First, the good news. Time travel isn't necessarily forbidden by the laws of physics. In fact, the mathematics underpinning Albert Einstein's theories of relativity offer up possibilities via the likes of wormholes and curved spacetime that might allow someone — or more likely something — to make a journey back into the past. Not that anyone really knows how you'd do it — or if it's even possible at all.
Luckily for sci-fi writers, there's rarely any need to worry about backing up their ideas with facts. Over the years, they've come up with numerous ingenious ways to negotiate the space-time continuum, from iconic cars and spaceships slingshotting around stars, to heavily-modded telephone boxes and even — we're sorry to say — a time machine built into a hot tub.
