Spacetime travel. Artwork showing the journey through spacetime within a wormhole. A wormhole is a tunnel through the four-dimensional spacetime linking one black hole to another one located elsewhere, maybe in another universe, or in another time. Wormholes, which are legitimate solutions of Einstein's equations, could therefore allow an observer to travel from one point to another either in a time direction (past- future) or in a space direction at a speed greater than the speed of light. Unfortunately for spacetime travellers, the life of a wormhole spans a tiny fraction of a second making a practical use impossible.