“Bigger on the Inside”: Black Hole Behavior Gives Weight to Tardis Trick from Dr. Who

Are black holes, like elder statesmen, simply fading away? Do they pop like hyperdimensional balloons? Possibly, or perhaps they traverse a cosmical Rubicon, reversing their very essence and becoming inverse aberrations which cannot be entered through their event horizons but instead constantly expel energy and matter back into the universe.

In his recent publication, White Holes, physicist and philosopher Carlo Rovelli directs his attention and extensive expertise toward mystifying phenomena of the cosmos, delving beyond the event horizon to explore their theoretical mechanisms and theorizing what might lie at the core of these unimaginably miniscule yet endlessly captivating gravitational focal points. In this week’s Hitting the Books excerpt, Rovelli discusses a scientific division splitting the astrophysics community regarding where all the information—currently understood to be indestructible under the rules of our universe—goes once it is ensnared within a black hole that is impossible to escape.

In 1974, Stephen Hawking made an unexpected theoretical discovery: black holes must emit heat. This is a quantum tunnel effect, but a simpler one than the bounce of a Planck star: photons trapped inside the horizon escape thanks to quantum physics that enables everything to “tunnel” beneath the horizon. So black holes emit heat, like a stove, and Hawking computed their temperature. Radiated heat carries away energy, causing the black hole to gradually lose mass (mass is energy), becoming ever lighter and smaller. Its horizon shrinks, and eventually, the black hole “evaporates.”

Heat emission is the most characteristic of the irreversible processes, the ones that occur in one direction and cannot be reversed. A stove emits heat and warms a cold room, but the reverse never holds true. When heat is produced, the process is irreversible. Therefore, heat distinguishes past from future. Consequently, at least one clearly irreversible aspect of a black hole’s life is the gradual shrinking of its horizon.

However, the shrinking of the horizon does not indicate that the interior of the black hole becomes smaller; the interior largely remains unchanged, and its volume actually expands. The shrinking solely pertains to the horizon. Therefore, an old black hole has a peculiar geometry: an enormous interior that continues to grow, encompassed by a minuscule, fade away horizon. The gravity of a Planck star generates a massive distortion, enclosing a voluminous space within a small sphere.

The existence of vast volumes within small horizons has sparked disagreements among scientists. The scientific community is divided and is embroiled in a dispute over this topic. One side contends that when there is limited energy and the horizon is substantially reduced, minimal information can be retained inside, while the other side, including Rovelli, argues to the contrary.

These disagreements, though common in the history of science, can last a long time. Scientists can split, quarrel, and vehemently argue their points of view. Yet, over time, clarity emerges. Personally, Rovelli believes that his colleagues who believe a small horizon can retain only a small amount of information are mistaken, despite their convincing arguments. Even if their first argument suggests that as the horizon gets smaller, the fewer elemental components it contains and regardless of their second argument stating that the number of elemental components decreases as the horizon shrinks, Rovelli argues that it is a serious mistake.