Galileo (1564-1632)
A popular image of Pisa in the early seventeenth century, is of Galileo
Galilei at the top of the Leaning Tower, dropping different sized
cannonballs to prove they all fall at the same rate.
As it happens, very few historians think that Galileo actually
performed this experiment. Rather, Galileo was puzzled by a
paradoxical consequence of Aristotle’s belief that heavy things
fall faster than light ones. What would happen if a heavy thing
and a light thing were tied together? On the one hand, accord-
ing to Aristotle’s philosophy, the heavier weight will fall faster;
and since the lighter one will be holding the heavier up, the
string will be pulled tight, and overall the falling speed should
be something between the speed of the two weights separately.
On the other hand, since the two weights are joined, they and
the string are effectively one thing with a combined weight, so
the combined speed should be faster than the individual speeds.
Those two outcomes can’t both be right. Although Galileo may
or may not have dropped weights from the Tower, he did do,
and recorded, a lot of experiments rolling different weights
down slopes whose results flatly contradicted Aristotle’s claim
that more weight equals more speed.
Then there were the observations Galileo made with his
telescope. They didn’t rule out the possibility that the Earth
was at the centre, but they did clearly show that the universe
was not as Aristotle had described it. Galileo’s discovery of the
moons of Jupiter, for instance, demonstrated that not every-
thing revolves around the Earth. You could get round this by
positing that there are ethereal spheres centred on Jupiter. But
if you keep making up stuff to explain awkward new facts, are
you making claims about how the world actually works, or
about your explanation?Newton (1643-1727)
On 28 November 1660 in London, a group of natural philoso-
phers announced the formation of a ‘College for the Promot-
ing of Physico-Mathematical Experimental Learning’. Hear-
ing of the plan, King Charles II gave his approval, and within
two years a charter was signed creating the Royal Society of
London. The motto of the Royal Society is Nullius in verba,
which can be translated as “Don’t take anyone’s word for it.”
In 1660, that ‘anyone’ still largely meant Aristotle.
In 1687 the Royal Society published the Philosophiæ Natu-
ralis Principia Mathematica (The Mathematical Principles of Natu-
ral Philosophy) by one of its fellows, Isaac Newton. In this book
Newton described his law of universal gravitation.
The legend of the apple falling on Newton’s head has about
as much historical credibility as that of Galileo dropping can-
nonballs off the Tower of Pisa. Nevertheless, something made
Newton realise that the reason stones fall to the ground is the
same reason that moons go around planets and planets go round
the Sun. In other words, there aren’t two forces at work here,
there’s only one, the force of gravity. Physicists like simplic-
ity, and they particularly like unifying forces. Newton demon-
strated that instead of it being the nature of earth to move
towards the centre and air to move away, every particle in the
universe is attracted to every other. And you can forget about
ethereal spheres.
Although Newton and others recognised that a lot of evi-dence was needed to corroborate his law, as time progressed it
became clear that it was extremely successful in accounting for
the position of the planets, which at the time were known only
far as Saturn. Almost a century later, in 1781, William Her-
schel recognised that a point of light which earlier astronomers
had mistaken for a star was a planet, which he called Uranus,
after the god of the heavens. However, by 1845 – by which
time Uranus had completed most of an orbit – it was clear that
it was not behaving as Newton’s law demanded. Yet by then
such was the confidence in Newton’s theory that mathemati-
cians in Paris and Cambridge began calculating the mass and
position of another body that could account for the anomalies.
Using the results of Urbain Le Verrier as his guide, Johann
Gottfried Galle identified the planet Neptune, which, like
Uranus, had previously been mistaken for a star. So Newton’s8 Philosophy Now ●August/September 2019Science
SIMON&^
FINN©MELISS
AFELDER2019PLEASEVISIT
SIMONAN
DF
INN.COMby Melissa Felder