While such strange solutions to the equations of general relativity first appeared in the 1949 work of Austrian-American logician Kurt Gödel, it is commonly thought that they must be a mathematical artefact yet to be explained away.
Black and white holes
In 1964, two Americans, the writer Ann Ewing and the theoretical physicist John Wheeler, introduced the term “black hole”. Subsequently, in 1965, the Russian theoretical astrophysicist Igor Novikov introduced the term “white hole” to describe the hypothetical opposite of a black hole.
The argument was that if matter falls into a black hole, then perhaps it is spewed out into our universe from a white hole.
This idea is partly rooted in the mathematical concept known as an Einstein-Rosen bridge. Discovered (mathematically) in 1916 by the Austrian physicist Ludwig Flamm, and re-introduced in 1935 by Einstein and the American-Israeli physicist Nathan Rosen, it was later termed a “wormhole” by Wheeler.
In 1962, Wheeler and the American physicist Robert Fuller explained why such wormholes would be unstable for transporting even a single photon across the same universe.
Fact and fiction
Not surprisingly, the idea of entering a (black hole) portal and re-emerging somewhere else in the universe – in space and/or time – has spawned countless science fiction stories, including Doctor Who, Stargate, Fringe, Farscape and Disney’s Black Hole.
Ongoing productions can simply claim that their characters are travelling to a different or a parallel universe to our own. While it appears to be mathematically feasible, there is of course no physical evidence to support the existences of such universes.
But this is not to say that time travel, at least in a limited sense, is not real. When travelling at great speed, or perhaps falling into a black hole, the passage of time does slow down relative to that experienced by stationary observers.
Clocks flown quickly around the world have demonstrated this, displaying time lags in accordance with Einstein’s theory of special relativity.
The 2014 movie Interstellar played on this effect around a black hole, thereby creating a sense of travelling forward in time for astronaut Cooper (played by Matthew McConaughey).
Despite the strangely endearing name, the phrase “black hole” is perhaps somewhat misleading. It implies a hole in space-time through which matter will fall, as opposed to matter falling onto an incredibly dense object.
What actually exists within a black hole’s event horizon is hotly debated. Attempts to understand this include the “fuzzball” picture from string theory, or descriptions of black holes in quantum gravity theories known as “spin foam networks” or “loop quantum gravity”.
One thing that does seem certain is that black holes will continue to intrigue and fascinate us for some time yet.