Geoscience Reference
In-Depth Information
which theory of gravity was right. As shown by John Greenberg (1995) and other
historians of science, this widely held view was not true - the Cartesian theory has
little to say about the shape of the Earth.
The two theories were very different in their basic philosophy. Newton's theory
of gravity contained the remarkable concept of “action at a distance” for the force
of gravity. Newton made the bold step linking the force of gravity at the surface of
the Earth with the force that holds the planets in orbit. According to legend, he saw
an apple fall from a tree and suddenly realized that this force must propagate into
space to hold the Moon in orbit around the Earth. Even if the legend is untrue (it
appears in print for the first time long after it was supposed to have happened),
something must have triggered this thought in Newton's mind: the force that causes
a pendulum to oscillate is the force that dominates in the solar system. It was as
extraordinary to think of a force that extended from the Earth to an apple as one that
reached to the Moon and beyond, meaning that gravity was “action at a distance.”
It was logical, however, to think that the effect of the force would diminish with
distance, and Newton postulated that the force of gravity between any two bodies
varies as the inverse square of the distance between them. If two bodies are moved
twice as far apart, the force of gravity between them diminishes by a factor four; if
moved three times further the force diminishes by a factor nine.
With the aid of this law of gravity, and his other laws of motion, Newton was
able to derive Kepler's laws about the motion of the planets. Kepler had discovered
his laws by years of trial and error: Newton was able to derive them theoretically
from more fundamental principles. This was a brilliant success that linked together
seemingly quite separate issues - Kepler's laws, pendulum oscillations and the
shape of the Earth - into one, unifying framework of calculations.
Philosophically Newton's theory of gravity left something to be desired. In parti-
cular, how could a push or a pull propagate across empty space, enabling one body
to affect another? What carried the force? The alternative theory of gravity due to
René Descartes had more appeal to common sense.
People
René Descartes (1596-1650)
Descartes was a French philosopher who settled in Holland and published in both mathe-
matics and mathematical philosophy (science), including meteorology, optics and astron-
omy. Since he was philosophically unconvinced by the notion of “action at a distance,” or
the Newtonian concept of the force of gravity, he developed a theory of vortices in the
Universe and this theory carried the planets on their motions around the Sun. He is some-
times described as the first modern philosopher, the common x-y axes of a graph at right
angles to each other, “Cartesian coordinates,” are named after him, and “Cartesian logic”
is the application of ruthlessly logical and consistent principles to an analysis.
DESCARTES ENVISIONED the Universe to be full of a material called the
plenum.
There was no space between planets and stars so therefore no vacuum could exist.
The plenum had been set in motion at the Creation. If there were no empty spaces,
how did matter move? Each body, reasoned Descartes, moved instantaneously into
any space that had been vacated by a contiguous body. A body pulled or pushed
another through a chain of motions of plenum material lying between them. The
result was that the plenum rotated in vortices and acted on material bodies pushing
