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Black holes

Black holes are the most enigmatic objects in the universe and are being used as a valuable resource to help physicists resolve fundamental issues in physics. Scientists have made significant strides in this area over the past few years, including capturing the first image of a black hole and detecting gravitational waves from black hole mergers. These advancements have provided scientists with more tools to test theoretical predictions.

The information from the gravitational wave

Recently, physicists used gravitational wave observational data to confirm Hawking’s black-hole area theorem. This theorem states that the surface area of a black hole never decreases and is derived from a combination of black hole physics and the second law of thermodynamics.

The second law of thermodynamics

The second law of thermodynamics explains that the entropy of a system never decreases, which means the randomness of a system always increases. Entropy is typically proportional to the temperature of a system, with higher temperatures resulting in more energetic elements and increased randomness.

The “entropy” of black hole

To determine the entropy of a black hole, scientists first need to define its temperature. This temperature is determined by observing the thermal Hawking radiation emitted by the black hole, similar to how the temperature of the sun is determined. Interestingly, the black hole’s entropy is directly proportional to its surface area, and combining this with the second law of thermodynamics leads to the conclusion that the surface area of a black hole never decreases.

The study published in Physical Review Letter on July 1, 2021, analyzed the data from the first observed gravitational wave event, GW150914, from the Laser Interferometer Gravitational-Wave Observatory (LIGO). The researchers used a high-accuracy numerical simulation of a GW150914-like system to extract the area information, and their results showed that the black hole’s surface area increased after the merger with 97% credibility, confirming Hawking’s theorem.

This confirmation provides physicists with confidence in their current theoretical foundation and opens up new avenues for exploring black holes through observational tools such as gravitational waves. With the progress of these tools, we can expect a more comprehensive understanding of black holes in both theory and observation in the near future.