Metastability denotes the phenomenon when an isolated system spends an extended time
in a configuration other than the system's state of least energy.
During a metastable state of finite lifetime all state-describing parameters reach and
hold stationary values.
While in isolation: the state of least energy is the only one
the system will indefinitely inhabit; the system will spontaneously leave any other
state to eventually return to the least energetic state.
A single particle analogy may be drawn with a ball resting in a hollow on a slope.
With some perturbation the ball may start rolling again to lower levels.
Isomerisation is another common example, where higher energy isomers are long lived as they
are prevented from rearranging to the ground state by barriers in the potential energy.
The metastability concept originates in the physics of first-order phase transitions later
to acquire new meanings in the study of aggregated subatomic particles or in molecules, macromolecules
or clusters of atoms and molecules.
Later on it was borrowed for the study of decision-making and information transmitting
systems.
Many complex natural and man-made systems can demonstrate metastability.
Metastability is common in physics and chemistry - from an atom to statistical ensembles of
molecules at molecular levels or as a whole.
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 like electronic circuits, signal trafficking, decisional systems and
neuroscience - it is the time-invariance of the active or reactive patterns with respect
to the external influences that defines stability and metastability.
Here the equivalent of the thermal fluctuations is the "white noise" affecting the signal
propagation and the decision-making.
Statistical physics and thermodynamics Non-equilibrium thermodynamics 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 being kinetically persistent.
The particular motion or kinetics of the atoms involved has resulted in getting stuck, despite
there being preferable alternatives.
States of matter Metastable states of matter range from melting
solids, boiling liquids and sublimating solids to supercooled liquids or superheated liquid-gas
mixtures.
Extremely pure, supercooled water stays liquid below 0 °C and remains so until applied vibrations
or condensing seed doping will initiate crystallization centers.
This is a common situation for the droplets of atmospheric clouds.
Condensed matter and macromolecules Metastable phases are common in condensed
matter.
For example, diamond is a metastable form of carbon at standard temperature and pressure.
It can be converted to graphite, but only after overcoming an activation energy - an
intervening hill.
Martensite is a metastable phase used to control the hardness of most steel.
The bonds between the building blocks of polymers such as DNA, RNA and proteins are also metastable.
Metastable polymorphs of silica are commonly observed.
In some cases, such as in the allotropes of solid boron, acquiring a sample of the stable
phase is difficult.
Generally speaking, emulsions/colloidal systems and glasses are metastable.
Sandpiles are one system which can exhibit metastability if a steep slope or tunnel is
present.
Sand grains form a pile due to friction.
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 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 are found to have many distinguishable states.
Of these, one is indefinitely stable: the ground state or global minimum.
All other states besides the ground state have higher energies.
Of all these other states, the metastable states are the ones having lifetimes lasting
at least 102 to 103 times longer than the shortest lived states of the set.
A metastable state is then long-lived but not eternal.
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.
One mechanism for this to happen is through tunnelling.
Nuclear physics Some energetic states of an atomic nucleus
are much longer-lived than others.
E.g.
Technetium-99m.
Atomic and molecular physics Some atomic energy levels are metastable.
Rydberg atoms are an example of metastable excited atomic states.
Transitions from metastable excited levels are typically those forbidden by electric
dipole selection rules.
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 away from a metastable state are not impossible, the electron will
eventually be able to decay to a less energetic state by spontaneous emission.
This property of the electron is used in lasers.
When light of suitable wavelength falls on atoms, their electrons jump to a higher energy
state.
When the incoming radiations are removed, the excited electron goes back to its original
level, typically within a duration of around 10−8 seconds.
However, when an electron goes to a metastable state, it remains there for a relatively longer
duration of 10−3 seconds.
This phenomenon leads to accumulation of electrons in the metastable state, since the rate of
addition of electrons to the metastable state is higher than the rate of their de-excitation.
This leads to the phenomenon called population inversion, which forms the basis of lasing
action of lasers.
Chemistry In chemical systems, a system of atoms or
molecules involving a change in chemical bond can be in a metastable state, which lasts
for a relatively long period of time.
Molecular vibrations and thermal motion make chemical species at the energetic equivalent
of the top of a round hill very short-lived.
Metastable states that persist for many seconds are found in energetic valleys which are not
the lowest possible valley.
A common type of metastability is isomerism.
The stability or metastability of a given chemical system depends its environment, particularly
temperature and pressure.
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 recommends referring to these as transient rather than metastable.
Metastability is also used to refer to specific situations in mass spectrometry and spectrochemistry.
Electron systems in biochemistry The evolution of a many-body quantum system
between its characteristic set of states may be influenced by the following external actions:
The environment may act chaotically onto the system and add uncertainty to all state energies
— as in the spectral line broadening.
Just as well, resonant exterior actions may nudge the system into a lower cohesive energy
state while making it release an intrinsic amount or quanta of its energy — as in the
stimulated emissions.
Alternatively, external catalytic fields of forces may briefly flatten some of the barriers
in the potential landscape of the system and help it tunnel through to lower energy states.
Last but not least, under the impact of thermal or directional external actions, some systems
may wander for extremely long periods of time among a certain sub-group of their states.
As such the enzymes will enter a biochemical reaction sequence with an initial configuration,
perform through its many steps as catalysts while continuously contorting, and eventually
leave that reaction sequence in the same configuration as they have entered it, ready to perform
again.
Electronic circuits Metastability in electronics is usually seen
as a problem.
A changing circuit is supposed to settle into one of a small number of desired states, but
if the circuit is vulnerable to metastability, it can get stuck in an undesirable state.
Computational neuroscience Metastability in the brain is a phenomenon
which is being studied in computational neuroscience to elucidate how the human mind recognizes
patterns.
The term metastability here is used rather loosely.
There is no lower-energy state, but there are semi-transient signals in the brain which
persist for a while and are different than the usual equilibrium state.
See also False vacuum
Hysteresis References