, with the properties of a gas but the density to act as a solvent, is used to extract caffeine in making decaffeinated coffee.

• 4. Order Without Position

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  Transition metal atoms often carry magnetic moments from the net spin of unpaired electrons that never form chemical bonds. Magnetic states do not depend on where atoms sit in space; they depend on how those intrinsic spins align. Even in a solid where atoms are locked in position, the spins can organize in distinct ways. In a ferromagnet such as solid iron, the magnetic moment on each atom points the same direction within a domain, and if the domains align too, the result is a permanent magnet. That magnetization vanishes when the magnet is heated to the Curie point, which for iron is 768 C. An antiferromagnet holds two networks of equal and opposite moments that cancel to zero net magnetization; in nickel(II) oxide, half the nickel atoms point one way and half the other. A ferrimagnet has two opposite but unequal networks, so cancellation is incomplete and a net magnetization remains, as in magnetite. Then there is the quantum spin liquid, a solid whose magnetic order stays inherently disordered even at very low temperatures. Its domains are neither parallel nor antiparallel but randomly oriented, often because geometrically frustrated moments cannot settle uniformly. When the system must choose, and the options share similar energy, it picks one at random, leaving strong short-range order but no long-range magnetic order.

• 5. Between Solid And Liquid

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  Para-azoxyanisole is a long rod-like molecule, and in its nematic range it flows like a liquid while every molecule points the same direction within a domain. Liquid crystal states sit between mobile liquids and ordered solids: they flow, yet they hold long-range order, and unlike a liquid they react to polarized light. Several types matter technologically, including in liquid crystal displays. Glass takes a different middle path. It is a non-crystalline, amorphous solid that shows a glass transition when heated toward the liquid state, and it can be built from inorganic networks, metallic alloys, ionic melts, aqueous solutions, molecular liquids, or polymers. Thermodynamically a glass is metastable with respect to its crystalline counterpart, but the conversion rate is practically zero. Disorder can also be frozen into otherwise ordered structures. A plastic crystal has long-range positional order while its molecules keep rotational freedom; freeze that freedom and you get an orientational glass. A spin glass freezes magnetic disorder the same way. Copolymers add yet another route: because their blocks are covalently bonded, they cannot demix the way oil and water do, so they separate into nanometre-sized periodic structures instead, as in the styrene-butadiene-styrene block copolymer.

• 6. Closest To Absolute Zero

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  Bose-Einstein condensation was predicted in 1925 by Albert Einstein, building on particle statistics he developed with Satyendra Nath Bose. When bosonic particles are cooled close to absolute zero, a large fraction suddenly drops into the same lowest energy quantum state. In 1937, helium-4 was found to form a superfluid below the lambda temperature of 2.17 K, flowing with zero viscosity and no friction. A superfluid will try to climb out of its container, carries infinite thermal conductivity so no temperature gradient can form, and produces quantized vortices when its container spins. In the gas phase, the condensate stayed an unverified prediction for years. Then in 1995, Eric Cornell and Carl Wieman of JILA at the University of Colorado at Boulder made the first such condensate with rubidium atoms, while Wolfgang Ketterle independently made one in a gas of sodium atoms the same year. Fermions can join the club too. The Pauli exclusion principle bars individual fermions from the same quantum state, but paired fermions behave like bosons and can condense. Superconductors are one example, along with the superfluid phase of helium-3 and ultracold lithium-6. A superconductor has zero electrical resistivity and excludes all magnetic fields from its interior, the Meissner effect, which is why superconducting magnets serve as electromagnets in magnetic resonance imaging machines. Superconductivity was discovered in 1911 and, for 75 years, known only in some metals and alloys below 30 K. High-temperature superconductivity arrived in 1986 in certain ceramic oxides, since observed as high as 164 K.

• 7. Crushed And Accelerated

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  Inside white dwarf stars, electron-degenerate matter forms when electrons stay bound to atoms yet can transfer to adjacent ones. Under the extreme pressure inside dead stars, ordinary matter transitions to exotic states collectively called degenerate matter, supported mainly by quantum mechanical effects and the Pauli exclusion principle. Such matter expands little when heated, because there are simply no momentum states left, so degenerate stars collapse to very high densities. More massive degenerate stars are smaller, since gravity grows but pressure does not keep pace. In neutron stars, vast gravitational pressure forces electrons to combine with protons via inverse beta-decay, leaving a superdense mass of neutrons; free neutrons normally decay with a half life of about 10 minutes, but here inverse decay overtakes that. Quark matter goes further. In regular cold matter, the strong force confines quarks into hadrons of 2-4 quarks, such as protons and neutrons, but quark matter deconfines them at extremely high densities or temperatures. Strange matter, suspected inside some neutron stars near the Tolman-Oppenheimer-Volkoff limit of roughly 2-3 solar masses, manifests part of its energy as strange quarks, a heavier analogue of the down quark. Quark-gluon plasma frees quarks into a sea of gluons; theorized in the late 1970s and early 1980s, it was first detected in the laboratory at CERN in the year 2000, and unlike ordinary plasma it flows like a liquid. Faster still, a nucleus traveling near light speed appears length-contracted, its gluons forming a gluonic wall whose density climbs at very high energies. That wall is the color-glass condensate, an intrinsic property observable at the Relativistic Heavy Ion Collider and possibly the Large Hadron Collider. Theory pushes even past these: a Hagedorn temperature near 10 to the power 30 K where superstrings are copiously produced, and the Planck temperature near 10 to the power 32 K where gravity grows significant between individual particles. No current theory can describe such states, but the universe may have passed through them in the Big Bang.