So, another area of interest in which general relativity might be useful is the enommingly of the black hole. Black holes exist all throughout the universe, but mysteriously no one can actually see them. So what exactly are black holes then? Where do they come from? How do they affect their environment? What do Einstein's laws of relativity tell us about them? What would happen if someone were to float straight into a black hole? We will find out this and more in today's episode of Space Time and Matter. Let's get started. Black holes were first predicted in the late 18th century by geologists John Mitchell and separately by mathematician Pierre Simmelon-Oploss. Using the known formula for escape velocity, they found that beyond a certain mass, an object could have such a strong gravitational force that not even light could escape. Let's see how this works. The formula for escape velocity is a square root of the quantity of two times gravitational constant times the mass of the object, all over its radius. The gravitational constant is equal to 6.67 times 10 to the negative 11th meters cubed per kilogram second squared. If we set it escape velocity equal to the speed of light, we will know at what threshold an object's gravitational pull becomes inescapable. The proportion of the mass to the radius of the object comes out to be 6.75 times 10 to the 26th kilograms per meter. Whoa. Which brings us to the main part of this section on black holes. Consider now what you know about black holes. What have you always wondered? Is there anything that makes you go, ee-e-e-e-e-e? Keep these questions in mind so we can address them at the end of this presentation. Thank you for listening and ee-e-e-e-e-e. How are black holes created? While they usually come from stars, a star size is determined by the balance of its gravitational pull inward on itself and its outward pushing explosive forces caused by the fusion of hydrogen atoms. Eventually, the fuel for the fusion processes in the star runs out, and the gravitational pull becomes the compressive star into its core. The star heats up during this compression process and blasts out debris and radiation, leaving only the core. After this, the research has shown that if the core is mass is at least 2.5 to 3 times the mass of the sun, there will be enough gravitational force to turn the core into a black hole. Otherwise, the core is referred to as a neutron star. What are some properties of black holes? How do they relate to relativity? First off, there is a border around the black hole called the Shorthold Radius or Eventhorizon. According to General Relativity, the presence of matter deforms the fabric of space time. When there is a large amount of matter in one place, the deformation becomes very intense. When an object passes into the Shorthold Radius, this curvature of space time is so great that there are no paths by which it can escape. Not even light can escape, hence the name of Enterizon. Since light cannot escape, no information from the inside of the eventhorizon can reach the outside. So the Eventhorizon signifies the limit from which information from near the black hole can no longer reach the outside world. How about the size of a black hole? Well, black holes usually come in two sizes. First, there are stellar black holes that are 10 to 24 times the mass of the sun. There are hundreds of millions of these in the Milky Way alone. Second, there are supermassive black holes, which are hundreds of millions more times mass of the sun, and they are thought to be at the center of almost all large galaxies. Black holes are typically divided into two categories. The first is called the Swarchild Black Hole. It is a simpler case, consisting of only a singularity at the point like extremely dense center in the eventhorizon. The second more common category is known as Kerr. A Kerr black hole rotates due to conservation of angular momentum from the original star. Like a Swarchild Black Hole, it has a singularity in eventhorizon. However, because it is rotating, it also has what is known as an Ergosphere. Einstein's theory of general relativity predicts that a rotating object will have a frame dragging effect on the spacetime around it, much like a rotating marble in a jar of peanut butter, which drags some of the peanut butter with it. So, an object falling toward the black hole may feel like it is descending directly towards the center, but it will actually be dragged along in the direction of rotation, along with the rest of the surrounding spacetime. The Ergosphere is a region in which a frame dragging effect can be felt. Finally, the static limit denotes the boundary between the Ergosphere and space. And one last thing about the properties of a black hole. We have already seen that they have mass and angular momentum. They can also be assigned a charge. Now I know what you are thinking at this point. This is mind blowing stuff. You probably think that was enough information on black holes to last you a lifetime. Oh ho, but we are not done. Oh no no no. We are going to equip you with some practical knowledge about what would happen if you found yourself floating towards a black hole. And don't doubt it. Better safe than sorry. Now that we know some of the properties of black holes, let's consider a scenario where a wild ashian I named Sir Nevinca is approaching a black hole. We will walk you through what happens along each step of the way. First, general relativity dictates that strong gravitational fields will dilate time relative to areas with weaker gravitational fields. Almost like traveling at high speeds dilates time relative to objects at rest. As Nevinca drifts towards a vectorizing, time slows down more and more for him relative to us observers on the spaceship. He appears to be approaching the horizon more and more slowly, taking an infinite amount of time to reach. Another prediction of general relativity is called gravitational redshift. Electromagnetic radiation in a strong gravitational field will be reduced in frequency and intensity to a viewer in a weaker gravitational field. As Nevinca takes his sweet time reaching the horizon, he will also begin to fade and become redder with each passing moment. From Nevinca's perspective, everything is proceeding normally and he passes the event horizon. Now there is no turning back. He is doomed. He is doomed. Now a horrifying process will begin to happen. The technical term that scientists use to describe this process is... Spaghettification. What's like a difference in gravitational pull due to the distance from the Sun causes tidal forces, Spaghettification happens due to the difference between gravitational pull on Nevin's head to not his feet. Since his feet are closer to the singularity, they feel a stronger gravitational pull and experience more acceleration. Thus, Nevinca experiences a stretching effect as his feet accelerate ever farther from his head. In the final step, Nevin meets his maker. He reaches a singularity, a point of infinite density and infinite curvature in space time, where he is compressed into nothing and he is slain. Errrr. He is no match for the fabric of the car's most. Okay, so we just learned that black holes are scary phenomena when one is close up to them. How do we detect these things if they let no light escape? Although it appears that there is nothing there. Black hole does exert a gravitational influence on nearby astronomical bodies as any other object with the same mass would. Scientists also look for the gravitational lens effect, where strong gravitational feels distort light in the way similar to a lens's effects. Finally, black holes do emit some radiation. Objects approaching black holes are super-heated, and as a result, release the recorway radiation before they cross the event horizon. One less thing. There is one quirk that Einstein and his boss are relatively could not predict. Quantum mechanics actually plays a key role in our understanding of black holes. By the classical model, no particle can escape from a black hole. The quantum mechanical model, however, predicts that something called quantum vacuum fluctuations will create particle and antiparticle pairs just outside the sort child radius of a black hole. This is called hocking radiation. These pairs usually annihilate each other. However, right outside the event horizon, the antiparticle is instead pulled towards the black hole while the particle escapes. Through this process, a black hole evaporates and loses mass over time. Amazingly, it has been found that the radiation emitted by black holes is identical to that which would be emitted by a black body. A black body is a hypothetical object that is capable of absorbing 100% of incoming electromagnetic radiation. This is some of what we know so far about black holes, but there is undoubtedly so much more to find out. In the scientific world, general relativity has conventionally been used for objects in a larger scale. While quantum mechanics has been used for objects in a micro scale, black holes are an enigma in that they are extremely massive, calling for the use of general relativity, but they are also extremely small, compressed with point-like singularity. Well, that's all for today, folks.