The '''Planck length''' is defined as
:√(<s><i>h</i></s> ''G'' / ''c''<sup>3</sup>),
the [[square root]] of h-bar ([[Planck's constant]] divided by 2 [[Pi]]) multiplied by the [[gravitational constant]] ''G'' and divided by the cube of the [[speed of light]] ''c''. Its numerical value is approximately [[1e-35 m|1.6 × 10<sup>-35</sup>m]].
[[Max Planck]] defined this in his 1899 paper and gave a value close to the one in use today. His paper says nothing about its being "the smallest meaningful length in quantum mechanics" although some contemporary physicists talk like that. In 1899 quantum mechanics had not been invented yet. It might or might not be helpful to say "two points separated by less than the Planck length are indistinguishable from each other". This is an issue for today's physicists irrelevant to the original definition of the Planck length a hundred years ago. Similarly, the amount of time it takes a photon to travel one Planck length is Planck time: 0.54 × [[1e-44 s|10<sup>-43</sup> seconds]]. This is the natural time unit proposed by Planck in 1899. It might or might not turn out to be useful to think of it as "the smallest meaningful division of time." One hears speculation about that, but the jury is still out.
The Planck length and Planck time are used as the fundamental units in the system of [[Planck units]].
Max Planck: 'Über irreversible Strahlungsvorgänge'. ''Sitzungsberichte der Preußischen Akademie der Wissenschaften'', vol. 5, p. 479 (1899).
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