The once-impossible black holes that could break thermodynamics
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- Christoph Kehle (MIT) and Ryan Unger (UC Berkeley) proved in 2024 that extremal black holes — long treated as unattainable mathematical ideals — can form dynamically through two physical mechanisms without producing a naked singularity.
- The third law of black hole physics held that no black hole could reach zero temperature or stop emitting Hawking radiation, mirroring thermodynamics' rule that no system can be cooled to absolute zero; Kehle and Unger showed extremal black holes do achieve zero surface gravity and zero Hawking temperature.
- The two formation pathways Kehle and Unger identified are gradually adding charged matter to a black hole, or firing a charged-particle beam into empty space that collapses directly into an extremal black hole.
- Chiara Toldo (Free University of Brussels) called the result significant because it shows extremal black holes are 'not just mathematical limits' but 'can genuinely arise within classical gravity,' making them usable as physical models rather than pure idealisations.
- Extremal black holes have zero temperature but retain non-zero entropy, making them unusually clean systems for probing what black hole microstates actually are — a central question that string theory has long pursued in idealised form.
- Researchers showed last year that extremal horizons would leave a distinctive lingering fingerprint in radiation reaching distant observers, though actually detecting one 'remains a distant prospect' per the article.
- Kehle himself said the work made him realise 'I understand black holes better than I understand classical thermodynamics,' flagging that the gap exposed may be in thermodynamics itself, not just in black hole physics.
Why it matters: Thermodynamics has survived every major upheaval in physics — quantum mechanics, relativity, the expanding universe — but the collapse of its third-law analogy with black holes suggests the correspondence between black holes and thermodynamic systems may not be as exact as assumed. Kehle's own admission that he now understands black holes better than classical thermodynamics opens the unsettling possibility that the venerable theory of heat itself has a gap.
