Along with black holes, wormholes have never been directly observed, but have been so widely theorized that many physicists are confident in their hypothetical existence. A wormhole is essentially a "short-cut" through space-time, where matter falls into a black hole and is then ejected from the event horizon (one-way boundary) of a white hole, its theoretical time reversal. Some scientists speculate that there is a white hole at the 'other end' of every black hole, potentially spewing out all of the matter that entered the black hole into another region of the cosmos or even another universe entirely (Mastin, 2009).
In 1916, Austrian physicist Ludwig Flamm proposed an alternate solution to Einstein's General Theory of Relativity after studying Karl Schwarzschild's solution, a Schwarzschild black hole (i.e., static black holes) (Mastin, 2009). He theorized the existence of white holes, which, as aforementioned, are essentially theoretical time reversals of black holes. They eject any matter that crosses its event horizon. While calculating, Flamm also noticed that both Schwarzschild's and his own theory hypothesized that two different regions of space-time could be mathematically connected when matter entered a black hole and exited the white hole on the supposed 'other side'. Einstein, along with the physicist Nathan Rosen, elaborated on this theory and created a solution known as an Einstein-Rosen bridge, aka a Lorentzian wormhole, aka a Schwarzschild wormhole (Mastin, 2009).
A simplified version of a wormhole sets space as a two-dimensional surface. In this case, a wormhole can be pictured as a hole in that surface that leads into a 3D tube (the inside surface of a cylinder). This tube then re-emerges at another location on the 2D surface with a similar hole as the entrance. This "wrinkle" could be analogous to pulling a needle through a piece of fabric when sewing.
In 1916, Austrian physicist Ludwig Flamm proposed an alternate solution to Einstein's General Theory of Relativity after studying Karl Schwarzschild's solution, a Schwarzschild black hole (i.e., static black holes) (Mastin, 2009). He theorized the existence of white holes, which, as aforementioned, are essentially theoretical time reversals of black holes. They eject any matter that crosses its event horizon. While calculating, Flamm also noticed that both Schwarzschild's and his own theory hypothesized that two different regions of space-time could be mathematically connected when matter entered a black hole and exited the white hole on the supposed 'other side'. Einstein, along with the physicist Nathan Rosen, elaborated on this theory and created a solution known as an Einstein-Rosen bridge, aka a Lorentzian wormhole, aka a Schwarzschild wormhole (Mastin, 2009).
A simplified version of a wormhole sets space as a two-dimensional surface. In this case, a wormhole can be pictured as a hole in that surface that leads into a 3D tube (the inside surface of a cylinder). This tube then re-emerges at another location on the 2D surface with a similar hole as the entrance. This "wrinkle" could be analogous to pulling a needle through a piece of fabric when sewing.
A rendering of a wormhole. Retrieved from http://www.physicsoftheuniverse.com/topics_blackholes_wormholes.html
Although noted physicists such as Stephen Hawking have theorized that wormholes could be used as methods of time-travel as well as travel through space, the general consensus is that wormholes are highly unstable and would probably collapse before any matter that entered it could get out the other side. However, wormhole theory provides a fascinating view into the realm of possibility of cosmological exploration.