A black hole collision, the strongest yet
On 10 September, physicists announced the strongest gravitational wave signal yet recorded, the collision of two black holes in a distant galaxy so violent that it confirms one of the most celebrated predictions in the field: that the area of a black hole can only grow, never shrink. The observation vindicates Einstein’s general relativity and a theorem derived from it by Stephen Hawking.
Almost a decade after the first detection of gravitational waves, the ripples in space itself predicted a century ago, physicists have announced the strongest such signal ever recorded. The event behind it, reported on 10 September, was the collision of two black holes in a distant galaxy, a merger so violent that it produced a gravitational wave signal more powerful than any the detectors have seen. And beyond the drama of the signal itself, the observation carries a confirmation that reaches to the foundations of the theory: the merger of the two black holes obeyed the area theorem, the celebrated rule, derived from general relativity by Stephen Hawking, that the area of a black hole can only grow and never shrink.
The area theorem has the status of a near-sacred prediction. It follows from the structure of general relativity and from Hawking’s reasoning about the behaviour of black hole horizons, and it has been tested indirectly for decades; the merger of two black holes into one is the natural laboratory in which the theorem can be checked directly, because the two initial horizons merge into one, and the total area of the final horizon must be at least the sum of the two originals. The new observation, the strongest collision yet seen, measures the areas before and after with enough precision to confirm the theorem with the cleanest evidence so far: the area grew, as the rule demands.
The event also stretches the reach of the detectors themselves. A signal this strong came from a collision so massive that its ripples travelled across a substantial fraction of the observable universe before reaching the instruments, and the detection is a demonstration of how sensitive the gravitational wave network has become, and of how much of the universe’s most violent history it can now hear. The strongest collision is not merely the loudest event; it is the farthest reach of the technology.
The confirmation carries a particular satisfaction because of the two names attached to the theorem’s two halves. Einstein’s relativity predicted that gravitational waves exist and that black holes behave in specific, extraordinary ways; Hawking’s theorem turned part of that behaviour into a crisp, testable statement. A single observation that verifies both halves, that measures the ripples Einstein described and checks the growth Hawking derived, is the kind of event that the field lives for: a theoretical prediction, decades old, confirmed by a collision too distant to have been imagined when it was written, and confirmed in the most direct way the theory allows.
The signal is now in the catalogue, the strongest one yet, awaiting the next. The detectors will keep listening, the mergers will keep arriving, and each one tests the theory a little more severely than the last. The area theorem has been confirmed again, on the strongest evidence so far, and somewhere in a distant galaxy, two black holes merged as relativity said they must, their horizons growing as Hawking’s theorem required, the loudest proof yet of the oldest theory’s reach.