Western governments have criticised social media platforms including Facebook, YouTube and Twitter for being slow to stop the spread of terrorist material. But few critics have as much technical expertise as Hany Farid, a professor of computer science at Dartmouth College in the US. Mr Farid developed an algorithm to detect child pornography, work that is being adapted to curb the proliferation of terrorist videos. Prof Farid is planning to launch a new technology alongside the Counter Extremism Project, a US non-profit organisation, to help online platforms identify extremist content and take it down in an instant.