The Laser Interferometer Gravitational-Wave Observatory (LIGO) is commemorating a decade of groundbreaking discoveries in gravitational wave science. This remarkable achievement not only confirms the predictions of renowned physicists like Albert Einstein, Stephen Hawking, and Roy Kerr, but it also paves the way toward a potential theory of quantum gravity. LIGO accomplished this milestone by successfully detecting gravitational waves, which are tiny ripples in the fabric of spacetime.
The concept of gravitational waves was first introduced by Einstein in his 1915 theory of gravity, known as general relativity. The latest detected ripples were produced by the collision of two black holes, each with a mass approximately 32 times that of the sun. On September 14, LIGO will celebrate exactly ten years since it made its inaugural detection of gravitational waves, designated GW150914. This groundbreaking signal traveled around 1.3 billion years to reach Earth, marking the dawn of a new era in astronomy — one where we can 'hear' the echoes of the universe rather than solely rely on visual observations through electromagnetic radiation.
Since the first detection, LIGO, along with its partners Virgo and the Kamioka Gravitational Wave Detector (KAGRA), has identified multiple gravitational wave signals from various cosmic events, including additional black hole collisions, neutron star mergers, and even rare mixed mergers involving both a neutron star and a black hole. Maximiliano Isi, a member of the LIGO-Virgo-KAGRA collaboration from the Flatiron Institute's Center for Computational Astrophysics, expressed that this is the clearest insight yet into the nature of black holes, providing compelling evidence that astrophysical black holes align with Einstein’s predictions.
The recently detected signal, GW250114, stands out as one of the clearest gravitational wave signals recorded to date. Geraint Pratten, another member of the LIGO-Virgo-KAGRA collaboration and a researcher at the University of Birmingham, described this event as the loudest gravitational wave detection so far, likening it to a whisper evolving into a shout. This significant finding allowed scientists to rigorously test Einstein's theories, bolstering one of Hawking's key predictions regarding black hole mergers: when black holes merge, the total area of their event horizons can only increase.
The event horizon is the light-trapping boundary surrounding a black hole, where the gravitational pull is so immense that even light cannot escape. The size of the event horizon, also known as the Schwarzschild radius, is directly related to a black hole's mass; the greater the mass, the larger the event horizon. In 1971, Hawking and physicist Jacob Bekenstein proposed that during black hole mergers, the surface area of the resulting black hole's event horizon would surpass the combined areas of the original black holes' event horizons. Their theory suggested that this area would correlate with the level of disorder, or entropy, of the system.
In the case of GW250114, it was revealed that the progenitor black holes had a total surface area of approximately 93,000 square miles (240,000 square kilometers), roughly equivalent to the size of the entire United Kingdom. In contrast, the daughter black hole formed from the merger exhibited a surface area of 154,000 square miles (400,000 square kilometers), akin to the size of Sweden.
Another significant verification stemming from this research involves New Zealand mathematician Roy Kerr, who formulated Kerr geometry to describe the spacetime around a rotating black hole. Following black hole mergers, these systems enter a phase known as ringdown, where the daughter black hole vibrates and emits gravitational waves at specific frequencies, analogous to the changing tones of a voice. Kerr predicted that the characteristics of a black hole could be described solely by its mass and spin, setting black holes apart from other celestial objects.
This clarity of the signal produced by GW250114 allowed researchers to identify two distinct 'tones' from the black holes' vibrations, confirming that they behaved in accordance with Kerr's predictions and providing unprecedented evidence for the Kerr nature of black holes found in nature. Gregorio Carullo, a member of the LIGO-Virgo-KAGRA collaboration and a researcher at the University of Birmingham, noted the significance of this discovery.
The recent detection is notably similar to the original signal detected by LIGO back in 2015, GW150914. The identification of a black hole binary with parameters akin to those of GW150914, but three times more intense, is a testament to the remarkable technological advancements of LIGO's instruments. Patricia Schmidt, an Associate Professor at the University of Birmingham and LIGO-Virgo-KAGRA collaboration team member, emphasized how regular improvements have been pivotal in LIGO's journey.
LIGO consists of two detectors located in Washington and Louisiana, capable of measuring distortions in spacetime as small as 1/10,000 the width of a proton, or 700 trillion times smaller than a human hair’s width. Kip Thorne, a Nobel Prize-winning physicist and one of LIGO's proponents, reflected on the initial skepticism surrounding the project, stating the technical challenges involved were monumental and required innovative technology.
As we celebrate this pivotal milestone, Aamir Ali, a program director in the National Science Foundation's Division of Physics, highlighted that LIGO's discoveries represent just the beginning of a new chapter in our understanding of the universe. Future enhancements may include the addition of a fourth detector in India, which aims to refine the precision with which LIGO-Virgo-KAGRA can identify gravitational wave sources. The legacies of Einstein, Hawking, and Weiss—who passed away just last month—would undoubtedly resonate with the continued validation of their work, particularly Hawking's predictions regarding black hole mergers.
