Horizon Season 46, Episode 17: How Long is a Piece of String?

Why Are Thin People Not Fat?

The world is affected by an obesity epidemic, but why is it that not everyone is succumbing? Medical science has been obsessed with this subject and is coming up with some unexpected answers. As it turns out, it is not all about exercise and diet. At the centre of this programme is a controversial overeating experiment that aims to identify exactly what it is about some people that makes it hard for them to bulk up.

60 min

Cannabis: The Evil Weed?

Cannabis is the world's favourite drug, but also one of the least understood. Can cannabis cause schizophrenia? Is it addictive? Can it lead you on to harder drugs? Or is it simply a herb, an undervalued medicine? Addiction specialist Dr John Marsden discovers that modern science is finally beginning to find answers to these questions. John traces the cannabis plants' birthplace in Kazakhstan; finds the origins of our sensitivity to cannabis in the simple sea squirt; and finds out just what it does to our brains. He meets people who have been changed by this drug in drastically different ways - from those whose lives have been shattered to those who lives have been revived.

60 min

Why Do We Dream?

Horizon uncovers the secret world of our dreams. In a series of cutting-edge experiments and personal stories, we go in search of the science behind this most enduring mystery and ask: where do dreams come from? Do they have meaning? And ultimately, why do we dream? What the film reveals is that much of what we thought we knew no longer stands true. Dreams are not simply wild imaginings but play a significant part in all our lives as they have an impact on our memories, the ability to learn, and our mental health. Most surprisingly, we find nightmares, too, are beneficial and may even explain the survival of our species.

60 min

Can We Make a Star on Earth?

Professor Brian Cox takes a global journey in search of the energy source of the future. Called nuclear fusion, it is the process that fuels the sun and every other star in the universe. Yet despite over five decades of effort, scientists have been unable to get even a single watt of fusion electricity onto the grid. Brian returns to Horizon to find out why. Granted extraordinary access to the biggest and most ambitious fusion experiments on the planet, Brian travels to the USA to see a high security fusion bomb testing facility in action and is given a tour of the world's most powerful laser. In South Korea, he clambers inside the reaction chamber of K-Star, the world's first super-cooled, super-conducting fusion reactor where the fate of future fusion research will be decided.

60 min

The Secret Life of Your Bodyclock

Why are you more likely to have a heart attack at eight o'clock in the morning or crash your car on the motorway at two o'clock in the afternoon? Can taking your medication at the right time of day really save your life? And have you ever wondered why teenagers will not get out of bed in the morning? The answers to these questions lie in the secret world of the biological clock.

60 min

What's the Problem with Nudity?

What is wrong with nudity? Why are people embarrassed about their bodies? How and why did they get the way they are? Horizon takes a group of volunteers and subjects them to a series of psychological and physical tests to challenge attitudes to the naked human form. The questions raised strike at the heart of human physical and social evolution. Human beings are the only creatures that can be 'naked' - but why, how and when did people lose their fur? That question takes Horizon around the world to meet scientists from Africa to Florida, and they are finding answers in unexpected places: the chest hair of Finnish students, the genetic history of lice, and the sweat of an unusual monkey. It turns out that something everyone takes for granted may hold the key to the success of the entire human species.

60 min

How to Survive a Disaster

When disaster strikes who lives and who dies is not purely a matter of luck. In every disaster, from those people face once in a lifetime, to those they face every day, there are things that can be done to increase the chances of getting out alive. Horizon has gathered a team of leading experts to produce the ultimate guide to disaster survival. Through controversial experiments, computer simulations and analysis of hundreds of survivor testimonies from plane crashes to ferry disasters and even 9/11, they will reveal what happens in the mind in the moment of crisis and how the human brain can be programmed for survival.

60 min

Who Do You Want Your Child to Be?

David Baddiel, father of two, sets out to answer one of the greatest questions a parent can ask: how best to educate your child. Taking in the latest scientific research, David uncovers some unconventional approaches: from the parent hot-hosing his child to record-breaking feats of maths, to a school that pays hard cash for good grades. David witnesses a ground-breaking experiment that suggests a child's destiny can be predicted at four, and hears the three little words that can ruin a child's chance of success for good. He also uncovers the neurological basis for why teenagers can be stroppy and explosive and has his own brain tinkered with to experience what it is like to struggle at school. Through it all, David's quest remains true: to maximise his child's potential for success and happiness.

60 min

Why Can't We Predict Earthquakes?

Last century, earthquakes killed over one million, and it is predicted that this century might see ten times as many deaths. Yet when an earthquake strikes, it always takes people by surprise. So why hasn't science worked out how to predict when and where the next big quake is going to happen? This is the story of the men and women who chase earthquakes and try to understand this mysterious force of nature. Journeying to China's Sichuan Province, which still lies devastated by the earthquake that struck in May 2008, as well as the notorious San Andreas Fault in California, Horizon asks why science has so far fallen short of answering this fundamental question.

60 min

Alan and Marcus Go Forth and Multiply

Ever since he was at school, actor and comedian Alan Davies has hated maths. And like many people, he is not much good at it either. But Alan has always had a sneaking suspicion that he was missing out. So, with the help of top mathematician Professor Marcus du Sautoy, Alan is going to embark on a maths odyssey. Together they visit the fourth dimension, cross the universe and explore the concept of infinity. Along the way, Alan does battle with some of the toughest maths questions of our age. But did his abilities peak 25 years ago when he got his grade C O-Level? Or will Alan be able to master the most complex maths concept there is?

60 min

How Violent Are You?

What makes ordinary people commit extreme acts of violence? Michael Portillo investigates the dark side of human nature, and discovers what it is like to inflict pain.

60 min

Do I Drink Too Much?

Do I Drink Too Much? Alcohol is by far the most widely used drug - and a dangerous one at that. So why are so many of us drinking over the recommended limits? Why does alcohol have such a powerful grip on us? How much of our relationship with this drug is written in our genes? What are the real dangers of our children drinking too young? Addiction expert John Marsden, who likes a drink, makes a professional and personal exploration of our relationship with alcohol. He undergoes physical and neurological examinations to determine its impact, and finds out why some people will find it much harder than others to resist alcohol. Even at the age of 14 there may be a way of determining which healthy children will turn into addicts. John experiments with a designer drug being developed that hopes to replicate all the benefits of alcohol without the dangers. Could this drug replace alcohol in the future?

60 min

The Secret You

With the help of a hammer-wielding scientist, Jennifer Aniston and a general anaesthetic, Professor Marcus du Sautoy goes in search of answers to one of science's greatest mysteries: how do we know who we are? While the thoughts that make us feel as though we know ourselves are easy to experience, they are notoriously difficult to explain. So, in order to find out where they come from, Marcus subjects himself to a series of probing experiments. He learns at what age our self-awareness emerges and whether other species share this trait. Next, he has his mind scrambled by a cutting-edge experiment in anaesthesia. Having survived that ordeal, Marcus is given an out-of-body experience in a bid to locate his true self. And in Hollywood, he learns how celebrities are helping scientists understand the microscopic activities of our brain. Finally, he takes part in a mind-reading experiment that both helps explain and radically alters his understanding of who he is.

60 min

Fix Me

60 min

Who's Afraid of a Big Black Hole?

Black holes are one of the most destructive forces in the universe, capable of tearing a planet apart and swallowing an entire star. Yet scientists now believe they could hold the key to answering the ultimate question - what was there before the Big Bang? The trouble is that researching them is next to impossible. Black holes are by definition invisible and there's no scientific theory able to explain them. Despite these obvious obstacles, Horizon meets the astronomers attempting to image a black hole for the very first time and the theoretical physicists getting ever closer to unlocking their mysteries. It's a story that takes us into the heart of a black hole and to the very edge of what we think we know about the universe.

60 min

Why Do We Talk?

Talking is something that is unique to humans, yet it still remains a mystery. Horizon meets the scientists beginning to unlock the secrets of speech - including a father who is filming every second of his son's first three years in order to discover how we learn to talk, the autistic savant who can speak more than 20 languages, and the first scientist to identify a gene that makes speech possible. Horizon also hears from the godfather of linguistics, Noam Chomsky, the first to suggest that our ability to talk is innate. A unique experiment shows how a new alien language can emerge in just one afternoon, in a bid to understand where language comes from and why it is the way it is.

60 min

How Many People Can Live on Planet Earth?

Naturalist Sir David Attenborough investigates whether the world faces a population crisis and if it is the duty of individuals to change the way they live.