How gravitational waves can ‘see inside’ black holes
Black holes are among the most mysterious objects in the universe. This is partly because the equations of general relativity that we use to understand it break down on study black holes“Superdense centers.” However, a new paper shows how astronomers may one day overcome this challenge by using gravitational waves to “see” inside compact black holes, and learn what they are actually made of.
In Einstein’s general theory of RelativityBlack holes are objects that prevent light from escaping due to their enormous gravity. The boundary of a black hole is known as the event horizon – if you cross that threshold, you will never be able to get out of it. Relativity also predicts that the centers of black holes are points of infinitely high density, known as singularities.
The existence of singularities means that the equations themselves break apart; Infinities begin to appear in mathematics, which prevents further arithmetic operations. So we know that general relativity is incomplete. There must be a more fundamental theory, perhaps related to quantum physics of the subatomic scales, that can correctly describe what happens at the center of a black hole.
We don’t yet have a complete quantum theory of gravity, but we do have several candidates. For example, there string theory, which predicts that all particles in the universe are in fact composed of tiny, oscillating strings. There’s also loop quantum gravity, which says space-time itself is made up of tiny, indivisible bits like pixels on a computer screen.
Both methods could replace the traditional singularity at the center of the black hole with something else. But when you replace the singularity, you usually also cancel out the event horizon. That is because the event horizon is caused by the infinite gravitational force of the individual. Without a singularity, gravity is incredibly strong, but not infinite, and so you can always escape from the vicinity of a black hole as long as you escape fast enough.
In some variations of string theory, singularity and event horizons are replaced by tangled webs of tangled nodes of space-time. In loop quantum gravity, the singularity becomes an extremely small and extremely dense solid mass of exotic matter. In other models, the entire black hole is replaced by a thin shell of matter, or by clumps of new types of speculative particles.
gravity microscopes
The mystery of the black hole
with the The closest known black holes thousands of light years Far from it, these models are hard to test. But sometimes black holes send us important information, especially when they merge together. When they do, they unleash floods of gravitational waves, which are ripples in space-time that can be detected with sensitive instruments on Earth. a landsuch as the Laser Gravitational-Wave Observatory (LIGO) and the VIRGO experiments.
So far, all observations of black hole mergers agree with the vanilla black hole model predicted by general relativity. But that may change in the future as new generations of gravitational-wave observatories come online, published November 30 in Preprint. arXiv She suggests.
The key is not the gravitational waves emitted during the merger itself, but those emitted immediately afterwards, according to the paper. When the merger ends and the two black holes become one body, the new merged mass vibrates with an intense amount of energy, like a bell being struck. This “loop” phase has a distinct gravitational wave signature.
By studying these signatures, researchers may one day be able to tell which black hole theories hold up and which don’t. Each black hole model predicts differences in the gravitational waves emitted during the ring phase, which stem from differences in the black hole’s internal structure. With different black hole compositions, different types of gravitational waves appear.
Astronomers hope that the next generation of gravitational-wave detectors will be sensitive enough to detect these expected small changes in the ring’s signature. If they do, they will radically change our understanding of black holes and move us forward in solving their deepest mysteries.
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