Physicists detect strongest gravitational wave yet, confirm Hawking’s black hole theorem

Physicists detected strongest gravitational waves yet, confirming Hawking’s black hole area theorem, validating Einstein’s relativity.

The universe still holds its secrets, which it yields unexpectedly. Almost 10 years after the first detection of ripples in space itself, called gravitational waves, physicists have announced the strongest such signal yet.

Produced when two black holes in a distant galaxy spiralled into each other, the merger enabled researchers to test Albert Einstein’s theory of gravity, general relativity (GR), in new ways. In particular, as reported in Physical Review Letters, analysis of the event confirmed a theorem derived by Stephen Hawking that a black hole’s surface area can only grow and never shrink.

“That’s such a fundamental, pure theorem of general relativity that to see it verified is really fantastic,” says Clifford Will, a gravitational theorist at the University of Florida. “It’s one of the things that allows you to say we are really looking at black holes.”

The signal was detected on January 14 by the Laser Interferometer Gravitational-Wave Observatory (LIGO), which comprises two L-shaped interferometers in Louisiana and Washington state. Laser light resonating along the interferometers’ 4-km-long arms is used to detect passing gravitational waves, which stretch each arm by less than 1/100th the width of a proton.

Comparing the latest signal with models shows it was generated when distant black holes with masses 33.6 and 32.2 times that of the Sun spiralled into each other and merged.

The black holes had nearly the same masses as those that produced the first LIGO event, which was detected on September 14, 2015. However, since then, improvements to the LIGO detectors have made them three times more sensitive. So, the new signal stood out far more clearly, with a signal-to-noise ratio of 80, which enabled researchers to examine it in unprecedented detail.

After a merger, the final black hole’s event horizon—the boundary marking where gravity becomes so strong that not even light can escape reverberates fleetingly like a sharply struck bell. In this case, that “ring down” had a frequency of 247 cycles per second and lasted for about 10 milliseconds.

The main oscillation can have overtones of slightly different, generally lower frequencies that fade faster and are key to testing GR. According to the theory, a black hole is so featureless that it can be characterised by just two numbers: mass and spin. That implies the frequencies and decay rates of the overtones aren’t independent, but must be related mathematically. By comparing an overtone with the primary ring-down tone, physicists can test the relationships predicted by GR.

The hard part is spotting an overtone, which can fade in less than a single cycle of oscillation. “Black holes don’t ring, they thud,” says Scott Hughes, a theorist at the Massachusetts Institute of Technology. Other black hole mergers, including the first LIGO event, had offered hints of an overtone.

In the new event, the overtone stands out clearly, says Katerina Chatziioannou, a physicist and LIGO member at the California Institute of Technology. “This is definitive.” Sure enough, the frequencies of the tone and overtone conformed to the predictions of GR.

One caveat is that LIGO identifies and characterises signals with the help of simulations that assume GR is correct—a possible source of bias. To avoid it, researchers ignored most of the signal and analysed just the ring-down. They could do that only because the signal was so strong. “That is what enables all of this,” says Maximiliano Isi, a LIGO physicist at Columbia University. However, Frans Pretorius, a gravitation theorist at Princeton University, cautions that some amount of bias may still be baked into the analysis.

LIGO researchers then used the mass and spin of the final black hole to calculate the area of its event horizon, which is approximately 4 lakh square kilometres, roughly the size of Japan. Data from earlier in the event, when the initial black holes were spiralling together, revealed their mass, spin, and area as well.

The area of the final black hole exceeded the total area of the initial ones—even though the final object was less massive than the sum of the initial black holes, as some energy, and hence mass, radiated away in gravitational waves. That result jibes with Hawking’s mathematical theorem, which says no matter how a black hole’s mass and spin evolve, its area can only grow.

With further improvements to LIGO or future observatories, scientists should be able to detect multiple modes and overtones, and obtain a precise enough measurement to identify where an anomaly begins, Pretorius notes.

Of course, that depends on squeezing better performances out of LIGO. In June, the White House called for shuttering one of the LIGO stations as part of a plan to cut the budget of the National Science Foundation by 57%. Such a closure would dramatically reduce LIGO’s sensitivity—and dampen hopes of hearing further black hole harmonies.

(The author is an academic)