Thermal fluctuations

Thermal fluctuations are spontaneous variations in a system’s physical properties caused by random thermal motion of its particles. Their size depends on temperature and on how many particles participate.

Statistical mechanics: The framework that derives macroscopic behavior from the statistical properties of microscopic states. It connects random particle motion to fluctuations in observable quantities.
Temperature: A physical quantity that characterizes thermal state and determines the direction of spontaneous heat transfer. Higher temperature generally increases the energy available for thermal fluctuations.
Brownian motion: The irregular motion of particles suspended in a fluid, produced by collisions with molecules of the surrounding medium. It is a visible consequence of unresolved thermal fluctuations in a fluid.
Critical opalescence: Strong light scattering near a fluid’s critical point, caused by large density fluctuations. It makes the growth of thermal fluctuations near a critical point visible.
Quantum fluctuations: Variations associated with quantum uncertainty that persist even in a system’s ground state. Unlike thermal fluctuations, quantum fluctuations need not disappear as temperature approaches zero.
Boltzmann distribution: The probability distribution of energy states for a system in thermal equilibrium at a specified temperature. It determines how likely different energy fluctuations are at equilibrium.
Boltzmann constant: The physical constant relating temperature to energy, equal to about 1.381 × 10⁻²³ joules per kelvin. It sets the energy scale kT that governs thermal fluctuation amplitudes.
Johnson–Nyquist noise: Random voltage fluctuations generated by thermal agitation of charge carriers in an electrical conductor. It is the electrical manifestation of thermal fluctuations in resistive circuits.
Superparamagnetism: A magnetic state in sufficiently small particles where thermal energy randomly reverses their net magnetic moment. Thermal fluctuations can erase stable magnetization in nanoscale particles.
Cosmic microwave background anisotropy: Small directional variations in the temperature and intensity of the cosmic microwave background. These primordial variations are not ordinary thermal motion within a present-day material system.

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