A wormhole is a hypothetical shortcut through spacetime, a tube of curved space which connects two distant locations. In principle, a wormhole could allow us to cross interstellar distances faster than light, or even travel backwards in time. Although these ideas sound like science fiction, they are in fact mathematically possible within Einstein's theory of gravity; what we do not know, however, is whether these strange warped spacetimes can actually exist in reality, or whether they are forbidden by the laws of physics.
There is one very good reason to be sceptical about the existence of wormholes: they require a source of negative energy, something which is impossible according to classical physics. Without this exotic type of energy, a wormhole will collapse before anything could travel from one side to the other.
Fortunately for us, there is a quantum phenomenon, the Casimir Effect, which can produce negative energy. The Casimir effect is usually achieved by placing two metal plates very close together. The plates limit the quantum fluctuations that can occur between them, and consequently the energy of the region between the plates becomes lower than the energy of the space outside. Considering that the space outside has zero energy, the energy between the plates must be negative.