A ghost in the equations
On a quiet street in Princeton, New Jersey, is the Institute for Advanced Study, the academic home of Albert Einstein from 1933 until his death in 1955. It was within this intellectual environment that the deepest implications of his 1915 theory of general relativity were explored. A strange concept to emerge from its mathematics is the white hole.
The idea arises directly from the solutions to Einstein's field equations that describe a black hole. In 1916, Karl Schwarzschild was the first to solve these equations for a single, non-rotating ball of mass, giving birth to the black hole in theory. Later, physicists noticed that if you reverse the arrow of time in the mathematics, you get the opposite of a black hole: an object that nothing can enter, and from which matter and light can only escape.
This theoretical object is connected to a black hole through a mathematical construct called an Einstein-Rosen bridge, more popularly known as a wormhole. The full geometry, which describes an "eternal black hole," was mapped out in 1960 by Martin Kruskal and George Szekeres. Their coordinate system removed the mathematical artifacts at the event horizon and revealed a maximal spacetime that includes a black hole, a white hole, and two separate universes connected by the wormhole.
An unstable, unseen fountain
Despite their mathematical validity, no white hole has ever been observed. Most physicists believe they cannot form in our universe for several reasons. A major obstacle is the second law of thermodynamics, which states that entropy, or disorder, in an isolated system can only increase. A white hole would be a source of spontaneous and uncaused order, violating this fundamental principle.
White holes would also be incredibly unstable. Theory suggests that even a single photon of light approaching a white hole's event horizon would be blueshifted to have such enormous energy that it would cause the entire structure to collapse into a black hole. The process would happen almost instantly.
So why do physicists still talk about them? One speculative idea is that the Big Bang itself was a white hole. This theory suggests our entire observable universe emerged from a single, spontaneous pulse of matter and energy from a singularity. Other, more fringe ideas have proposed that short gamma-ray bursts (GRBs) could be the signature of microscopic white holes finally exploding. The anomalous GRB 060614, detected in 2006, was briefly considered a candidate, though this remains highly speculative. For now, the white hole is a concept in Einstein's beautiful equations, a theoretical possibility waiting for evidence that may never come.