Editing Fluctuation–dissipation theorem (section)

==Qualitative overview and examples==

The fluctuation–dissipation theorem says that when there is a process that dissipates energy, turning it into heat (e.g., friction), there is a reverse process related to [[thermal fluctuation]]s. This is best understood by considering some examples:
; [[Drag (physics)|Drag]] and [[Brownian motion]]: If an object is moving through a fluid, it experiences [[Drag (physics)|drag]] (air resistance or fluid resistance). Drag dissipates kinetic energy, turning it into heat. The corresponding fluctuation is [[Brownian motion]]. An object in a fluid does not sit still, but rather moves around with a small and rapidly changing velocity, as molecules in the fluid bump into it. Brownian motion converts heat energy into kinetic energy—the reverse of drag.
; [[Electrical resistance and conductance|Resistance]] and [[Johnson noise]]: If electric current is running through a wire loop with a [[resistor]] in it, the current will rapidly go to zero because of the resistance. Resistance dissipates electrical energy, turning it into heat ([[Joule heating]]). The corresponding fluctuation is [[Johnson noise]]. A wire loop with a resistor in it does not actually have zero current, it has a small and rapidly fluctuating current caused by the thermal fluctuations of the electrons and atoms in the resistor. Johnson noise converts heat energy into electrical energy—the reverse of resistance.
; [[Absorption (electromagnetic radiation)|Light absorption]] and [[thermal radiation]]: When light impinges on an object, some fraction of the light is absorbed, making the object hotter. In this way, light absorption turns light energy into heat. The corresponding fluctuation is [[thermal radiation]] (e.g., the glow of a "red hot" object). Thermal radiation turns heat energy into light energy—the reverse of light absorption. Indeed, [[Kirchhoff's law of thermal radiation]] confirms that the more effectively an object absorbs light, the more thermal radiation it emits.