Hello, my name is Federico Bianco. I am an Assistant Professor at the Department of Physics and Astronomy and at the Joe Biden School of Public Policy and Administration. And I will be talking about the significance of the Nobel Prize in physics. This year's Nobel Prize in Physics was awarded to Sir Roger Penrose for the discovery that black hole formation is a robust prediction of the general theory of relativity. And to Dr. Rainer Genzel and Dr. Andrea Ghez, for the discovery of a supermassive compact object at the center of our galaxy. To understand the significance of these discoveries, I will describe what black holes are. And I will talk about the theoretical contribution of Sir Roger Penrose and the observation on contributions of astrophysicists, Dr. Ghez and Dr. Genzel. The story of this Nobel Prize begins in the early 19 hundreds with Albert Einstein's formulation on the special and general theory of relativity. There are four core pillars of these theories that need to be described in order to understand the significance of the discoveries that we recognize with these prize. In 1905, Albert Einstein showed that light is made of particles. Those particles which we call photons are massless, they have 0-mass, and yet they're affected by gravity, like any mass in the universe, the massive stars or planets. A year later, Herman Minkowski and Albert Einstein formulated the special theory of relativity, and with that, they propose a revolutionary concept than the space and time are not disjoint properties of the universe. our universe, but rather than they should be thought of as inextricably linked. Three spatial dimensions and one temporal dimensions that form the space-time continuum. Ten years later, Albert Einstein developed the general theory of relativity and with it he describes a new way to think about gravity. Gravity is no longer thought of as a force that acts from a massive body to another, but as a warping of spacetime team due to the presence of a massive body. In other words, we stop thinking about gravity as a force that pulls the earth towards the sun and causes the earth to orbit around the sun. Instead, we think about the Sun with its mass itself deforming the space-time continuum, curving it so that there's no straight path that the earth could follow and it has to orbit around the sun. We usually demonstrate this with diagrams like the one on the right, where you see a grid that represents space and you see it curved by the presence of an object, in this case a star. The curvature is proportional to the mass that generates the curvature. So a more massive star, in this case a neutron star, an object that is much more compact than a star like our sun, will produce a more significant warping. While these diagrams might make understanding this space curvature a little more intuitive, it is still hard to think about the curvature of time. So you should think about it as a slowing down of time in proximity of a heavy mass. One year later. Karl Schwarzchild solved the equations propose by Albert Einstein's general theory of relativity to calculate a particularly interesting quantity. What is the distance from an object that is warping space-time such that the curvature is so significant that nothing that enter that area could escape it? We call this the "Schwarzschild radius". And we call the surface that lies at the Schwarzschild radius of an object, the "event horizon". Particles, even light particles that travel as fast as physically possible, if they entered the Schwarzschild radius can no longer escape the gravity of the object. Now for most objects, the Schwarzchild radius lies well inside of the boundaries of the body. And it's tiny compared to the, to the boundary of the body itself. But Schwarzchild also proposed that there could exist an object that's so dense for which the Schwarzschild radius lies outside of the object. And what would happen then? Than the entire object will have to collapse under its own gravity into what we call a "singularity". An infinitely small point in the space time continuum. Appointing the has no finite spatial size, and we're time actually stops. This object will be called, many years later, a black hole. Black, because if light enters the Schwarzschild radius it can no longer escape. However, at the time, everybody, including Albert Einstein and Schwarzchild, thought that this was just a mathematical novelty. They didn't really think that such objects existed in the universe. Furthermore, because light would be trapped inside the Schwarzschild radius, these objects have to be dark and therefore they could not visible. The contribution of the three scientists that were awarded the Nobel Prize, demonstrated in fact that these objects not only exist, but are in fact quite common. Sir Roger Penrose is a mathematical physicist. He's known for its theoretical work in the in the general theory of relativity, as well as in topology. He was, for example, a friend of MC Escher. He developed the, the description of the topology, called the Penrose ladder together with his father Lionel. This ladder is the one that you see in, in Escher's work as the ladder that goes neither up nor down, or rather both up and down at the same time. In 1960 observational astrophysicists discovered an unusual a new class of objects that may Penrose quite curious. In a conference in 1960, Allan Sandage presented an object call 3C 048. They call these objects, this objects and the entire family of objects like it, "quasars": quasi stellar radio sources. 3C 048 is shown in this image at the center, and it's pointed to by white lines. In this image, this object looks just like all the other objects. And the other objects are stars within our own galaxy. However, scientists proposed that there was the possibility that this object was much, much farther away than the stars in our own galaxy. Stars in our own galaxy, our galaxy are typically 10 thousand light-years away. This object is 4 billion light years away. Yeah, it appears just like all the other objects in the image. It appears to us about as bright as a star. For an object to be that far away and to be appear as bright as a star, it's intrinsic brightness have to be enormous. In fact, they had to be so bright that an entire new mechanism for producing brightness had to be developed. Because nothing that we knew could make an object that bright. So Penrose thought of the spacetime singularity and develop the theory of how a star may collapse onto itself and shrink beyond its Schwarzschild radius inside its event horizon. This is the original diagram published in 1964. This is a simplified modern version of that diagram. And what you see here is a star collapse in time. Time moves upward this diagram. The matter of the star collapses, it reaches and the Schwarzschild radius, and it continues to collapse into a singularity. At the event horizon (at the Schwarzschild radius) the laws of physics dramatically change and time essentially stops. Of course, many more theoretical developments were needed before we could truly understand how black holes function and form. And we're still learning about them. Among the many scientists I want to recognize explicitly the work of the Reba Kay Williams, the first Black-American woman to receive a PhD in astrophysics. But I did tell you that black holes are dark by definition because nothing, not even light particles can escape their gravity. Yet, I told you that quasars are incredibly luminous and we had to invoke black holes to explain this luminosity. What is the light that we see coming from faraway quasars? What we see is actually the matter in the galaxy that hosts the supermassive black hole. That attracted by the gravity of the black hole is spiraling inward towards the Schwarzschild radius of the black hole. As it's spiraling in, gravitational and frictional forces compress the material, raise its temperature. And this is what causes the light emission that we see. Essentially, what we are seeing is not the black hole itself, but the black hole, devouring its host galaxy. Quasars are very far away galaxies and in astrophysical images they look just like point sources because they are so far away, 4 billion light years away. They're also much younger, 4 billion years younger. 4 billion years younger. So now we know that galaxies in the early universe typically host a supermassive black hole at the center that is eating up the material from the galaxy. Yet we don't see these devouring black holes in galaxies in our neighborhood, in galaxies in the universe there is contemporary to us. Yet, what happens to these black holes between back then and today? Black holes could still be in the galaxies. But perhaps they're not eating up material from the galaxy anymore. They're not "active". So that Dr. Ghez and Dr. Genzel decided to look for the presence of a supermassive black hole at the center of our own galaxy. This is an image of a galaxy that we think is similar to our own. We cannot look at our galaxy like this because we are inside of it. This is an image of a our own galaxy as seen from Hawaii, the site of the Keck telescopes, the telescopes that Dr. Ghez used, forebear observations. Keep in mind those two bright beems. Those are not Photoshopped and I will tell you what they are in a second. They point towards the galactic center, which is the location of the observation that Dr. Ghez and Dr. Genzel wanted to make. What you see as these very high... the streak in the sky is the very high density of the Milky Way, the disc of our own galaxy. If we want to look at the center of the galaxy, we have two problems. We're looking through the disc of the Milky Way. So we're looking through a very large amount of stars and dust. Dust is what makes these black streaks that look like clouds. In order to avoid the dust and be able to look at stars in the galactic centers through it. the observations are conducted not in the optical wavelengths, the wavelengths at which our eyes see, but in the near infrared and radio wavelengths. But there's another problem. When we look at stars in the sky. They look fuzzy. When we look at stars in the sky, we look through layers and layers of atmosphere, which is turbolent. The turbulence, deforums the look of the sky. It's the same process as if you were looking at if we add a river bed through the water. But if when we want to look at very faint stars, we need to take very long exposures and the changes in the way in which the stars look over time get integrated and conspire to make the images look fuzzy. Doctor Gez and Dr. Genzel wanted to look at far away individual stars and measure the position accurately. I will tell you why in a second. But they couldn't do that because of the turbulence of the atmosphere that render the images fuzzy. So they developed a methodology called "adaptive optics". The way they used, the way they counteract the effects of the turbulent atmosphere is by using deformable mirrors. The mirrors respond, are deformed by pistons and respond to the changes in atmosphere to correct for the effect of the atmosphere. We point lasers to the sky, that's what the two bright beems where in the previous image, to understand how the turbulence make the sky look in real time and in real time we deform the mirrors to compensate for those changes. And that's how we obtain sharp images in spite of turbolent, of turbulent atmosphere. Having developed this technique, Dr. Ghez and Dr. Genzel embark in a 20-year observational campaigns of the position of stars as close as possible to the center of the galaxy. They measure the position over time and then measure the orbital motion of the stars around the galactic center. Some of those stars move so fast that they only take 15 years to complete an orbit. Compare that to the 230 million years that it takes our own Sun to go around the galaxy. Here you see a diagram of the position of sties as measured by the UCLA group led by Dr. Ghez. On the right, you see the actual images collected with the Very Large Telescope in Chile by the Max Plank group lead by Dr. Genzel. Measuring the orbital motion of stars around the galactic center, both groups concluded that the object at the center of the galaxy, which we call Sagitarius A*, has 4 million times the mass of our own Sun included inside of a radius, that is about the size of our solar system. And an object so dense can only be a black hole. So they've demonstrated in the presence of a supermassive black hole at the center of our own galaxy. Today we think that supermassive black holes are in fact at the center of most gases and are crucial in galaxy formation and evolution. Now want to show briefly a couple of other observational pieces of evidence of the existence of black holes all around us. If you're an aficionado of this symposium series, you may remember the 2017 Nobel Prize in Physics was awarded to the LIGO experiment that measure gravitational waves, the very warping of space time continuum that we described at the beginning, as caused by the merger of two black holes. These observations requires such precision that it is comparable to measuring the width of a hair at the distance of the moon Another breakthrough was this image produced by the Event Horizon Telescope in 2019. This is as resolved the image as we can make of a black hole. We see the black hole in the center and the accretion disk of a near, of a black hole near our own galaxy. shining around, around the Schwarzchild radius. The Event Horizon Telescope is in fact not a telescope, but a network or a telescopes that had to work in coordination and develop breakthrough data analysis techniques to produce that image. I have to end this talk on a bit of controversy. And in fact, it's a multifold controversy, sort of controversy in a controversy. First off, it should be noted that Andrea Ghez is only the fourth woman to receive a Nobel Prize in physics. That is a fraction of less than 2%, which is far below the representation of women in the field, even if the field itself is very male dominated. There are many examples of women who contributed and some who even lead projects that were awarded the Nobel Prize. Yet they were left out of the Nobel Prize of the Nobel Prize recipients. For example Lise Meitner, Chien-Shiung Wu, and Dame Jocelyn Bell Burnell. Dame Jocelyn delivered a colloquium at the Department of Physics and Astronomy just a couple of weeks ago. And she described how she discovered a pulsar, a rotating neutron star and as part of a thesis. And this discovery was awarded the 1974 Nobel Prize, except the Nobel Prize was awarded to her advisor. So I want to give a shout out to Dr. Ghez for being a strong advocate and a mentor for many women astronomers. She has a delightful young adult book, called "You can be a woman astronomer". And she is not only an astronomer but an experimentalist. She works in a niche of astrophysics that is far more male-dominated. than astrophysics itself, which itself is male-dominated.e She developed cutting-edge instrumentation as a junior faculty. She embarked in a ten-plus year research program with minimal resources, knowing that she was going to compete with the German group that had already establish significant resources for the same projects. And it was a project that many doubted it could even ever succeeded because of the significant technological advances that it required. Dr. Ghez is very gracious when she describes this competition, I heard recently in a colloquium saying that the competition was "great because it kept us on our toes" and secure that it we would continuously making, make progress. However, and this is the controversy in the controversy, I have to point out how that Dr. Genzel made some unpleasant remarks in a recent interview to the Der Spiegel German magazine, Dr. Genzel suggested that the reason why the Nobel Prize was awarded to Dr. Ghez, and in fact the Nobel Prize for these discoveries was awarded altogether, was that Dr. Ghez is a woman and there is a significant pressure to award Nobel Prizes to women. He suggested that to respond to the gender imbalance, women are chosen for the selection committee and that that slashed the pool of available women in half. Therefore implicitly suggesting that only about ten women are worthy of consideration for this prize. He also dismissed other the research in the area of black holes, including the event horizon images that I described earlier. The astrophysics community has responded pretty much unanimously, dismissing his comments as inappropriate and simply not accurate. However, I have to mention them in this talk, as unpleasant as they are, because I think it is important to call out bigotry when we see it. Nonetheless, the Nobel Prize this year in physics was awarded to Sir Roger Penrose, Doctor Rainer Genzel, and Dr. Andrea Ghez for discoveries that advanced our understanding of black holes, unique objects that are governed by exotic laws of physics that are entirely different than the laws of physics that govern the rest of the universe. Thank you for your attention.
Physics Talk - Dr. F. Bianco, Department of Physics and Astronomy
From Federica Bianco November 08, 2020
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A brief description and discussion of the discoveries awarded the 2020 Nobel Prize in Physics: Sir Roger Penrose, Dr. Andrea Ghez and Dr. Reinhard Genzel were awarded the Nobel Prize for theeretical and observational discoveries that advanced our understanding of the extreme physics of Black Holes.
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