♪♪ -Connectivity is a human right.

-Many of the details being spread online are simply not true.

-False claims -- there was no knife, like many online blogs are saying.

-They're fake news.

[ Gunshots ] -You may have been startled or confused by what you've seen online recently -- fake news, extreme views... -This has got to stop.

They are not compatible with our culture.

They hate us.

They don't want to be Americans.

-...polarization, information wars, an epidemic of trolling and so-called hate speech.

All of this has been made possible by one thing -- the exponential growth and exploitation of online social networks -- the networks that all of us are increasingly entangled in and use, and sometimes abuse, in our daily lives.

♪♪ [ Cars honking ] Online social networks, which connect billions of people at speeds measurable in seconds, are the biggest and fastest networks ever.

♪♪ To the youthful architects of this hyper-connected world, like Mark Zuckerberg, we're witnessing the birth of a true global community where we're all equipped to take the authorities to task.

Really?

You could be forgiven for thinking just the opposite -- that these global networks are enriching a tiny elite by enticing the rest of us to give up our privacy in ways that could actually undermine democracy.

In this series, I'm going to argue that you can't understand the vast disruptive power of modern networks without looking to the past, 'cause there's nothing new about social networks.

When we understand these insights, suddenly, the whole history of mankind looks different, and we begin to understand that our present and future Networld might not be the connected utopia we were promised, but an increasingly polarized and unstable place where the truth itself is at a disadvantage.

♪♪ [ Beeping ] [ Electricity buzzing ] ♪♪ -Some of the most familiar stories from history look very different when you understand them as network phenomena, like the events that happened here in Boston, Massachusetts, on April the 18th, 1775.

Most people know the story of Paul Revere's Midnight Ride, how he became one of the heroes of the American Revolution by riding just 13 miles from Boston to Lincoln and warning of the Redcoats' approach.

But his news spread far further and faster than he could possibly have ridden.

This was achieved by no other technology than word of mouth.

The question is why his message was so readily believed and passed on.

You'll find the answer here at the corner of Tremont and Boylston in Boston.

The Grand Lodge of Massachusetts, for Paul Revere, was networked.

[ Bell tolling ] Like Benjamin Franklin, George Washington, and Lafayette, he was a Freemason... ...just one of the associations that made him one of Boston's best-known and best-trusted citizens.

It wasn't being a fast rider that made Revere crucial to the American Revolution.

It was the networks to which he belonged.

The key lesson from this famous story is that networks play a pivotal role in history.

In the case of human networks, they decide which ideas and behaviors spread through society.

[ Indistinct shouting ] And they can explain which ideas go viral and which die in obscurity.

[ Beeping, message chimes ] In this program, we'll see why understanding networks is a fundamental basis for understanding life itself.

[ Indistinct conversations ] Actually, even though networks are all around us, for most of history, people didn't think of themselves as belonging to networks.

People assumed we were designed to be organized completely differently -- in hierarchies with a few people on top and everybody else down below, knowing their place and doing as they were told.

[ Clock ticking ] Which takes us back 700 years, to the Medieval Era in Europe.

♪♪ This magnificent tower looms over Siena's Piazza del Campo.

Inside the Palazzo Pubblico, the spectacular frescoes of Ambrogio Lorenzetti illustrate the ideal of peace or good government.

It's a strictly hierarchical order with a huge patriarch who rules over citizens and soldiers alike.

What we see here is a chain of top-down command.

True, people in medieval Siena were also connected together in networks.

Out in the piazza, they met, played, traded, and gossiped in just the kind of social network you see there today.

Friends, neighbors, social clubs, and businesses.

But the structure of power was hierarchical.

And that structure, with one man at the top and the ordinary folk down below following orders, was the dominant form of government for most of history, from the empires of the ancient world to the totalitarian regimes of the 20th century.

In the same way, the great corporations of the Industrial Age were hierarchies depicted in pyramid-like org charts.

-For every one prime contractor, there can be numerous subcontractors and sub-sub and sub-sub-sub.

-Workers, managers, directors, and stockholders all knew their places in the corporate hierarchy.

Yet there have been a few eras in history when the tables have been turned and the emperors, kings, and directors at the top of their hierarchies have been challenged from below by decentralized networks.

I want to focus on two of the biggest disruptions in history -- the great network revolution of our own time and the one that happened 500 years ago.

[ Wheels rumbling, bell tolling in distance ] ♪♪ Almost exactly half a millennium ago, Western Europe was ruled over by the sacred hierarchy of the Pope and his clergy and the secular hierarchies of hereditary rulers -- kings, princes, dukes.

It had been like this since the fall of the Roman Empire 1,000 years earlier.

Then, in 1517, this long-established order came under a sustained assault.

It started with one man... ♪♪ ...a German priest named Martin Luther, who dared to challenge the corrupt practices of the Roman Catholic Church.

With astonishing speed, his critique of the Church spread across much of Europe.

A network of reformers, who came to be called Protestants, rose up and overthrew papal rule.

The result was a true network revolution.

[ Metal scraping ] Luther wasn't a cardinal -- he was an obscure, disgruntled priest.

But what made him different from previous critics of the Church was a revolutionary new technology -- the printing press, invented by Johannes Gutenberg in 1439.

Previous religious reformers in Europe had been burnt at the stake as heretics.

Luther's message was able to travel with unprecedented speed because by 1517, the printing press had drastically cut the cost of reproducing words on paper.

Luther had revolutionized not only Western Christianity, but also the use of the new medium of print itself.

[ Bell tolling ] It didn't take long for these ideas to reach England.

-...this day our daily bread and forgive us our trespasses... -They captured the imaginations of priests like Hugh Latimer, who stood here in St Edward King and Martyr Church in Cambridge to relay Luther's message.

-...almighty God.

-The printing press enlarged, accelerated, and empowered social networks in much the same way as the Internet and the smartphone have done in our time.

If you look at this first graph, you can see how a 99% decline in the price of a computer led to an extraordinary increase in the volume of computers produced.

Now look at the way a comparable decline in the price of a book led to an exponential increase in the volume of books printed.

As people bought and sold, shared, and swapped books, a tidal wave of new data swept through Europe's social networks, unprecedented in history.

The power of networks to amplify new ideas wasn't limited to Luther's Protestantism.

Networks transmitted other revolutionary ideas, like those of liberty and equality that swept through Europe and across the Atlantic during the 18th-century Enlightenment, inspiring revolutionaries like Paul Revere in America and later in France.

[ Indistinct shouting ] To understand this process of network revolution, we need some lessons from network theory.

We need to see how networks bring like-minded people together, how they link us into small and shrinking worlds, and how they can spread ideas contagiously.

Let's start with the idea that networks are at the root of all biological and social systems, that they are universal.

The theoretical physicist Geoffrey West is obsessed with networks, their beauty and their meaning.

He believes that at the heart of the natural and man-made world are fundamental principles, laws of networks.

-Here's an elephant, here's a mouse, and here am I.

What is remarkable about the three of us is that at sort of the 80%, 90% level, these are scaled versions of each other.

[ Elephants rumbling ] -For example, the number of heartbeats an animal has in a minute is directly proportional to its size.

[ Mouse squeaking ] -If you double the size, the heart rate is actually 75% slower every time you make this doubling.

So this question arises, where in the hell did all these simple laws come from?

These come from the fundamental property of all complex systems, that they are network systems.

-And that leads us to some very complex mathematics.

-So, the mathematics of networks.

Here's a network, and it begins here, the heart pumping, and it keeps branching all the way down.

This should be the aorta.

These would be cells.

The cells would be here.

It's complicated.

-Indeed.

But what Geoffrey West realized is that networks, whether the veins in our bodies, the synapses in our brains, or the water pipes in a city, all follow the same rules.

-...minimize the time to get from any point A to any point B.

So if you put all that together, you would predict, again, that cities, in terms of their infrastructure, should be very similar to biology.

So, that allows one to make the following extraordinary speculation -- the structure of a city is a scaled-up version of what's going on inside the average brain.

[ Heart beating ] -A human brain isn't just a bit smarter than a mouse brain -- it's a lot more.

Just as a big river system isn't just a bit more dynamic than a small stream -- it's a lot more.

What Geoffrey West and other network scientists have discovered is that all these different kinds of network, from a fractal river basin to an urban transport system, behave in remarkably similar ways.

They have universal mathematical properties.

-To understand these properties, to understand the way networks grow and function, we need to look at two key words we'll be hearing a lot about -- nodes and links.

While working at the court of Catherine the Great in the early 18th century, the mathematician Leonhard Euler heard of a conundrum that was baffling the citizens of the Prussian city then known as Koenigsberg.

The Koenigsbergers liked to take an evening promenade over the town's seven connecting bridges.

But was there a way of taking a walk that crossed each of the bridges just once?

In solving this puzzle, Euler came up with what proved to be the foundation of network theory.

One of the world's leading network scientists today sees the roots of his own work in Euler's Koenigsberg breakthrough.

He is László Barabási.

-The question that the citizens of Koenigsberg were wondering about is that, could you find a way to cross all the bridges only once, yet reach all the four pieces of land in Koenigsberg?

And what he said is that, really, we're dealing with four pieces of land -- this piece, that piece, that land, and then there is this other island here -- that are separated by bridges.

So he marked each of the areas with what we call today a node.

One down here, and then one there.

And they said, "What kind of paths do we have to them?"

And then there is one path here.

There is one bridge and a path there.

And then you could also go here, and you could also go there.

And then he also realized that there is also a path here, and then there is a path here through a bridge that is not visible because the lady is covering it up.

And there is one more link here, as you can see, through that particular bridge, and the network consists of one, two, three, four nodes and as many links as there are actually bridges for each of them.

-The key to solving the Koenigsberg conundrum was the number of links connecting the nodes.

If it was an odd number, it would be impossible for the promenaders to cross each bridge just once.

-And when he looked at the map, he realized that in this particular map, this has three, this has three, this has three, and this has five links.

So each of them have odd number of links, hence the citizens of Koenigsberg will never find a solution.

[ People crying, "aww"ing ] -Euler's discovery of nodes and links underlies all network theory.

The insight is that the natural and social worlds can be represented as an infinite variety of networks.

These social networks can often affect us in subtle ways.

For example, by spreading bright ideas or bad habits between like-minded friends.

This is the second big idea of network theory -- birds of a feather flock together.

This fundamental property of social networks was discovered by a sociologist working in New York in the 1930s... ...with a little help from one of America's greatest singers.

[ Bells ringing ] In 1932, there was an epidemic of breakouts at the reformatory New York Training School for Girls, including the future Queen of Jazz, Ella Fitzgerald.

[ Bell ringing ] In her school report, she was deemed ungovernable and delinquent.

The baffled governors called in the sociologist and pioneer of group psychotherapy Jacob Moreno.

♪♪ Someone who's been deeply influenced by Moreno's pioneering ideas is the Yale network scientist Nicholas Christakis.

-He had this very novel and interesting way of beginning to think about groups of people, and he was desperate to find a visual way, a way of describing how people were arranged.

And so he took these girls at the New York State Training School for Girls, and he looked at which buildings they lived in and which girl liked which girl.

♪♪ And he drew the buildings like little clusters of dots on a page according to which buildings the girls were in, and then he drew lines representing the friendships of the girls within a building, and then lines representing friendships of girls across buildings, and drew this picture in 1932.

And it was the first sociogram, the first way of rendering networks.

♪♪ When you go and look at a group of people and you see who's friends with who or who spends time with who... ...you very quickly will come to the recognition that people who spend more time together or people who are friends with each other resemble each other.

They might have a similar religion or be a similar gender -- girls will be friends with girls, and boys will be friends with boys, for example -- or a similar height or a similar athletic ability.

[ Flamingos squawking ] -Moreno's work pointed the way to the first law of network science -- birds of a feather flock together.

[ Squawking continues ] ♪♪ It's what sociologists came to call homophily.

People with similar personalities or skin color or attitudes tend to gravitate towards one another.

That's why even the network of friends at a typical high school isn't a regular and uniform matrix, but tends to subdivide into different clusters.

Homophily hasn't gone away.

When look at modern-day social networks, we see the same principle of self-segregation at work, but on a vastly larger scale.

And with algorithms exaggerating and accelerating the process, today's network platforms do more than just reveal our innate divisions.

[ Gunshots ] When people on social media start discussing really contentious topics like gun control... -Whoo!

-Yay!

-...or same-sex marriage or global warming, this is what happens.

We sort ourselves into tribes, crowding into echo chambers that reinforce our prejudices.

The scale of this polarization really hits home when you watch and measure it as it happens.

Filippo Menczer is a network scientist at Indiana University in Bloomington.

By tracking social media activity, he can show how networks tend to polarize society.

-When we visualize who shares information with whom on Twitter, conservatives are more likely to retweet tweets from other conservative people, and liberal people are more likely to retweet other liberal people, but there is very little stuff that goes across between the two.

-It's as if social media, by increasing our tendency to flock together, reduces our ability to communicate and inflames our differences.

-If you are fringe or an extremist or somebody who believes in conspiracy, it is very easy for you to find other people like you and to form critical mass.

So the network will give me the impression that most people, those that I'm connected with, agree with me.

-To illustrate this, Menczer built a model of a social network based on real data from tweets.

-Slowly but surely, the conservative people tend to connect to other conservative people, and the liberal people tend to connect to other liberal people.

And not only the structure of the network changes towards this kind of echo chamber scenario, but also, inside each group, the opinions become more homogeneous.

On social media, they are sped up and amplified and facilitated.

Basically very, very quickly, with a few interactions, it is kind of inevitable that we will end up in a situation where we are completely segregated away and shielded from information that, you know, doesn't agree with us.

-In short, modern social networks, enlarged and accelerated by the Internet, have the capacity to exacerbate our natural divisions.

The fascinating thing is that a very similar process occurred when the printing press allowed a religious reformation to sweep Europe.

-And cast the unprofitable servant into the outer darkness.

There will be weeping and... -When birds of a feather flock together, a network grows, and with it, the number of people who can share a new idea.

In the early modern Reformation, Martin Luther needed to turn Protestantism from a core group of disgruntled priests and theologians into his dream of a priesthood of all believers with each individual in a direct relationship to God, able to read the Bible in the vernacular.

This was the 16th-century equivalent of Mark Zuckerberg's global community today.

-...at this scale and this price without Samsung's experience and excellence in hardware and systems... [ Organ music playing ] ♪♪ -How could this happen?

How could you synchronize a bunch of local communities into a world-shaking movement?

Well, it turned out that these groups of dissatisfied churchgoers were not as isolated as they thought.

Which leads us to the next big lesson from network theory -- It's a small world.

[ Insect chirping ] We all know that feeling of bumping into a friend in an unlikely location.

Network theory explains why the world seems so much smaller than we expect.

Steven Strogatz and Duncan Watts provide the answer, with the help of a lot of crickets.

-Every night in the summer, from, say, around June to October, you can go outside and hear these vast choruses of the little... well, they're just... they're crickets.

[ Chirping continues ] And what's interesting about it is that the crickets are, themselves, not particularly clever creatures, and yet they're chirping in sync, so you hear this deafening sound of the crickets at night.

-I found this whole notion of the emergence of synchronization from a bunch of sort of fairly dumb or completely unintelligent entities, you know, really just fascinating.

-You know, our intuition when we think about people getting in sync... [ Instruments tuning ] ...there's a bandleader.

There's a conductor for the orchestra.

And so is there some master cricket that they're all following?

Is there a maestro?

Or is it somehow a more democratic process where, cooperatively, collectively, they all find the rhythm together?

[ Classical music playing ] -The big question for Strogatz and Watts was, how did crickets manage to synchronize their chirping with no lead cricket playing the part of conductor and when the individual crickets are often quite far apart?

-So, we go out on campus at Cornell at night.

You know, they only chirp at night.

-Well, they've got to be here somewhere.

-What were you doing, just literally climbing on the tree?

-I was just climbing up the tree, yeah.

-Did you have any kind of belt on?

-No, no.

-Nothing.

Okay.

[ Cricket chirping ] -Oh, there he is.

-In all of those hours of sitting there in the dark waiting for the crickets to chirp, I had a lot of time to think about what was going on in the trees.

We can imagine some sort of stylized extreme alternatives, and the first one is this sort of, you know, perfect one-dimensional lattice where everybody is kind of standing in a ring holding hands, and you can only hear the people who are on your left and right, you know, up to some distance.

Your message can kind of hop around this big ring.

-In other words, the structure of a network determines who gets a message and when.

-And then at the other extreme, you have this totally random, messy arrangement where, you know, you're just as likely to be best friends with someone who lives in, you know, the Western Australian desert as you are the person that you grew up with or went to school with.

So, this is the sort of totally random world.

And neither of them seems like a very good representation of the world that we actually live in.

-And then Duncan says to me one time that the whole situation reminded him of something his father had said to him once, which was, "Do you realize you're only"... -Six handshakes.

-..."six handshakes from the president?"

-Yeah.

[ Camera shutter clicking ] -The idea of six degrees of separation dates back to a short story entitled "Chains," published by the Hungarian author Frigyes Karinthy in 1929.

"One of us suggested performing the following experiment to prove that the population of the Earth is closer together now than they've ever been before.

We should select one person from the 1.5 billion inhabitants of the Earth -- anyone, anywhere at all.

He bet us that using no more than five individuals, one of whom is a personal acquaintance, he could contact the selected individual using nothing except the network of personal acquaintances."

-So, Duncan's question to me was, "Well, what if the crickets were connected like that?"

-If you start with one of these big worlds where everybody is very far apart, and all of a sudden, one person, they're linked to somebody on the other side of the lattice, right?

So, very far away.

So now that person is clearly closer to this person.

They have gone from being very far away to just one hop.

But also, all of that person's friends are now closer to all of this person's friends.

-If we could understand this, we could understand how diseases spread, we could understand, you know... -Financial crises.

-Financial crises.

-How failures could cascade through the large... -How failures could cascade, the blackouts in the power grid.

-Yeah, yeah.

-It's a simple, but profoundly important idea.

When we're all as interconnected as Strogatz and Watts' crickets, suddenly, things can spread through our network much faster than we expect.

[ Crickets chirping ] -What Strogatz and Watts did was to show just how small our networked world really is.

We might think of something like a disease in Africa as being very far away, but a single link can bring it right to our doorstep.

It's these weak ties to casual acquaintances that are essential for diffusing things from inner circles to the wider population.

This explains how Luther's ideas, born here in the German town of Erfurt, could spread to hundreds of thousands of Europeans who were dissatisfied with the established Roman Catholic order.

But to understand how and why his ideas could spread so far and wide, you have to understand another big idea from network theory -- Contagion.

Any social network is susceptible to contagion as a virus or an idea is passed from node to node.

All sorts of things can spread this way, far beyond our closest family and friends -- social fads like the Ice Bucket Challenge... ...or even seemingly noncontagious things like obesity.

When an obesity epidemic swept across America in the 1990s and early 2000s, Nicholas Christakis thought it seemed as if it was behaving like a contagious disease.

But to prove his theory, he needed a database.

And back in 2003, he discovered a treasure trove.

-In partnership with my long-term collaborator at that time, James Fowler, had been wanting to study social networks, and our idea was to find an existing cohort of people.

So I went to the sort of most famous longitudinal study that was then existing in the Boston area, which was the Framingham Heart Study.

I suddenly realized that the social network data that we needed had already been collected and was existing for 30 years.

We could go back 30 years, and every four years, we had all this information about the people -- what their blood pressure was, how much they weighed, whether they were smokers, and so forth.

And I called James on my phone, and I said, "You're not gonna believe this.

The data that we want, or a variant of the data we want, already exists."

So I was ecstatic.

I mean, it was just a kind of eureka moment.

So, this is a short, like, 30-second video animation that took five years of my life and like a million dollars to make.

We're going to start taking daily cuts through this network for 32 years, beginning in the year 1971.

You're gonna see, first of all, dots appear and disappear as people are born and die.

You're gonna see marriages form and break as people and marry and divorce each other.

And also, you're gonna see dots get bigger and smaller as people gain and lose weight, and change color if they cross the threshold of being obese.

Mostly, you're gonna see yellow dots become very numerous because this period of time includes the obesity epidemic in the United States.

-By tracking real-time changes in people's lives, Christakis made a profound discovery.

Obesity is contagious.

-So, if your friends were obese, you had a 40%, approximately, higher chance of being obese yourself.

And if your friends' friends were obese, you had about a 20% higher chance of being obese yourself.

And if your friends' friends' friends are obese, you've got a 10% higher chance of being obese yourself.

-Christakis has also shown how benign forms of behavior, like people just being friendly to one another, can spread in this way.

-We showed that if one person is nice to another person, then that person will be nice to a second person, who, in turn, will be nice to a third person.

-In this extraordinary way, your behavior can spread laterally through networks to affect the behavior of another person you don't even know.

It's as if a social network takes on a life of its own, becoming a kind of superorganism.

And this raises a rather profound question.

If we're all just nodes and networks, how much individual autonomy do we really have?

-Many people have looked at our work and been troubled by it because they think it kind of delivers a whack to free will.

But I would say, equally, what it does is it shows the importance of free will, because what we've shown is that when you lose weight or when you quit smoking or when you are kind to other people or when you improve your mood, it affects other people.

In fact, dozens or even hundreds of other people can come to be affected by positive changes you make in your life.

[ Laughter, chatter ] -Birds of a feather... small worlds... the contagiousness of ideas -- all of these network rules came into play in the Protestant Reformation.

[ Bell tolling ] It had probably seemed before 1517 that there were just a few isolated clusters of people like Luther spread across Europe and disgruntled with the state of a corrupt Church ruled from Rome.

But with the cost of spreading his message reduced by 99% by the printing press, it could spread easily along weak ties from one local hub of would-be reformers to another, creating a network of Protestants right across Northern Europe.

♪♪ -[ Speaking foreign language ] And yet, within just a few years, Luther's dream of a priesthood of all believers would be shattered.

As the Catholic Church fought to re-establish its hierarchical authority, Europe was plunged into what became 130 years of intermittent religious warfare.

♪♪ It was a time of gruesome violence with no clear distinction between soldiers and civilians.

[ Thunder crashes ] The worst-affected areas suffered death and depopulation.

The problem was, once again, homophily -- birds of a feather.

Often, the inhabitants of Northern European cities were attracted to Luther's message.

[ Bell tolling ] But the country dwellers of Southern Europe swiftly formed an equal and opposite cluster.

The Counter-Reformation was born as the Catholics fought back.

In 16th-century Europe, despite the mass executions of the Counter-Reformation, it proved impossible for Catholic rulers entirely to destroy Protestant networks.

Why?

To have a lasting effect, revolutionary social networks have to be able to survive the attacks of counterrevolutionary hierarchies even when the leaders of the revolution are being burnt at the stake.

To understand how they can do this, we need to turn to what science teaches us about the ability of networks to survive attack.

Another key term in network theory is resilience.

-Resilience is really about the systems, the nodes' and the links' ability to rearrange and reformulate themselves when some of the other key nodes are there in the system.

♪♪ You remove one or two nodes, and the activity of all the nodes around that change, and they find a new equilibrium.

Like, you take a leader out, another leader steps in.

You take a key component out, somebody learns the job and steps in.

-The network of the Protestant Reformation in England was just like this.

It was very resilient, even under sustained attack in the five years of Mary the First's reign from 1553 until 1558, even when the central nodes in the network were killed, like John Bradford, canon of St. Paul's Cathedral, who was burnt at the stake on Mary's orders.

[ Bell tolling ] The nodes and links that ensure the network's resilience have recently been revealed by the historian Ruth Ahnert and her physicist husband, Sebastian.

-This is a very dynamic network because people were consciously being taken out, and they were being imprisoned and then executed and burnt at the stake.

So this was a network under attack.

It was sort of being violently kind of pulled apart.

And what we expected was that it would be completely fragmented, but the result was quite different.

-If you want to attack a network effectively, you want to not remove the hubs, but you want to remove the people who bridge different parts of the networks.

And those people might not actually have many connections, but they might just be in that crucial position where they form links with otherwise disjointed parts of the networks.

-Mm, and those are the kinds of people that have been written out of history.

They're letter couriers, and they're financial sustainers.

♪♪ They were people who supported the prisoners in very practical terms, sending them money, food, clothes, and other goods, but also sending them letters of support, trying to sort of boost their morale.

And these were often women, and they were often key women in their communities, and they were written to by the martyrs who appreciated their value.

This letter talks about a very specific and tense moment in time.

"You for our comfort run up and down and who beareth your costs, God knoweth."

And it kind of captures exactly why the couriers were figures who were keeping this community together.

They were literally creating links between the nodes, the people spread across England, and holding them together at a time of persecution.

-This was how the Protestants were able to prevent Roman Catholic rulers from stamping out the Reformation, and how similar networks have resisted attack ever since.

One lesson of the 16th and 17th centuries is that social networks are crucial to revolutions.

Another lesson is that once you've created a new information network, whether with the printing press or the Internet, ideas can go viral extremely fast.

By early 2011, the pro-democracy movement we know as the Arab Spring had spread from Tunisia to Egypt, then under the rule of the longtime president Hosni Mubarak.

[ All chanting in foreign language ] The movement gathered pace so fast, it was as if a virus was spreading through the population.

The focus for the demonstrators was Tahrir, or Martyr, Square.

[ Chanting continues ] -Most people now have this notion of a tipping point, which is that in the beginning, it's very few people... and then other people start getting it... [ Chanting continues ] ...and very quickly, then, you get the explosive growth phase.

[ Cheers and applause ] So, we have this tipping point, colloquially, or a phase transition, which borrows an idea from physics.

[ Water bubbling ] In physics, you have water that's boiling, and it's getting hotter and hotter and hotter, but it's still liquid, and then all of a sudden, it reaches a critical temperature... [ Kettle whistling ] ...and now the water evaporates, and it converts from liquid to gaseous form.

It's a very sudden change from one phase to another phase.

And the same kind of thing can happen in social groups, like a sudden, radical change in public opinion about a topic.

And now what happens is, the social pressure that previously was suppressing adoption now flips and starts encouraging adoption.

[ Crowd chanting, clapping ] A mob is moving.

You get a very rapid transition.

[ Indistinct shouting ] -Monitoring Twitter hashtags became a way of anticipating demonstrations as revolution swept through Egypt during the Arab Spring.

In the excitement of those days, some observers drew the inference that social media was going to force other authoritarian regimes to change their ways or be swept away.

But it didn't quite turn out that way.

[ Gunshots in distance ] -Authoritarian regimes were replaced not by the hoped-for democracies, but in most cases, by anarchy or new forms of despotism.

The problem is that networks don't necessarily prefer good ideas to bad ones.

Both kinds can go viral.

As Jonathan Swift said, "Falsehood flies, and truth comes limping afterwards."

It's often said that lies spread and take hold faster than the truth.

Now there's scientific evidence to prove it.

In this graph, you can see how tweets containing true news, in green, are fighting a losing battle against the Death Star of fake news.

♪♪ It's the work of the network scientist Deb Roy.

At the Massachusetts Institute of Technology's Media Lab, he tracked 126,000 stories tweeted by roughly 3 million people.

Then he put these stories through multiple fact-checks to decide which were true and which were false.

-Our question was really simple, which is, are there clear and significant differences between how the false stories and the true stories propagate?

-You are fake news.

-And we found that there was an elevated level of disgust and surprise that were expressed in association with false stories -- you know, in the moment, you read the story, and some elements of surprise and disgust lead you to retweet -- that it suggests the kind of recipe for making a story that's more likely to go viral.

So false news propagates much further, much faster, is much more likely to be retweeted.

-They make it up.

It is so dishonest.

It is so fake, and you know, I have come up with some pretty good names for people.

I think one of the best names is... You know, I've really started this whole "fake news" thing.

-Really?

Try turning back the clock by 500 years.

Fake news is nothing new.

The Reformation began with Martin Luther criticizing the Roman Catholic Church for selling indulgences so that people's souls would spend less time in purgatory.

Yet, Luther wrote in his "Commentary on Galatians," "When I was a child, there were many witches, and they bewitched both cattle and men, especially children."

Later, Luther also wrote, "For where God builds a church, there the devil would also build a chapel."

♪♪ Such falsehoods can have fatal consequences once they go viral via social networks.

Promoted by the printed word, witch hunting spread virally through Europe during the Counter-Reformation as competing Catholics and Protestants sought to attract new followers by demonstrating their prowess at exposing witches.

[ Bird squawking ] ♪♪ In England, the witch craze reached its horrifying climax between 1645 and 1647 here in the Puritan heartland of Cambridgeshire.

Matthew Hopkins, known as the Witchfinder General, claimed the lives of 200 men and women here.

Clearly, Hopkins believed in his God-given mission, but he was also on a bonus plan.

The more witches he found, the more he'd be paid.

[ Rope creaking ] ♪♪ He even wrote a book about it, a sort of beginner's guide, which included a number of surefire ways of identifying witches, like spotting a devil's mark -- a mole or birthmark.

[ Hooves clopping ] In 1647, Hopkins plied his trade on a gruesome journey from Cambridge to Ely.

He picked off witches along the way.

In May, Hopkins reached the small village of Stretham.

Here, Margaret Moore was arrested after three farmers blamed her for misfortunes they'd suffered.

She confessed to being a witch, quite probably after being tortured.

♪♪ Accused of having surrendered her soul to the devil, she was tried and convicted.

Margaret Moore was hanged here on the green in front of Ely Cathedral.

With books like Hopkins' spreading stories of witchcraft, over the course of a century and a half, some 40,000 people, mostly women, were put to death on bogus charges.

This kind of persecution mania has been reprised throughout history.

For example, by this man -- Senator Joseph McCarthy, whose Cold War witch hunt resulted in 2,000 government employees losing their jobs as suspected communists.

The Reformation illustrates as well as anything in our own time that social networks not only tend to self-polarize, but also act as conduits for bad as well as good ideas.

The printing press was just as good at mass-producing tracts about witchcraft as it was at spreading the Gospel.

The question is, are we turning back the clock of civilization to a time when hearsay alone is enough to cost someone their life?

Or to a time when we have no way of knowing if the information that comes to us on social networks has any truth at all?

-We're entering an era in which our enemies can make it look like anyone is saying anything at any point in time, even if they would never say those things.

So, for instance, they could have me say things like, "President Trump is a total and complete dip [bleep]."

Now, you see, I would never say these things, at least not in a public address.

But someone else would -- someone like Jordan Peele.

This is a dangerous time.

Moving forward, we need to be more vigilant with what we trust from the Internet.

[ Keyboard clacking ] -So, we've seen how social networks can spread behaviors and ideas.

Sometimes, the results can be revolutionary.

But they can also cause chaos because of the tendency for conspiracy theories and fake news to travel further and faster than the boring old truth.

In the next episode, we're going to focus on how a decentralized network called the World Wide Web experienced what network scientists call a phase transition, how it came to be dominated by a handful of network platforms selling our attention for billions of dollars to the world's advertisers.

These networks have the power and the reach to disrupt human society just as much as the printing press did half a millennium ago.

♪♪ -To order "Niall Ferguson's Networld" on DVD, visit Shop PBS or call 1-800-PLAY-PBS.

This program is also available on Amazon Prime Video.

♪♪