In 2017, a gravitational wave was detected in the Laser Interferometer Gravitational-Wave Observatory (LIGO) for the very first time. Recently, astronomers have looked back on the records of the wave in order to search for confirmation on a relatively old astrophysics proposal.
From the 1970s, physicists have claimed that black holes have “hair.” Many intellectuals like Stephen Hawking, Malcolm Perry, and Andrew Strominger agreed. However, after studying the wave from 2017, astrophysicists now realize that the answer is much closer and potentially different than they thought. Together, they formed the “no-hair theorem” which Stephen Hawking famously doubted.
When physicists discuss the existence of “hair” of black holes, they are referring to the simplicity of astrophysical objects. The only difference between each black hole is its mass, electric charge, and angular momentum. In reality, however, black holes are similar in electric charge, meaning they really only differ in angular momentum (rate of spin) and mass. Black holes are said to be “bald,” and physicists call them “Kerr black holes.”
As Maximiliano Isi asserted to Live Science, this aspect of black holes differentiates them from basically every known object in the universe. When a real bell rings, it emits sound waves and faint gravitational waves that can’t be detected with modern technology. Using the sound waves, you can determine the material and the shape of the bell with some background knowledge in acoustics. If you could somehow detect the bell’s gravitational waves, you could also theoretically determine the composition and shape.
This concept can be applied to black holes. Gravitational waves cannot determine the composition and shape of black holes because they are all uniform singularities and have the same shape, but the wave provides information about the collision of black holes that caused it.
“The pattern of this stretching and squeezing,” Isi explains, “encodes information on the source, the thing that made this gravitational wave.”
The recording was not very detailed, however. LIGO was able to detect a change in distance caused by a warp in space-time, but the wave was not intense enough for the detectors to record the details. As a result, astrophysicists can’t be certain about many things. Isi said, “It’s like we’re listening from very far away.”
Luckily, researchers were able to spot a wave in the “ringdown” process which occurred two weeks after the black holes merged into a larger one. The discovery of overtones of gravitational waves confirmed data on the ringing black hole.
The overtone proved that the black hole is close to a Kerr black hole, and consequently the “no-hair theorem” can predict the overtones. Astrophysicists require a clearer overtone or a more sensitive instrument to gather data beyond this conclusion.
Even though it’s difficult to be certain in the astronomical world, black holes continue to help us by speaking to us. Astrophysicists need more information, but they are slowly answering the questions that could not be answered for centuries.
As Isi states, “Physics is about getting closer and closer.”
by Sanghyun Kim