Book Reviewed: White Holes by Carlo Rovelli

When spacetime breakdown White holes are theoretical objects predicted by the mathematical solutions to the equations of general relativity. One way to think about the gravitational dynamics of white holes is to consider them as time-reversed versions of black holes, the black holes pull matter/energy inward, but white holes are known to expel matter/energy outward, i.e., matter and light can only escape from them, but they can’t enter. White holes are the final stage in the evolution of black holes. According to the author’s arguments, space itself is made up of individual grains or quanta, and when matter inside a black hole reaches these incredibly tiny scales, a quantum repulsive force causes it to bounce back, transforming the black hole into a white hole. The white holes are not visible because nothing can enter them including light. This means that no information, including light or any electromagnetic radiation is coming out of a white hole. Therefore, they are invisible to outside observers. White holes also decrease entropy by expelling matter and energy in a highly ordered manner which is in violation of the second law of thermodynamics. This is a small pocket-sized book of 176 pages which doesn’t discuss how white holes could be detected if they exist but builds the work on the known ideas. If white holes are expelling matter and energy, they could potentially create unique patterns like cosmic ray anomalies that could be measured. Black holes are theorized to emit Hawking radiation due to quantum effects near the event horizon, and white holes may absorb radiation from the surrounding universe due to quantum effects resulting in cosmic ray anomalies. Merging black holes produce detectable gravitational waves, and they have been detected, similarly, merging white holes should produce distinct gravitational wave signatures. Theoretical considerations allow white holes to connect with black holes through wormholes, hypothetical tunnels in spacetime that could potentially allow for travel between distant cosmic points or different universes. The extreme gravitational effects near a black hole’s event horizon cause time to pass much more slowly compared to a distant observer, meaning that the transformation of a black hole into a white hole could take billions of years from our perspective on earth. However, primordial black holes which were formed shortly after the Big Bang, about 13.8 billion years ago, may have resulted in white holes. The author claims that the mysterious dark matter which makes up 27% of the universe are white holes. But this is a wild speculation.