&Bullet; physics 14, p47

An analysis of the gravitational waves emitted by black hole fusions confirms that black holes are the fastest known distributors of information.

The extreme nature of black holes means that they offer unique opportunities to test the limits of the laws of physics. One law that researchers wanted to test in this way describes the maximum rate at which information can flow out of a system. Until recently, however, this black hole test was not possible due to the lack of suitable candidates. That changed with the first measurements of gravitational waves. An analysis of the gravitational waves found in eight black hole fusions now confirms that the law applies to these extreme objects [1] .

Every disturbed object outputs information about its condition until it is again in equilibrium. The theory predicts a limit to the rate of this information emission, this limit depending on the temperature of the object and its relaxation time (how quickly it regains equilibrium). For newly merged black holes, these parameters are encoded in the emitted gravitational waves.

Of the around 50 black hole fusions to date, researchers from the University of Pisa (Italy) and the University of Glasgow (Great Britain) selected eight from which they could reliably measure the relaxation times. For each of these fusions, the team calculated the maximum average information emission rate per unit of energy. They found that these rates are the fastest for any known object: approximately

$2.2×1{0}^{34th}$

Bits per second per joule or 75% of the theoretical maximum. At this extreme rate, disturbed black holes send information at a rate about 11 orders of magnitude higher than objects with “everyday” room temperature, which are about three feet wide.

The result confirms that black holes obey the basic principles of general relativity, information theory, and thermodynamics – a finding that the team says is not guaranteed to be true. Future extensions of the general theory of relativity would also have to obey this bound information.

–Christopher Crockett

Christopher Crockett is a freelance writer based in Arlington, Virginia.

## References

1. G. Carullo et al.“Bekenstein-Hod, universally tied to the information emission rate, is obeyed by binary remnants of the black hole of LIGO-Virgo.” Phys. Rev. Lett.126161102 (2021).

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