In Space, No One Can Hear You Scream
Nearly four years after the monumental observation of gravitational waves, scientists have detected tones in the “ringing” of a black hole for the first time, corroborating yet another one of Einstein’s theoretical predictions. According to Einstein’s theory of general relativity, a black hole formed from the collision of two other black holes produces “ringing” gravitational waves. However, what is special about the ringing of these gravitational waves is that they encode specific information about the nature of the black hole, namely their mass, spin, and electric charge.
On September 14, 2015, scientists first detected gravitational waves using LIGO, the Laser Interferometer Gravitational-wave Observatory. Gravitational waves are waves that transport energy in the form of gravitational radiation and propagate through a gravitational field, creating a ripple-like effect in space in the process. In order to study gravitational waves, scientists have translated the signals into sound waves, which crescendo over time. When black holes merge, the gravitational waves translate into a “chirping” sound. The chirp is loudest at the moment when the black holes fuse. The infant black hole that is created from the collision of the two parent black holes also produces gravitational waves that “ring;” however, physicists previously assumed that the noise created by the infant would be too faint to interpret amid the loud noise from the initial collision of the parents. Therefore, traces of this ringing have to be studied sometime after the peak, when the signal becomes too faint to be of much significance.
The team that published the findings, led by Maximiliano Isi of MIT’s Kavli Institute for Astrophysics and Space Research, found a way to extract a black hole’s reverberation from instants after the signal’s peak. In previous work led by Isi’s co-author, Matthew Giesler of Caltech, the same team showed that the signal, particularly the portion immediately after the peak, contained “overtones,” which are loud, short-lived tones.
“We detect an overall gravitational wave signal that’s made up of different frequencies, which fade away at different rates, like the different pitches that make up a sound,” Isi said for the MIT news. “Each frequency or tone corresponds to a vibrational frequency of the new black hole.”When the team took the overtones into account, they were able to identify a ringing pattern specific to a newly-formed black hole.
“This was a very surprising result. The conventional wisdom was that by the time the remnant black hole had settled down so that any tones could be detected, the overtones would have decayed away almost completely,” noted Saul Teukolsky, the Robinson Professor of Theoretical Astrophysics at Caltech and adviser to Giesler.
Using these ringing patterns, physicists were able to use the equations Einstein outlined in his theory of general relativity to calculate the mass and spin of the black hole. These measurements concurred with previous measurements. If they had not matched, then the ringing patterns would have encoded more information than the mass, spin, and electrical charge of the black hole. However, the extra information would have violated Einstein’s No-Hair Theorem, stating that the only characteristics needed to define a black hole are mass, spin, and electrical charge, without any “excess hairs.” The alignment of these measurements with Einstein’s theory is significant as it paves the way for more advanced work in ringing patterns in the future with better equipment.
As LIGO improves its resolution techniques and more sensitive instruments are developed, scientists will be able to use the ideas put forth by Giesler and the Isi team to “hear” the ringing of other newborn black holes.
“In the future, we’ll have better detectors on Earth and in space, and we will be able to see not just two, but tens of modes, and pin down their properties precisely,” Isi remarked. “If these are not black holes as Einstein predicts, if they are more exotic objects like wormholes or boson stars, they may not ring in the same way, and we’ll have a chance of seeing them.”
Clearly, the future of ringing patterns is abuzz with potential.
References
Blair, D., Dr. (2018, March 12). Why you can hear gravitational waves when things collide in the universe. Retrieved October 23, 2019, from The Conversation website: https://theconversation.com/explainer-why-you-can-hear-gravitational-waves-when-things-collide-in-the-universe-92356
Chu, J. (2019, September 12). Scientists detect tones in the ringing of a newborn black hole for the first time. Retrieved October 13, 2019, from MIT News website: http://news.mit.edu/2019/ringing-new-black-hole-first-0912
Clavin, W. (n.d.). First ‘overtones’ heard in the ringing of a black hole. Retrieved October 13, 2019, from Phys.org website: https://phys.org/news/2019-09-overtones-heard-black-hole.html
Inspiral Gravitational Waves. (n.d.). Retrieved October 23, 2019, from LIGO Scientific Collaboration website: https://www.ligo.org/science/GW-Inspiral.php
Mathur, S. (n.d.). The no-hair theorem [Lecture transcript]. Retrieved October 13, 2019, from Ohio State University Department of Physics website: https://www.asc.ohio-state.edu/mathur.16/gian/bhdrivershortnh.pdf