Black holes are some of the most mysterious objects in the universe. This is partly because the equations of general relativity that we use to understand them break down when studying the super-dense centers of black holes.
However, a new paper shows how astronomers might one day overcome this challenge by using gravitational waves to “see” merging black holes, and learn what they’re really made of.
in Einstein’s general theory relativityBlack holes are objects that prevent light from escaping due to their extremely strong gravity. The boundary of a black hole is known as the event horizon – if you go beyond that boundary, you’ll never be able to get out. Relativity also predicts that the centers of black holes are points of infinitely high density, known as singularities.
The presence of singularities means that the equations themselves are breaking down; In maths infinity starts appearing which prevents further calculation. So we know that general relativity is incomplete. There must be a more fundamental theory, possibly one involving quantum physics of subatomic scales, that can properly describe what is happening at the center of a black hole.
We don’t have a complete quantum theory of gravity yet, but we do have several candidates. For example, there is string theory (opens in new tab), which predicts that all particles in the universe are actually made of ultra-small vibrating strings. There’s also loop quantum gravity, which says that space-time itself is made up of tiny, indivisible chunks, like pixels on a computer screen.
Both of these approaches may replace the traditional singularity at the center of the black hole with something else. But when you replace the singularity, you usually also remove the event horizon. This is because the event horizon is caused by the infinite gravitational pull of the singularity. Without a singularity, the gravitational pull is only incredibly strong, but not infinite, and so you are always able to escape the vicinity of a black hole as long as you escape with sufficient velocity.
In some forms of string theory, the singularity and event horizon are replaced by an interconnected network of tangled knots of spacetime. In loop quantum gravity, the singularity turns out to be a few extremely small, extremely dense nuggets of exotic matter. In other models, the entire black hole is replaced by a thin shell of matter, or by clusters of new types of speculative particles.
black hole mystery
with him nearest known black hole (opens in new tab) thousands light years Away, these models are hard to test. But sometimes black holes send us important information, especially when they merge together. When they do, they release a flood of gravitational waves, which are ripples in space-time that can be detected with sensitive instruments. EarthLike the Laser Interferometer Gravitational-Wave Observatory (LIGO) and VIRGO experiments.
So far, all observations of black hole mergers agree with the vanilla black hole model predicted by general relativity. But this may change in the future as new generations of gravitational wave observatories come online, a paper published November 30 in the preprint journal arXiv (opens in new tab) gives suggestions.
The key is not the gravitational waves emitted during the merger, but those emitted right after, according to the paper. When the merger is over and the two black holes become a single object, the newly merged mass is vibrating with intense amounts of energy, like a struck bell. This “ringdown” phase has a distinctive gravitational wave signature.
By studying those 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 ringdown phase, which arise from differences in the black hole’s internal structure. Different structures of black holes emit different types of gravitational waves.
Astronomers hope that the next generation of gravitational wave detectors will be sensitive enough to detect these predicted tiny changes in the ringdown signature. If they do, they will fundamentally change our concept of black holes and lead us closer to solving their deepest mysteries.
originally published on livescience (opens in new tab),