In
physics, thermalisation is the process of physical bodies reaching
thermal equilibrium through mutual interaction. In general the natural tendency of a system is towards a state of
equipartition of energy and uniform
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied on ...
that maximizes the system's
entropy. Thermalisation, thermal equilibrium, and temperature are therefore important fundamental concepts within
statistical physics,
statistical mechanics
In physics, statistical mechanics is a mathematical framework that applies statistical methods and probability theory to large assemblies of microscopic entities. It does not assume or postulate any natural laws, but explains the macroscopic ...
, and
thermodynamics
Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws o ...
; all of which are a basis for many other specific fields of
scientific understanding and
engineering application.
Examples of thermalisation include:
* the achievement of equilibrium in a
plasma
Plasma or plasm may refer to:
Science
* Plasma (physics), one of the four fundamental states of matter
* Plasma (mineral), a green translucent silica mineral
* Quark–gluon plasma, a state of matter in quantum chromodynamics
Biology
* Blood pla ...
.
* the process undergone by high-energy
neutrons
The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behave ...
as they lose energy by collision with a
moderator
Moderator may refer to:
Government
*Moderator (town official), elected official who presides over the Town Meeting form of government Internet
*Internet forum#Moderators, Internet forum moderator, a person given special authority to enforce the ...
.
The hypothesis, foundational to most introductory textbooks treating
quantum statistical mechanics, assumes that systems go to thermal equilibrium (thermalisation). The process of thermalisation erases local memory of the initial conditions. The
eigenstate thermalisation hypothesis is a hypothesis about when quantum states will undergo thermalisation and why.
Not all quantum states undergo thermalisation. Some states have been discovered which do not (see below), and their reasons for not reaching thermal equilibrium are unclear .
Theoretical description
The process of equilibration can be described using the
H-theorem or the
relaxation theorem Relaxation stands quite generally for a release of tension, a return to equilibrium.
In the sciences, the term is used in the following ways:
* Relaxation (physics), and more in particular:
** Relaxation (NMR), processes by which nuclear magnetiza ...
, see also
entropy production.
Systems resisting thermalisation
Some such phenomena resisting the tendency to thermalize include (see, e.g., a
quantum scar
Quantum scarring refers to a phenomenon where the eigenstates of a classically chaotic quantum system have enhanced probability density around the paths of unstable classical periodic orbits. The instability of the periodic orbit is a decisive po ...
):
* Conventional quantum scars, which refer to eigenstates with enhanced probability density along unstable periodic orbits much higher than one would intuitively predict from classical mechanics.
* Perturbation-induced quantum scarring:
despite the similarity in appearance to conventional scarring, these scars have a novel underlying mechanism stemming from the combined effect of nearly-degenerate states and spatially localized perturbations,
and they can be employed to propagate quantum wave packets in a disordered quantum dot with high fidelity.
* Many-body quantum scars.
*
Many-body localisation
Many-body localization (MBL) is a dynamical phenomenon occurring in isolated many-body quantum systems. It is characterized by the system failing to thermalization, reach thermal equilibrium, and retaining a memory of its initial condition in lo ...
(MBL), quantum many-body systems retaining memory of their initial condition in local observables for arbitrary amounts of time.
Other systems that resist thermalisation and are better understood are quantum
integrable systems and systems with
dynamical symmetries.
References
Thermodynamics
{{Quantum-stub