In February 2016, scientists at the Laser Interferometer Gravitational wave Observatory (LIGO) announced the first detection of gravitational waves (GW). These ripples in space time, originally predicted by Einstein’s Theory of General Relativity, are caused by the merger of massive objects (like neutron stars and black holes). Since then, gravitational wave observatories like LIGO, VIRGO, and the Kamioka Gravitational Wave Detector (KAGRA) have detected around 300 gravitational wave (GW) events.
Improved technology and international collaborations have also led to increased accuracy and higher rates of detection. Recently, scientists at LIGO and the Flatiron Institute’s Center for Computational Astrophysics announced the detection of a black hole merger that provides the greatest insight yet into the fundamental nature of black holes and space time. The discovery also offers clues as to how quantum physics and General Relativity fit together, essentially confirming predictions made by Einstein, Stephen Hawking, and Roy Kerr in one event.
The findings were reported on September 10th in the Physical Review Letters in a paper by the LIGO-Virgo-KAGRA Collaboration, titled “GW250114: Testing Hawking’s Area Law and the Kerr Nature of Black Holes.” According to their analysis, the merger created a black with the mass of 63 Suns spinning at a rate of 100 revolutions per second.
During mergers, the process begins when the co-orbiting black holes lose angular momentum and finally come together producing gravitational waves. As the newly-formed black hole settles into a new state, it produces reverberations that are also visible as a faint “ringing.” For previous events, scientists have had difficulty capturing the final reverberations since the ringing of the black hole would be very faint. As a result, scientists couldn’t separate the ringing of the collision from that of the final black hole itself. With these new signals, the team was provided with a complete picture of a binary black hole merger, from the moment they collided to the final reverberations.
Maximiliano Isi and Will Farr, two research scientists at the Flatiron Institute’s CCA, led the analysis of the event. As Isi explained in a CCA press release, this latest event was a milestone in GW astronomy:
This is the clearest view yet of the nature of black holes. We’ve found some of the strongest evidence yet that astrophysical black holes are the black holes predicted from Albert Einstein’s theory of general relativity. The new pair of black holes are almost twins to the historic first detection in 2015. But the instruments are much better, so we’re able to analyze the signal in ways that just weren’t possible 10 years ago.
Their work builds on previous research led by Isi. In 2021, he and his colleagues demonstrated a cutting-edge method for isolating certain frequencies using data from the first black hole merger. Back in 2015, GW measurements weren’t clear enough to confirm key predictions about black holes, but more precise measurements have since become available. This allowed Isi, Farr, and their colleagues to successfully isolate the ten millisecond signal emitted by the post-merger black hole, which enabled more accurate tests of several theories by prominent astrophysicists.
This includes the Kerr metric, proposed by physicist Roy Kerr in 1963, which uses Einstein’s General Relativity to mathematically show that black holes are simple objects that can be described by only two characteristics: spin and mass. The new measurements showed that the merged black hole is definitely a simple object defined by these two characteristics, and also confirmed Stephen Hawking’s area theorem. This theory states that the size of a black hole’s event horizon (it’s outer boundary from which nothing can return) can only grow over time.
Credit: Lucy Reading-Ikkanda/Simons Foundation
By confirming Hawking’s theorem, the results also indicate a possible connection to the second law of thermodynamics. This law, which is fundamental to our understanding of physical systems, establishes that disorder in any system (aka. entropy) must increase, or at least remain constant, over time. In this respect, the results of this study could lead to a greater understanding of the thermodynamics of black holes. This, in turn, could lead to breakthroughs in other areas of physics, including attempts to unify General Relativity with quantum physics (quantum gravity theory). Said Isi:
It’s really profound that the size of a black hole’s event horizon behaves like entropy. It has very deep theoretical implications and means that some aspects of black holes can be used to mathematically probe the true nature of space and time. For so long this field has been pure mathematical and theoretical speculation. But now we’re in a position of actually seeing these amazing processes in action, which highlights how much progress there’s been — and will continue to be — in this field.
Within the next decade, new detectors will become operational that are expected to have ten times the sensitivity of current observatories. This includes NASA’s Laser Interferometer Space Antenna (LISA) that is expected to be deployed by 2035. In the same way that space telescopes offer a more pristine view of the cosmos by avoiding atmospheric interference, LISA will enable more sensitive detections of GWs. Along with proposed ground-based observatories, these improved detectors will mean astronomers can listen more closely to the tones emitted by black hole mergers and test the characteristics of black holes more rigorously.
Even more exciting is in how this could trigger another revolution in astronomy and astrophysics, potentially leading to a Theory of Everything and a new understanding of the properties of spacetime itself. For Einstein, Kerr, and Hawking, such discoveries would be a fitting tribute.
Further Reading: Simons Foundation, Physical Review Letters
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