Where does temperature collide?

The physical quantity that measures how hot a body is on an absolute scale, tied to the average energy of the random motion of its particles: formally, its reciprocal is the rate at which a system's entropy changes as its energy grows, so two bodies at the same temperature exchange no net heat when placed in contact. Fischer and Ullrich's 2016 The new system of units described how the kelvin was redefined by fixing the Boltzmann constant, anchoring temperature directly to energy rather than to the freezing and boiling of water. This referent is a genuine thermodynamic quantity carried by matter, measured in kelvin, not a star's inferred surface value, a dimensionless knob in an algorithm, or the hue label on a lamp.[1]

Unit

One value on the absolute kelvin scale, fixed through the Boltzmann constant

Recovery means

Not applicable

Measured by

Read by primary thermometry that relates a measurable quantity, such as gas pressure or thermal noise, to particle energy, or in practice by a calibrated thermometer

A single number that characterises a star's surface by the temperature a perfect black body would need to radiate the same total power from the same area: it is not the actual temperature of any one layer but a summary set by the star's luminosity and radius through the Stefan-Boltzmann law. Prsa and colleagues' 2016 Nominal values for selected solar and planetary quantities adopted a nominal solar effective temperature of 5772 kelvin as a fixed reference for the field. This referent is a model quantity that stands in for a whole glowing atmosphere, useful for comparing stars, and it differs from the true thermodynamic temperature of matter, from an algorithm's control parameter, and from the hue rating of a light source.[2]

Unit

One effective temperature per star, in kelvin, derived from its luminosity and radius

Recovery means

Not applicable

Measured by

Inferred by fitting the star's measured brightness and spectrum to model atmospheres, or computed from its luminosity and radius via the Stefan-Boltzmann law

A dimensionless scalar borrowed from statistical physics that sets how much randomness governs a choice: it scales the exponents in a Boltzmann or softmax distribution, so a high value spreads probability across many options while a low value concentrates it on the best. Kirkpatrick, Gelatt, and Vecchi's 1983 Optimization by simulated annealing introduced this quantity as a control parameter that is lowered on a schedule to let a search escape poor solutions early and settle later, and the same knob now tunes how adventurously a language model samples its next word. This referent carries no heat and no energy at all: it is a tuning number, unrelated to a real thermodynamic temperature, a star's radiated value, or the hue of a lamp.[3]

Unit

A dimensionless scalar set by the user or by an annealing schedule

Recovery means

Not applicable

Measured by

Not measured but chosen: read from the cooling schedule of a search or from a sampler's setting, then applied to the distribution's exponents

A rating of the visible hue of a light source, given as the temperature of the black body whose glow most closely matches it: a warm reddish light carries a low value near 2700 kelvin, and a cool bluish light a high one near 6500 kelvin, which is why the label runs opposite to everyday talk of warm and cool. Ohno's 2014 Practical use and calculation of CCT and Duv set out how this correlated color temperature is computed from a source's measured color coordinates. This referent describes the appearance of light, not the heat of anything: the lamp itself need not be that hot, and the quantity is distinct from a body's true thermodynamic temperature, a star's effective temperature, or an algorithm's randomness parameter.[4]

Unit

One correlated color temperature per light source, in kelvin

Recovery means

Not applicable

Measured by

Computed from the source's measured chromaticity coordinates as the nearest point on the black-body locus, reported as a correlated color temperature