Extremism challenge for YouTube

The streaming service has vast power but will it contain the problem of extremist videos? We ask Chris Stokel-Walker, author of the new book YouTubers. Plus, Facebook outlines a business shift towards privacy and communication within smaller groups. But pressure is growing on founder Mark Zuckerberg to relinquish some of his power. And we discover how techniques from astronomy could help improve life in cities. Presented by Rory Cellan-Jones, with BBC tech reporter Chris Fox, and special guest Rhiannon Williams from the i Newspaper.

(Image: Stock photo of a teenager looking disturbed at something on his phone, Credit: Getty Images).