Sol Tukosky solved a seemingly purely hypothetical problem in the early 1970s while he was still studying for a doctorate in theoretical physics. We can imagine a black hole as an infinitely small point formed by the burning and collapse of massive stars, with unparalleled gravity. Suppose you disturb the black hole like ringing a big clock, how will the black hole react?
Tukosky, then a PhD student at Caltech, analyzed the problem with pencil, paper and Einstein’s theory of gravity, general relativity. He found that black holes are like a big clock, oscillating at one main frequency and multiple universal frequencies. When the black hole releases gravitational waves, these oscillations will disappear quickly.
Also Read This:
Today, Tukosky is the head of the physics department at Cornell University. He said that this is a very interesting question, but until five years ago, the study of this problem was completely abstract.
In February 2016, researchers reported for the first time on gravitational wave signals observed by the Laser Interferometry Gravitational Wave Observatory (LIGO). According to calculations, the gravitational wave originated from two black holes that surrounded each other and approached gradually. They were about 1.3 billion light-years away from the earth and had a mass of about 29 times and 36 times that of the Sun, respectively. LIGO can even detect the “ring down” (also known as “draging waveform”) stage after the merger of the two black hole system into a larger black hole, which is equivalent to the disturbance caused by the merger process.
It is estimated that the black hole they combined formed is about 62 times the mass of the Sun, and about 3 times the mass of the Sun’s energy is released in the form of gravitational waves in less than a second. As a result, Tukosky’s early papers suddenly became cutting-edge physics.
Although it sounds incredible, scientists can now study black holes as real objects. Since LIGO’s breakthrough discovery, gravitational wave detectors have discovered more than 40 black hole merger events. In April 2019, an international cooperation project called the Event Horizon Telescope (EHT) took the first image of a black hole.
EHT combines multiple radio telescopes around the world to form a virtual telescope with the diameter equivalent to the diameter of the earth. The researchers aimed it at the Virgo A galaxy (M87) near the Milky Way and photographed the hot gas ring around its central supermassive black hole “shadow”. At the same time, astronomers are also tracking stars that are fast approaching the black hole at the center of the Milky Way, whose trajectory may provide clues to the nature of the black hole itself.
In the view of astrophysicists, these observations have challenged assumptions about how black holes form and how they affect the surrounding environment. The researchers analyzed the Virgo Interferometer (Italy) and showed that some of the smaller black holes detected by LIGO before may be heavier and more diverse than expected. This makes it more difficult for astrophysicists to understand the massive stars that may form these black holes.
In our Milky Way, the environment around supermassive black holes seems surprisingly “rich” and full of young stars, which, according to previous conjecture, will not form in such “vortexes”. There are also some scientists who ask a more attractive fundamental question: do we really see the black hole predicted by Einstein’s theory?
Some theoretical physicists say that the answer is likely to be bland “yes”. Robert Wald, a gravity theorist at the University of Chicago, said: “From these results, I don’t think we can learn more about general relativity or black hole theory.” But others don’t think so.
“Is the real black hole exactly the same as the black hole predicted in general relativity, or are they two completely different things?” Clifford Weir, a gravity theorist at the University of Florida, said, “This will be the focus of future observations.” Any abnormal phenomenon requires a rethinking of Einstein’s theory. Physicists suspect that Einstein’s theory is not the final conclusion of gravity, because it is incompatible with another cornerstone of modern physics, quantum mechanics.
Andrea Gates, an astrophysicist at the University of California, Los Angeles, said that researchers have obtained different and complementary views on black holes through many means. Gaze won the 2020 Nobel Prize in Physics for speculating the existence of supermassive black holes in the center of the Milky Way. “We still have a long way to go to piece together a full picture, but we’ll definitely find more pieces of the puzzle,” she said.
Black holes full of contradictions
Black holes are composed of pure gravitational energy, which is full of contradictions. It contains no matter, but like a bowling ball, it has mass and can rotate; it has no surface, but has size; it behaves like a grand and weighty object, but it is actually just a special area in space.
This is exactly what Einstein said in his general relativity published in 1915. As early as two centuries ago, Isaac Newton proposed that gravity is a force that somehow passes through space and attracts massive objects to each other. Einstein’s view is deeper. He believes that gravity is caused by massive objects such as stars and planets distort space and time – that is, spacetime – which causes freely falling objects to bend their trajectory, such as throwing balls falling below parabolic lines.
The prediction of early general relativity is only slightly different from Newton’s gravitational theory. Newton predicted that planets should orbit their stars in stable elliptical orbits; general relativity predicted that each orbit would rotate slightly in a certain direction at the time of operation, which is called orbital precess.
In the first victory of general relativity, Einstein proved that the theory can explain the precession of Mercury’s orbit. Previously, the values predicted by classical mechanics did not match the precession of Mercury’ perihelion, and general relativity eliminated the difference between observation and theory. Just a few years later, physicists realized that Einstein’s theory also implied something more subversive.
In 1939, theoretical physicist Robert Oppenheimer and his colleagues calculated that when a star of enough mass burned out, no known force could prevent its core from collapsing into an infinitesimal point, and its gravitational field was like a permanent bottomless pit in space and space. Within a certain range from this point, gravity is so powerful that even light cannot escape. In 1958, David Finkelstein, a theoretical physicist at Caltech, proposed that any object that crosses this distance would be isolated from the rest of the universe, which is called the “event horizon”.
The event horizon is not a physical surface, and astronauts who fall into it (if possible) will not see any special phenomena. Finkelstein died on January 24, 2016, just days before LIGO announced that it had detected gravitational waves; he left a corollary that the event horizon would be like a one-way membrane that could allow objects to fall in, but nothing inside could escape.
According to general relativity, these objects—which were eventually named “black holes” by the famous theoretical physicist John Archibald Wheeler — should exhibit striking similarities. In 1963, New Zealand mathematician Roy Kerr calculated how rotating black holes of a certain mass can distort space-time.
Other researchers quickly proved that in general relativity, mass and spin are the two basic characteristics that black holes can have, which means that Kerr’s mathematical formula, the Kerr metric, can describe every black hole that exists in the universe. Wheeler called this result the “no-hair theorem” to emphasize that two black holes with the same mass and spin are as difficult to distinguish as baldness. Tukosky pointed out that Wheeler himself is bald, “This is probably the pride of bald people.”
Sean Carroll, a theoretical physicist at Caltech, said that some physicists doubt that black holes may not exist, but just what theorists imagine. These skeptics believe that black holes may be only artificial products of the exquisite mathematical system of general relativity, or they can only be formed under unrealistic conditions, such as the collapse of a perfect spherical star.
However, in the late 1960s, Roger Penrose, a theoretical physicist at Oxford University, dispelled these doubts with rigorous mathematics, and he shared the 2020 Nobel Prize in Physics. “Penrose has proved accurately that even a massive object collapses into a black hole as long as the density becomes high enough,” Carroll said.
How to detect black holes? Einstein’s general relativity predicts that when explored with enough mass, it will leave a self-sustaining gravitational field, which is strong enough that no object can escape, even light. But is black hole really as incredible as general relativity predicts? Observing physicists now have the tools to find the answer:
1. Tracking stars. Tracking the orbits of stars around the black hole at the center of the Milky Way can reveal whether the black hole distorts space and time as predicted by general relativity;
2. Take pictures. Images of supermassive black holes will provide us with clues to determine whether it has an event horizon rather than a physical surface, as predicted by general relativity, and to verify whether a black hole has only two basic characteristics: mass and spin.
3. Capture gravitational waves. When two smaller black holes surround and merge, gravitational waves are emitted. Observations of gravitational wave signals can reveal whether these black holes are really material entities. If the merged black hole oscillates in the form of main frequency and universal frequency, it can verify whether its basic attributes are only mass and spin.
Soon, astronomers began to detect signs of the existence of black holes. They found tiny X-ray sources orbiting stars, such as Cygnus X-1. Astrophysicists speculate that these X-rays come from the gas flowing out of the star, and when it falls on a mysterious object, the temperature will continue to rise.
The details of gas temperature and orbit indicate that this X-ray source is too massive and has an extremely small energy range, so it is impossible to be anything but a black hole. Similar reasoning suggests that distant quasars—active galactic nuclei that can radiate huge energy—are also powered by supermassive black holes at their center.
No one can be sure that these black holes are actually as described by theoretical physicists, notes Fiyal Ozer, an astrophysicist at the University of Arizona in the United States. What we have made so far is rarely able to determine the existence of the event horizon,” she said. “This is an unresolved issue.”
Now, through a variety of ways to observe black holes, scientists can start testing their understanding of black holes and look for new discoveries that could revolutionize physics.” While this possibility is very small, it would be of great significance if we could find any results that deviate from the general relativity predictions,” Carroll said. “It’s a high-risk, high-return problem.”
Observations on black holes
Scientists hope to answer three very specific questions: Does the black hole we observed really have an event horizon? Are they really not as other features as the glabrous theorem says? And, will they distort space-time as predicted by the Kerr metric?
Perhaps the simplest tool to answer these questions is the tool developed by Andrea Gates. Since 1995, she and her colleagues have been using the 10-meter Keck telescope in Hawaii to track the stars around Sagittarius A* (Sgr A*). Sagittarius A* is an extremely bright and dense radio source located in the center of the Milky Way. In 1998, they observed the stars moving at high speed, indicating that they were orbiting an object with a mass of about 4 million times the Sun. Because Sagittarius A* has so much mass in such a small volume, it must be a supermassive black hole according to the prediction of general relativity. Astrophysicist Reinhard Genzel of Max Planck Institute of Extraterrestrial Physics independently tracked the stars, reaching the same conclusion and won the Nobel Prize with Andrea Gates.
Most of this information comes from a star called S02 by Gates (genzel denoted “S2)”, which revolves around Sagittarius A* every 16 years and has a very flat ellipse orbit. Just as Mercury precurs on the Sun, the orbit of S02 should have this phenomenon. Gaze and his colleagues try to find this precession effect from extremely complex data.” We are very close,” Gaz said. “We found a signal, but we are still convincing ourselves that it is real.”
In April 2020, the Gentzel team made a major discovery: Years of observation showed that the orbit of S02 stars did not remain static, but slowly underwent regular rotations – that is, the Schwarzschild precession – showing It’s like the running trajectory of rose knots.
If lucky, researchers such as Gates and Gentzel are expected to find other anomalies to finally determine the nature of supermassive black holes. The spin of a black hole should change the precession of the orbits of stars near them, and the specific way of change can be predicted by Roy Kerr’s mathematical description. Clifford Weir said: “If a star is closer than the star that has been observed (near the black hole) – nearly 10 times, for example, it can be tested whether the Kerrmetric is completely correct.”
Star tracking technology may never detect places very close to the sight of the Sagittarius A* event. It is estimated that the diameter of Sagittarius A* is about 44 million kilometers, which is only slightly shorter than Mercury’s closest distance to the Sun (46 million kilometers). In contrast, the Event Horizon Telescope (EHT) combines data from 11 radio telescopes or arrays around the world to form a huge virtual telescope that can observe another supermassive black hole at close range. This giant in Virgo A galaxy has a mass of 6.5 billion solar masses.
Two years ago, the EHT team released a famous image of a black hole, which looks like a burning circus ring, but in fact, the content contained in the image is much more complicated. The bright aura comes from a high-temperature gas, and the black part surrounded by it is not the black hole itself; instead, it is the “shadow” projected by the light emitted by the gas in front of it by the gravitational distortion (gravitational lens effect) of the black hole. However, the edge of the shadow is not the boundary of the event horizon, but exceeds about 50% of the distance; within this distance, space-time is twisted enough to make the passing light rotate around the black hole, neither escaping nor falling into the black hole.
Even so, the image retains clues related to the center of this supermassive black hole. For example, the spectrum of the halo can reveal whether the object has a physical surface or an event horizon. Fayar Ozer explains that matter that hits the physical surface emits brighter light than the material that slides into a black hole (so far, researchers have not found spectral distortion). The shape of the shadow can also test the classic image of a black hole, that is, the event horizon of a spin black hole should protrude at the equator. However, other effects in general relativity may counteract this effect of shadows.” Because the squeeze in different directions is very strangely offset, the shadow still looks round,” Ozer said. “This is why the shape of the shadow can directly verify the hairless theorem.”
Some researchers question whether EHT can obtain black hole images with enough accuracy to carry out these verifications. Samuel Gralla, a theoretical physicist at the University of Arizona, suspected that EHT might not have “seen” the shadow of the black hole, but looked down at the disk-shaped gas rotating around the black hole from top to bottom. If so, the black dot in the center of the picture is just the eye of an astrophysical hurricane. But Ozer said that even with limited resolution, EHT still made a significant contribution to verifying that general relativity is in unknown conceptual fields around black holes.
In contrast, the information transmitted by gravitational waves comes directly from the black hole itself. When black holes spiral together at half the speed of light, these space-time ripples will pass through ordinary matter unhindered. At present, in the black hole merger events already detected by LIGO and Virgo, the mass of black holes ranges from three to 86 times that of the Sun.
Frank Ohm is a gravitational theorist and a member of the LIGO team at the Max Planck Institute for Gravitational Physics. “Through these combined events, we can detect black holes in many ways,” he said. Assuming that these are classical black holes, researchers can calculate how the gravitational wave buff signal generated by the merger accelerates, reaches its peak, and how it weakens based on general relativity. If these massive objects are actually larger physical entities, they will distort each other when approaching, thus changing the peak of the signal. So far, researchers have not found any such changes.
This merger also produces a disturbed black hole, as described in Tukosky’s early theory, which provides another way to verify general relativity. The merged black hole will have short and strong oscillations, presenting as a main frequency and multiple shorter general frequencies. According to the hairless theorem, these frequencies and their duration depend only on the mass and spin of the final black hole. Frank Ohm said, “When you analyze each pattern separately, they must point to the same black hole mass and spin, otherwise there will be problems.”
In September 2019, Tukosky and his colleagues identified the main frequency and the general frequency in a particularly strong merger event. Frank Ohm said that if the experimenters could improve the sensitivity of the detector, they might find two or three universal frequencies, which is enough to verify the hairless theorem.
Future black hole research
Future detection instruments may make such verification easier. Andrea Gaez said that the 30-meter optical telescope being built in Chile and Hawaii will have a resolution 80 times higher than the existing instrument, can observe the area near Sagittarius A* more carefully, and may detect stars closer to the black hole. Similarly, researchers at EHT have added more radio telescope arrays to their networks, which will enable them to take more accurate images of the black hole at the center of the Virgo A galaxy. On the other hand, they are also trying to observe Sagittarius A* and image it.
Meanwhile, gravitational wave researchers have set out the development of the next generation of more sensitive detectors, including the Laser Interferometer Space Antenna (LISA), which will consist of three satellites millions of kilometers apart to form an equilateral triangle in space. Nicholas Eunice, a theoretical physicist at the University of Illinois at Urbana-Champaign, said that the LISA probe has extremely high detection sensitivity and will be launched in the 1930s. It may find an ordinary stellar mass black hole spiraling close to a supernatural in a distant galaxy. The process of massive black holes and eventually merger with them.
A smaller black hole can act as some kind of accurate detector to reveal whether the space-time distortion around a larger black hole is exactly as predicted by the Kerr metric. Eunice pointed out that a positive result will consolidate the prediction of general relativity about black holes, “but you must wait for the progress of LISA”.
Meanwhile, the fact that black holes suddenly become observable has changed the lives of gravity physicists. General relativity and black holes, once only existed in thought experiments, or could only perform elegant and abstract calculations like Tukosky, have suddenly become the hottest fields in basic physics. To test general relativity, scientists designed experiments that would require up to billions of dollars.” I really feel this transformation,” Frank Ohm said. “It’s a very small circle, but with the detection of gravitational waves, everything has changed.”V #duzline #Blackhole