White holes: What we know about black holes' neglected twins

White holes are theoretical cosmic regions that function in the opposite way to black holes. Just as nothing can escape a black hole, nothing can enter a white hole.

White holes were long thought to be a figment of general relativity born from the same equations as their collapsed star brethren, black holes. More recently, however, some theorists have been asking whether these twin vortices of spacetime may be two sides of the same coin.

To a spaceship crew watching from afar, a white hole looks exactly like a black hole. It has mass. It might spin. A ring of dust and gas could gather around the event horizon — the bubble boundary separating the object from the rest of the universe. But if they kept watching, the crew might witness an event impossible for a black hole — a belch. "It's only in the moment when things come out that you can say, 'ah, this is a white hole,'" said Carlo Rovelli, a theoretical physicist at the Centre de Physique Théorique in France.

What are white holes?

White holes emerge from the solutions of Einstein's theory of general relativity devised by Karl Schwarzschild in 1916, just a year after the theory was first published.

Schwarzschild was the guy who wrote down the mathematics that describes black holes in the universe as completely collapsed objects. He had to make a choice, there is nothing in general relativity that dictates which direction time flows in.

You could point the clock in this particular direction, and that's what gives you this picture where things fall into a black hole. That's where gravity is attractive, but you could also choose the opposite direction in which time flows and get the opposite effect. So as well as a black hole, the mathematics of Schwarzschild also gives us a white hole by just flipping the way that time works.