# nLab Planck length

Contents

### Context

#### Gravity

gravity, supergravity

# Contents

## Idea

The fundamental physical unit of length.

In comparison to macorscopic physical units such as the meter, the approximate value of the Planck length is $\sim 1.6 \;10^{-35}$ meter.

## Definition

Two important physical units of length induced by a mass $m$ are

1. $\ell_m \coloneqq \frac{2 \pi \hbar}{m c}$
2. $r_m \coloneqq 2 m G/c^2$

where

Solving the equation

$\array{ & \ell_m &=& r_m \\ \Leftrightarrow & 2\pi\hbar / m c &=& 2 m G / c^2 }$

for $m$ yields the Planck mass

$m_{P} \coloneqq \tfrac{1}{\sqrt{\pi}} m_{\ell = r} = \sqrt{\frac{\hbar c}{G}} \,.$

The corresponding Compton wavelength $\ell_{m_{P}}$ is given by the Planck length $\ell_P$

$\ell_{P} \coloneqq \tfrac{1}{2\pi} \ell_{m_P} = \sqrt{ \frac{\hbar G}{c^3} } \,$
• Max Planck, Über irreversible Strahlungsvorgänge, Sitzungsberichte der Königlich Preußischen Akademie der Wissenschaften zu Berlin. 5: 440–480. pp. 478–80, 1899, (10.1002/andp.19003060105)