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Black Hole Theory, Observation and Nobel Prize 2020

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Black hole theory, observation and Nobel Prize 2020


The 2020 Nobel Prize in Physics was awarded for theoretical research and observations on black holes. What came out of that study?

Roger Penrose, Andrea Ghez and Reinhard Gengel won the Nobel Prize in Physics in 2020. Their Nobel Prize-winning research has resulted in an excellent combination of theory and observation, intellectual activity, and perseverance in the impossible - a brilliant combination of these two unique human characteristics. In this article I made an effort to discuss their research work in simple language.

Theoretical possibilities of black holes

We've all heard that a 'black hole' is an object whose gravitational pull is so great that even light can't get out of it. If a lot of mass is condensed in a very small space - such as if the whole earth were to shrink and become the size of a meridian and its mass remain the same - then the force of gravity could be so great.
This possibility lies in Newtonian gravitational theory, although it was not widely discussed in the scientific community before Einstein published his famous general theory of relativity in 1915. In a word, general relativity is the combination of some equations related to ‘spacetime’ [1]. Together they are called Einstein equations. A black hole is a so-called 'singularity' in space-time. Simply put, its density is infinite.

Shortly after Einstein published his theory, the scientist Carl Schwarzschild showed in 1917 that the black hole was a solution to Einstein's equations. That is, according to Einstein's equations, unique points can be formed in space-time under special conditions. Schwarzschild's solution further showed that the central mass in which this unique point would be formed would have a spherical surface at a certain distance around which matter and light could enter but not exit. This surface is called the 'event horizon'.

After Einstein's theory of general relativity was published, the theoretical possibility of a black hole became apparent.

But even then there was the idea among other scientists, including Einstein, that although there was such a solution in the pen, there was no possibility of black holes forming in nature. Because the conditions that Schwarzschild used to solve Einstein's equations, such as 'spherical symmetry' or spherical symmetry, would never create such a simplified situation in nature. For some time after that there was no significant progress in this regard. The development of quantum theory in the 1920s brought a new revolution in the world of physics. In the 1930's, research into the division of the nucleus and nuclear energy began. Then World War II. Most physicists in the world were preoccupied with these.

In the early 1960's, astronomers discovered a new type of cosmic object called the quasar. Observations and analyzes show that they are not much larger in size than our solar system, but that their luminosity is the same as that of a galaxy, about a trillion times that of the Sun. How can such a huge amount of energy be radiated from a small space? Even the method of ‘nuclear fusion’ or nuclear fusion, which is the source of energy for stars, is not so effective.

Then some people said that if there was a black hole in the center of these quasars then the objects around it would radiate a lot of energy before falling into it due to the attraction of that black hole and in that case so much radiation is possible from a very small space. Also the Big Bang theory of the universe was becoming acceptable among a section of astronomers at that time. But according to this theory the universe is gradually expanding. So at some point in the past, our universe started from an infinitely dense state, that is, from a single point. For all these reasons, scientists began to think anew about the possibility of the presence of a single point in the universe.

One of them was Roger Penrose, a mathematician and physicist from Oxford University in England. In 1975, he demonstrated for the first time that black holes could form in space-time, in addition to the conditions assumed by Schwarzschild. Not only that, but he also showed with perfect arithmetic what the religion of space-time would be on the horizon of the black hole or just outside and inside it. To make this kind of explanation more effective, he used a special kind of diagram, later known as the Penrose Diagram. This whole theoretical result of Penrose is called the singularity theorem in one word. After his research was published, many in the scientific community agreed that it would be surprising that black holes did not exist in the universe. Roger Penrose won half of the 2020 Nobel Prize for this work.

Two more scientists are certainly noteworthy in this context. One is Stephen Hawking. Within a few months of Penrose publishing that research paper in 1975, Hawking took his theory a step further and showed that our expanding universe has the potential to have unique points in the past as well. Over the next few years Hawking, along with a few other collaborating scientists, wrote numerous research papers proving that our universe must have had a unique point in the past. Finally, in 1960, Hawking and Penrose, in another paper, elaborated on what should be the condition for having a single point in space-time, which is still considered the most authoritative version of the singularity theorem. If Stephen Hawking were alive, he would undoubtedly be a contender for this Nobel Prize.

In another study, Hawking and Penrose elaborated on what should be the condition for having singularity in space-time.

The second is Amal Kumar Roychowdhury, the proverbial professor in the Department of Physics at the then Presidency College, Calcutta. The early 1950s. AKR passed his MSc a few days ago. While at Ashutosh College and later at the Indian Association of Cultivation of Science, he began research on whether there could be a single point in space-time. He did the mathematical calculation of what should be the ‘curved spacetime’ of space-time. Through an equation from that calculation, he showed that if space-time adhered to Einstein's general theory of relativity, those trajectories would move toward each other and meet at a point. From this it is possible to indirectly show that there can be unique points in space-time.

That equation is known as the 'Raychowdhury Equation' and is now an integral part of any textbook on general relativity. The most important result of his work was that it was possible to mathematically show the existence of a single point of space-time without the condition of spherical symmetry. This is said to be an early step in the singularity theorem. The Roychowdhury equation is one of the foundations of all the research that Roger Penrose and his later scientists have done on the Singularity Theorem and its various aspects. This work of AKR is one of the foremost researches in theoretical physics in post-independence India. Penrose, Hawking, and their associates knew him by name, and later Roger Penrose met him in Calcutta and exchanged greetings. It is rare in the history of modern science that Amal Kumar Roychowdhury has achieved such an important result of world-class research without any previous experience of research and without any advisor.

Observation of black holes

Although the possibility that a black hole could actually exist in the universe through Penrose's research is evident, it is beyond the definition of science to accept anything without experimenting with experimentation and observation. So then began the various attempts to directly observe the existence of black holes. But observing black holes is not easy. Because no light or electromagnetic waves are emitted from there.

The only way is to indirectly prove its existence by observing its gravitational effects on other objects near the black hole. Meanwhile, the newly discovered quasars have black holes at their center, and their huge amount of radiation, as well as their spectrum and other religions, can be easily explained. So the theory that there really is a black hole at their center became somewhat accepted in the scientific community.

Then, in the 1980's, scientists analyzed and showed that black galaxies could be at the center of all galaxies, not just quasars. In the case of Koesar, many objects around him are attracted to him and fall into the black hole, radiating huge amounts of energy before reading. But in the case of the rest of the galaxies, there is not enough material around the black hole. So there is not as much radiation from the quasars as there is. So the black hole is in the background. So is there a black hole in the center of our Milky Way galaxy?

The way to prove the existence of a black hole is to indirectly prove its gravitational effect on other nearby objects.

The center of the Milky Way galaxy is 27,000 light-years away. This distance is negligible in terms of astronomy. Cosmic objects millions of miles away from him are easily captured by telescopes. If you observe the center region of the Milky Way galaxy once, you will find indirect evidence of the existence of that black hole.

Two more scientists are certainly noteworthy in this context. One is Stephen Hawking. Within a few months of Penrose publishing that research paper in 1975, Hawking took his theory a step further and showed that our expanding universe has the potential to have unique points in the past as well. Over the next few years Hawking, along with a few other collaborating scientists, wrote numerous research papers proving that our universe must have had a unique point in the past. Finally, in 1960, Hawking and Penrose, in another paper, elaborated on what should be the condition for having a single point in space-time, which is still considered the most authoritative version of the singularity theorem. If Stephen Hawking were alive, he would undoubtedly be a contender for this Nobel Prize.

In another study, Hawking and Penrose elaborated on what should be the condition for having singularity in space-time.

The second is Amal Kumar Roychowdhury, the proverbial professor in the Department of Physics at the then Presidency College, Calcutta. The early 1950s. AKR passed his MSc a few days ago. While at Ashutosh College and later at the Indian Association of Cultivation of Science, he began research on whether there could be a single point in space-time. He did the mathematical calculation of what should be the ‘curved spacetime’ of space-time. Through an equation from that calculation, he showed that if space-time adhered to Einstein's general theory of relativity, those trajectories would move toward each other and meet at a point. From this it is possible to indirectly show that there can be unique points in space-time.

That equation is known as the 'Raychowdhury Equation' and is now an integral part of any textbook on general relativity. The most important result of his work was that it was possible to mathematically show the existence of a single point of space-time without the condition of spherical symmetry. This is said to be an early step in the singularity theorem. The Roychowdhury equation is one of the foundations of all the research that Roger Penrose and his later scientists have done on the Singularity Theorem and its various aspects. This work of AKR is one of the foremost researches in theoretical physics in post-independence India. Penrose, Hawking, and their associates knew him by name, and later Roger Penrose met him in Calcutta and exchanged greetings. It is rare in the history of modern science that Amal Kumar Roychowdhury has achieved such an important result of world-class research without any previous experience of research and without any advisor.

Observation of black holes

Although the possibility that a black hole could actually exist in the universe through Penrose's research is evident, it is beyond the definition of science to accept anything without experimenting with experimentation and observation. So then began the various attempts to directly observe the existence of black holes. But observing black holes is not easy. Because no light or electromagnetic waves are emitted from there.

The only way is to indirectly prove its existence by observing its gravitational effects on other objects near the black hole. Meanwhile, the newly discovered quasars have black holes at their center, and their huge amount of radiation, as well as their spectrum and other religions, can be easily explained. So the theory that there really is a black hole at their center became somewhat accepted in the scientific community.

Then, in the 1980's, scientists analyzed and showed that black galaxies could be at the center of all galaxies, not just quasars. In the case of Koesar, many objects around him are attracted to him and fall into the black hole, radiating huge amounts of energy before reading. But in the case of the rest of the galaxies, there is not enough material around the black hole. So there is not as much radiation from the quasars as there is. So the black hole is in the background. So is there a black hole in the center of our Milky Way galaxy?

The way to prove the existence of a black hole is to indirectly prove its gravitational effect on other nearby objects.

The center of the Milky Way galaxy is 27,000 light-years away. This distance is negligible in terms of astronomy. Cosmic objects millions of miles away from him are easily captured by telescopes. If you observe the center region of the Milky Way galaxy once, you will find indirect evidence of the existence of that black hole.

Astronomers embarked on that endeavor in the 1990's. But there are many obstacles along the way. The density of stars is very high in the central region of the Milky Way, and at some point in the life cycle of stars, large amounts of dust come from the stars through the medium around them, i.e. the interstellar medium. This cosmic dust obstructs the movement of light. As a result, only one-tenth of the visible light emanating from a star in the Milky Way galaxy can reach our telescope.

But all of them also radiate at ‘infrared wavelengths’, and cosmic dust cannot block the rays of light of that wavelength very much. So astronomers first realized that this observation could not be made with visible light. Light with infrared wavelengths and very powerful telescopes should be used to detect even the faintest light from a star in the central galaxy.

The most powerful instrument for detecting visible and infrared wavelength radiation on Earth at the time was the Cake 1 and Cake 2, a pair of telescopes located on the Mauna Kea Mountains in the Hawaiian Islands. They are so powerful that if all the people in a city like Kolkata look up at the sky together, only the amount of light that enters their eyes per second can detect the same amount of light per cake 1 and cake 2 per second. Not only that, this telescope will be able to detect two objects that are only a few centimeters apart on the moon from the earth. Many scientists, technologists and staff have been working in the observatory throughout the year to keep the two giant machines running, and the daily cost is estimated at one million US dollars.

Andrea Ghez, an astronomer at the University of California, Los Angeles, or UCLA, and her colleagues began observing the central region of the Milky Way in 1995 using the Cake Telescope. But even there the earth's atmosphere was obstructed. We know that without the atmosphere, there would be no life on earth. Various movements are going on to protect the atmosphere properly, many laws have been passed abroad. But the atmosphere is a special barrier to astronomical observations. As light passes through the atmosphere, some changes occur, and the resulting reflection in the telescope is distorted. This happens mainly because the air layers in our atmosphere are not stable. They change due to airflow.

Astronomical observations interfere with the atmosphere because the air layers in the atmosphere are not stable, resulting in some changes in the nature of light rays.

It may seem that we are in this atmosphere. When we see distant things, there is no distortion? The reason is that we do not look as closely as a telescope. But if we dive into the water of a pond or swimming pool and look from the bottom of the water, people or plants outside look distorted to our eyes, especially if the water is not stagnant. This difficulty has to be encountered in any telescope on the surface where it is observed in the light of visible or near wavelengths. This difficulty was especially evident in the case of Andrea Ghez because they wanted to understand their orbits by observing multiple stars that were very thin and very close to each other. The way then?
Picture 1 - Adaptive Optics Used in Chile's VLT Telescope (Source - European Southern Observatory)

To overcome this obstacle, they used two technical techniques in astronomical observations, the use of which began only a short time ago. Since the main cause of distortion of reflections is the change in the air layer over time, they created the unaltered reflection by computer-assembling them in a special way with many short-term reflections instead of individual long-term reflections. This method is called 'spectal imaging'. In the 2000s, they used a more sophisticated method that is expected to change the outline of astronomical observations in the future. Its name is 'Adaptive Optics'. In this method, a laser beam is projected from the observatory into the sky (Fig. 1). That laser beam is reflected in a special layer of the atmosphere and acts as a star-like but artificial light source. The reflection of the artificial star is determined by the computer by analyzing how much it is being distorted by the atmosphere and the reflection of the real star is determined accordingly. Using these two methods, Andrea Ghez and her colleagues regularly observed the central region of the Milky Way for fifteen years and were able to determine the movements of many of the stars there.

What did you know after so long and tactical observation? At the center of our galaxy is an object or mass of mass 40 million times greater than the Sun. But no light is coming from there. Not only that, the size of this object is not larger than our solar system. Keep in mind that if we put the size of our solar system side by side, about ten thousand suns can be caught there. But according to the laws of physics, it would not be possible for them to remain stationary in such a small space for an object of mass equal to forty million suns. They will meet at a point under the unmistakable effect of their combined gravity. That is, a unique point or black hole will be created. How exactly did scientist Ghez and his colleagues calculate the mass and volume of the object from their observations? See picture 2 for him. The image shows the trajectory of the stars in the central region of the Milky Way. And the black circle in the middle of the picture is the center of the galaxy. Two of the most notable stars are S0-2 and S0-18. The first has completed its orbit. In other words, once around the center, the whole AS0-2 star has turned. From there, the mass of the central object can be easily determined using Kepler's law of Newtonian law of gravitation. The S0-16 star followed its orbit and passed very close to the center. From this it is understood that the volume of the heavy object in the center is not large. If it had happened, ESK-16 would have pushed there.
Picture 2 - The Orbits of the Stars in the Central Region of the Milky Way (Source - UCLA Galactic Center Research Group, Modification - Author)

At the end of a long and tactical observation, it was discovered that at the center of our galaxy is an object or mass of mass forty million times greater than the Sun from which no light is coming.

But scientists cannot readily accept the interpretation of any cosmic event, relying solely on the results of this technically sophisticated and complex method of observation and analysis. They want those results to be verified in a unique way.

Fortunately, that opportunity was also available. Reinhard Gengel, co-director of the Max Planck Institute for Extraterrestrial Physics in Garking, Germany, and his colleagues have been making such observations since the 1980s. They first calculate the total mass of a large region in the direction of the Milky Way galaxy by determining the motion of gaseous substances and ions in the interstellar space. They then used the European Southern Observatory's 'New Technology Telescope' and speckled imaging methods to determine the motion of stars in the Milky Way's central region in the 1990s to get the total mass of a specific small region very close to the center. Because the telescope was relatively less powerful, the stars were not very close to the center, but the mass they were determining from the motion of stars some distance from the center was the same. Subsequently, by observing with a more powerful telescope called the VLT, they also determined the total mass of an area equal to the size of our solar system at the center of the Milky Way and obtained consistent results.

After two different teams of scientists using two different telescopes and their own analysis methods to get the same results, there is no doubt that at the center of our galaxy is an object 40 million times heavier than the Sun, not larger than our solar system. Even with the most powerful telescope in the world, no radiation can be detected from there. That is to say, the nature of this object coincides exactly with what is known from theoretical physics about the properties of black holes. Since 2010, most scientists have acknowledged that there is a black hole in the center of the Milky Way. In this context, it is noteworthy that the mass of the Sun, which is tens of millions of times or more in the central hundreds of light years of the Milky Way galaxy, was somewhat known from various observations of the 1960s and 90s. But the mass that is in such a small volume, with a radius of a few light-hours or less, was not known before the subtle and precise observations of Ghez and Genzel. And that's why Andrea Ghez and Reinhard Gengel won a quarter of the 2020 Nobel Prize, respectively.

Last word

The singularity of Roger Penrose The myriad branches of the theoretical subject matter and their application are scattered throughout modern physics and mathematical theory. Observations by Andrea Ghez and Reinhard Gengel are still ongoing. A new kind of cosmic object has been discovered in the central region of the galaxy. Some small gaseous bodies that are shattered by the effect of the central mass during their orbit. Just like if there is a black hole in the center. These have been named ‘Tidal Disruption Events’. Many astronomers around the world have been observing these objects through various telescopes and practicing the theory of how the flow of liquids or gases to a black hole can begin, which was not possible before. This theory is further confirmed by the fact that there is a black hole at the center of all constellations. It is believed that their central black hole has a special role in the formation and evolution of constellations. This idea has revolutionized the current theory of the recent evolution of the universe. These far-reaching effects of the research of Roger Penrose, Andrea Ghez and Reinhard Gengel are their greatest achievements, no less than the Nobel Prize.

One more thing cannot be finished. In the 120-year history of the Nobel Prize in Physics, 218 people have received this award. Andrea Ghez is only the fourth woman among them. Isn't it sometimes feared that half of the members of human society - that is, women - may not be able to fully participate in scientific activities due to historical and social factors? We have to ask ourselves this question. And we all know that to be inspired you need to be a role model. After reading the news of Andrea Ghez's Nobel Prize, if a few schoolgirls from around the world look out of the window and imagine the invisible orbits of the stars and the next morning if they are encouraged to pay more attention to their studies or if their parents insist on not dropping them out of school. It will be a bigger prize than the Nobel.

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