Horizons are weird. They delimit what we can see in the distance, but they are also always personal: Walk 10 miles to the west and your horizon line moves 10 miles to the west with you.
Remarkably, this local, personal character of horizons also extends to black holes, the most enigmatic objects in the cosmos. Going beyond that horizon towards a new understanding of space, time and black holes is the principal goal of physicist Carlo Rovelli's wonderful new book White Holes.
"What happens at the center of a Black Hole?" is one of those questions I get whenever I tell someone I'm an astrophysicist — and it's the question that propels this book. Rovelli is unique among modern scientists who write for popular audiences in his ability to capture the purest essence of his science with both precision and lyricism. White Holes, like Rovelli's other works, is remarkably short — less than 200 pages. But the clarity of his explanations is unparalleled. As a scientist who is also a popularizer, I often find myself marveling at the acuity of his passages. More than just an ability to explain cutting edge ideas in physics, Rovelli's erudition and sensitivity lets him make contact with the broadest human yearnings for making sense of the world. This capacity is put to good use in White Holes, where the descent into a black hole is often narrated via quotes from Dante who made his own journey "down there in the blind world below."
The science question at the heart of Rovelli's new book comes from his own research into the intersection of Einstein's General Theory of Relativity and Quantum Mechanics. The former identifies gravity with the shape of space and time. The latter determines the behavior of the nanoworld — i.e., atoms and their constituents. Black holes are a crossing point for these two great theories because they're places where gravity is so strong that space and time become distorted on quantum scales.
Black holes form when so much matter accumulates in one location that no force can stop its contraction via gravity. Imagine a star that has used up its nuclear fuel and no longer produces energy to support itself against its own weight (the own "shining" thing is a just a consequence of this battle). As the fuel runs out, gravity squeezes the star down to ever smaller sizes. At some point the dead star is so small and dense that light emitted from its surface cannot escape gravity's pull. In the language of Einstein, the curvature of surrounding space-time is too extreme for light to escape. That's when a horizon forms around the black hole. This "event horizon" marks the point of no return. Observers outside the horizon can never get any information about what's inside the horizon (that's why it's called a horizon).
Black holes are real. They have been observed in a number of ways including direct images using the entire Earth as a telescope. But even though physicists have seen black holes and developed many remarkable and sophisticated ideas about them, the eventual fate of matter falling into one remains a stubborn scientific mystery.
That's where Rovelli and White Holes comes in. His answer to the question "What happens?" is that black holes eventually become white holes where everything that fell into event horizon emerges again. To demonstrate how this is possible, Rovelli takes the reader on a fascinating exploration of what a horizon means for time. One of the most remarkable aspects of Einstein's relativity is that your time is not my time. The flow of time is relative. It can change. In particular it depends how fast observers (i.e., us) move in relation to each other or how close we are to massive bodies (like a black hole). So, to an outside observer, someone falling towards a black hole seems to have their clocks slow down until they stop entirely at the event horizon's edge. Understanding how this works, how the flow of time is both personal and relative, represents some of Rovelli's best work in the book. As he writes "... if we approach the [event] horizon and go beyond it, our watches do not slow and nothing strange happens to the space around us, just as nothing peculiar happens to a ship when it crosses the line of the horizon and disappears from our view." From these observations Rovelli then builds a path for us towards a new theory of black holes and their fate.
I won't spoil the ending by telling you what Rovelli says happens when black holes turn into white holes. I will, however, tell you that taking the journey with Rovelli is more than worth the price of the book. Dante gave us his tour of the underworld. We could not do better than having Rovelli as a guide into the dark world of black holes.
Adam Frank is an astrophysics professor at the University of Rochester. His newest book is The Little Book of Aliens. You can find more from Adam here: @adamfrank4.
