In particle physics and physical cosmology, Planck units are a system of units of measurement defined exclusively in terms of four universal physical constants : c, G, ħ, and k B (described further below). Expressing one of these physical constants in terms of Planck units yields a numerical value of 1. They are a system of natural units, defined using fundamental properties of nature (specifically, properties of free space) rather than properties of a chosen prototype object. Originally proposed in 1899 by German physicist Max Planck, they are relevant in research on unified theories such as quantum gravity.