Astronomy 1021 Chapter 14: Black Holes
29 May 2018
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Astronomy 1021
Chapter 14
Black Holes
•Black hole: “black” b/c nothing (even light) can escape from a black hole b/c it has an escape
velocity of speed of light, therefore can’t see them (Really high gravity); “hole”- Einstein
discovered that space and time are not distinct but 4-dimensional spacetime, what we perceive as
gravity arises from curvature of spacetime
-Rubber sheet analogy- 2-dimensional representation of spacetime is flat; gravity causes curvature of
sheet, curvation increases as we approach black hole (black hole is a bottomless put in spacetime)
-Black hole has a huge mass which causes great curvature in spacetime- creates a ‘hole’
•The only measurable properties are mass, charge, and angular momentum
•Mass Warps Spacetime- Einstein: objects always travel in the straightest possible paths, but mass
changes what a ‘straight path’ means, instead get a curved path; light from farther stars bends
towards the Sun b/c Sun bends spacetime
•The Event Horizon- boundary b/w inside of a black hole and the universe; the point of no return,
escape velocity=speed of light here, size of black hole is size of its event horizon
-Escape velocity- speed you need to escape the gravity of an object, depends on mass and size of
object; increasing mass but keeping radius the same increases escape velocity; keeping mass the
same but decreasing radius of object increases escape velocity
-Schwarzchild radius- size of sphere (Event horizon), depends only on black hole’s mass, ex. black
hole of Sun’s mass has S radius of 3 km, greater the mass, greater the S radius
-Collapsing stellar core becomes a black hole when it shrinks to a size smaller than its S radius, can’t
see the event horizon or anything that falls in it
•How can Black Hole kill you? 1) If you go near one, and 2) if you fall into one- can never escape,
if you’re a bit farther away you can orbit around it and escape, escape is possible as long as you
don’t cross the event horizon
-If you fall in, everything becomes darker and you don’t see anything; you will see the view longer if
you fall into a supermassive black hole
-3) Spaghettification- if fell in feet first, feel stronger pull on feet than on head, so body would get
stretched (b/c immense gravity of black hole)
-4) Fried by the light
•Testing Einstein’s theory of relativity- time should run more slowly as force of gravity grows
stronger, light from gravitational field should be redshifted (gravitational redshift) b/c gravity
-If you go towards the black with a clock, time won’t speed up or slow down for you as you enter the
event horizon, you would rapidly accelerate into the black hole; but to an outsider, time slows down
near the black hole; near the event horizon outsider will think time has stopped (even if the person
falls in)- Time dilation
-This is b/c huge gravitational redshift of light- really strong tidal forces near the event horizon
-Supermassive black holes have weaker tidal forces
Document Summary
Rubber sheet analogy- 2-dimensional representation of spacetime is flat; gravity causes curvature of sheet, curvation increases as we approach black hole (black hole is a bottomless put in spacetime) Schwarzchild radius- size of sphere (event horizon), depends only on black hole"s mass, ex. black hole of sun"s mass has s radius of 3 km, greater the mass, greater the s radius. If you fall in, everything becomes darker and you don"t see anything; you will see the view longer if you fall into a supermassive black hole. 3) spaghettification- if fell in feet first, feel stronger pull on feet than on head, so body would get stretched (b/c immense gravity of black hole) 4) fried by the light: testing einstein"s theory of relativity- time should run more slowly as force of gravity grows stronger, light from gravitational field should be redshifted (gravitational redshift) b/c gravity.
