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11. Greek Science
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Astronomia: Explorando oTempo e Espaço
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Science and History
Science combines logic and evidence to increase our understanding of the natural world, including remote and inaccessible regions of space and time.
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Chris ImpeyDistinguished Professor
Astronomy
Science as we understand it, starts with the ancient Greeks.
This extraordinary culture that lived in the Aegean part of the Mediterranean
about 2500 years ago.
Over a span of a few hundred years,
a small number of extraordinary thinkers, philosophers of the time
came up with strikingly prescient ideas about nature and the universe.
They didn't always agree, and they weren't observational scientists,
they were philosophers, so we can see striking divergence in the ideas they had.
Consider this, on the possibility of life in the universe.
Epicurus saying, there must be infinite worlds both like and unlike this world.
And going on to speculate about creatures and
perhaps intelligent creatures on those worlds.
That's a strikingly modern statement that fits completely with modern astrobiology.
And yet from the same time we also have the statement of Aristotle
that this world must be unique, the Earth, there can be no other world.
And the dominant philosopher of the time, his opinion held.
And so, forward into history, the idea of the Earth is unique and
esteemed amongst objects, and of course, the center of the universe held sway.
>> [SOUND] More than 25 centuries ago, among the Greek islands
here at the vibrant crossroads of Africa, Asia, and Europe,
philosophers began to devise rational theories about the world around them.
[SOUND] The wondrous waves and foams of nature,
they said, could be understood.
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[SOUND]
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One Greek thinker suggested that the Earth actually moved around the sun.
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Another thought that everything, the work of man and
nature was made of particles too small to see.
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Others estimated the sizes of the Earth and the moon and
the distances between them.
And reasoned that both were spheres.
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But it would be many centuries before we had the tools to extend our vision.
And confirm the wisdom of these early thinkers.
In the meantime, people around the world gazed on the stars and gave them names.
Most assumed that the Earth was the center of an unchanging universe.
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>> We will talk about the contribution of the ancient Greeks to science.
But they made an extraordinary set of contributions to other fields as well.
Obviously to philosophy and logic,
the foundational pieces of the scientific method.
But they also gave us strikingly modern ideas on systems of government, of law,
and of the city-state.
They had ideas in theater and music and
the arts that are still used to the present day.
While their ideas on medicine were perhaps a little archaic,
they also contained some rudiments of modern ideas.
In mathematics, they came up with the ideas of geometry,
algebra, the notion of infinity, and even the concept of irrational numbers.
And then strikingly, for people who may never have traveled more than a few dozen
miles in their lives, any educated ancient Greek knew that the Earth was round,
suspected that the sun was larger than the Earth, and
had the idea of a vast universe, much larger than the Earth itself.
What allowed a small set of people,
basically a set of interrelated city-states in the Aegean peninsula,
to come up with such radical ideas, so advanced for their time.
Nobody really knows, but unlike the fixed civilizations of Greece,
Mesopotamia, and Babylon, the Greeks were mobile.
They lived by trade and by their wits.
They traveled the Aegean, carrying things like the Antikythera mechanism,
trading in ideas and artifacts and technologies that they knew had value.
Because they were mobile and lived by trade and commerce,
they were open to ideas.
They had to be, that's how they survived.
They also esteemed thought, and rational thought above all things.
And they implemented it in their society in every way that they could.
They were also a small, nimble society, not too large.
They did indeed depend on slaves for
that small set of thinkers to be able to think the way they did, but otherwise,
they were democratic in their implementation of the rule of law.
Key to what the Greeks did in science and why they did something that other
cultures before them didn't was they made mental models of nature.
Other cultures were willing to track the motions in the night sky and
indeed see patterns, but they never went to a general principle or
an abstract idea of what they saw.
They just took it to be the way they saw it.
The Greeks were making mental models and they were using mathematics to understand
the patterns of nature in an abstract way and rather than a literal way.
This is the rudiment of the scientific method.
One example will show this in mathematics.
The Egyptians used to survey the Egyptian Nile valley,
which flooded every year, because during the floods all the lines and
demarcations with property were obliterated, and so they developed
the surveying techniques to re-survey the plots of land in the Nile Delta.
To layout these right angle triangles, they kept enormous loops of rope knotted
in increments that corresponded to perfect triangles.
For instance, three squared plus four squared equals five squared.
So they would keep the triad of numbers that lead to perfect triangles,
meaning they could stretch these ropes out into right angle triangles of different
sizes and put a rectilinear grid in place every year.
But the Greeks while maintaining warehouses full of these loops of ropes
never went to the general case.
That fell to Pythagoras, who invented the Pythagorean theorem,
which as we know from The Wizard of Oz, defines intelligence.
A squared plus b squared equals c squared.
What he's doing is abstracting into algebra the general case.
Pythagoras knew this was a big invention, an innovation.
Legend has it that he sacrificed 100 oxen to the gods when he came up with his
Pythagorean theorem, and he wasn't a rich man.
That was a lot of oxen.
So the mathematics that was applied to the natural world, and
that became the basis of the scientific method, dates back to these times.
Now it wasn't always esteemed in those times,
Pythagoras ran what in modern age we would probably characterize as a cult.
He and his followers lived in an island in the Aegean Sea.
They had been hounded off the Greek homeland, and
there was an indication he would have been killed if he'd gone back.
His followers were like acolytes, or cult followers.
Nothing Pythagoras wrote survives to us,
everything we know about him comes from his followers and their descendants.
And Pythagoras went further, he went to the limit,
suggesting that the universe itself is a mathematical entity.
As we'll see when we get to cosmology, that's a strikingly modern idea
with the mathematical theories of general relativity and
perhaps string theory underlying nature at the deepest and
most profound level in modern scientific theorizing.
Pythagoras may have been right.
A great example of the application of both logic and
mathematics to the everyday world and the astronomical world comes in the fact
that the average educated Greek knew that the Earth was round and how large it was.
Think how striking that is.
Millennia ago, when none of these people would have traveled more than a few dozen
miles in their lives.
The logic behind this was fairly simple.
It started with the observation of a lunar eclipse
where the Earth's shadow travels across the moon.
Understanding the illumination of the Earth, sun, and the moon,
the Greeks knew that, that was the situation during a lunar eclipse.
And so they could see that the Earth projected a circular shadow onto the moon.
And they knew that the only object where regardless of its orientation it casts
a circular shadow is a sphere.
So for them logically it was self evident that the Earth was a sphere.
As for the size of this sphere they make another observation.
Knowing that on a certain day of the year the sun casts no shadow
at a certain time in a certain place.
They would then make the same observation in a place a known distance away,
several hundred miles.
And at that distance the Earth casts a shadow with an angle that can be measured.
And then it is simple geometry of similar triangles to infer the diameter or
circumference of the Earth, knowing only the angle cast by that shadow and
the distance between those two points on the Earth's surface.
They actually got the size of the Earth accurate to about 5%,
which may have been fortuitous because their linear distance measurement
on the Earth was perhaps not that precise, but the principle was valid.
So, the Greeks absolutely knew the size of the Earth.
A separate piece of logic lead to the inference
that the Earth was larger than the moon and smaller than the sun.
And that lead one Greek thinker,
2000 years before Copernicus to speculate on a heliocentric model.
So, they were strikingly modern in their views of space and time.
They were also asking profound questions, and
two examples will give a sense of this.
Plato had a colleague called Archytas.
Archytas wondered about the universe.
Was it finite or infinite?
It's a dichotomy and it's very profound.
In the analogy he used, he imagined they a warrior, a spear carrier, who
went to the edge of the universe as far as they could travel, and hurled their spear.
What do you imagine Archytas said should happen?
Should the sphere travel forever, and into what?
Or will the sphere hit a boundary or a barrier or the limit, and
then what is that substance, and what's beyond that substance?
So in this logic, Archytas knew that if there was a boundary, or a barrier, or
a container, then something has to contain the container.
And so, in his logic, it was more rational to imagine that the universe was infinite.
And so in Greek philosophy, at the time,
the dominant idea was of an infinite universe.
A hundred or so years later, Democritus made an analogous thought experiment,
this time going in the downward direction.
He imagined a sharp knife in a stone.
You cut the stone into smaller and
smaller pebbles until it becomes a grain of sand, and
then perhaps you can't do the experiment, but you imagine cutting it in half and
half and half again, until it's almost too small to see on the tip of your finger.
Now you can't do the experiment but the thought experiment continues, and
how does it end?
To Democritus there are only two outcomes.
Either you reach a fundamental, invisible, indivisible object, he called it an atom,
or you carry on the process forever generating infinitely small sub-units,
and to him it was apart logically that an infinite progression would continue,
and so he imagined an indivisible element called an atom.
He hypothesized atoms and also inferred
that the properties of normal material like its texture, or its smell, or its
taste would be secondary properties not held by the individual atoms themselves.
Another strikingly modern idea,
2000 years before we had the ability to prove that atoms exist.
These were the powerful thought processes that Greeks used to address each
situations where they had no apparatus, no equipment to do the observation, and
of course, no telescope.
As I've mentioned,
the dominant philosopher of the Greek age was Aristotle.
Aristotle believed that the Earth was unique, motionless, spherical, and
the center of the universe.
Why did he believe that?
Because the alternative to put the Earth in motion around the sun,
which was speculated by a couple of Greek philosophers,
would mean the Earth was in motion, and yet we felt no motion.
If we think about it, knowing the size of the Earth, if the Earth is in motion,
then a degree of latitude, the Earth must be spending about 700 miles an hour.
No such motion was obvious, so
they are startle the self-evident that the Earth could not be moving.
And if it was stationary, it was more sensibly, the center of the universe.
So that is called the geocentric model that came to us from the Greeks.
But there's a fly in the ointment of this model.
And it comes from those moving objects in the night sky,
the planets, especially Mars.
When you observe Mars carefully over a period of years there are sometimes when
in its normal steady motion through the fixed pattern of the stars,
it appears to reverse course for a few months and then resume its normal motion.
That so called retrograde motion was a strange anomaly also observed for
Jupiter and Saturn.
And it had no natural explanation in a geocentric model where the Earth is
the center of the universe and
the planets simply orbit on crystalline spheres around us.
So this retrograde motion stayed as an enigma in the Greek time, and
would eventually come back into prominence at the Copernican Revolution.
The astronomical knowledge of the ancient world,
of the Greek world, was codified by Ptolemy, a thinker from about 100 AD.
He wrote up the astronomical knowledge of the ancient world in a 47 volume
encyclopedia called the Almagest, which is Arabic for the greatest.
And this was the knowledge that went forward through the dark ages in medieval
times that followed when scientific progress essentially ground to a halt.
The Ptolemaic model is based on geocentric cosmology with the Earth
stationary at the center of the universe.
And to explain what are actually non-uniform motions of the planets in
the night sky, you can't do it with simple spherical objects.
A reminder of why we're looking to explain this with spherical objects was
the dominant influence, not only of Aristotle, but of Pythagoras.
And for Pythagoras the sphere is the most perfect shape.
As a circle is the most perfect shape, and so it was natural that
celestial objects would follow circular trajectories, but they don't.
And so to make that happen in a Ptolemaic or
geocentric model, you have to add offset or retrograde spheres.
Spheres within spheres, if you like.
And in the end, it took 47 spheres in the Ptolemaic model,
to explain the motions of only 7 objects.
King Alfonso the 10th of Spain in the 11th century,
when presented this by the court astronomers of the time,
said, if his almighty had asked me, I would've suggested something simpler.
So this was considered a kludge, an inelegant solution for cosmology.
But there are things rested through the medieval times.
An extraordinary set of ideas emerged from ancient Greek
philosophers about 2500 years ago, including the notion of atoms,
the idea of a heliocentric universe, the size of a universe that's far larger
than the Earth, and other concepts that would only be proven in the 20th century.
The Greeks applied logic and mathematics to their understanding of nature.
And they were the first culture to use mental models and
abstraction as a way of understanding the world around them.
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