Trip to a Black Hole I

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Extraordinary Concepts in Physics
Lecture 4
Trip to a Black Hole I
by  Robert J. Nemiroff  Michigan Tech
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Physics X About This Course

• Officially "Extraordinary Concepts in Physics"
• Being taught for credit at Michigan Tech
• Light on math, heavy on concepts
• Anyone anywhere is welcome
• No textbook required
• Wikipedia, web links, and lectures only
• Find all the lectures with Google at
• "Starship Asterisk" then "Physics X" 
• http//bb.nightskylive.net/asterisk/viewforum.php?
  f39

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Trip to a Black Hole Overview

• Schwarzschild black hole only
• It's the easiest
• It's what I know best
•  
• Based partly on my paper
•  "Visual distortions near a neutron star and
  black hole"
• American Journal of Physics 1993, 61, 619
• And my web page  
• Virtual Trips to Black Holes and Neutron Stars

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Trip to a Black Hole Key Distances

• r infinity space is flat aN GM/r2
• Near the black hole a aN / (1 - rs/r)1/2
• r 3 rs last stable orbit
• r 1.5 rs photon sphere
• r rs event horizon
• r rc Compton radius
• r 0 GR singularity

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Trip to a Black Hole Far Away

•  
• r -gt infinity space is "flat"
• all (1 - rs/r) terms go to 1.
• gravity is Newtonian  aN GM/r2
•  black holes attract the same as normal matter
• curved universe NOT flat as r -gt infinity
• can see lensing effects with a telescope 
• orbiting the same as spinning in place

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Trip to a Black Hole Approaching

• black hole appears black
• excludes evaporation effects
• blackness everywhere inside photon sphere
• cannot see to the event horizon
• outside, average surface brightness unchanged
• appears fuzzy as star images merge
• distant universe 
• speeds up
• appears bluer

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Background Gravitational Lensing

• gravity bends light "null geodesics"

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Gravitational Lensing Einstein Ring

• Actual Einstein ring APOD 2008 July 28

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Gravitational Lensing Einstein Ring

• Detailed image of Einstein ring from AJP paper

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Trip to a Black Hole Orbiting Nearby
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Trip to a Black Hole Orbiting at 10 rs 

• Stars CANNOT cross the Einstein ring
• Einstein ring mapped point behind BH center
• Einstein ring divides complete image sets
• Angular speeds diverge at the Einstein ring
• One can see oneself 
• All stars have two discernible images
• One outside the ER, one inside
• Observers also have two images
• Actually, an infinite number of images exist

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Trip to a Black Hole Orbiting at 10 rs 

• Distant stars appear slightly bluer
• Distant clocks appear to run faster
• This also occurs for stars that appear next to
  the black hole
•  
• Objects ACTUALLY nearer to the black hole
• Appear redder
• Clocks appear to run slower

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Trip to a Black Hole Orbiting at the Photon
Sphere
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Trip to a Black Hole Orbiting at the Photon
Sphere

• Everything below you is black
• because those light paths fall into the BH
• The whole sky appear above you
• because those light paths escape the BH
• The Einstein ring appears above the horizon
• Stars still CANNOT cross the Einstein ring
• Stars still speed up near the Einstein ring

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Trip to a Black Hole Orbiting at the Photon
Sphere

• Other image sets between other Einstein rings
• "The" Einstein ring actually "First Sky Einstein
  ring"
• There are infinitely many Sky Einstein rings
• Every radius from the BH has 
• its own infinite set of Einstein rings
• its own redshift (or blueshift)

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Gravitational Lensing Einstein Ring

• Einstein rings near a black hole