Fluctuations and noise

Fluctuations are random variations in physical quantities; noise is their irregular influence on measurements or system behavior. Their statistics reveal underlying dynamics and limits to precision.

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Thermal fluctuations: Random variations in a system’s physical quantities caused by thermal energy. They arise from molecular motion and connect temperature to observable randomness.
Probability distribution: A mathematical description of the possible values of a random variable and their probabilities. It specifies the range and likelihood of fluctuation amplitudes.
Johnson–Nyquist noise: Thermal voltage noise generated by the random motion of charge carriers in an electrical resistor. It makes thermal fluctuations measurable in a familiar circuit component.
Thermal noise: Random electrical or physical variation produced by thermal agitation in a system. It contrasts equilibrium thermal randomness with noise from discrete events.
Quantum fluctuations: Variations in quantum observables associated with quantum states, including vacuum states. They extend fluctuation analysis beyond classical random motion.
Brownian motion: The irregular motion of particles suspended in a fluid, driven by collisions with surrounding molecules. It provides a direct mechanical picture of fluctuations in a fluid.
Mean: The arithmetic average of values in a dataset or probability distribution. Fluctuations are commonly defined as deviations from this average.
Noise-equivalent power: The input optical power that gives a detector signal equal to its noise level in a one-hertz bandwidth. It expresses how detector noise limits the weakest resolvable signal.
1/f noise: Noise whose power spectral density increases approximately in inverse proportion to frequency over a range. Its broad, slow variations differ from ideal white noise.
Vacuum fluctuations: Variations in quantum fields present even in their lowest-energy vacuum state. They raise questions about the physical meaning and measurable effects of noise in empty space.