Planck units: a natural system of physical units that at the same time represents the limit of the divisibility of both ►space and ►time. The Planck units define the smallest possible length and duration allowed for by the ►laws of nature as they are currently understood. The ►standard model of physics, which describes the behavior of elementary particles, is only valid at or above the limits these units represent. Below the limits space and time lose their familar properties. If our space has hidden coiled ►dimensions, their order of magnitude will be beneath the Planck length. Planck units constitute a "natural" system of units for length, time, mass, and other factors that can be defined solely on the basis of the fundamental ►natural constants. This is why, as contrasted with more familiar units such as meters and seconds, they are valid at all times and in all cultures (including extraterrestrial cultures). Physicists sometimes call them "God's units".
These units are based on the ►speed of light, the gravitational constant, and the Planck action quantum, from which they derive their name. The action quantum was discovered by the physicist Max Planck in 1899 as a natural constant and proposed as the basis for a universal system of units. The Planck length is roughly 10^{20} times smaller than an atomic nucleus. The Planck time is the time required by light to travel along the Planck length. Such minuscule lengths and times cannot be directly observed in our universe. Due to the ►uncertainty principle, any object smaller the Planck length would be of such high speed and energy that it would at once collapse to become a ►black hole. A particle accelerator able to dissolve a structure of the Planck length would confer on the particles a gigantic amount of energy — an amount available in nature only shortly after the ►big bang. To accomplish this it would have to be roughly as large as our solar system.
