The BBC's Horizon series returns for its monumental 54th series this year, exploring the cutting edge of scientific research and development. The series takes us from the latest breakthroughs in medicinal science to feats of engineering which would have only ever have been thought to exist in the pages of science fiction novel before, standing on the precipice of the unknown and speaking to the pioneers of their respective fields who are breaking new ground every day. In this episode, Horizon explores the exciting concept and practical possibilities of time travel. While time travel itself isn't theoretically forbidden by the laws of nature, our current knowledge and grasp on technology isn't enough to put such ideas into practice and so it's up to the those working behind closed-doors to find a way to subvert and manipulate the boundaries which prevent us from making this dream a reality. With progress being made every day, Horizon visits the men and women at the forefront of this branch of research; from building wormholes to manipulating entangled photons and engineering fully functioning time crystals. For these researchers, tomorrow could soon be an idea of the past. Horizon: How to Build a Time Machine - Tuesday 9:00pm on BBC2.