The once-impossible black holes that could break thermodynamics

The fundamental laws of thermodynamics have long been an unshakable cornerstone in the field of physics, surviving through major shifts and revolutions in scientific understanding. However, a new challenge may be arising in this field regarding black holes – the once-impossible black holes that could break thermodynamics.

The story begins in the early 1970s, when Jacob Bekenstein, a graduate student at Princeton University, observed that matter falling into a black hole seemed to vanish from the observable universe, taking its entropy with it. Entropy refers to the number of microscopic configurations that give rise to the same observable state, and it is a central concept in thermodynamics. The second law of thermodynamics states that the total entropy of an isolated system always increases over time.

If matter was disappearing from the universe when it fell into a black hole, it seemed that entropy would also be disappearing, seemingly violating the second law. To resolve this apparent contradiction, Bekenstein proposed that black holes must have their own entropy. This idea, however, posed a problem – if a black hole had entropy, it must also have a temperature, and anything with a temperature should radiate.

Physicist Stephen Hawking initially disagreed with Bekenstein's proposal, suggesting that black holes should not have a temperature or radiate. However, further calculations revealed that black holes do emit a faint glow, now known as Hawking radiation, and thus possess a temperature. This discovery, combined with the striking correspondence between the laws governing black holes and the laws of thermodynamics, led physicists to believe that the third law of thermodynamics, which states that a system cannot reach absolute zero temperature, may also apply to black holes.

In 2024, Christoph Kehle at MIT and Ryan Unger at UC Berkeley proved that extremal black holes – those at the limit of the theory of general relativity – could indeed exist. This finding suggests that the third law of black hole physics may not be as strict as previously thought, potentially allowing black holes to violate some thermodynamic laws.

This discovery has significant implications for our understanding of black holes and the fundamental laws of physics, challenging long-held beliefs about the behavior of these enigmatic cosmic entities.