Jim Al-Khalili examines how the Islamic world advanced science. He tells the story of physicist Ibn al-Haytham, who helped establish the science of optics.
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Every now and then, an idea takes form that changes everything -
it revolutionises the way we see and understand the world around us.
I believe that just such an idea
took form in the medieval Islamic world.
It's the idea that everything,
from the stars above to the working of our own bodies,
is not arbitrary or whimsical, but subject to certain systematic rules.
And what's more, that we humans can work out what those rules might be
and then, we can refine and test our theories
through observation and experiments.
In other words, it's the idea we now call the scientific method.
'For me, the story of the scientific renaissance that took place in
'the medieval Islamic world is a personal one.
'This is my cousin Samir's house in the Iranian capital, Tehran.
'I haven't seen some of the relatives
'on my father's side of the family in over 30 years.'
This is my not so tall, but very beautiful Auntie Anis.
'The Al-Khalili family is originally
from the city of Najaf in Iraq, south of Baghdad.
'In fact, I grew up in Iraq.
'But when Saddam Hussein came to power, the family split.
'Many of the Al-Khalilis fled here to Iran.
'As my mother's English, I came to Britain with my parents.'
There, I pursued my passion for science
and am now a professor of physics at the University of Surrey.
But now, I find that my own scientific work
and my Arabic and Islamic heritage are intertwined.
On my journey through the Middle East, I discovered that
an astonishing leap in scientific knowledge
took place here 1,000 years ago
under a powerful and flourishing Islamic Empire.
Wealthy, powerful, successful cultures
will produce enormous advances
in understanding and in technique,
and that's just what we find in Islam, in Baghdad,
under a series of successful, powerful,
wealthy and self-confident Islamic regimes.
Over 1,000 years ago, the Islamic Empire was the largest in the world.
It governed an estimated 60 million people -
that was over 30% of the world's population.
I found an archaeological fragment of this glorious past
in a suburb of Tehran, not far from my cousin's house.
These ancient walls tucked behind a backstreet
on the outskirts of southern Tehran are literally all that remain
of the ancient city of Ray.
The city that the great Persian geographer Al-Muqaddasi described
as one of the glories of Islam.
Of course, Ray was just one of a number of cities
that flourished under early Islamic rule.
From Baghdad, its capital,
the empire spread across thousands of miles
from North Africa through to central Asia.
Cities like Al-Askar, Basra, Merv, Gurganj, Bukhara,
each powerful and thriving cities.
Each would have been rich in trade, alive with culture.
Each would have had its own libraries, its own academies.
These were powerhouses of the new science.
This really was a Golden Age.
Think of that span of land.
This is larger than any empire human civilisation
had ever known. Within that span of land,
you can plug in the Roman Empire and it will fill
just maybe one-third of it, one-half of it or something like that.
CHANTING IN ARABIC
Reminders of this great Islamic Empire
are everywhere in the Arab world today.
This football match in the Syrian capital, Damascus, is being played
at the Abbasid Stadium. That's the name of the family
who ruled the Islamic Empire from 750 to 1258 AD.
This large territory allowed them to raise enormous tax revenues
to fund a search for knowledge and scholarship
which became known as the Translation Movement.
They sent scholars around the known world to gather up great books
and have them translated into Arabic.
It's a legacy that's still alive in the minds of most modern Arabs.
For medieval Islamic leaders, scientific knowledge
was crucial to successfully running a vast empire.
They did have a big and sophisticated governmental administration,
and that needed knowledge. If you wanted to be an administrator
and had to assess taxes, you needed to know about mathematics.
It also wants to be able to build monumental buildings. That requires a knowledge of architecture,
and mathematical skills to construct fine buildings safely.
Medicine just to keep the elite happy and healthy.
Those are the areas of knowledge
which are first translated from other languages into Arabic.
The legacy of the medieval Islamic Empire
is scattered across a vast region.
There's architectural masterpieces,
like the Ummayyad Mosque in Damascus,
the Jame Mosque in Isfahan,
and Al-Azhar University and mosque in Cairo.
And then there are many ruins that still hint at past glories,
like this, a crumbling 8th-century palace deep in the Syrian Desert.
And this, a huge Muslim palace called Madinat Al-Zahra,
currently being excavated in Southern Spain.
These are the impressive ruins of Madinat Al-Zahra,
the fantastic palace city built outside Cordoba
in the 9th century by Abd al-Rahman III,
who was the greatest of all the Andalucian caliphs.
At the time that it was ruined, Cordoba was in fact
the largest and most important city in Europe,
a rival to Baghdad in the east
for a centre for Islamic scholarship and science.
And as I travelled, I saw how science,
especially numerical record-keeping and measurement,
was crucial to dealing with the challenges of running a vast empire.
This is the mighty River Nile
as it flows through the Egyptian capital, Cairo.
Since antiquity, its unpredictable floods have determined the fate
of Egypt's people, bringing years of lean and plenty.
By the 8th century, Cairo was part of the Islamic Empire
and the new rulers took the first step
to understanding this mighty river in a scientific way.
They built a device to measure it.
It's an amazing structure, right?
'Dr Nader El-Bizri of the Institute of Ismaili Studies,
'is showing me the Nileometer. It's basically a huge colonnade
'that was built in a chamber connected by tunnels to the river.
'As the water rose or fell,
'its height could be read from the central column.'
The central colonnade here is ultimately a measuring instrument.
It is very precise. It's almost one inch between a marking and another.
Presumably they need to know seasonal variations in the height.
And to try to have some sort of record,
so that they could measure against certain years,
-where a year was known for a high level of flood...
..versus another year known for its drought.
-Then they might perhaps take some precautions.
'The data collected from the Nileometer had one practical use.
'By creating an objective record of the river's behaviour,
'it allowed the rulers of the time to calculate
'how much tax to levy on Egypt's farmers.
'But whatever its uses, what I love about the Nileometer
'is how it shows that to understand the world,
'you have to build devices to measure it.'
If you think very hard, it's never obvious
that measurement can make sense of the world around us.
The world appears, as a Western philosopher once put it,
like a buzzing, blooming confusion, and the idea that we as a group
have tools which are reliable, which have sufficient integrity,
which have an intellectual grip that can make sense
of the basic phenomena we see around us, that's an astonishing idea.
'And one medieval Islamic ruler made measurement a personal obsession,
'giving it a scale and ambition that was truly unprecedented.'
His name was Al-Ma'mun,
and he became the caliph, or ruler, of the Islamic Empire in 813 AD.
Al-Ma'mun lived in a culture without portraiture,
so all we have are later impressions of what he might have looked like.
Al-Ma'mun funded a range of scientific research,
but one particular project was a personal favourite of his.
And given that he ruled over such a large territory,
it's hardly surprising what it was - map-making.
In the second decade of the 9th century AD,
Al-Ma'mun commissioned a new map of the world,
and his scientists did a pretty impressive job.
It was a vast improvement on all maps that had come before.
What we see here is that they've really got the Mediterranean,
its shape and how it links in with the Black Sea, the Middle East,
even the whole of Asia as far as China and Japan.
They've even got the Indian Ocean and the East coast of Africa.
It all looks pretty impressive for the known world at the time.
Of course, what Al-Ma'mun ultimately wanted to know
was how much of the Earth as a whole did he possess.
And this begged the question, just how big is the Earth?
It's a sign of amazing ambition that groups of scholars
and craftsmen together can, as it were, capture the world.
Where does that ambition and that confidence come from?
Part of it comes from religious faith.
Because the world was made
by someone a bit like us, but much smarter,
if we're smart enough, the thought was,
we could probably make sense of a bit of what he did.
And that's very clear as a motivation
in a lot of Islamic, as in a lot of Christian, science.
And more specifically, the practice of Islam demanded
that its followers have a very clear idea
of the size and shape of the world.
This is crucial information for Muslims, because,
wherever they are in the world, they need to know
the direction to Mecca for their prayer. This is known as al-qibla.
Now, over such a large territory,
finding the direction to Mecca is not a trivial problem.
This problem was wonderfully illustrated
when a mosque was built recently in Washington DC.
Some worshippers were confused,
because the direction they were told to face when praying
was slightly north and not south-east as they expected.
After all, Mecca is south-east of Washington
and, on a flat map, it does appears to lie in that direction.
But on a curved sphere, the shortest distance
between any two points follows what's called a great circle.
So, for example, this great circle line between Washington and Mecca
is quite different to what you might expect,
so the direction to Mecca from Washington
actually points slightly north-east rather than south-east.
Of course, this is complicated stuff, but the key point
for Islamic scholars is that knowing the direction to Mecca
requires a knowledge of how steeply the Earth curves,
and that means knowing how big it is.
So Al-Ma'mun commissioned his very best scientists to measure it.
Nice to meet you.
'To understand how they did it, I'm meeting up
'with Professor Sami Chaloubi from Aleppo University in Syria,
'who's an expert in early Islamic science.
'Professor Chaloubi began by explaining the measuring technique,
'which Al-Ma'mun's scientists first used
'and which they had inherited from the Greeks.'
We're now talking about this,
the earlier Eratosthenes technique of measuring the circumference.
It was repeated by the Abbasid astronomers.
It was to measure the distance between two points
and then look at the angle of inclination of the sun.
So in Egypt, in Aswan down in the south, they regard the sun
as being vertical - this is near to the equator -
and they worked out how far away from the vertical the sun was
if they measured it from the north of Egypt,
in Alexandria, which is on the Mediterranean coast.
'Al-Ma'mun's astronomers repeated the Greek experiments
'in Syria and Iraq by measuring the angle of the sun in the sky at noon
'at one known location.
'They then walked due north to a second location,
'carefully measuring the distance they travelled.'
At the second location,
they once again measured the angle of the sun at noon.
This angle would have been slightly smaller than the first one.
With these figures, Al-Ma'mun's astronomers
were able to estimate the Earth's circumference.
They got a value of 24,000 miles -
within 4% of the correct value. Not bad, you might think.
But this method was flawed and ultimately unreliable.
The main problem was that measuring the distance between two locations
was incredibly difficult. It could only be done
by the unreliable method of counting paces
as you walked through the burning desert.
A more reliable and sophisticated method for estimating the Earth's size was needed,
and two centuries after Al-Ma'mun died, it came.
What made it possible was a great leap of imagination
and the fact that, by 900 AD,
much of the world's mathematical knowledge
had been translated into Arabic,
so scholars could scrutinise and improve on it.
Out of this obsession with scholarly learning
came a true mathematical visionary -
Abu Rayhan Muhammad Ibn Ahmad Al-Biruni.
And like all Islamic scholars of the time,
Al-Biruni was obsessed with the science and mathematics
of the ancient Greeks, Babylonians and Indians.
And because of the success of the Translation Movement,
he had literally on his desk the great work on geometry by Euclid,
Ptolemy's Almagest, the Indian text the Sindhind,
and the famous work on algebra by Al-Khwarizmi.
CONVERSATION IN ARABIC
'Professor Chaloubi has brought along the book
'in which Al-Biruni describes how he combined algebra and geometry
'with some very simple and practical measurements
'to solve the epic problem of how to calculate the size of the Earth.'
-And this his...?
-The Masoodi Canon.
This is Biruni's Canon, which I've been trying to get hold of,
where he describes this fantastic experiment.
Oh, you've found the page.
'Having read Al-Biruni's description of how to
'estimate the size of the world, I wanted to try it for myself.'
First, he had to find a fairly high mountain
from the top of which he could see a flat horizon -
in this case, the sea.
What I love about this story is that,
with a few simple measurements around this small mountain peak,
you can work out the size of the whole world.
Al-Biruni's first step was to work out the height of the mountain.
He did this by going to two points at sea level a known distance apart
and then measuring the angles from these points to the mountain top.
So, to measure the angle to the mountain top,
Biruni had to use a device like this, called an astrolabe.
It's basically a giant protractor.
It has the angles in degrees marked around the outside
and a pointer to help him determine his line of sight.
So, if we try now and determine the angle to the top,
it has to hang freely. And then... OK, so if you let it hang...
'I'd like to stress, if you haven't noticed already, that Al-Biruni
'would have made his measurements more meticulously than I am.
'He did them again and again to get consistently reliable results.'
OK, that's about it.
And that is 24.5 degrees.
OK, so now, we've determined one angle,
we now have to go and pick our second spot along the beach.
'The distance from the first to the second point
'must be measured accurately - in this case, it's 100 metres -
'and the two points must be in a straight line with the mountain.
'I measured the second angle to be about 26.5 degrees and now
'had enough information to calculate the height of the mountain.
'Using trigonometry and algebra, Al-Biruni used a formula
'that relates the height of the mountain to what are known
'as the tangents of the angles he measured. Using my measurements,
'I get a figure for this mountain of about 530 metres.
'I now need only one more measurement
'to get the size of the Earth, and to get that,
'I have to climb to the top of the mountain.'
What Biruni did next was measure
the angle of the line of sight to the horizon
as it dips below the horizontal.
We're going to try and reproduce that,
so if you can lift it up so that it's hanging...
..and if I locate the horizon...
..which is about half a degree, about the value that Biruni got.
Now, here's the really ingenious part.
Biruni had measured four quantities -
three angles and a distance. He used two of the angles and the distance
to work out the height of the mountain.
Al-Biruni now had everything he needed.
In essence, Al-Biruni imagined a huge right-angled triangle,
which has as its three corners
the mountain top, the horizon and the centre of the Earth.
Trigonometry told him that the angle he had measured
and the height of the mountain are related to the radius of the Earth,
and algebra allowed him to calculate it.
With this formula, Biruni is able to arrive
at a value for the circumference of the Earth
that's within 200 miles of the exact value which we know it to be today,
about 25,000 miles.
That's to within an accuracy of less than 1%.
A remarkable achievement for someone 1,000 years ago.
For me, Biruni's experiment is an early dramatic example
of a scientist using mathematical reasoning
to extend humanity's reach.
He really pushes the idea that abstract geometrical rules
governing idealised shapes like perfect circles and triangles
can help us to comprehend the real world.
Einstein used precisely the same approach,
admittedly with much more advanced mathematics,
when he developed his General Theory of Relativity
almost 1,000 years after Biruni.
But both Einstein and Biruni were united by a single common idea -
with mathematics, humanity can embrace the universe.
In this story of the birth of the scientific method,
the Islamic scholars' ability to master sophisticated mathematics
is the first crucial ingredient.
The second crucial ingredient is the use of experiment in science.
Without experiment, theory remains meaningless and sterile.
It's experimentation that allows theory
to be held up against the real world.
It gives it physical meaning.
But whereas sophisticated mathematics
grew out of the Empire's obsession with the world's learning through the Translation Movement,
practical experiment came from the daily needs
of a powerful and expanding civilisation.
The driving force of the expanding medieval Islamic Empire was trade.
It boomed from around 700 AD onwards,
creating a massive demand for metalworkers, glass-blowers,
tile-makers, craftsmen of every possible kind.
When this collided with scholarly tradition,
symbolised by the Translation Movement,
it had seismic consequences for science.
The sciences absolutely depend -
astronomy is a wonderful example, chemistry is another -
on really intense relationships between craft traditions
of instrument making, of working with metal and fire,
of working with medicines, drugs, plants, and scholarship -
highly sophisticated literary and mathematical analysis.
And the Islamic world is just such a place.
By around 800 AD, the great cities of the Islamic Empire
dominated the world's trade.
To its markets came silks, spices, drugs, fruit,
perfumes and gold from as far afield
as India and China in the east and Spain in the west.
Anything that could be traded was.
A wonderful relic of this medieval trade boom
are the great Caravanserais,
like this one in the Syrian capital, Damascus.
This huge vaulted building was designed as a resting place
for all the traders and their animals who visited the city.
On their ground floors were wide spaces for animals and goods
and, above, there were rooms for the rich merchants
to refresh themselves before another day of haggling.
One 10th-century traveller talks of
the "riches and beauties of the bazaars",
and that the income of the provinces and localities
was between 700 and 800 million dinars.
Markets like this in the Egyptian capital, Cairo,
still capture the intensity of medieval trade.
And still surviving in the modern world of the internet
and the mobile phone is a fantastic example
of how traders 1,000 years ago communicated across a vast empire.
THEY SPEAK ARABIC
So this is a carrier pigeon.
Its base is here, so wherever you took it all over Egypt,
it would make its way back to this guy.
There's a famous story that a rich Cairo merchant
by the name of Al-Nawr wanted to grow cherry trees,
so he sent a message by carrier pigeon
to a contact of his in Damascus, asking for some seeds.
His contact sent back 500 birds,
each one carrying a small bag with seeds in it.
The whole process took just three days.
Sort of a medieval FedEx, really.
By 700 AD, the Islamic Empire
was taking the first steps towards mass production.
And in this world where knowledge of materials, metals
and how they're worked became increasingly important,
one practice flourished.
It's the practice that was inextricably linked with magic -
specifically the dream to turn base metals into gold.
The mysterious practice of alchemy.
The ancient art of alchemy was a mystical system of belief
based on spells, symbols and magic.
But I believe it took Islamic scholars to turn this quasi-religion
into something much more scientific - chemistry.
Increasingly, the knowledge of the alchemists
found more and more practical applications.
For instance, when during the last decade of the 7th century,
the ruler of the Islamic Empire, Abd al-Malik,
made the bold decision to create a common currency
for all his dominions, he turned to alchemists for help.
The proportion of gold to other alloyed metals
that you have to put into the dinar to make the dinar useable,
otherwise pure gold will become very soft and you can't use it -
that proportion is adjusted by, believe it or not,
in this period, the alchemists.
It is the alchemists who knew how to combine metals together
and how to get the proportions of this gold to silver
and gold to bronze and so on.
'I hunted down tangible evidence
'of the skill of medieval Islamic alchemists
'in the old market in the Syrian capital, Damascus.'
This is an Islamic dinar.
The date of this is 128 after Hijri.
-So the middle of the 8th century?
'This 1,300-year-old coin, made of an alloy of different metals,
'isn't just durable - it's also malleable enough
'to be inscribed with intricate Arabic writing.'
"No God instead of Allah" and then...
'Coin-making is one of the many examples
'of how the practical needs of a booming economy
'began to turn the magical practice of alchemy into modern chemistry.'
What's striking about chemistry in the medieval Islamic world
is the sheer quantity of manuscripts that deal with the subject.
There are literally thousands that survive dealing with subjects
as varied as metallurgy, glass-making,
tile-making, dyeing, perfumery, weaponry.
There's even a description on how to distil alcohol.
All this activity clearly points to a bustling economy,
with consumers, soldiers, engineers, architects
all demanding innovation and all demanding new technology.
A great example of applied chemistry in the medieval Islamic world
was the manufacture of soap.
This stuff - solid soap that you can really clean yourself with -
was virtually unknown in Northern Europe until the 13th century,
when it started being imported from Islamic Spain and North Africa.
By that time, the manufacture of soap in the Islamic world
had become virtually industrialised.
The town of Fez boasted some 27 different soap makers,
and cities like Nablus, Damascus and, of course, Aleppo
became world-renowned for the quality of their soaps.
A 12th-century document
has the world's first detailed description of how to make soap.
It mentions a key ingredient and it's a substance
that became crucial to modern chemistry - an alkali.
Now, alkaline substances are crucial to soap-making.
But what's interesting is that our word "alkali"
derives from the Arabic "al-qali", which means "ashes".
That's because, back then, alkalis were manufactured from the ashes
of the roots of certain plants like saltworts.
Islamic chemists' new understanding of alkalis and other new chemicals
gave another industry a lift, too - glass-making.
The Islamic chemists discovered
that they could change the colour of glass
using newly discovered chemicals like manganese salts.
And they built industrial furnaces, some several storeys high,
to manufacture glass in huge quantities.
The legacy of their skills
can still be seen in beautiful stained-glass windows.
Islamic chemists also developed many other colours, pigments and dyes
using their new alkalis and metals like lead and tin.
These helped architects to decorate mosques,
like this one in the Iranian city of Isfahan,
in a glorious range of colours and designs.
'Chemistry was also driven by the booming market in perfumes.'
'In the main market of Damascus, traders still make up
'your favourite scent as they would have 1,000 years ago.'
So it basically has a base of alcohol and then he adds to it
the oils from the plants you want - jasmine and rosewater and mint.
But these days, they'll use...
-Yeah, I think I'll buy some of that.
'Perfumiers pushed chemists
'to come up with ever more ingenious techniques
'for extracting subtle and fragile fragrances from flowers and plants.
'They responded by refining and really establishing a technique
'that all chemists would instantly recognise today - distillation.'
Many of the techniques originate with Islamic scholars, or even earlier.
'Dr Andrea Sella, a chemist from University College London,
'shows me how distillation was used.'
Distillations would have been done in devices related to these.
This is what's now called a retort. We don't really use them any more,
but "retort" comes from the word "to bend" - in other words,
a flask which has been bent over, and that's crucial.
'The shape means that a gas produced in the flask
'is forced to condense in the spout,
'and it's the main way of extracting scents from flowers and plants.
The idea here is you heat at this end and you collect at the other.
We should actually take a look and see if we can do
a quick distillation with rose petals.
First, we need to just put in a little bit of water.
The water and steam will essentially control the temperature.
What we don't want is for this to get too hot.
'The trick with this kind of distillation
'is to use heat to release the scent molecules,
'but at the same time making sure
'that these delicate substances
'aren't destroyed in the process.'
You actually use the steam to control the temperature, and the steam
will carry those smells over.
You can see the liquid coming up, condensing in the long tube
-and there is already liquid coming through...
..and that should be carrying with it some of the rose water smell.
Mmm, yes, you can really smell it.
This picture shows a 14th-century perfume distillery.
Middle Eastern perfumes
where known to have been sold as far away as India and China.
The Islamic chemists also played a pivotal role
in another more gruesome industry - weaponry.
Historical records during the Crusades talk in terrified tones
of how the Muslims would attack the Christians
with burning missiles and grenades,
striking fear into the hearts of the defenders.
Many of these used a substance known as Greek Fire.
Islamic chemists improved on Greek Fire
by using and refining a naturally occurring resource - petroleum.
They developed the idea of distilling petroleum, or naft,
to create a lighter, extremely flammable oil which they mixed
with other volatile chemicals to make them burn furiously,
and the result was clearly terrifying.
What all these medieval Islamic texts on chemistry have in common
is their great attention to detail,
which is clearly based on careful experimentation.
In fact, the whole idea of a laboratory,
where chemical and industrial processes can be tried out,
really takes hold at this time.
The ingenuity of medieval Islamic chemists is impressive.
But I wanted to know something deeper.
What contribution did they make to our modern understanding
of the principles behind chemistry?
This is the centrepiece of modern chemistry - the periodic table.
It lists all the known elements.
Its key idea is to group substances with similar properties together.
On the far right, for instance, are the inert gases.
On the far left are the volatile metals.
The periodic table is triumph of classification,
giving scientists a way of organising
their knowledge of the material world.
Classification is simply a way to think clearly.
What you need when you have some ideas about how the world works is
that gives you a schema and you chop the world into categories,
and that helps you to understand, to make sense of what's around you.
People had been trying to classify the material world
since ancient times. The Greeks, for instance, thought there were just
four worldly elements - air, earth, fire and water.
But this idea was a philosophical one and had little practical value.
And that's what medieval Islamic chemists really changed.
They used experimental observations
to classify the stuff the world is made of.
At the forefront of this was a medieval Islamic doctor and chemist
called Ibn Zakariya Al-Razi, who was born here in the city of Ray,
just outside the Iranian capital Tehran in 865 AD.
Al-Razi's classification was very different from the Greek one.
He argued, for instance, that minerals -
roughly stuff we dig out of the ground -
should be classified into six groups,
depending on their observed chemical properties -
the same guiding principle that lies behind the modern periodic table.
Now, I've brought materials from his classification scheme.
We have here what he called the spirits,
we have the metallic bodies, we have the stones,
then we have the attraments, the salts and finally the boraxes.
'Each of Al-Razi's groups
'had a profoundly different experimental behaviour.
'For instance, spirits were flammable.
'The metals were shiny and malleable.
'Salts dissolved in water.
'Of course, these classifications are not the way we do it today,
'but the point is that, for the first time,
'Al-Razi was grouping substances on the basis
'of experimental observations, not philosophical musings.'
We've come over 1,000 years since the work of Al-Razi.
What sort of debt does modern chemistry
owe to him for his classification?
Well, I think with Razi, we start to see the first classification
which really leads on to further experiments,
the first schema which allows people to start doing rational work.
And so, really, he lies at the start of almost formal chemistry,
which ultimately leads to our periodic table.
I believe that what we see
in the work of the Islamic chemists and alchemists
is the first tentative steps to a new science.
Yes, by our standards, it contained a lot of magic and mumbo jumbo,
but it placed an emphasis on experimentation
that was truly revolutionary.
But bigger and better was to come,
because Islamic mathematics and the experimental techniques
of Jabir Ibn Hayyan and Al-Razi were about to be welded together
in a completely innovative way that would revolutionise their work
and create the modern scientific age.
Until the 9th or 10th centuries,
ideas about science and how the natural world worked
were dominated by the Greek philosopher Aristotle,
and they were very different from ours today.
He believed that mathematics was concerned
only with an abstract world of perfect forms,
of idealised shapes like circles, squares and triangles.
It had no power to explain what we observe in the world around us,
a world characterised by irregular, wonky shapes and constant change.
"Physics" is a Greek word meaning "the science of change",
and for the classical Greek tradition,
there was a strong sense in which
the science of change was in contradiction with mathematics.
Mathematics dealt with perfect knowledge,
with the unchanging world of mathematical forms.
And it seemed, in principle, extremely unlikely
that processes of coming into being and passing away,
of growth and of decay,
of qualitative change,
could be captured with the beauties of geometry and mathematics.
The story of how humanity shook off this idea
and began to see that mathematics is actually an incredibly powerful way
of describing the world around us is long and complicated.
But for me, Islamic scientists played a crucial role,
and I believe one man really led this movement to turn mathematics
from a language of abstract thought into a truly practical science.
He was, like me, from Iraq, and his name was Ibn Al-Haytham.
What Al-Haytham and his contemporaries argued for
was the possibility in a way of a single science,
which would be both mathematical and philosophical,
which would link together a physics - a science of change -
with a mathematics - a science of quantity.
And that seems to me to be radical and crucial
for the construction of new forms of reliable knowledge.
Ibn Al-Haytham was born in 965 AD in the southern Iraqi town of Basra,
and other scholars regarded him as a prodigy.
He shot to scientific fame just after the turn of the first millennium
and was an incredibly innovative and brilliant scholar.
His reputation as an intellect spread throughout the empire.
But it was this reputation that'd almost cause him to lose everything
when he took up the poisoned chalice
of trying to tame one of the world's greatest rivers.
There's a wonderful, if suspiciously apocryphal, story
about how Ibn Al-Haytham's career as a scientist was transformed.
It concerns the Nile and how, just after the turn of the millennium,
Ibn Al-Haytham was asked by the ruler of Egypt
to find a way of controlling it. Could he prevent
its unpredictable and potentially devastating floods and droughts?
But it didn't take Ibn Al-Haytham long to realise
that the Nile was way too large to control.
On hearing this, the Caliph flew into a terrible rage
and ordered Ibn Al-Haytham's execution.
Ibn Al-Haytham responded by feigning madness.
The execution was called off and he was placed under house arrest.
There, with time on his hands to contemplate, the story goes,
Ibn Al-Haytham considered deep and fundamental questions in physics,
and he began with a truly enigmatic and universal problem.
He asked if the wonderful and entirely mysterious nature of light and vision
could be explained by mathematics and geometry.
Under house arrest, or perhaps here in the rooms
of Al-Azhar University in Cairo, Ibn Al-Haytham carried out
a series of experiments that created the modern science of optics.
'I'm with Dr El-Bizri,
'who has carefully studied Ibn Al-Haytham's work.
'He explained that Ibn Al-Haytham first considered
'the Aristotelian explanation for how we see,
'an explanation that was completely un-mathematical.
'Aristotle argued that we when we look at, say, a tree,
'its essence or form emanates from it
'and then mysteriously flows into our eyes.'
So if I'm, for instance, now looking at the buildings
and the trees on the banks of the Nile, I'm receiving the forms
of these buildings and these trees in the eye
abstracted from their matter.
'According Dr El-Bizri,
'Ibn Al-Haytham found this idea deeply unsatisfactory.
'He wanted a mathematical explanation.
'And looking back at existing Greek writings,
'he found one, although it was obscure and bizarre.
'This idea claimed that we see,
'because light rays come out of the eye.'
Ultimately, it says that vision occurs by way of the emission
from the eye of light that is shaped in the form of a pyramid or a cone.
This cone-shaped beam illuminates what we're looking at
and is defined by nice geometric straight lines.
It seems Ibn Al-Haytham liked this mathematical approach,
but immediately spotted its flaws.
If we see, he asked, because light comes out of the eye,
why does it hurt when you look at a bright object like the sun
but not hurt when you look at something dim?
Or at night, can light from our eyes
really be lighting up distant objects in the sky?
So, in an inspired piece of thinking,
Ibn Al-Haytham combined the two Greek ideas
and defined our modern understanding of light and vision.
Light, he said, does travel in straight lines that obey geometric laws.
But instead of them coming out of the eye, these rays travel into it.
It is the development of an entirely new theory, and also methodologically
it is the beginnings of mathematising physics.
What Ibn Al-Haytham did was take the principles of geometry,
with its rules governing straight lines,
and applied them to the real world. He then designed experiments
to test whether the real world measured up to his mathematics.
In about 1020, Ibn Al-Haytham published
his ground-breaking geometric explanation of light
in his Kitab al-Manazir, or Book of Optics.
And what really marks this book out as science
is that Ibn Al-Haytham carefully justifies his theories
with detailed experiments that others can repeat and verify.
He starts from first principles to find out how light travels.
For his first experiment, Ibn Al-Haytham
wanted to test the idea that light travels in straight lines.
To do this, he took a straight tube on which he'd drawn a straight line
down the side and a ruler with a straight line down the length of it.
And by matching the two together,
he was convinced then that the tube was straight.
If he uses it to look at an object - in this case, a candle -
he can see the candle through the tube, which is good evidence
that the light is travelling up in a straight line. But to be sure,
he then blocked the end of the tube.
And then, by looking at the candle again, he can't see it,
because what this does is confirm the light doesn't travel to his eye
via any other route in a curved path outside the tube.
Proof that light only travels in a straight line.
Now, this might sound quite trivial and obvious to us,
but Ibn Al-Haytham was starting from first principles.
Then, through experiment, he extends
his "light travels in straight lines" idea to many other phenomena.
He explains how mirrors work, by arguing that the angle
the ray comes in at is the same as the angle it bounces off at.
He explains what we now call refraction,
why objects look kinked in a glass of water - arguing that light rays
bend when they move from one medium to another.
And then he tackles the nature of vision.
Ibn Al-Haytham wanted to understand
how an object makes an image on the retina of the eye.
So he built what he believed was a stripped down version of the eye,
which is basically a black box with a tiny hole in it.
This is what we call today the camera obscura.
He next took his subject, in this case Anna, who's very brightly lit,
and we now go inside the box to see what the image looks like.
Now that I'm inside the camera obscura and I've allowed my eyes
to get used to the dark, we can open the hole.
And there we clearly see the image of Anna waving on the screen.
But the image is inverted, because light travels in straight lines,
so the light from her head has to move diagonally downwards
to hit the bottom of the screen
and light from her feet travels diagonally upwards to hit the top.
But, more importantly, what this proved to Ibn Al-Haytham is
there's a one-to-one correspondence between every point on the object -
on Anna - and every point on her image on the screen.
Just like a modern scientific paper,
the attention to detail in the Kitab al-Manazir is incredible.
His book isn't just a dry scientific treatise -
it's a manual for future generations.
In his work, he constantly justifies
his theories about light with experimental observation
and he describes his experiments in great detail,
so that other people can repeat them and confirm his ideas.
His message is, "Don't take my word for it, see for yourself."
I believe that Ibn Al-Haytham was one of the very first people
to ever work like this. This, for me,
is the moment that science itself is summoned into existence
and becomes a discipline in its own right.
What I find so impressive about Ibn Al-Haytham is how,
once he arrives at his mathematical theories,
he then uses them to extend our knowledge of the real world.
So, for instance, he used his new ideas about light to deduce
that the Earth's atmosphere is of a finite thickness,
and he even estimated what that thickness is.
He did it basically by measuring how long twilight lasts.
He rightly assumed that the reason it continues to be light
after the sun has dropped below the horizon
must be because its rays bend as they enter the Earth's atmosphere.
The length of twilight and an educated guess
for what we today call the air's refractive index
gave Ibn Al-Haytham a way
of estimating the thickness of the Earth's atmosphere.
He came up with a figure of around 40 kilometres -
about half of the modern value. That's pretty impressive.
It really shows how mathematics
extends the power of science to explain.
On my journey so far, I've been overwhelmed by
the sheer intellectual ambition of medieval Islamic scientists.
When their leaders asked them to find out the size of the world,
scholars like Al-Biruni used mathematics in startling new ways
to reach out and describe the universe.
And as trade and commerce boomed, scientists like Al-Razi
responded by developing a new kind of experimental science - chemistry.
But if there's one Islamic scientist we should remember above all others,
it is, in my view, Ibn Al-Haytham,
for doing so much to create what we now call the scientific method.
The scientific method is, I believe,
the single most important idea the human race has ever come up with.
There is no other strategy that tells us how to find out
how the universe works and what our place in it is.
It's also delivered technologies that have transformed our lives.
So, the next time you jet off on holiday or use a mobile phone
or get vaccinated against a deadly disease,
remember Ibn Al-Haytham, Ibn Sina, Al-Biruni
and countless other Islamic scholars 1,000 years ago
who struggled to make sense of the universe
using crude mirrors and astrolabes.
They didn't get all the right answers,
but they did teach us how to ask the right questions.
In the next episode, I travel to Syria and Northern Iran
to find out about the great Islamic scientists
who revolutionised astronomy,
making it a truly modern science.
And I'll also discover how the man many consider
to be the father of the European scientific renaissance, Copernicus,
borrowed from Islamic astronomical theories.
And I'll unravel the mystery of how
the Golden Age of Islamic science came to an end.
Subtitles by Red Bee Media Ltd
Physicist Jim Al-Khalili travels through Syria, Iran, Tunisia and Spain to tell the story of the great leap in scientific knowledge that took place in the Islamic world between the 8th and 14th centuries.
Al-Khalili travels to northern Syria to discover how, a thousand years ago, the great astronomer and mathematician Al-Biruni estimated the size of the earth to within a few hundred miles of the correct figure.
He discovers how medieval Islamic scholars helped turn the magical and occult practice of alchemy into modern chemistry.
In Cairo, he tells the story of the extraordinary physicist Ibn al-Haytham, who helped establish the modern science of optics and proved one of the most fundamental principles in physics - that light travels in straight lines.
Prof Al-Khalili argues that these scholars are among the first people to insist that all scientific theories are backed up by careful experimental observation, bringing a rigour to science that didn't really exist before.