The equations of physics suggest time should be able to go backwards as well as forwards. Experience suggests, though, that it cannot. Why? And is time travel really possible?
Visiting a hidden location buried beneath the hills of Scotland, Helen experiences some of the most extreme acoustics in the world. Here she learns just how much information can be carried by sound. She discovers how sound has driven the evolution of truly incredible biological systems and complex relationships between creatures that exploit sound for hunting - and escaping from predators. Helen demonstrates how sound waves diffract (bend around objects) and in doing so help us sense danger and locate it. Helen explains how we are not limited to passively detecting sound waves; we can also use them to actively probe the world.
It's called the speed limit of the universe. Einstein blew all of our minds when he worked out the Theory of Relativity, and showed that space and time were malleable substances. He also theorized that we as humans can never travel faster than the speed of light, which leaves the stars and other galaxies almost impossibly out of our reach. But the dreams of Star Wars and Star Trek are not dead. In fact, there could be ways to travel faster than the speed of light - and some of them are already being tested in labs around the world.
Engineer Jem Stansfield looks back through the Horizon archives to find out how scientists have come to understand and manipulate the materials that built the modern world. Whether it's uncovering new materials or finding fresh uses for those we've known about for centuries, each breakthrough offers a tantalising glimpse of the holy grail of materials science - a substance that's cheap to produce and has the potential to change our world. Jem explores how a series of extraordinary advances have done just that - from superconductors to the silicon revolution.