Energy rate density
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Energy rate density is the amount of free energy per unit time per unit mass (in CGS metric units erg/s/g; in MKS units joule/s/kg). It is terminologically (but not always numerically) equivalent to power density when measured in SI units of W/kg. Regardless of the units used, energy rate density describes the flow of energy through any system of given mass, and has been proposed as a measure of system complexity. The more complex the system, the more energy flows per second through each gram. 
Energy rate density is actually a general term that is equivalent to more specialized terms used by many different disciplinary scientists. For example, in astronomy it is called the luminosity-to-mass ratio (the inverse of the mass-luminosity ratio), in physics the power density, in geology the specific radiant flux (where “specific” denotes per unit mass), in biology the specific metabolic rate, and in engineering the power-to-weight ratio. Interdisciplinary researchers prefer to use the general term, energy rate density, not only to stress the intuitive notion of energy flow (in contrast to more colloquial connotations of the word "power"), but also to unify its potential application among all the natural sciences, as in the cosmology of cosmic evolution. When the energy rate density for systems including our galaxy, sun, earth, plants, animals, society are plotted according to when, in historical time, they first emerged, a clear increase in energy rate density over time is observed. 
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