August/September 2019 ●Philosophy Now 9idea of a force of gravity explained the motion of the planets,
it was supported by a wealth of evidence, and it had been use-
fully applied in the discovery of a new planet.
But does saying “There is a force” tell us any more than saying
“There is a nature”? The ink was barely dry on the first edition
of the Principia before people started objecting that Newton
had introduced a force without a mechanism: for all the explana-
tory power of the idea of ‘the force of gravity’, there was (and
is) no explanation for how gravity works. Much of the challenge
came from followers of René Descartes (1596-1650). Descartes
had also been interested in the movement of the planets, but
his main concern was to give a explanation of the orbits. This
he did by invoking the idea of vortices, according to which space
is composed of infinitesimal ‘corpuscles’ that behave like a fluid.
These are swept around the Sun a little like water is dragged
around a plughole, and they in turn pull the planets along with
them. When in 1713 Newton published a second edition of the
Principia, he felt compelled to add an essay called ‘The General
Scholium ’ in which he directly challenged the idea of vortices.
Newton pointed out that the orbits of comets are too eccentric
to fit the model, and that they cut across planetary vortices with
no apparent effect: “And therefore the celestial spaces, through
which the globes of the planets and comets move continually
in all directions freely and without any sensible diminution of
motion, are devoid of any corporeal fluid”.
Having dismissed Descartes’ explanation of how gravitational
attraction works, Newton included a passage known by a phrase
that occurs in it: hypotheses non fingo– ‘I make no hypotheses’.
He writes: “But hitherto I have not been able to discover the
cause of those properties of gravity from phenomena, and I
make no hypotheses. For whatever is not deduced from the phe-
nomena, is to be called an hypothesis; and hypotheses, whether
metaphysical or physical, whether of occult qualities or mechan-
ical, have no place in experimental philosophy.” To Newton,
an explanation of how something works isn’t essential to sci-
ence; as long as the mathematical model gives us the power to
map, predict, and manipulate our environment, the job of
physics is done. As the passage concludes: “And to us it is
enough, that gravity does really exist, and act according to the
laws which we have explained, and abundantly serves to account
for all the motions of the celestial bodies, and of our sea.” The
explanation of why it works isn’t that important to science. As
Osiander had said, what matters is, can you use the theory?
Newton could just as easily have called the force of gravity
the ‘nature’ of gravity. The real difference between Aristotle’s
‘nature’ and Newton’s ‘force’ is not so much in the explanation:
it is in the quality and therefore usefulness of the mathematics,
and the abundance of evidence for it. However, if being right
were a criterion for science, then we’d have to throw Newton
out along with Aristotle.
Einstein (1879-1955)
Uranus was not the only planet that appeared to be breaking
Newton’s laws of motion. In fact Le Verrier, instrumental in
the discovery of Neptune, had been working on anomalies in
Mercury’s orbit since 1840. However, when his predictions were
tested by observations of the transit of Mercury across the face
of the Sun in 1843, they didn’t match. But with the success of
Neptune behind him, Le Verrier returned to the problem, once
again calculating the mass and position of another planet near
Mercury that could explain Mercury’s erratic behaviour. So
confident was he that he even gave this planet a name – Vulcan.
Astronomers began looking for Vulcan. Some claimed to have
found it: Edmond Modeste Lescarbault was even awarded the
Legion D’Honneur for doing so. But on closer inspection all
the claims proved unfounded. There is no Planet Vulcan. The
Newtonian explanation was not supported by the evidence.
Something else was causing the discrepancy.
There is also another story, concerning how Einstein dis-
covered relativity. In 1865 the Royal Society published James
Clerk Maxwell’s A Dynamical Theory of the Electromagnetic Field.
Maxwell’s elegant equations described electromagnetism as a
wave that travels through space at the speed of light. But waves,
as a rule, require a medium; after all, a wave on the ocean isn’t
a wave if there’s no ocean. Against the advice of Newton,
Maxwell was prepared to offer an explanation: that “light and
magnetism are affections of the same substance, and that light
is an electromagnetic disturbance propagated through the field
according to electromagnetic laws.” The hypothetical substance
through which light waves propagated – the explanation of the
observed behaviour – became known as the ‘luminiferous
aether’. Unlike the swirling corpuscular medium proposed by
Descartes, this was believed to be static, and something that the
Earth and all other celestial bodies were moving through: more
like a fog than a whirlpool.
Given that light’s speed through the aether was supposed to
be constant, and that the Earth was supposed to be moving through
the aether, the speed of light measured here on Earth should vary
according to whether it is moving in the same direction as the
Earth or perpendicular to it. In 1887 two American physicists,
Albert Michelson and Edward Morley, devised a sensitive experi-
ment to compare the time taken for light to travel two equal paths
at right angles to each other. From this they expected to be able
to prove the existence of the aether and calculate the speed and
direction of the Earth’s movement through it. To their surprise,
they found there was no difference in the time taken for the light
to travel the two paths. Either the Earth was stationary – and by
this time everyone knew from astronomy that it was not – or else
the speed of light for an observer on Earth is the same regardless
of the Earth’s motion. How could this be?
For a while, physicists scratched their heads and produced
explanations for how the luminiferous aether could produce
such baffling results. Then, in 1905 Albert Einstein put forward
his special theory of relativity. He jettisoned the luminiferous
aether in favour of empty space, and created a mathematical
description that accurately accounts for the evidence: that the
speed of light is the same for all observers, regardless of their
own velocity, because time passes more slowly for the observer
the faster they are travelling.
However, Einstein took a different view of space when he pub-
lished his general theory of relativity in 1915. General relativity
explains gravity by imagining that rather than being simply a
vacuum, space is a medium which is warped by the presence of
mass. Space is like a stretched rubber sheet – if you put an iron
ball on it, it will make a dip in the sheet, and any smaller balls
nearby will tend to roll down into the dip. Einstein offers noScience