Enlarge /. Image of the night sky over Paranal, Chile, on July 21, 2007 showing the galactic center of the Milky Way. The laser creates a guide star for the telescope.
The 2020 Nobel Prize in Physics was awarded to Roger Penrose "for discovering that the formation of black holes is a robust prediction of general relativity". He shares it with Reinhard Genzel and Andrea Ghez "for the discovery of a supermassive compact object in the center of our galaxy".
Penrose, Rouse Ball's retired professor of mathematics at Oxford University, will receive half of the award of 10 million Swedish kronor (more than $ 1.1 million). He helped solidify the theoretical foundation for black hole physics in the 1960s by providing seminal mathematical evidence that black holes were a direct consequence of general relativity.
Genzel is acting director of the Max Planck Institute for Extraterrestrial Physics in Germany and professor at the University of California at Berkeley, while Ghez is a professor at the University of California in Los Angeles. You will each receive a quarter of the prize money. Genzel and Ghez each lead astronomy groups that have mapped the orbits of stars closest to the center of our Milky Way – a region known as Sagittarius A * – and provide us with the best evidence that it is in the center of our galaxy there is a supermassive black hole. This work has been immensely supported by the development of advanced adaptive optics tools to counteract the distortion effects of the Earth's atmosphere.
"The discoveries made by this year's award winners have broken new ground in the study of compact and supermassive objects," said David Haviland, chairman of the Nobel Committee on Physics, in an official statement. "But these exotic objects still raise many questions that ask for answers and motivate future research. Not only questions about their internal structure, but also questions about how we can test our theory of gravity under extreme conditions in the immediate vicinity of a black hole." "
Black holes have a precursor in the "dark stars" adopted by John Michell in 1783 and Pierre-Simon LaPlace in 1796. In a 1783 article in the Royal Society's Philosophical Transactions, Michell argued that, according to Classical (Newtonian) mechanics, a star with the same density as our Sun but a radius 500 times larger would create such a strong pull that that Light itself would be captured. LaPlace made similar calculations in his own 1799 work.
Our modern concept of a black hole dates back to 1916, when Albert Einstein's general theory of relativity was brand new and revolutionized our understanding of gravity. Einstein envisioned a space-time that is curved, not flat, and therefore gravity is less a force than space-time brought out of shape by the presence of mass or energy. How much mass or energy there is determines the degree of curvature. The stronger the curvature, the stronger the attraction. Since space and time are one, what happens to space also affects time: when space is warped, time is stretched or compressed accordingly. Hence, time slows down in direct proportion to the strength of a gravitational field, and the strength of that field depends on the distance.
Enlarge /. Last year the Event Horizon Telescope captured the first image of the supermassive black hole at the center of the Messier 87 galaxy.
Einstein's equations opened up a whole new range of theoretical possibilities. A physicist named Karl Schwarzschild began toying with various solutions under heavy fire at the front lines during World War I, shortly after Einstein published his seminal paper – his way of distracting himself from the horrors of war. Schwarzschild eventually encountered a roadblock where the equations "exploded", and his work provided an early description of a black hole. (Robert Dicke coined the term in 1960, and John Wheeler later helped popularize it.) An extremely heavy mass can cut off a piece of space to form a black hole surrounded by an event horizon – a hypothetical point of no return, beyond which nothing can escape (not even light). The larger the mass, the larger the black hole and the larger the diameter of its event horizon.
Initially, physicists viewed these exotic objects as purely theoretical, although Robert Oppenheimer and his student Hartland Snyder made some early calculations that showed that massive stars, many times more massive than our sun, could dramatically collapse and form black holes. "The star therefore tends to isolate itself from any communication with a distant observer; only its gravitational field remains," they concluded. However, the general consensus was that this was not a realistic model of something that could actually form in our universe. Then, in the 1960s, physicists discovered quasars, the brightest known objects in the universe. The source of all this radiation, the scientists concluded, had to be matter that fell into a massive black hole. Black holes could be "real" after all.
Roger Penrose decided to tackle the problem of demonstrating the realistic creation of black holes, and later recalled the moment when he reached his key findings in the fall of 1964. While strolling through London with a colleague, he imagined a "trapped surface": a closed, two-dimensional surface that directs all rays of light onto an infinitely dense center – what we now call a singularity, where time and space are in a black hole end up. Penrose went on to show, among other things, by means of his Penrose diagrams of the same name, that nothing can stop the inevitable collapse in the direction of singularity if such an enclosed surface has formed under general relativity.
Journey to the center of the Milky Way
However, creating a solid theoretical basis for the existence of black holes was not the same as observing one directly. Our Milky Way is a flat disk about 100,000 light years across, and our Sun is only one of several hundred billion stars in it. Physicists had long thought that there might be a supermassive black hole at its center, aided by the discovery of radio waves from the central region known as Sagittarius A *. So it seemed like an ideal candidate for further investigation.
But how exactly do you "observe" an object from which no light can escape? This must be done indirectly by measuring the effects of gravity that such an object would exert on objects in the vicinity – for example, on the orbits of neighboring stars. This must be done with earth-based telescopes that perform near infrared observations, as any light in the optical spectrum would be obscured by interstellar gas and dust. The finer the scale in which one can follow these movements, the easier it is to make the necessary calculations.
From the 1990s onwards, Genzel's team relied on the telescopes of the European Southern Observatory in Chile, in particular the Very Large Telescope Array. Meanwhile, Ghez's team relied on the Keck Telescope in Hawaii.
The work was arduous, time consuming, and hampered by the turbulent effects of the earth's atmosphere. Genzel and Ghez and their respective teams developed a technique called "Speckle Imaging" to meet this challenge. This involves taking multiple, highly sensitive, short exposures of a particular star and stacking this data to get a sharper image. However, this only proved effective for the brightest stars orbiting Sagittarius A *, and it also took years to get the necessary information about the speeds of a handful of these stars.
The advent of adaptive optics in the late 1990s turned out to be groundbreaking. This involves using a "guide star" as the first point of observation – either an actual star or an artificially created point source, which can be achieved by using a powerful laser to excite sodium atoms in the upper atmosphere. Once the position and brightness of the guide star has been determined, this information can be used to calculate the effects of atmospheric turbulence. This, in turn, allows astronomers to use quickly deformable mirrors to compensate for the distortion.
The use of adaptive optics allows longer exposures so that more stars can be observed at a much greater depth of field. The two teams were able to monitor the movement of about 30 bright stars near Sagittarius A * over a much shorter period of time. Both were able to image and analyze a star particularly close to the galactic center – S2, which completes an orbit in just under 16 years (compared to the 200 million years it takes our sun to complete its orbit around the center of the Milky Way) . and their dates matched perfectly. Conclusion: The object in the center of the Milky Way is a supermassive black hole.
Enlarge /. NuSTAR captured these first focused views of the supermassive black hole in the heart of the Milky Way in high-energy X-rays.
NASA / pubic domain
We stand ready to make even more exciting black hole discoveries in the future. For example, it is very likely that we will soon have an actual image of the black hole at the center of our galaxy, courtesy of the Event Horizon Telescope, which made headlines last year for being a stunning image of the black hole at the center of the galaxy shows Messier's 87 galaxy, about 55 million light-years from Earth. In the ongoing cooperation between LIGO and VIRGO, gravitational waves are recorded, among other things, which are caused by black hole fusions, among other things.
"Research on the dark universe, once an exotic topic, is becoming more mainstream," said Giovanni Losurdo of INFN, spokesman for the Virgo Collaboration, in a statement responding to the price announcement. "In fact, the discovery of gravitational waves announced in 2016 was also the first direct discovery of a black hole. Since then, Virgo and Ligo have discovered dozens of binary systems of black holes, so we can take a closer look at physics. This year's Nobel Prize encourages us to follow the path we have already chosen To continue research. "
Editor's Note: After last year's pricing, we've started a pricing discussion. The end result was we decided to report faster on the day of the award and see if there are facets of the work that deserve deeper coverage that could be done later. Currently we don't see anything about this year's Physics Award, which would suggest that more detailed coverage would be informative for our readers.