![Meta-stability](https://upload.wikimedia.org/wikipedia/commons/a/a0/Meta-stability.svg)
In
chemistry and
physics
Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...
, metastability denotes an intermediate
energetic state within a
dynamical system
In mathematics, a dynamical system is a system in which a function describes the time dependence of a point in an ambient space. Examples include the mathematical models that describe the swinging of a clock pendulum, the flow of water in ...
other than the system's
state of least energy.
A ball resting in a hollow on a slope is a simple example of metastability. If the ball is only slightly pushed, it will settle back into its hollow, but a stronger push may start the ball rolling down the slope.
Bowling pin
Bowling pins (historically also known as skittles or kegels) are the target of the bowling ball in various bowling games including tenpins, five-pins, duckpins and candlepins.
Tenpins
Pin specifications are set by the United States Bowling ...
s show similar metastability by either merely wobbling for a moment or tipping over completely. A common example of metastability in science is
isomerisation. Higher energy isomers are long lived because they are prevented from rearranging to their preferred ground state by (possibly large) barriers in the
potential energy.
During a metastable state of finite lifetime, all state-describing parameters reach and hold stationary values. In isolation:
*the state of least energy is the only one the system will inhabit for an indefinite length of time, until more external energy is added to the system (unique "absolutely stable" state);
*the system will spontaneously leave any other state (of higher energy) to eventually return (after a sequence of transitions) to the least energetic state.
The metastability concept originated in the physics of
first-order phase transitions. It then acquired new meaning in the study of aggregated
subatomic particles (in atomic nuclei or in atoms) or in molecules, macromolecules or clusters of atoms and molecules. Later, it was borrowed for the study of decision-making and information transmission systems.
Metastability is common in physics and chemistry – from an
atom
Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons.
Every solid, liquid, gas, ...
(many-body assembly) to statistical ensembles of
molecule
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...
s (
viscous fluid
In condensed matter physics and physical chemistry, the terms viscous liquid, supercooled liquid, and glassforming liquid are often used interchangeably to designate liquids that are at the same time highly viscous (see Viscosity of amorphous m ...
s,
amorphous solid
In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid, glassy solid) is a solid that lacks the long-range order that is characteristic of a crystal.
Etymology
The term comes from the Greek ''a'' ("wi ...
s,
liquid crystals,
mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...
s, etc.) at molecular levels or as a whole (see
Metastable states of matter and
grain piles below). The abundance of states is more prevalent as the systems grow larger and/or if the forces of their mutual interaction are spatially less uniform or more diverse.
In
dynamic systems (with
feedback) like electronic circuits, signal trafficking, decisional, neural and immune systems, – the
time-invariance of the active or reactive patterns with respect to the external influences defines stability and metastability (see
brain metastability below). In these systems, the equivalent of
thermal fluctuations
In statistical mechanics, thermal fluctuations are random deviations of a system from its average state, that occur in a system at equilibrium.In statistical mechanics they are often simply referred to as fluctuations. All thermal fluctuations b ...
in molecular systems is the "
white noise
In signal processing, white noise is a random signal having equal intensity at different frequencies, giving it a constant power spectral density. The term is used, with this or similar meanings, in many scientific and technical disciplines ...
" that affects signal propagation and the decision-making.
Statistical physics and thermodynamics
Non-equilibrium thermodynamics
Non-equilibrium thermodynamics is a branch of thermodynamics that deals with physical systems that are not in thermodynamic equilibrium but can be described in terms of macroscopic quantities (non-equilibrium state variables) that represent an ext ...
is a branch of physics that studies the dynamics of statistical ensembles of molecules via unstable states. Being "stuck" in a thermodynamic trough without being at the lowest energy state is known as having kinetic stability or being kinetically persistent. The particular motion or
kinetics of the atoms involved has resulted in getting stuck, despite there being preferable (lower-energy) alternatives.
States of matter
Metastable
states of matter
In physics, a state of matter is one of the distinct forms in which matter can exist. Four states of matter are observable in everyday life: solid, liquid, gas, and plasma. Many intermediate states are known to exist, such as liquid crystal, ...
(also referred as
metastates) range from melting solids (or freezing liquids), boiling liquids (or condensing gases) and
sublimating solids to
supercooled liquids or
superheated
A superheater is a device used to convert saturated steam or wet steam into superheated steam or dry steam. Superheated steam is used in steam turbines for electricity generation, steam engines, and in processes such as steam reforming. There are ...
liquid-gas mixtures. Extremely pure, supercooled water stays liquid below 0 °C and remains so until applied vibrations or condensing seed doping initiates
crystallization centers. This is a common situation for the droplets of atmospheric clouds.
Condensed matter and macromolecules
Metastable phases are common in condensed matter and crystallography. Notably, this is the case for
anatase
Anatase is a metastable mineral form of titanium dioxide (TiO2) with a tetragonal crystal structure. Although colorless or white when pure, anatase in nature is usually a black solid due to impurities. Three other polymorphs (or mineral form ...
, a metastable polymorph of
titanium dioxide
Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. It is a white solid that is insolub ...
, which despite commonly being the first phase to form in many synthesis processes due to its lower
surface energy, is always metastable, with
rutile
Rutile is an oxide mineral composed of titanium dioxide (TiO2), the most common natural form of TiO2. Rarer polymorphs of TiO2 are known, including anatase, akaogiite, and brookite.
Rutile has one of the highest refractive indices at visib ...
being the most stable phase at all temperatures and pressures.
As another example,
diamond
Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the chemically stable form of carbon at room temperature and pressure, ...
is a stable phase only at very high pressures, but is a metastable form of carbon at
standard temperature and pressure. It can be converted to
graphite
Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on lar ...
(plus leftover kinetic energy), but only after overcoming an
activation energy
In chemistry and physics, activation energy is the minimum amount of energy that must be provided for compounds to result in a chemical reaction. The activation energy (''E''a) of a reaction is measured in joules per mole (J/mol), kilojoules p ...
– an intervening hill.
Martensite
Martensite is a very hard form of steel crystalline structure. It is named after German metallurgist Adolf Martens. By analogy the term can also refer to any crystal structure that is formed by diffusionless transformation.
Properties
M ...
is a metastable phase used to control the hardness of most steel. Metastable
polymorphs of
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...
are commonly observed. In some cases, such as in the
allotropes
Allotropy or allotropism () is the property of some chemical elements to exist in two or more different forms, in the same physical state, known as allotropes of the elements. Allotropes are different structural modifications of an element: th ...
of solid
boron, acquiring a sample of the stable phase is difficult.
The bonds between the building blocks of
polymer
A polymer (; Greek '' poly-'', "many" + ''-mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
s such as
DNA,
RNA, and
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
s are also metastable.
Adenosine triphosphate
Adenosine triphosphate (ATP) is an organic compound that provides energy to drive many processes in living cells, such as muscle contraction, nerve impulse propagation, condensate dissolution, and chemical synthesis. Found in all known forms o ...
is a highly metastable molecule, colloquially described as being "full of energy" that can be used in many ways in biology.
Generally speaking,
emulsion
An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloids. Altho ...
s/
colloidal systems and
glass
Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling ( quenching ...
es are metastable. The metastability of silica glass, for example, is characterised by lifetimes on the order of 10
98 years (as compared with the lifetime of the universe, which is thought to be about years).
Sandpile
The Abelian sandpile model (ASM) is the more popular name of the original Bak–Tang–Wiesenfeld model (BTW). BTW model was the first discovered example of a dynamical system displaying self-organized criticality. It was introduced by Per Bak, ...
s are one system which can exhibit metastability if a steep slope or tunnel is present.
Sand grains form a pile due to
friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction:
*Dry friction is a force that opposes the relative lateral motion of ...
. It is possible for an entire large sand pile to reach a point where it is stable, but the addition of a single grain causes large parts of it to collapse.
The
avalanche
An avalanche is a rapid flow of snow down a slope, such as a hill or mountain.
Avalanches can be set off spontaneously, by such factors as increased precipitation or snowpack weakening, or by external means such as humans, animals, and eart ...
is a well-known problem with large piles of snow and ice crystals on steep slopes. In dry conditions, snow slopes act similarly to sandpiles. An entire mountainside of snow can suddenly slide due to the presence of a skier, or even a loud noise or vibration.
Quantum mechanics
Aggregated systems of
subatomic particles described by
quantum mechanics
Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistr ...
(
quarks
A quark () is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. All commonly ...
inside
nucleons
In physics and chemistry, a nucleon is either a proton or a neutron, considered in its role as a component of an atomic nucleus. The number of nucleons in a nucleus defines the atom's mass number (nucleon number).
Until the 1960s, nucleons we ...
, nucleons inside
atomic nuclei
The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden gold foil experiment. After the discovery of the neutron ...
,
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no ...
s inside
atom
Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons.
Every solid, liquid, gas, ...
s,
molecule
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...
s, or
atomic clusters) are found to have many distinguishable states. Of these, one (or a small
degenerate set) is indefinitely stable: the
ground state or
global minimum
In mathematical analysis, the maxima and minima (the respective plurals of maximum and minimum) of a function, known collectively as extrema (the plural of extremum), are the largest and smallest value of the function, either within a given ran ...
.
All other states besides the ground state (or those degenerate with it) have higher energies. Of all these other states, the metastable states are the ones having
lifetimes lasting at least 10
2 to 10
3 times longer than the shortest lived states of the set.
A ''metastable state'' is then long-lived (locally
stable with respect to configurations of 'neighbouring' energies) but not eternal (as the global
minimum
In mathematical analysis, the maxima and minima (the respective plurals of maximum and minimum) of a function, known collectively as extrema (the plural of extremum), are the largest and smallest value of the function, either within a given r ...
is). Being excited – of an energy above the ground state – it will eventually decay to a more stable state, releasing energy. Indeed, above
absolute zero, all states of a system have a non-zero probability to decay; that is, to spontaneously fall into another state (usually lower in energy). One mechanism for this to happen is through
tunnelling.
Nuclear physics
Some energetic states of an
atomic nucleus
The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden gold foil experiment. After the discovery of the neutron ...
(having distinct spatial mass, charge, spin,
isospin
In nuclear physics and particle physics, isospin (''I'') is a quantum number related to the up- and down quark content of the particle. More specifically, isospin symmetry is a subset of the flavour symmetry seen more broadly in the interactions ...
distributions) are much longer-lived than others (
nuclear isomers
A nuclear isomer is a metastable state of an atomic nucleus, in which one or more nucleons (protons or neutrons) occupy higher energy levels than in the ground state of the same nucleus. "Metastable" describes nuclei whose excited states have ha ...
of the same
isotope
Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numb ...
), e.g.
technetium-99m
Technetium-99m (99mTc) is a metastable nuclear isomer of technetium-99 (itself an isotope of technetium), symbolized as 99mTc, that is used in tens of millions of medical diagnostic procedures annually, making it the most commonly used medical ra ...
. The isotope
tantalum-180m
Natural tantalum (73Ta) consists of two stable isotopes: 181Ta (99.988%) and (0.012%).
There are also 35 known artificial radioisotopes, the longest-lived of which are 179Ta with a half-life of 1.82 years, 182Ta with a half-life of 114.43 days, ...
, although being a metastable excited state, is long-lived enough that it has never been observed to decay, with a half-life calculated to be least years, over 3 million times the current
age of the universe
In physical cosmology, the age of the universe is the time elapsed since the Big Bang. Astronomers have derived two different measurements of the age of the universe:
a measurement based on direct observations of an early state of the universe, ...
.
Atomic and molecular physics
Some atomic energy levels are metastable.
Rydberg atom
A Rydberg atom is an excited atom with one or more electrons that have a very high principal quantum number, ''n''. The higher the value of ''n'', the farther the electron is from the nucleus, on average. Rydberg atoms have a number of peculi ...
s are an example of metastable excited atomic states. Transitions from metastable excited levels are typically those forbidden by electric dipole
selection rule
In physics and chemistry, a selection rule, or transition rule, formally constrains the possible transitions of a system from one quantum state to another. Selection rules have been derived for electromagnetic transitions in molecules, in atoms, in ...
s. This means that any transitions from this level are relatively unlikely to occur. In a sense, an electron that happens to find itself in a metastable configuration is trapped there. Of course, since transitions from a metastable state are not impossible (merely less likely), the electron will eventually decay to a less energetic state, typically by an electric quadrupole transition, or often by non-radiative de-excitation (e.g., collisional de-excitation).
This slow-decay property of a metastable state is apparent in
phosphorescence
Phosphorescence is a type of photoluminescence related to fluorescence. When exposed to light (radiation) of a shorter wavelength, a phosphorescent substance will glow, absorbing the light and reemitting it at a longer wavelength. Unlike fluo ...
, the kind of
photoluminescence seen in glow-in-the-dark toys that can be charged by first being exposed to bright light. Whereas spontaneous emission in atoms has a typical timescale on the order of 10
−8 seconds, the decay of metastable states can typically take milliseconds to minutes, and so light emitted in phosphorescence is usually both weak and long-lasting.
Chemistry
In chemical systems, a system of atoms or molecules involving a change in
chemical bond
A chemical bond is a lasting attraction between atoms or ions that enables the formation of molecules and crystals. The bond may result from the electrostatic force between oppositely charged ions as in ionic bonds, or through the sharing of ...
can be in a metastable state, which lasts for a relatively long period of time. Molecular vibrations and
thermal motion
A thermal column (or thermal) is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically. Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example ...
make chemical species at the energetic equivalent of the top of a round hill very short-lived. Metastable states that persist for many seconds (or years) are found in energetic ''valleys'' which are not the lowest possible valley (point 1 in illustration). A common type of metastability is
isomerism.
The stability or metastability of a given chemical system depends on its environment, particularly
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer.
Thermometers are calibrated in various Conversion of units of temperature, temp ...
and
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
. The difference between producing a stable vs. metastable entity can have important consequences. For instances, having the wrong crystal
polymorph can result in failure of a drug while in storage between manufacture and administration. The map of which state is the most stable as a function of pressure, temperature and/or composition is known as a
phase diagram. In regions where a particular state is not the most stable, it may still be metastable.
Reaction intermediates are relatively short-lived, and are usually thermodynamically unstable rather than metastable. The
IUPAC
The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
recommends referring to these as ''transient'' rather than metastable.
Metastability is also used to refer to specific situations in mass spectrometry and spectrochemistry.
Electronic circuits
A digital circuit is supposed to be found in a small number of stable digital states within a certain amount of time after an input change. However if an input changes at the wrong moment a digital circuit which employs feedback (even a simple circuit such as a
flip-flop) can
enter a metastable state and take an unbounded length of time to finally settle into a fully stable digital state.
Computational neuroscience
Metastability in the brain is a phenomenon studied in
computational neuroscience
Computational neuroscience (also known as theoretical neuroscience or mathematical neuroscience) is a branch of neuroscience which employs mathematical models, computer simulations, theoretical analysis and abstractions of the brain to u ...
to elucidate how the human brain recognizes patterns. Here, the term metastability is used rather loosely. There is no lower-energy state, but there are semi-transient signals in the brain that persist for a while and are different than the usual equilibrium state.
See also
*
False vacuum
In quantum field theory, a false vacuum is a hypothetical vacuum that is relatively stable, but not in the most stable state possible. This condition is known as metastable. It may last for a very long time in that state, but could eventually ...
*
Hysteresis
*
Metastate
In statistical mechanics, the metastate is a probability measure on the
space of all thermodynamic states for a system with quenched randomness. The term metastate, in this context, was first used in by Charles M. Newman and Daniel L. Stein in ...
References
{{Reflist
Chemical properties
Dynamical systems