&Bullet; *physics* 14, p87

By comparing the size of black holes before and after a merger, researchers tested Hawking’s theorem about the areas of black holes.

Black holes may be the strangest cosmic creatures, but they should follow simple laws dictated by thermodynamics. Stephen Hawking formulated such a law, which states that the area of the event horizon of a black hole – similar to entropy – cannot decrease over time (at least on timescales that are much shorter than the age of the universe). It is currently impossible to test this law by sizing individual black holes, but gravitational wave observations now allow researchers to do it using black hole mergers. Maximiliano Isi of the Massachusetts Institute of Technology and colleagues report a direct test of the theorem obtained by analyzing the first fusion ever discovered, GW150914. The comparison of the size of the black holes before and after the merger by the team shows that the theorem is not violated [1] .

When applied to a fusion, the area theorem implies that the combined area of the two precursor black holes cannot exceed that of the post-merger remainder. Inspired by a question from the physicist Kip Thorne, Isi and co-workers developed a method to calculate these areas from the gravitational wave signal. The method involves extracting the “before” and “after” regions of the black hole by analyzing the “inspiral” and “ringdown” components of the signal in the time domain. The team compared methods that included or excluded different parts of the signal – such as overtones or a narrow window around the moment of coalescence – as each of these analyzes required different assumptions. They found that the theorem holds for this particular blend with a probability of at least 95% (or 2-sigma confidence). By analyzing other detections – past and future – the researchers can improve this accuracy and also check the validity of the theorem on a large population of black holes or candidates for black holes.

– Matteo Rini

Matteo Rini is the editor of *physics*.

## References

- M. Isi
*et al.*, “Testing the Black Hole Area Law with GW150914”, Phys. Rev. Lett.**127**, 011103 (2021).

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