May 23, 2019, Written by Ashley Hamer

Einstein made some rock-solid predictions. His theories predicted that the collision of massive objects would create ripples in spacetime called gravitational waves, which we detected in 2015. They also predicted the existence of black holes, one of which we imaged this year. But they also made predictions for phenomena that haven’t turned out to be real — at least, not so far. One of those is the white hole, the theoretical opposite to a black hole. While they’ve never been shown to exist, some scientists think they could explain some of the universe’s most unexplainable characteristics.

How Deep the Rabbit Hole Goes

A black hole is an object that possesses a gravitational pull so great that not even light can escape from it. Once something crosses the invisible, spherical boundary surrounding the black hole known as the event horizon, it can never return. Black holes form when a star many times more massive than our sun dies and its core shrinks until it’s so dense that nothing can escape its gravity. Inside of that point of no return, the star continues to collapse into an infinitely dense point.00:1101:00

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That point at the center of a black hole is known as a singularity, and it’s one of the places black holes get messy, mathematically speaking. Einstein and fellow physicist Nathan Rosen fixed that math in 1935 by extending the point into a path that leads to a second location. This path was called an Einstein-Rosen Bridge, but you probably know it as a wormhole. But if a black hole is at one end of a wormhole, what’s at the other end? A white hole, naturally. The same math that predicts the existence of a black hole also predicts the existence of its mirror opposite: a white hole. While anything that enters a black hole can never escape, anything that escapes a white hole can never return.

Theoretically. While we know that black holes exist, so far, white holes (and wormholes, for that matter) exist only in pages of physics papers. “In all likelihood, white holes are just fancy math,” writes Universe Today’s Fraser Cain. “And since fancy math rarely survives contact with reality, white holes are probably just imaginary.”

Keeping the Faith

And yet, physicists keep coming back to them. Some say it’s possible that every time a black hole forms, an entirely new universe forms in the white hole at the opposite end of the wormhole. And in 2014, theorists Hal Haggard and Carlo Rovelli used quantum theory to show that black holes could actually transform into white holesvia something called loop quantum gravity. That theory says that the fundamental building blocks of spacetime are shaped like tiny loops, and since those loops have a finite size, a dying star can’t actually collapse into a point of infinite density. Instead, right before it reaches infinity, a dying star experiences a “quantum bounce” that exerts an outward pressure and turns it into a white hole. (If true, this could be a solution to the black hole information paradox, which says that even though anything that falls into a black hole can never escape, black holes gradually emit radiation until they disappear completely. That’s a problem because information cannot be destroyed. A white hole would deliver that information safely out the other side.)

The black-to-white transformation would happen in a few thousandths of a second, but because time dilates in the presence of gravity, an outside observer might perceive an average-sized black hole to take billions of years to turn into a white hole. That could be a reason we haven’t seen one yet: The universe is just too young, and most black holes are too big.

But hypothetically, fluctuations in density caused by the Big Bang could have concentrated matter into numerous points that collapsed into tiny black holes. Because these primordial black holes would be so small, Haggard and Rovelli think it’s possible that they could become white holes in our universe’s lifetime. Rovelli and other scientists have even proposed that these tiny black holes, and the invisible white holes they may have turned into, could be the secret ingredient of dark matter, but recent tests have cast serious doubt on that theory.

White holes sure would be a convenient way to explain many of the universe’s most dogged mysteries, but they’re probably not a thing. Still, some of Einstein’s wilder predictions turned out to be true — maybe we just need to wait another few billion years to find out.

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