Friday, May 25, 2018

Friday Science: Hawking 6 (Black Holes)

Friday reviews of Stephen Hawking's A Brief History of Time so far.
Chapter 1: Heliocentric
Chapter 2: Spacetime
Chapter 3: Expansion of the Universe
Chapter 4: Uncertainty Principle
Chapter 5: Elementary Particles and the Forces of Nature

Chapter 6: Black Holes
I hate it when I don't finish things. I'm about half way through Hawking's book but got sidetracked by the end of the semester.
  • "Black hole" is a term coined by John Wheeler in 1969.
  • The concept goes back even further. In 1783 John Mitchell suggested a sufficiently massive and compact star would not allow light to escape its gravitational pull.
  • The Marquis de Laplace suggested the same thing just a few years later.
  • In 1928 Subrahmanyan Chandrasekhar, on a boat to Cambridge to study with Sir Arthur Eddington, calculated how big a star could get after burning out.
  • The principle here is the balance between the Pauli exclusion principle and the fact that nothing can move faster than the speed of light. When a star gets sufficiently dense, the repulsion of the exclusion principle becomes less than the gravitational attraction and the cold star would collapse in on itself.
  • A cold star about 1.5 times the size of our sun would do so, a mass now known as the Chandrasekhar limit. (Lev Davidovich Landau came to similar conclusion about about the same time.)
  • Stars less than the limit become white dwarfs, with a radius of a few thousand miles.
  • Slightly more massive neutron stars can be supported by the exclusion repulsion between neutrons and protons. Radius of about 10 miles. 
  • A pulsar is a special kind of neutron star that emits regular pulses of radio waves. Pulsars were discovered in 1967 by Jocelyn Bell at Cambridge.
  • Above the Chandrasekhar limit, might such stars reduce to a singularity, a point. Chandresekhar faced strong opposition to the idea from people like Eddington and Einstein. 
  • Oppenheimer before WW2 did some work on this in relation to light. At a boundary known as the event horizon, light cannot escape the black hole.
  • "God abhors a naked singularity." In cosmic censorship, we can't see what goes on inside a black hole (Penrose).
  • Roger Penrose and Stephen Hawking did a lot of work on black holes between 1965-1970, which are a little like space before the Big Bang. 
  • Some solutions to the general relativity equations suggest wormholes through black holes to the other side of the universe or perhaps even passage back in time. But a human probably wouldn't survive.
  • In 1967, Werner Israel, some non-rotating black holes end up spherical (a solution Karl Schwarzchild suggested in 1917). Penrose and Hawking showed that all non-rotating stars collapsing into a black hole end up spherical.
  • In 1963 Roy Kerr postulated Kerr black holes, rotating black holes that end up in various shapes.
  • "A black hole has no hair." Their final shape depends only on mass and rate of rotation, not on the shape the star had before collapse. As they collapse, they give off gravitational waves until they settle down.
  • Black holes are detected by the gravitational pull they have on other visible stars under certain conditions (e.g., the emission of massive amounts of energy from the nearby star). Cygnus X-1 is a system that seems to have such a black hole. There's probably one at the center of our galaxy as well.
  • Quasars are systems that emit enormous amounts of energy and may have massive black holes at their center.
  • There could be some primordial black holes out there, formed in the early universe, smaller than our sun.
  • Hawking discovered that black holes glow like a hot body. See next chapter.

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