Date: February 15, 2011
Title: Black Holes
Podcaster: CMU Astronomy Class – Brandon Wirakesuma, Karolos Waldron, Yanko Yanchev
Organization: Carnegie Mellon University Physics Department – http://www.cmu.edu/physics/
Description: Although the term “black hole” was not publicly used until 1967 by John Wheeler, the concept of an area of space in which nothing can escape even including light is not new. As scientific theory and experimentation has become more advance in the past couple of centuries, there have been significant improvements in the models used to capture how black holes function. This podcast aims to cover three things, the history of the black hole and the science behind it, the properties of a black hole and how it interacts with other objects across space and time, as well as the formation and evolution of a black hole.
Bios: Karolos Dimitrios Waldron is a student at Carnegie Mellon University majoring in Economics.
As a senior, Karolos was given the chance to pursue electives and learn more about interesting subjects that he knew little. Karolos grew up in Greece and has had an interest in Astronomy ever since learning about the accomplishments of Ancient Greeks in the field of Astronomy. However, in his education he did not have the opportunity to study astronomy in depth and learn about contemporary issues. So this is an opportunity for both him and the public to learn more about black holes.
Brandon Wirakesuma is currently a senior at Carnegie Mellon majoring in economics. Brandon grew up in Indonesia, a country with a long history of myths and superstitions related to celestial activity, and decided to take up the chance to pursue astronomy in order to uncover fact from fiction. In his free time, Brandon enjoys surfing, travelling and reading about behavioral finance.
Yanko Yanchev was born in 1988 in Bulgaria and spent quite a lot of his childhood outside looking at the sky or hiking in the mountains. Nature, stars and astronomy always fascinated him fueling his strong interest in physics and mathematics during high school years. One of the most memorable experiences he ever had was to operate his own Celestron CGE-1400 telescope back at home, which opened a new world for him and made amateur astronomy a hobby for a lifetime.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Ian Harnett.
By Brandon Wirakesuma, Karolos Waldron, Yanko Yanchev
John Mitchell as an early pioneer in the field
Amazingly, the history of black holes goes all the way back to the 18th century when the English geologist John Mitchell first realized that in theory it would be possible for gravity to be so overwhelmingly strong that nothing — not even light traveling at nearly 300,000 kilometers per second — could escape. Of course, he was really thinking big as in order to generate such a strong gravity, an object must have very large mass and unimaginable density. Mitchell didn’t believe “dark stars” (as he called them) existed in the real world due to the extreme conditions they required. Most of what we know today about Mitchell’s ideas is from two French editions of an astronomy guide published by the famous mathematician and philosopher Pierre Laplace. Unfortunately, the idea of “dark stars” was dismissed soon after the publications and scientists needed almost two hundred years and Einstein’s groundbreaking discovery of the general theory of relativity in order to revisit the subject.
Einstein’s general theory of relativity and its consequences
First to apply Einstein’s equations to define a “black hole” was Karl Schwarzschild in 1916. He was also the first to introduce the term black hole we know today. Schwarzschild calculated that our Sun would have to shrink to less than two miles in radius in order to trap light within its gravitational field. Another interesting conclusion Schwarzschild made was that if our Sun was to collapse the planets’ orbits and paths will be unchanged due to the mass and position of the star remaining the same. Back then, there was no clear idea of what would be the process and how in fact the Sun can become this compact. This was probably one of the main reasons why black holes had so many skeptics during the early 20’s and 30’s of the 20th century. Even Einstein himself, along with Sir Arthur Eddington, opposed the black hole theory.
Only around 1935, and through the work of several brilliant scientists such as Chandrasekar,
Baade, Zwicky and Oppenheimer, it was realized that during the collapse of a star, atoms are stripped of their electrons, thus turning it into a neutron star, which is only 10-15 miles in diameter with a density of about a billion tons per cubic inch. This proved that “unimaginable densities” do exist in our Universe. More interestingly, scientists proved that in order to make a black hole the collapsing star needs to be at least 3.9 times more massive than our own Sun. In the seventies the now world famous Steven Hawking came up with theoretical arguments demonstrating that black holes aren’t entirely black and that it is possible for them to evaporate because they emit radiation. His theory was supported later on by the discovery of the first black hole Cygnus X-1. Both of these discoveries answered so many questions, yet a lot more remained open.
Properties of a Black Hole
The Schwarzschild black holes, which are the simplest black holes, have mass but no electric charge, nor angular momentum. According to Birkhoff’s theorem, there is no observable difference between the gravitational field of such a black hole and that of any other spherical object of the same mass. The popular notion of a black hole “sucking in everything” in its surroundings is therefore only correct near the black hole horizon; far away, the external gravitational field is identical to that of any other body of the same mass. Rotating black holes are called Kerr whereas charged black holes are called Reissner- Nordström. A black hole that is rotating and charged is called Kerr-Newman. Black holes are commonly classified according to their mass, independent of angular momentum J or electric charge Q. Black holes with mass up to that of the moon are called micro black holes. Those with mass approximately 10 times the mass of the sun are stellar black holes. Intermediate mass black holes have mass approximately equal to 103 times the mass of the Sun and any black holes with larger mass are supermassive black holes.
As an object collapses, its density and strength of its surface gravity increase; and if an object collapses to zero radius, its density and gravity become infinite. Such a point is called a singularity. For a non-rotating black hole this region takes the shape of a single point and for a rotating black hole it is smeared out to form a ring singularity lying in the plane of rotation.
Behavior of objects inside a black hole
Objects cannot avoid falling into a Schwarzschild black hole. They will be torn apart by the growing tidal forces in a process called spaghettification or the noodle effect. Any attempt to escape the singularity will only decrease the time to get there. When it reaches the singularity, the object is crushed to infinite density and its mass is added to the total of the black hole. On the other hand, it is possible to avoid the singularity of a Reissner–Nordström or Kerr black hole. Extending these solutions as far as possible reveals the hypothetical possibility of exiting the black hole into a different spacetime with the black hole acting as a worm hole. It also appears to be possible to follow closed timelike curves around the Kerr singularity, which lead to problems with causality like the grandfather paradox. It is expected that none of these peculiar effects would survive in a proper quantum mechanical treatment of rotating and charged black holes.
The appearance of singularities in general relativity is commonly perceived as signaling the breakdown of the theory. This breakdown, however, is expected; it occurs in a situation where quantum mechanical effects should describe these actions due to the extremely high density and therefore particle interactions. To date, it has not been possible to combine quantum and gravitational effects into a single theory. It is generally expected that a theory of quantum gravity will feature black holes without singularities.
Formation of a Black Hole
At present astrophysicists have come up with a couple of theories and models of how black holes come into formation, grow and later evaporate. One popular explanation behind the formation of a black hole is known as a stellar collapse, which as its name suggests relates to a star’s nuclear core degenerating and forming into a black hole as it collapses. This occurs when the Chandrasekhar limit for nuclear degeneration is exceeded. Although this model can take into account why large stars collapse, it does not take into account the formation of white dwarfs into black holes. A potential way in which a white dwarf could form into a black hole could be through a stellar companion such as a supergiant or a stellar star. This occurs when matter that is being ejected from the larger stellar companion is released via strong solar wind or Lagrangian overflow to the point where the Chandrasekhar limit is exceeded. Once this happens, this sets the conditions necessary for the formation of a black hole.
Black Holes and the Big Bang
When astronomers and astrophysicist alike view the center of the galaxy, there appears to be large bulges which can be explained by the presence of giant black holes. The current theory used to describe the giant black holes was that these black holes came about along with the formation of the galaxy, and have since played an important part in the formation of the galaxies shape and structure. Scientist currently hypothesize that these black holes were formed at the beginning of the universe when there were extremely large disparities in the distribution of density and mass throughout the universe.
Black holes grow by attracting matter and mass from nearby stars and objects in what is known as accretion. Accretion occurs when a black holes gravitational force causes objects to spin and release mass due to the angular momentum induced by the black hole. This process causes stars to release glowing hot gas that later swirls like a water wheel into the center of the hole. However, just like a water wheel, the black hole must gain and shed angular momentum at the same rate. At present, scientist are arguing how this is done, with some hypothesizing that momentum is released through friction between the gas particles and others hypothesizing that momentum is released through a magnetic field winds generated from the black hole itself.
Just as black holes can develop and grow, they also will evaporate over time. The theory behind black hole evaporation was predicted by Stephen Hawking who calculated that black holes, while absorbing energy and particles from other objects, also undergo a process where they release quantum mechanical particles. As energy is released, the black hole losses some of its mass, which is in accordance to Einstein’s equation where energy is equal to mass times the speed of light squared. Eventually as energy is completely released, the black holes will completely dissipate and shrink.
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End of podcast:
365 Days of Astronomy
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