# All Science Fair Projects

## Science Fair Project Encyclopedia for Schools!

 Search    Browse    Forum  Coach    Links    Editor    Help    Tell-a-Friend    Encyclopedia    Dictionary

# Science Fair Project Encyclopedia

For information on any area of science that interests you,
enter a keyword (eg. scientific method, molecule, cloud, carbohydrate etc.).
Or else, you can start by choosing any of the categories below.

# Planck length

The Planck length is the natural unit of length, denoted by $\ell_P$.

 Contents

## History

This unit was first developed by Max Planck who wished to create a system of measurement based on natural units. These are all based on the Planck mass. Although quantum mechanics and general relativity were unknown at the time that the units were proposed, it later became clear that at distances of the Planck length, gravity would begin to display quantum mechanical effects, requiring a theory of quantum gravity to predict what happens.

## Value

The Planck mass is a mass where its Schwarzschild radius and its Compton length are equal distances. This distance, called the Planck length, is equal to:

$\ell_P =(\hbar G/c^3)^{1/2} \cong 1.616 \times 10^{-35}$ metres

where:

$\hbar$ is Dirac's constant
G is the gravitational constant
c is the speed of light in vacuum

The estimated size of the Universe (7.4 × 1026 m) is 1.2 × 1062 Planck lengths.

## Consequences

By the Heisenberg uncertainty principle of standard quantum mechanics, an object whose position was accurate to the Planck length would have an uncertainty in momentum approximately 3.2629 kg m / s. What this means is, if one could determine the position of a baseball and be accurate to the Planck length, it would be impossible to distinguish a speed of zero (at rest) from a speed of 22.89 m/s (approximately 51 miles an hour).

Note that this doesn't say that you lose some of your sight. Most people can, at a glance, tell whether something is moving at 22.89 m/s or is at rest. What this really means, is that, because of this glance, you can't be accurate to the Planck length with the baseball's position.