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The Scientific Revolution: Some Key Changes
Heliocentrism
The most well-known change is the switch from a geocentric (earth-centered) to a
heliocentric (sun-centered) conception of the universe. It was first propounded in
modern times by the Polish astronomer Nicolas Copernicus (1473-1543) in his
posthumous book De revolutionibus orbium coelestium (On the revolutions of the
heavenly spheres) in 1543. As Copernicus himself admitted, the idea was not entirely
new. It was indeed known to Aristotle. The astronomer-philosopher Aristarchus of
Samos had proposed a heliocentric model with an orbiting earth around which the
moon orbited – the correct model! But it never caught on.
It is not too surprising that it didn’t. It doesn’t feel as if the Earth is moving. It was
Galileo who solved this puzzle by developed the concept of relative motion. When
you are on a ship moving away from shore, you are moving relative to the shore but
not to the ship. Indeed, you could feel yourself at rest on a moving ship. But,
importantly, not easily. Only today do we have forms of transport that move us so
comfortably that we can fail to notice we are moving. For someone riding a horse,
running or on the deck of a pitching ship, motion most definitely is something you
feel.
On top of this, there was scripture. Joshua commands the sun to be still during a
battle to give his people continued daylight enough to kill off the enemy. You can
hardly command a sun to stand still if it is not moving in the first place.
Based on his observations and the recorded observations of previous astronomers,
notably Ptolemy himself, Copernicus realised that a simpler model of the heavens
was gained with the sun at the centre.
The Heavens
From the earliest pre-historic star-gazers to Copernicus, all astronomers shared the
same tool for studying the heavens: their eyes. It was Galileo who first pointed a
telescope of his own invention up to the skies.i He discovered that the moon had
mountains and craters and was not a perfect sphere, as Aristotle had taught. He
discovered that Jupiter itself had four moons, thus providing further evidence that not
everything revolves around the Earth; that Venus had phases like the moon; and that
the sun had dark ‘sun spots’ on it, showing that it too was not a perfect object. (Diehard Aristotelians were not convinced and tried to blame the telescopes for producing
misleading images.)
A further challenge came in the form of supernovae observed in 1572 and 1604 and
studied by astronomers Tycho Brahe and Johannes Kepler respectively. A
supernova is an exploding star that shines with intense brightness, being visible even
in the daytime. Since the stars were supposed to be a fixed background, the
appearance of new stars was a problem.
Matter
There was a return to various forms of atomism or ‘corpuscularianism’ (‘corpuscle’ =
‘small body’) in the 17th century; a return, that is, to the idea of Greek atomism.
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Ordinary objects are composed of lots of small atoms and it is the properties of the
atoms and how they compose the larger object that determines the physical
properties of the larger object. Matter is therefore not infinitely divisible. Not everyone
agreed; Descartes, notably, rejected atomism in favour of infinitely divisible matter.
What united Descartes and atomists was the idea that matter was primary. Aristotle
regarded matter as a kind of general stuff out of which objects were composed with
the form of the object being that which explained the features of the object. The
matter is just ‘filling’. Descartes and others rejected the forms and chose the scientific
view that remains with us today, namely that the properties of bigger things are to be
explained in terms of the properties of their parts.
Elements
The Aristotelian four-element theory disappeared with the development of atomism.
Natural philosophers – scientists, as we would now call them – started identifying
other substances, such as water, iron and sulphur, as atomically basic or as
elemental. (It would take until the 19th century with Dalton to think of elements as
composed of atoms.)
Motion
Aristotle said that objects need a constant force to remain in motion. This view had
been opposed before. The atomist philosopher Lucretius said that the natural state of
matter was to be in motion. Galileo (1564-1642) developed the concept of inertia
which was refined by Newton in his laws of motion. ‘Inertia’ means ‘resistance to
change’. The thought was that an object will remain in the state it is in unless acted
upon. Whether it be moving or stationary, the default behaviour of an object is to
remain the same. There is therefore no need for a force to keep ‘pushing’ a moving
object.
Galileo also discovered that falling objects accelerate at a constant rate instead of
moving with uniform speed. He wrote that if two cannonballs of different weights were
dropped from a height, they would both hit the ground at the same time. This appears
not to hold true of all objects; try dropping a feather and a cannonball, for example.
Galileo explained that this was because the air provided resistance, not because the
feather was special. The legend (and it is only a legend) is that the two-cannonball
experiment was performed from the Leaning Tower of Pisa.
Aristotle’s law of uniform motion was also something that had been questioned
before. Indeed, one of Aristotle’s successors as head of his school, the Lyceum,
gave two clear reasons for opposing it. First, if one observes a continuous stream of
water falling from a roof, one observes that it does not remain continuous but breaks
up into drops that become further apart as they accelerate. Second, the force
imparted on impact is clearly proportional to the distance dropped: a stone dropped a
metre will make less of an impact than one dropped from a hundred metres. This is
hard to explain if one assumes constant velocity and a natural connection between
motion and degree of ‘oomph’ that is familiar from horizontal motion: e.g. if you want
to break a door down, it is easier to pick up a lot of speed.
Force
Aristotle’s distinction between the terrestrial and celestial spheres with its two types
of motion was dismantled. The Earth and the heavens are part of the same universe.
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The ‘natural’ form of motion, Newton argued, was rectilinear. Objects move in nonrectilinear paths as a result of forces acting on them. The planets’ move not in
circular but elliptical orbits because of gravitational forces. (It was the German
astronomer Kepler (1571-1630) who discovered that the planets have elliptical orbits;
Newton showed that the planets should move in elliptical orbits if his laws of motion
were correct, thus vindicating them.)
Newton’s concept gravitational force was revolutionary and extremely controversial.
For Newton was proposing that two objects could attract one another without
anything between them pulling them together. Scientists and philosophers alike
found it very hard to give up the principle of no action at a distance. Indeed, it took
until the 19th century before the concept of a force finally became an accepted
physical concept and until the 20th century for a deeper explanation of gravity.
Space
The two fundamental questions about space were whether it is finite or infinite and
whether it is full or has pockets of emptiness: vacua. In antiquity, opinion was divided
over the first question but pretty much agreed on the second. There cannot be vacua.
How could a pocket of nothing exist? The view was widespread later amongst
scholars in the Islamic world and in Christendom. How could nothingness exist in the
divinely-created totality that was the universe? The existence of vacua was finally
established in the 17th century with the invention of the barometer. (Earlier, evidence
for vacua was provided by the suction pump, which was invented at the start of the
13th century but they were never powerful enough to clinch the case.) Torricelli and
Pascal showed that a glass cylinder filled with mercury, open at one end, turned
upside down in a basin of mercury left an empty space between the top of the closed
end and the surface of the mercury.
Nevertheless, the experiment was contested. Descartes did not believe in vacua and
argued that the cylinder must have tiny holes that let in matter to fill the gap.
Descartes held the (standard) view that space was a plenum: entirely filled.
(A third and related question that become the subject of vigorous dispute in the 17th
century was whether space was absolute or relative. Two of the greatest minds of the
time, Newton and Leibniz took opposing views. Newton took the view that space was
absolute. This means that space was a something – an infinitely big something – that
existed independently of the matter in it. In theory, there could be an empty universe.
Leibniz argued for the relativist view. There is no such thing as space. There are
objects with three-dimensional bulk, so to speak. Space is, in a sense, created by
objects. To put it more accurately, space is a way of talking about the arrangement of
objects.)
Mathematics: Quantitative and Qualitative
Galileo famously wrote that the book of nature is written in the language of
mathematics. His point was some key properties needed to explain natural
phenomena were quantifiable. For example, mass is a quantifiable property as there
are different amounts of mass that can be represented by a numerical scale: e.g. a
scale of mass in grams. Also quantifiable are temperature, length, depth, breadth,
density and velocity, for example. By contrast, colour and flavour are not quantifiable.
There is no property of colour that comes in degrees that can be mathematically
represented.
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Where properties are quantifiable, laws can be stated that relate degrees of the
properties. In other words, you can write equations such as distance = velocity x
time. This leads us to…
Primary vs. Secondary Qualities
Very roughly speaking, the primary/secondary distinction is to capture the idea of the
difference between the way the world really is and the way it appears to us thanks to
the particular creatures we are. The shape of a red cube is a paradigm primary
quality. The shape results from the way the matter is arranged. The redness is a
paradigm secondary quality. It appears red because it causes us to have a sensation
of a certain sort. Other animals that are not sensitive to that part of the
electromagnetic spectrum might not see it as red. But no creature could perceive the
cube as anything other than cubic. Secondary qualities are thus observer-dependent
whereas primary qualities are not.
The view was that science should look for and deal with the primary qualities. The
secondary qualities are mere decoration that result from the interaction of our
sensory organs with the world. They are irrelevant to physics. The colour of a cube
has no bearing on how fast it moves if thrown; the mass does. Primary qualities are
quantifiable whereas secondary qualities are not. The mass of a cube can be
represented with a numerical value (relative to a scale); the colour cannot be.
Mechanism
The resulting scientific world-view came to be known as mechanism. Stripped of its
familiar sensory qualities, the universe is a collection of bits of stuff with certain
primary qualities, such as masses and shapes. These bits of stuff move in ways
determined by laws of motion, collide and react, be it by combining or bouncing off or
smashing each other into smaller bits. Precisely happens depends on the primary
qualities. The reactions are determined by laws of nature. There is no room for
chance or choice: if two bits of stuff collide in the same way twice, the same thing will
happen. In short: nature is a lot of bits of stuff colliding with one another in fixed
ways.
Why mechanism? A mechanism such as a watch or an engine is a collection of bits
of stuff that perform a function by ‘colliding’ with one another – by exerting forces on
one another.
In short, Aristotelian physics explained the behaviour of things in terms of their teloi.
Mechanism is non-teleological. Things behave as they do because there are laws of
nature that they ‘blindly’ obey.
To put things in a slight longer way that will become clearer when we turn to his
metaphysics, Aristotle’s physics sees a world of very many different substances with
their distinctive qualities. To explain natural phenomena, we look to the distinctive
qualities of the substances in question and ask after the goals they define. The
mechanistic world view cuts down on the number of substances. There is just one
type of stuff – physical stuff out of which all the distinct Aristotelian substances are
composed. It may be composed of different types of atoms but this is irrelevant. For
natural phenomena are explained by reference to a small class of primary properties
common to all types of atom, such as shape and mass. These properties are
quantitative properties rather than qualitative. There are mathematical relationships
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between them that are the laws of nature that govern their behaviour without
reference to any purpose or function.
Descartes’ Views (A Summary)
Descartes favoured heliocentrism. He rejected the theory of the spheres and the
fixed stars. He thought space was a plenum: entirely full of matter. Matter is infinitely
divisible. Nevertheless, we can speak of large objects being made out of smaller
atomic-like clumps of matter called corpuscles; where atoms genuinely cannot be
divided, corpuscles can. Descartes was a mechanist. The physical world is matter in
motion. It is the size, shape and matter of the parts that determines the organisation
and function of the whole. These are the primary qualities that science should deal in.
The secondary qualities of material objects trigger innate sensory ideas in us. The
physical world obeys strict laws of nature.
i
Galileo did not invent the telescope. Credit goes to a Dutch lens-maker, Hans Lippershey.
The telescopes he made had a low magnifying power (x3) and were being sold as toys. When
Galileo came across an example in 1609, he saw its potential and made a better one within a
day. He soon managed to make one with a twenty-fold magnifying power. (His improvements
were in part owed to an alteration of design. Lippershey had combined two convex lens in a
tube; Galileo combined a concave and a convex lens. This had the added benefit of keeping
the image the right way up.)
To say that Lippershey was a lens-maker of course implies that the idea of lenses used for
magnification was already known. Lenses were known in Ancient Greece but used as devices
to concentrate the sun’s rays to start fires. The first discussion of lenses as magnifying
devices and, importantly, the optical theory to explain their function is by Ibn al-Haytham
(a.k.a Alhazan) (965-1038) in his Kitab al-Manazir (Book of Optics). Spectacles were invented
in the 13th century in Europe. It is of interest to note that was known over a thousand years
earlier , Seneca (1 B.C.–65 A.D.) observes in his Natural Questions that a glass filled with
water had a magnifying effect that made letters bigger and easier to read.
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