Since 1783, geographer John Michel first proposed the concept of black hole , and people have opened a long way to explore this mysterious celestial body.
On April 10, 2019, the Horizon Telescope (EHT) cooperative organization released the first ever black hole image-the M87 supermassive black hole 55 million light-years away, human beings have been See the true face of the black hole.
One of the most well-known characteristics of black holes is that nothing can escape from a black hole, including light. Therefore, even a very small black hole will not give people any clues about its origin or evolution.
Source of the first M87 black hole photo: Royal Institution
It is inferred that the Milky Way galaxy is not only full of hundreds of millions of stars _span1111 strong_span11 has as many as 110 million strong_span strong_span Multiple black holes . However, since these black holes cannot be directly observed, it is difficult for people to distinguish them from the appearance.
is different from the difference between twins,Although twins have the same genes, we can distinguish them from appearance, hairstyle, and even temperament.
But according to Hawking’s "black hole no hair theory" , a black hole has only three identifiable characteristics- strong 19 span_span strong 19 span_span strong momentum and electric charge . If there are two black holes with the same value in these three properties, it is theoretically impossible to distinguish the two black holes.
Time comes to 2021. A recent study shows that is very likely to have "hairs" in the black hole with the approximate extreme value, that is, the black hole boundary may carry detectable information, _strong22 It is likely to break the famous "black hole no hair theory".
The history of the development of black hole theory
Black Hole (Black Hole) is a modern general relativity , a kind of density that exists in the space of the universe. span11span celestial body.Black holes are created by the gravitational collapse of stars with sufficient mass at the end of the nuclear fusion reaction after their energy is exhausted and dies.
The gravitational pull of a black hole is very strong, and even the speed of light as fast as 300,000 kilometers per second cannot exceed the escape velocity in its horizon (the boundary of the black hole).
Therefore, Any matter, including light, will be absorbed by the black hole and cannot escape when it approaches the black hole. This also explains why black holes cannot be directly observed. Way to prove its existence.
Black holes are not "black", but we cannot directly observe the source of the picture: pixabay
black hole research progress should start with Einstein in 1915, he proposed the general relativity The famous gravitational field equation was proposed in the general theory of relativity.
A few months later, German mathematician Schwarzschild calculated the first exact solution of the gravitational field equation, that is, the critical radius at which the celestial body we know now becomes a black hole- Schwarzschild Radius .When the stellar body collapses to its Schwarzschild radius , no force can stop the stellar body from collapsing further, and then the black hole is formed irreversibly.
After confirming the existence of black holes, Hawking proposed the "black hole hairless theory" in 1973: No matter what kind of black hole, its final properties consist of only a few physical quantities, namely mass, angular momentum, Electric charge is the only decision.
When the black hole is formed, only these three conserved quantities that cannot be turned into electromagnetic radiation are left. There are any other complex properties.
Extremal black holes may overthrow the "hairless theory"?
If two black holes have the same mass, angular momentum, and charge value, it means that they are exactly the same without the slightest difference. Paul Chesler, a theoretical physicist at Harvard University, said, “In the classical general theory of relativity, they are exactly the same. There is no way to distinguish them.” Scientists are extra cautious when demonstrating each theory based on the "hairless theory", so they always doubt whether all black holes strictly abide by this theory.
2012,Stefanos Aretakis, a mathematician who was working at the University of Cambridge and now working at the University of Toronto, proposed that some black holes may have instability in their horizon.
These instabilities will make certain areas on the horizon of the black hole have stronger gravity than other areas , the difference in this respect will make the black hole exactly the same. With distinguishability .
Extremal black hole "hair" simulation image
However, the equation solution given by him can only prove that his conclusion is true in extreme black hole 11span _strong strong , and extreme value black holes cannot exist in nature, because extreme value black holes will have a maximum value among the three parameters of mass, angular momentum, or charge.
Although strictly black holes do not exist, what if we have a black hole that approaches the limit infinitely, but does not actually reach the limit? Such a black hole can exist at least theoretically. Is it possible that it violates the "hairless theory"?Is there an unstable area that can be detected in the horizon?
Can the black hole "hair" explain the black hole information paradox
The paper published at the end of January 2021 confirms the rationality of the above conjecture. In addition, the paper also stated that the black hole "hair" "It can be detected by the Gravitational Wave Observatory.
University of Massachusetts and University of Rhode Island physicist Gallaf Conner was one of the participants in this study. He said: "Aretakis thinks that there are some in the horizon of black holes. The information that can be detected, and our research results provide the possibility to detect this information (also called "black hole hair")."
It is worth mentioning that the scientists believe that, Whether it is the residue produced when the black hole is formed, or the disturbance that appears in the later growth of the black hole, such as the matter falling into the black hole, may produce unstable gravitational attraction near or near the horizon of the extreme black hole. Connor said that they look forward to seeing gravitational signals on black holes that are close to their extreme values, which are very different from ordinary black holes.
Princeton Institute for Advanced Study astrophysicist Liya Medros said that if black holes do have "hair" and retain some historical information about their past, this may be It conflicts with the black hole information paradox proposed by the famous physicist Stephen Hawking.
A black hole not only "eats", but also spit out something, that is, thermal radiation. Source: Ashley Mackenzie for Quanta Magazine
What is the black hole information paradox? First of all, the basic requirement of quantum mechanics is information conservation , information will not appear out of thin air, nor will it disappear out of thin air. Secondly, Hawking proposed the concept of black hole radiation in his black hole research. Black holes generate external radiation when they swallow matter. However, thermal radiation itself hardly carries information .
This means that when a star falls into a black hole and is swallowed by it, the star evaporates in the form of Hawking radiation in the black hole, but the black hole and radiation do not carry information, so the original star’s Where did all the information go? The law of conservation of information is broken, and this is where the paradox lies.
This paradox condenses the fundamental contradiction between the two pillars of physics in the 20th century-quantum mechanics and general relativity. The existence of the black hole information paradox shows that we are likely to have a problem in a certain step when dealing with black holes, and even use the wrong theoretical basis.
Therefore,Medros said: "If one of the assumptions of the black hole information paradox can be proved wrong, you may be able to solve the paradox itself, and one of the assumptions of the information paradox is the hairless theory."
Source: Sakkmesterke/Science Source
When people detect the "hair" of a black hole, the consequences may be quite extensive.
"If we can prove that the actual time and space outside the black hole is different from what we expected, then I think this will have a huge impact on general relativity." Medros said. In October, Medros co-authored a paper discussing whether the geometry of the black hole he detected was consistent with the prediction.
The most exciting part of this newly published paper is that it provides a way to integrate black hole observations and basic physics.
Perhaps in the most extreme astrophysics laboratory in the universe, by detecting the "hair" on black holes, we can explore many theories in an unprecedented way, such as string theory and quantum gravity etc.
But Medros said,One of the main problems with string theory and quantum gravity is that it is difficult to verify the correctness of these predictions. Therefore, if there is any method that can verify these predictions, even remotely, it will be a major improvement.
Gravitational Wave Observatory detects black hole "hair"
But detecting black hole "hair" is not easy. First of all, we cannot determine whether the approximate extreme black hole really exists. Professor Chessler of Harvard University admitted frankly that there is still a 30% gap between the black holes simulated by the most advanced methods and the extreme black holes.
Secondly, even in the presence of approximately extreme black holes, we still don't know whether the sensitivity of existing gravitational wave detectors can detect gravitational instability in black hole hair.
Finally, and more importantly, this kind of "hair" is very difficult to capture, because its has a very short life span, which can only exist for less than one second.
However, theoretically, the results of this research are quite reasonable. Chessler said: "I don't think anyone in the scientific community will question it. This is not an unreasonable speculation, but Einstein's equations of relativity are so complicated that we can discover new features from them every year."
next stage,Researchers will focus on what kind of gravitational signal should look for on the gravitational wave detector.
These gravitational wave detectors include gravitational wave observatories such as LIGO and Virgo that are currently in use, or may be a collaboration between NASA and European Space Agency (ESA) Gravitational wave observatory in space-laser interference space antenna, LISA .
Human's first gravitational wave observatory in space LISA Source: New Scientist
Astrophysicist Helvey Witteke of the University of Illinois at Urbana-Champaign said: _span11 We should now continue to calculate the gravitational radiation frequency of this black hole'hair' on the basis of previous studies, and really figure out how we can measure and identify this gravitational radiation. And then, we This very excellent and important theoretical study will be extended to the exploration of the specific characteristics of black hole'hair'."
Scientists have every reason and motivation to continue to explore in this field. Although it is very slim to verify whether the theory of the existence of black hole "hair" is correct,But once information is found on the black hole horizon, will not only challenge Einstein's general theory of relativity, but also prove the existence of approximate extreme black holes.
Connor said, "We really want to know whether there is a naturally formed black hole that approximates the extreme value, which will have a tremendous impact on our research field."
Reference:
1.https://www.quantamaggazine-of-hair-span-20210
2.https://journals.aps.org/prd/abstract/10.1103/PhysRevD.103.L021502
3.https://www.newscientist.com listening-for-gravitational-waves-from-the-birth-of-the-universe/
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compilation | Daisy Zuo
layout | Xiao Gu
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