White Holes | Space Time

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Matt O'Dowd (1:02)

This episode is supported by LegalZoom. Lurking in the depths of the mathematics of Einstein's general relativity is an object even stranger than the mysterious black hole. In fact, it's the black hole's mirror twin, the white hole. Some even think that these could be the origin of our universe.

The astrophysical phenomenon of the black hole has captured the imagination of scientists and science enthusiasts alike for many decades. When the idea first emerged from Einstein's general theory of relativity, physicists wondered how seriously to take this mathematical description of an inescapable region of spacetime. Astronomers have since demonstrated that black holes are very real, with convincing evidence that quasars, x-ray binaries, even the center of our own Milky Way galaxy harbor these gravitational monstrosities.

But the mathematics that predicts the existence of the black hole also describes entities that are even stranger, but whose relationship with reality is still unclear.

One such entity is the white hole.

A white hole is the opposite of a black hole, in a very literal mathematical sense. In fact, it's a time-reversed black hole. A black hole is defined as a region of inward-flowing space-time with a one-way boundary called the event horizon, from inside of which nothing can ever escape. That makes a white hole a region of outward-flowing space-time. It also has an event horizon, but that horizon prohibits entry, not exit. Nothing outside a white hole can ever enter, and everything inside must be ejected. Not even light can leave a black hole, hence the whole black thing. But light can only leave a white hole. So these might be expected to radiate like crazy, and white would be an understatement. Now, before everyone gets too excited, white holes are probably a figment of mathematical imagination. But they're a fascinating one, and the idea may help us understand the origin of the universe.

White holes first emerged in the very earliest mathematical description of black holes. Only a few months after Einstein published his General Theory of Relativity, Karl Schwarzschild solved its equations for a very particular case, a single point of mass in an otherwise empty spacetime. The resulting Schwarzschild metric actually describes a black hole, the simplest black hole possible, one without spin, without charge, or without change. An eternal black hole that doesn't grow or shrink, and has always existed. We've talked quite a bit about the bizarre behavior of space and especially time at and below the event horizon of a black hole. Here's a little playlist if you want to refresh her, but here's the lowdown. The time that happens inside a black hole is not part of the past or future history of the outside universe. From the perspective of an outside observer, any events occurring at the event horizon, including falling into it, happen infinitely far in the future. Once you fall into the black hole, the Schwarzschild metric tells us that space and time switch their roles. The singularity no longer occupies a central location. It now occupies an inevitable future time. Now, a real black hole forms from the gravitational collapse of a massive star's core. After the collapse, the future singularity comes into being. And in the past, well, there's just a star.

But what does this idealized eternal black hole look like in the past? If we followed the Schwarzschild metric back in time, we find something very strange. We find the singularity again, lurking infinitely far in the past. From the point of view of the outside universe, the eternal black hole singularity exists both in the infinite future and in the infinite past. That may sound strange, but it gets stranger. To really understand what this eternal black hole looks like, we're going to need to use a tool that we've already played with, the Penrose diagram. To refresh your memory, in a Penrose diagram, the X and Y axes are redefined from space and time to merge space and time into new coordinates. They compactify space-time so that time bunches up towards the edges and the borders correspond to infinite past and future. Also, lines of constant distance and time curve so that light paths always travel on 45 degree paths.