Wormholes seem like they belong in sci-fi, but physicists were actually the first ones to hypothesise their existence. When Einstein published his theory of general relativity in 1916, Austrian physicist Ludwig Flamm realised that if black holes (which suck in matter) could exist, so could white holes (which spew out matter). Einstein and Nathan Rosen built upon the idea, theorising that two different points in space could be connected by space-time shortcut, or “bridge”, with a black hole as the entrance and a white hole as the exit. In theory, they could not only connect two points in our universe but also be a bridge between universes, and would allow you to travel between two points faster than the speed of light. However, wormholes would be highly unstable, existing for an instant and collapsing in on themselves if any matter attempts to pass through. Travelling through would only be possible if the walls could be held apart, and achieving this would require matter with a negative density and a large negative pressure. This hypothetical matter has been dubbed “exotic matter”—but it’s also been suggested that stable wormholes could be constructed using string theory techniques.