The Laser Interferometer Gravitational-Wave Observatory (LIGO) is celebrating 10 years of cutting-edge gravitational wave science by confirming predictions made by physics luminaries Albert Einstein, Stephen Hawking and Roy Kerr — and potentially revealing a path toward a theory of quantum gravity.
LIGO achieved this latest milestone by detecting gravitational waves, or tiny ripples in spacetime. The existence of gravitational waves was first predicted by Einstein in his 1915 theory of gravity, general relativity. The newly detected ripples resulted from the collision of two black holes, each estimated to have a mass around 32 times that of the sun.
In just four days, on September 14, LIGO will celebrate exactly 10 years since it made the very first detection of gravitational waves. This signal, designated GW150914, had traveled about 1.3 billion years to reach Earth. Its detection represented an entirely new method of astronomy — a way to “hear” spacetime ringing after some of the most powerful events in the cosmos rather than “see” them by relying on electromagnetic radiation. Since then, LIGO and its gravitational wave detecting partners, Virgo and the Kamioka Gravitational Wave Detector (KAGRA), have detected a multitude of gravitational wave signals from other black hole collisions, mergers between neutron stars, and even from two rare “mixed mergers” involving a neutron star and a black hole.
“This is the clearest view yet of the nature of black holes,” Maximiliano Isi, a member of the LIGO-Virgo-KAGRA collaboration from the Flatiron Institute’s Center for Computational Astrophysics, said in a statement. “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.”
Black hole theories validated by new gravitational wave signal
This newly detected signal, GW250114, stands among previous detections as one of the clearest gravitational wave signals ever.
“GW250114 is the loudest gravitational wave event we have detected to date; it was like a whisper becoming a shout,” Geraint Pratten, member of the LIGO-Virgo-KAGRA collaboration and a researcher at the University of Birmingham, said in a separate statement. “This gave us an unprecedented opportunity to put Einstein’s theories through some of the most rigorous tests possible — validating one of Stephen Hawking’s pioneering predictions that when black holes merge, the combined area of their event horizons can only grow, never shrink.”
Hawking’s prediction involves the light-trapping outer boundary of a black hole called the event horizon. This marks the point at which the gravitational influence of the black hole becomes so great that not even light moves fast enough to escape its grip. As gravity is related to mass, the size of an event horizon, also known as the Schwarzschild radius — after Karl Schwarzschild, the first physicist to solve the equations of general relativity and inadvertently predict the existence of black holes — also depends on the mass of a black hole.
The greater the mass, the wider the event horizon.
In 1971, Hawking, along with physicist Jacob Bekenstein, predicted that when black holes merge, the total area of the resultant daughter black hole’s event horizon would be larger than the area of the progenitor black holes’ event horizons combined; the duo said that event horizon would have an area proportional to its level of disorder or “entropy.”
GW250114 revealed that the progenitor black holes had a total surface area of around 93,000 square miles (240,000 square kilometers), which is around the size of the entire U.K. The daughter black hole created by the merger, however, has a surface area of 154,000 square miles (400,000 square kilometers), which is about the size of Sweden.
Another prediction verified by this research comes from New Zealand mathematician Roy Kerr, who developed Kerr geometry from general relativity, which describes empty spacetime around a rotating black hole, or a Kerr black hole.
Following mergers between black holes, these systems enter what scientists call a ringdown phase. This sees the daughter black hole vibrating and emitting gravitational waves at very specific frequencies, akin to the changing “voice” of the black hole. Kerr predicted the “voice” of the black hole could be described by two values alone: the mass of the black hole and its spin.
This really sets black holes apart from other celestial objects, like stars, that must be described using a vast range of characteristics. What is extraordinary about this is the fact that a black hole 1 billion times as massive as the sun can be completely characterized by just two numbers: mass and spin.
“Given the clarity of the signal produced by GW250114, for the first time, we could pick out two ‘tones’ from the black hole voices and confirm that they behave according to Kerr’s prediction, obtaining unprecedented solid evidence for the Kerr nature of black holes found in nature,” Gregorio Carullo, member of the LIGO-Virgo-KAGRA collaboration and a researcher at the University of Birmingham, said in the statement.
What is fitting about this new detection is the fact it is so similar to the signal that LIGO detected to make history on September 14, 2015, GW150914.
“The detection of a black hole binary with parameters similar to those of GW150914, but three times louder, only a decade after the breakthrough discovery, is owed to the tremendous technological improvements of our instruments, paving the path for precision astronomy with gravitational waves,” said LIGO-Virgo-KAGRA collaboration team member Patricia Schmidt, Associate Professor at the University of Birmingham.
Regular and purposeful improvements have been a constant factor in the life of LIGO, which consists of two detectors in Washington and Louisiana that can now measure distortions in spacetime that are 1/10,000 the width of a proton, or 700 trillion times smaller than the width of a human hair.
“Rai Weiss proposed the concept of LIGO in 1972, and I thought ‘this doesn’t have much chance at all of working,'” Kip Thorne, an expert on the theory of black holes who won the 2017 Nobel Prize in Physics with Weiss and Barry Barish for the development of LIGO, said in a separate statement. “It took me three years of thinking about it on and off and discussing ideas with Rai and Vladimir Braginsky [a Russian physicist], to be convinced this had a significant possibility of success. The technical difficulty of reducing the unwanted noise that interferes with the desired signal was enormous. We had to invent a whole new technology.”
Who could have predicted this technology could have been so successful in opening a new window to our study of the universe? Certainly not Einstein, who, when he theorized the existence of gravitational waves, predicted that no instrument of Earth would ever be sensitive enough to detect these spacetime ripples.
“Just 10 short years ago, LIGO opened our eyes for the first time to gravitational waves and changed the way humanity sees the cosmos,” Aamir Ali, a program director in the National Science Foundation (NSF) Division of Physics, which has supported LIGO since its inception, said in the statement. “There’s a whole universe to explore through this completely new lens, and these latest discoveries show LIGO is just getting started.”
Future improvements for LIGO could include the addition of a planned fourth detector, this time located in India, which will improve the precision with which LIGO-Virgo-KAGRA can localize gravitational wave sources.
As for this latest milestone, it is certain that Einstein, Hawking and Weiss, the latter of whom passed away just last month, would all have been thrilled to see LIGO further validate their work.
“If Hawking were alive, he would have reveled in seeing the area of the merged black holes increase,” Thorne added.
The team’s research was published on Wednesday (Sept. 10) in the journal Physical Review Letters.
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