Carlo Rovelli and Brian Greene on Black Holes and White Holes

The conversation begins with an exploration of white holes, theoretical entities that are the time reversal of black holes. It discusses the concept of time reversal in physics, where processes can occur in reverse order. The example of a glass shattering and then reassembling is used to illustrate this idea. The discussion introduces physicist Carla Relli, who suggests that white holes could be real and may hold the key to understanding dark matter, a persistent mystery in astrophysics.

The discussion moves to the historical aspect of black holes, from being considered non-existent to being detected and studied. The discovery of a strong signal from the Sagittarius constellation in the 1930s, which was initially unexplained, is highlighted. This signal was later understood to be from the supermassive black hole at the center of our Milky Way galaxy. The conversation emphasizes the journey from skepticism to confirmation in the scientific community regarding the existence of black holes.

Carla Relli provides a theoretical background on black holes and white holes, explaining that white holes are solutions of general relativity equations, just like black holes, but in reverse time. The conversation delves into the formation of black holes from collapsing stars and the lack of understanding of their death or transition into white holes. Relli introduces the idea that white holes could have been produced by the Big Bang, but this theory was not convincing until recent developments in quantum gravity.

The discussion shifts to the role of quantum gravity in understanding the nature of black holes. It is explained that at the center of a black hole, where the curvature and energy density are immense, quantum mechanics must come into play. The concept of singularities in black holes is addressed, noting that they signify a breakdown of classical general relativity and the need for a quantum theory of gravity. The conversation also touches on the process of black hole evaporation, as predicted by Stephen Hawking, and the implications for the understanding of black hole remnants.

Carla Relli discusses the idea that the death of a black hole could be the birth of a white hole, describing the transition as a quantum jump that occurs when a black hole becomes very small and enters the quantum regime. The conversation explores the possibility of white holes being remnants of black holes, emitting very slow radiation and having a mass at the Planck scale. Relli suggests that these white hole remnants could be candidates for dark matter, as they interact only gravitationally and could be abundant in the universe.

The conversation continues with a discussion on the role of quantum gravity theories, such as loop quantum gravity and string theory, in understanding the transition from black holes to white holes. Relli explains that loop quantum gravity provides a framework for calculating the transition amplitude, suggesting that small black holes are more likely to transition into white holes. The conversation also addresses the issue of singularities in black holes and how quantum gravity might resolve them. Relli expresses hope that the theoretical work on white holes could lead to a better understanding of dark matter.

The conversation concludes with a reflection on the speculative nature of white hole research and its potential implications for physics. Both participants agree that theoretical work is essential but must ultimately be connected to experimental evidence to be considered credible. The historical lack of evidence for phenomena such as black holes, gravitational waves, and the Big Bang is mentioned, emphasizing that scientific understanding progresses over time and that white holes may eventually be observed, just as these other phenomena have been.