Planck units measure the universe using nature's constants instead of metal bars
Most units start with an arbitrary object, a bar or a weight. In 1899 Max Planck proposed a set built only from universal constants: the speed of light, the gravitational constant, the reduced Planck constant and Boltzmann's constant. At their scale, around 10 to the minus 35 metres, known physics is expected to break down.
In Planck units, each of those four constants takes the numerical value 1. Because the units rest on properties of empty space rather than a chosen prototype, they count as natural units. Planck put them forward at the end of the 1899 paper that introduced what became the Planck constant, a year before quantum theory arrived, and he stressed their universality. The idea was not wholly new: in 1874 George Johnstone Stoney, noticing that electric charge comes in fixed amounts, built units in which gravity's constant, light speed and the electron's charge all equal 1.
The Planck scale refers to quantities close to these units: energies near 10 to the 19 GeV, times around 10 to the minus 43 seconds and lengths near 10 to the minus 35 metres, roughly the Planck mass, time and length. Down there, physicists doubt that general relativity, the Standard Model or quantum field theory still work, and expect gravity's quantum effects to dominate. The universe's first 10 to the minus 43 seconds after the Big Bang, some 13.8 billion years ago, are one such regime. That is why the units matter in work on unified theories.
A practical attraction is tidier equations. Express force, mass and distance as ratios to their Planck counterparts and Newton's law of gravitation loses its G, leaving force equal to the product of the masses over distance squared. Strictly, this is a statement about dimensionless ratios, not a claim that G literally equals one.
That subtlety worries some physicists. Paul S. Wesson called setting G and c to 1 a mathematically acceptable, labour-saving trick that physically discards information and invites confusion. Variants also exist that normalise the constants differently, and while the Planck length and Planck time have established names, that naming has not spread consistently to every quantity.
Source: Planck units
