Where does gravity collide?

The universal attraction between any two masses, and the field that describes it. Newton's 1687 Philosophiæ naturalis principia mathematica gave it as a force pulling masses toward each other in proportion to their masses and inversely with the square of the distance between them, while Einstein's 1916 Die Grundlage der allgemeinen Relativitätstheorie recast it as the curvature of spacetime that mass and energy produce. It is the weakest of the four fundamental interactions, yet it governs falling bodies, orbits, and the large-scale structure of the cosmos. The referent is a physical interaction between masses, measured in units of force or acceleration, and it has nothing to do with a statistical model of trade flows, a ratio of densities, or the seriousness of a wrongdoing.[1][2]

Unit

A force between two masses, in newtons, or an acceleration in metres per second squared

Recovery means

Not applicable

Measured by

Gauged by the weight and free fall of test masses, by the orbits of bodies, and by torsion-balance experiments that fix the gravitational constant

A workhorse empirical model in which the flow of trade or migration between two places rises with the economic or population "mass" of each place and falls with the distance between them, by analogy to Newton's law. Tinbergen introduced it for trade in 1962, and Anderson and van Wincoop's 2003 Gravity with Gravitas gave it a theoretical footing, showing that trade between two economies also depends on their barriers to trade relative to all other partners. The referent is a fitted statistical relationship between regions, estimated from trade or migration data, and here the word gravity is only a metaphor: it names no physical force, no ratio of densities, and no seriousness of an offense.[3][4]

Unit

A predicted bilateral flow of trade or migration between two places

Recovery means

Not applicable

Measured by

Estimated by regression of observed flows on the sizes of the two economies and the distance and barriers between them

Specific gravity, also called relative density, is the ratio of a substance's density to that of a reference such as water, a number with no units that tells how heavy a fluid is for its volume. The IUPAC Compendium of chemical terminology defines it under the name relative density. The referent is a pure ratio of densities, and despite the shared word it involves no attractive force between masses, no model of trade, and no judgement of wrongdoing; its "gravity" descends from the older meaning of weight or heaviness.[5]

Unit

A dimensionless ratio of a substance's density to a reference density

Recovery means

Not applicable

Measured by

Read with a hydrometer, refractometer, or reagent strip against a reference fluid at a stated temperature

The gravity of an offense is how serious a wrong it is, the weight given to it when a sentence is set or when the harm of an act is weighed against any justification. Rossi, Waite, Bose, and Berk's 1974 The Seriousness of Crimes found that people share a broadly ordered scale of how grave different offenses are, and measured that ordering across social groups. The referent is a moral and legal ranking of wrongdoing, expressed as relative seriousness rather than in physical units, and it bears no relation to a force between masses, a model of trade, or a ratio of densities.[6]

Unit

A relative ranking of how serious an offense is

Recovery means

Not applicable

Measured by

Elicited by asking people to rate or rank offenses to build a shared seriousness scale, and applied in sentencing guidelines