Birmingham, Sep 11 (The Conversation) Black holes are among the most enigmatic phenomena in the universe, but our understanding of them has grown significantly only recently. It's been just ten years since their existence was confirmed through the detection of gravitational waves—ripples in spacetime.
Since then, these waves, particularly those generated by colliding black holes, have provided a window into their hidden dynamics and the underlying theories. A landmark event occurred on January 14, 2025, when the strongest gravitational-wave signal to date, labeled GW250114, was captured by the Laser Interferometer Gravitational Wave Observatories (LIGO).
This extraordinary detection offered an unprecedented opportunity for our international team of scientists to test crucial aspects of Albert Einstein's general relativity: the nature of black holes and the theory known as Hawking's area law.
Our findings, which have been published in Physical Review Letters, represent a pivotal advance in our comprehension of gravity and black holes.
According to general relativity, black holes are formed when massive stars exhaust their nuclear fuel and collapse, provided the resulting mass surpasses the Chandrasekhar limit, which is about 1.44 times that of the sun.
The aftermath is a spacetime region sealed off from the rest of the cosmos by the event horizon, a boundary beyond which nothing, not even light, can escape. But if black holes cannot emit signals beyond this threshold, how can we confirm their presence and behavior?
The Promise of Gravitational Waves General relativity itself forecasted the existence of gravitational waves. Any massive object moving through spacetime creates tiny distortions that travel at light speed. These waves hold a trove of information about their source and gravity's nature.
Detectable gravitational waves are produced by systems where massive objects are subject to significant and sustained acceleration. The interaction between orbiting black holes—binary black holes—provides one of the most potent sources of gravitational waves. As these waves carry energy away, the orbit decays, culminating eventually in a merger that forms a single, larger black hole.
Analyzing gravitational waves from such black hole binaries allows us to verify if astrophysical black holes conform to general relativity's predictions.
Gravitational waves were first directly observed on September 14, 2015, by LIGO, when the merger of two black holes—GW150914—was detected.
With rapid advancements in detector technology, contemporary observations of binary black hole mergers now offer ultra-high definition views, supporting the most precise tests of general relativity and black hole physics yet.
Exploring Hawking’s Theorem Despite their complexity, black holes are deceptively straightforward entities, defined solely by mass, rotation, and possibly electromagnetic charge.
In a landmark 1972 paper, Stephen Hawking articulated the area law: when two black holes merge, the final event horizon's surface area exceeds the total of the initial black holes' surface areas.
This can be understood by considering how event horizon surface area is tied to a black hole's mass and spin. Doubling a black hole's mass results in an event horizon four times larger. Increasing rotation makes the horizon more oblate, reducing surface area. Hawking proved that, despite energy and angular momentum losses to gravitational waves, black hole mergers yield a larger final event horizon.
By modeling the “ringdown” phase after the merger, when the resultant black hole emits gravitational waves in a distinct pattern called quasi-normal modes—akin to a bell's tone—we confirmed Hawking’s predictions through GW250114. The emitted waves' analysis offered insights into the black hole’s mass and spin, and hence its event horizon's surface area.
This robust signal allowed us to execute a series of tests on Einstein’s theory. Each confirmed general relativity's predictions.
The GW250114 observation stands as a definitive validation of Einstein's theory, including Hawking’s area law. Yet, this is just the commencement of what promises to be a new era in gravitational and black hole physics. The Conversation) SKS SKS
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