For centuries, the precise workings of gravity have confounded the greatest scientific minds - from Newton to Faraday and Einstein - and the idea of controlling gravity has been seen as little more than a fanciful dream. Yet in the mid 1990s, UK defence manufacturer BAE Systems began a ground-breaking project code-named Greenglow. Nasa was simultaneously running its own Breakthrough Propulsion Physics Project". It was concerned with potential space applications of new physics, including concepts like 'faster-than-light travel' and 'warp drives'. Looking into the past and projecting into the future, Horizon explores science's long-standing obsession with the idea of gravity control. It looks at recent breakthroughs in the search for loopholes in conventional physics and examines how the groundwork carried out by Project Greenglow has helped change our understanding of the universe. Gravity control may sound like science fiction, but the research that began with Project Greenglow is very much ongoing, and the dream of flying cars and journeys to the stars no longer seems quite so distant.

The Internet is an uncontrollable labyrinth of content and imagery that operates under loose laws and partial anonymity - a new frontier where people can do anything and see anything, whether they should or not. In this lawless landscape, who's in control? The protectors? The abusers? In this episode meet the content moderators who watch the worst of the Internet so you don't have to; a convicted hacker whose punishment was the virtual prison of the unplugged; plus, a blackhat hacker who encrypts and holds computer files hostage.

We all project images of ourselves online - a curated identity that defines how we want to be perceived. But what temptations exist to embellish and what are the dark implications? In this episode, enter the world of Cyberbanging, where inner city youth are using social media to brag, conspire and incite violence; two African-American women bloggers who have turned the table on the trolls who threaten them; and the creator of Abdullah X, an online character who's trying to keep Muslim kids from falling under the spell of Islamic extremists.

In less than two years, Al Capone has risen from a humble bookkeeper to second-in-command of Johnny Torrio's criminal empire in Chicago. The empire is worth the modern-day equivalent of $35 million. Capone purchases a home for his family and moves them from New York to Chicago. Torrio and Capone expand their empire from Chicago's South Side to the North Side, which is controlled by Chicago's second largest gang, the Irish. Violence erupts as ruthless Irish gang leader Dean O'Banion protects his territory and bootlegging business from Torrio and Capone.

Syria Civil Defense (SCD), also known as the White Helmets, is a volunteer civil defense organization that operates in rebel-controlled Syria during the Syrian Civil War as a response to indiscriminate bombardment of civilian communities. This group of indomitable first responders risk their lives to rescue victims from the rubble.

The Bronze Age had the first large urban centers, powerful kingdoms and armies, writing, and trade routes across vast areas. What led to the collapse of the city-state entities and the end of the Bronze Age? Possibly a perfect storm of uncontrollable factors that are still being researched.