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MIT scientists captured the "ring" of a newly formed black hole, in the form of gravitational waves, depicted in this artist's illustration. Credit: Maximiliano Isi / NASA
Scientists detect tones in the ringing of a newborn black hole for the first time. The results support Einstein's theory and the idea that black holes do not have "hair".
If Albert Einstein's theory of general relativity turns out to be valid, then a black hole, born from the trembling collisions of two gigantic black holes, should itself "ring" as a result, producing gravitational waves a bit like a knocked bell reverberates sound waves. Einstein predicted that the particular height and decay of these gravitational waves should be a direct signature of the mass and rotation of the newly formed black hole.
Now the physicists of MIT and elsewhere have studied the ringing of an infant black hole and found that the structure of this ringtone actually predicts the mass and rotation of the black hole – further evidence that Einstein was always right.
The results, published on September 12, 2019, in Letters of physical examination, also favor the idea that black holes have no "hair" – a metaphor referring to the idea that black holes, according to Einstein's theory, should have only three observable properties: mass, spin and electric charge. All the other features, which physicist John Wheeler called "hair," should be swallowed by the black hole itself and would therefore be unobservable.
The team's findings today support the idea that black holes are, in reality, hairless. The researchers were able to identify the ring pattern of a black hole and, using Einstein's equations, calculate the mass and spin that the black hole should have, based on its ring pattern. These calculations corresponded to the measurements of the mass and the spin of the black hole carried out previously by others.
If the team's calculations deviated considerably from the measurements, he would then have suggested that the black-hole bell coded for properties other than mass, spin, and electric charge – an interesting proof of physics beyond what Einstein's theory can explain. But it turns out that the ring pattern of the black hole is a direct signature of its mass and rotation, thus confirming the idea that black holes are bald giants, devoid of superficial hair-like properties.
"We all expect that the general relativity is correct, but this is the first time we confirm it," says lead author of the study, Maximiliano Isi, NASA Einstein member of the Institute Kavli of MIT for astrophysics and space research. "This is the first experimental measurement that allows to directly test the theorem of the absence of hair. This does not mean that black holes can not have hair. This means that the image of black holes without hair is still alive one day. "
A tweet, decoded
On September 14, 2015, scientists performed the first-ever gravitational wave detection – infinitesimal undulations in space-time, emanating from distant and violent cosmic phenomena. The detection, named GW150914, was performed by LIGO, the gravitational wave laser interferometer observatory. Once the scientists eliminated the noise and zoomed in on the signal, they observed a waveform that quickly accelerated before fading. When they translated the signal into sound, they heard something that sounded like a "chirp".
Scientists determined that gravitational waves were triggered by the rapid inspiration of two huge black holes. The peak of the signal – the strongest part of the chirp – is linked to the very moment when the black holes met, melting into a single new black hole. While this infantile black hole emitted its own gravitational waves, physicists assumed that its signature sound would be too weak to be deciphered amid the clamor of the initial collision. Thus, the traces of this ring were only identified some time after the peak, where the signal was too weak to be studied in detail.
However, Isi and his colleagues found a way to extract the reverberation from the black hole from the moments immediately following the peak of the signal. In earlier work led by Isi co-author Matthew Giesler of Caltech, the team showed by simulations that such a signal, and in particular the portion just after the vertex, contains "harmonics" – a family of loud and short sounds. When they reanalyzed the signal, taking into account the harmonics, the researchers discovered that they could successfully isolate a specific type of ringing at a newly formed black hole.
In the team's new document, the researchers applied this technique to the actual GW150914 detection data, focusing on the last milliseconds of the signal, immediately after the peak of modulation. Taking into account the harmonics of the signal, they could discern a ring coming from the new black hole for baby. Specifically, they identified two distinct tones, each with a tone and a drop rate that they could measure.
"We detect a global gravitational wave signal composed of several frequencies that fade at different speeds, such as the different heights that make up a sound," explains Isi. "Each frequency or tone corresponds to a vibratory frequency of the new black hole."
Listen beyond Einstein
Einstein's theory of general relativity predicts that the pitch and decay of gravitational waves of a black hole should be a direct product of its mass and spin. That is to say that a black hole of a given mass and spin can only produce tones of a certain height and a certain decline. To test Einstein's theory, the team used the equations of general relativity to calculate the mass and spin of the newly formed black hole, as a function of the height and decay of the two detected tones.
They found that their calculations corresponded to the measurements of the black hole mass and the rotation previously performed by others. According to Isi, the results demonstrate that researchers can actually use the strongest and most detectable parts of a gravitational signal to discern the ringing of a new black hole, whereas before, the Scientists assumed that this ring could only be detected inside the much darker end. the signal of the gravitational wave and the identification of many tones would require much more sensitive instruments than what currently exists.
"It's exciting for the community because it shows that this type of study is possible now, not in 20 years," Isi said.
As LIGO improves its resolution and more sensitive instruments are connected in the future, researchers will be able to use the group's methods to "hear" the sound of other newly born black holes. And if they learn sounds that do not quite match Einstein's predictions, it could be an even more exciting prospect.
"In the future, we will have better detectors on Earth and in space, and we will be able to see not only two, but dozens of modes, and precisely define their properties," said Isi. "If they are not black holes, as Einstein predicted, if they are more exotic objects such as wormholes or boson stars, they may not sound the same way and we will have a chance to see them."
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This research was funded in part by NASA, the Sherman Fairchild Foundation, the Simons Foundation and the National Science Foundation.
Reference: "Test of the hairless theorem with GW150914" by Maximiliano Isi, Matthew Giesler, Will M. Farr, Mark A. Scheel and Saul A. Teukolsky, September 12, 2019, Letters of physical examination.
DOI: 10.1103 / PhysRevLett.123.111102
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