Last night, I headed down to the beautiful Bell House in the Gowanus section of Brooklyn for the latest Secret Science Club lecture, featuring Dr Caleb Scarf, director of Columbia University's Astrobiology Center and author of Scientific American's "Life, Unbounded" blog. Dr Scharf spoke to a packed house, standing room only.
In some respects, the lecture, which touched on topics in Dr Scharf's book Gravity's Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos, covered ground covered previous lectures by doctors Charles Liu and David Hogg. That being said, it was another tour de force, accompanied by some beautiful astronomical images.
Dr Scharf opened his talk with the assertion that science involves the telling of stories using algebra and images, as well as words. The particular story he was telling tonight began in the 18th Century. Black holes were a conception of the human mind before we knew they were possible- in 1783 John Michell, an English clergyman and scientist (now, that's a combination that is sadly improbable these days) conceived of objects so massive that light could not escape from them. In Michell's words:
"If the semi-diameter of a sphere of the same density as the Sun were to exceed that of the Sun in the proportion of 500 to 1, a body falling from an infinite height towards it would have acquired at its surface greater velocity than that of light, and consequently supposing light to be attracted by the same force in proportion to its vis inertiae (inertial mass), with other bodies, all light emitted from such a body would be made to return towards it by its own proper gravity."
Tragically, Michell's idea was forgotten for over a century. In the early 20th century, some guy named Al theorized that space was stiff, but flexible, and could be distorted by mass (here's a good visual representation). The speed of light in a vacuum is a constant. Measurements of distance, length and time are flexible depending on the position of the observer. Gravity poses a problem because it is dependent on distance, it is "stitched into the fabric of space itself". To sum this up, gravity is a consequence of the structure of space, which is distorted by mass.
Einstein's Theory of Relativity provides a mathematical framework, but no details. While serving on the Russian front in WW1, physicist Karl Schwartzchild sent two letters to Einstein. In one of these letters, Schwartzchild posited that a large mass could be shrunk to such a great density that it would stress space until nothing could escape its gravitational attraction- the Schwartzchild metric describes this gravitational field. Even light would be "stretched to nothingness" by such a strong "pull". In the vicinity of such an object, the curvature of space would be extremely distorted- expressed jocularly, the universe can "make holes in itself". The edge of this zone of distortion is known as the Event Horizon. While even light would not escape the gravity of a black hole, light would bend around the event horizon.
Einstein was stymied by the sheer ridiculousness of the notion of black holes- could nature compact matter until it popped out of normal existence? If the Earth were compressed into a 9mm ball, it would develop an event horizon. A small black hole, ten times the mass of our sun, would be 37 miles across, about the diameter of London's M25 Motorway, which is another inescapable object. A black hole with a mass equal to four million suns would have an event horizon the size of Mercury's orbit. With a mass equal to that of a billion suns, the event horizon would stretch to the distance of Neptune's orbit.
With improved methods of observation, we now know that black holes are real, and that every known galaxy has a supermassive black hole at its center. These supermassive black holes were discovered by observation of the gravitational effects they had on objects around them. Matter falling into black holes accelerates to rapid speeds and releases a vast amount of energy. The process is a very noisy one, the matter "gurgles" like water flowing down a drain, 57 octaves below the threshold of human hearing. Four percent of known galaxies have two supermassive black holes at their center, but these will probably merge into one black hole.
The energy released by the matter falling into a black hole is "turbocharged" by the spin of the black hole, which drags the fabric of space around it. The matter falling into a black hole emits light and subatomic particles (here are some nice conceptual images). Black holes also carry an electrical charge. The energy producing, light emitting region surrounding a black hole is known as the ergosphere. Black holes produce the most efficient conversion of matter to energy known to us, fifty times the efficiency of fusion.
Radio telescopes are used to "map out" electrons ejected from the vicinity of black holes. The Hubble space telescope has provided beautiful images, which were a highlight of Dr Scharf's lecture. Galaxy M87 has a huge "plume" emitting black hole in its center. The ejecta of Centaurus A's nucleus are particularly dramatic. The black hole at the center of Cignus A is a billion times the mass of the sun (but is compacted to a diameter smaller than that of our solar system), and ejects particles to a distance of 50,000 light years. Dr Scharf likened it to a bacterium blowing up a balloon that could envelop Brooklyn.
Perseus A is a galaxy "cohabiting" a galaxy cluster- between the galaxies in the cluster there is a tenuous "atmosphere" of hot gas. As the gas cools, it emits radiation. The Chandra X-ray Observatory has produced gorgeous images of Perseus A and its gas "envelop". The gas is not uniform in structure- ejecta from the black hole forms "bubbles", which are the dark patches in the linked image. These bubbles float outward and form ripples. While, in a typical situation, hot clouds of gas can cool and condense to form stars, the ripples produced by the black hole bubbles inhibit the formation of new stars.
Supermassive black holes are closely linked to the nature of galaxies. The ratio of mass between the supermassive black hole at the center of the galaxy and the cluster of stars around it tends to be 1:1000. The Milky Way seems to be an exception, it is a "bulgeless" galaxy, so it is difficult to determine the mass ratio between our black hole and our galaxy. Supermassive black holes affect the nature of galaxies, because they affect the formation of stars, which produce the elements that form planets, chemistry, and life. Galaxies which are still producing large amounts of stars are known as blue galaxies, they are still making stars- big, hot, young, short-lived stars. Red galaxies are not making new stars, which means they aren't making planets. Between the blue and red galaxies are green valley galaxies, among which is our Milky Way. Star formation in green valley galaxies is slow, the process is probably shutting down. The supermassive black holes in the centers of green valley galaxies tend to be the "busiest" of all black holes. The black hole at the center of the Milky Way has formed two enormous bubbles, as Dr Scharf quipped, "We don't live in a quiet place."
Dr Scharf ended his lecture with the assertion that black holes are not a peripheral subject, but a vital piece in the disposition of our universe.
Here's a link for an animation of how a black hole effects the development of galaxy 4c41.17, which is 12 billion light years from Earth (so the image is of a "teenaged" galaxy, 12 billion years before present), the link contains a bonus interview with Dr Scharf.
Dr Scharf also mentioned that, in the next 12-18 months, a gas cloud will "fall" into a black hole in the Milky Way, providing a unique opportunity to observe the process.
Once again, the Secret Science Club lecture was a rousingly entertaining, informative affair. As a side note, some bastard in the audience was interviewed by a reporter from the Associated Press. I'll post a link when the article pops up in the news.