Class notes
ASTRO 101: what is a black Hole
- Course
- ASTRO101 (ASTRO101)
- Institution
- University Of Alberta (UofA)
What is a Black Hole
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The basic idea behind a dark star only requires knowledge of 18th century physics. If a star isdense enough, its escape velocity will be the speed of light, making it impossible for light emitted
by the star to escape the stars gravity. The idea of a dark star, as proposed by John Michell is not
correct, but it is still important since it introduces some ideas that apply to black holes even in
modern theories. The problem is, that the concept of a dark star uses Newton's older theory of
gravity instead of Einstein's newer theory. That being said, Newton's theory of gravity is a pretty
good approximation to Einstein's theory of gravity, when gravity is weak. It was good enough for us
to plan a mission to the moon, for instance.
When I mean by weak is this, calculate the escape velocity from a planet or star, and compare the
value of its escape velocity to the speed of light. If the escape velocity is tiny compared to the
speed of light, then we say that gravity is weak, and Newton's theory of gravity is a good enough
approximation. For example, the escape velocity from the Earth is 11.2 kilometers per second, but
the speed of light is approximately 27,000 times larger.
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Since the escape velocity from Earth is so small compared to the speed of light, Newton's gravity is
good enough for most calculations near the surface of the Earth. But if the escape velocity is larger,
say 10 percent of the speed of light or larger, that means that Newton's theory of gravity is no
longer sufficient to calculate the strength of gravity. Since Albert Einstein's equations correctly
describe relativistic effects at high speeds, they improved on Newton's theory of gravity. This
means that we can predict what happens in situations with strong gravity. Einstein's theory of
gravity is called The Theory of General Relativity.
In general relativity, mass, energy, and angular momentum are all responsible for creating curvature
in space time. The curvature of space time then causes planets, stars, and light to travel on curved
paths.
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To create a dark star, we might start with a large star and compress it inwards to make it smaller
and denser while keeping the amount of mass unchanged. As the star shrinks in size, the escape
velocity from the surface becomes faster and faster until it becomes equal to the speed of light. At
this point, Newton's theory of gravity just predicts that light won't be able to escape from the star,
and it will appear dark. However, the predictions from Einstein's theory of gravity demonstrate a so
called dark star, would exert a much stronger force due to gravity than predicted by Newton. This
additional inwards gravitational force makes it impossible for a star to have a stable size. In order
for stars to exist, there is a delicate balance between its gas molecules, which exert a net outwards
pressure that is exactly balanced by the attraction of gravity, allowing stars to stay the same size
over time.
, Schwarzschild radius
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hebolas absorb light
dark mass brass mass but does not emir
light
WIMP weakly interacting massive punk
Dark energy bone
acting opposing energy
nothing to
do with darkholes but stars may
use it to hold itself from collapsing
expansion is speeding
up
the tidal force
holes stronger
Smaller the black
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