# Laplace, Pierre Simon de (1749–1827)

Pierre Laplace was a French physicist and mathematician who put the final capstone on mathematical
astronomy by summarizing and extending the work of his predecessors in his
five-volume *Traité de Mécanique Céleste* (Treaty on
Celestial Mechanics), published from 1799 to 1825. This work was important
because it translated the geometrical study of mechanics used by Isaac Newton to one based on calculus. In *Mécanique
Céleste*, Laplace proved the dynamical stability of the Solar System
(with tidal friction ignored) on short timescales. Over long periods, however,
this assertion has proven false because of the effects of chaos. Laplace
explained the long-term variations in the orbital speeds of Jupiter and Saturn (1786), and the Moon (1787). His nebular hypothesis of the
origin of the Solar System (1796) is similar to that of Immanuel Kant,
of which he was apparently unaware.

The Solar System, Laplace said, originated out of a gradually cooling cloud
of gas, with the planets most remote from the center condensing first. This
theory had a strong influence on subsequent speculation about the nature
of our neighboring worlds. It implied that the inner worlds were younger
and that, in particular, cloud-covered Venus might be an immature version of the Earth – a virgin world. By contrast,
planets further from the Sun, such as Mars,
would have formed later and therefore could be expected to be more highly
evolved. Laplace's theory also suggested that planets are a natural consequence
of the evolution of stars, so that many stars ought to have planetary retinues.
It therefore provided powerful support for the doctrine of pluralism.
After reading *Mécanique Céleste*, Napoleon Bonaparte is said to have
questioned Laplace on his neglect to mention God. In contrast to Newton's
view on the subject, Laplace replied: "Sir, I have no need of that hypothesis."

## Laplace's equation

Laplace's equation is a partial differential equation named after its discoverer. The solutions of Laplace's equation are important in many fields of science, notably electromagnetism, astronomy, and fluid dynamics, because they describe the behavior of electric, gravitational, and fluid potentials.

In three dimensions, the problem is to find twice-differentiable real-valued
functions *φ* of real variables *x*, *y*, and *z* such that

Solutions of Laplace's equation are called harmonic functions.