trip to a black hole i by robert j. nemiroff michigan tech extraordinary concepts in physics lecture...
Post on 26-Mar-2015
214 Views
Preview:
TRANSCRIPT
Trip to a Black Hole I
by Robert J. Nemiroff
Michigan Tech
Extraordinary Concepts in Physics Lecture 4
Physics X: About This Course
• Officially "Extraordinary Concepts in Physics"• Being taught for credit at Michigan Tech
o Light on math, heavy on concepts o Anyone anywhere is welcome
• No textbook requiredo Wikipedia, web links, and lectures onlyo Find all the lectures with Google at:
"Starship Asterisk" then "Physics X" o http://bb.nightskylive.net/asterisk/viewforum.php?f=39
TRIP TO A BLACK HOLE: OVERVIEW
• Schwarzschild black hole onlyo It's the easiesto It's what I know best
• Based partly on my paper:
o "Visual distortions near a neutron star and black hole"o American Journal of Physics 1993, 61, 619
• And my web page: o Virtual Trips to Black Holes and Neutron Stars
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
TRIP TO A BLACK HOLE: FAR AWAY
• r -> infinity: space is "flat"
o all (1 - rs/r) terms go to 1. o gravity is Newtonian: aN = GM/r2o black holes attract the same as normal matter o curved universe NOT flat as r -> infinityo can see lensing effects with a telescope o orbiting the same as spinning in place
TRIP TO A BLACK HOLE: APPROACHING
• black hole appears blacko excludes evaporation effects
• blackness everywhere inside photon sphereo cannot see to the event horizon
• outside, average surface brightness unchangedo appears fuzzy as star images merge
• distant universe o speeds upo appears bluer
BACKGROUND: GRAVITATIONAL LENSING
• gravity bends light: "null geodesics"
GRAVITATIONAL LENSING: EINSTEIN RING
• Actual Einstein ring: APOD 2008 July 28
GRAVITATIONAL LENSING: EINSTEIN RING
• Detailed image of Einstein ring from AJP paper
TRIP TO A BLACK HOLE: ORBITING NEARBY
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
o One outside the ER, one insideo Observers also have two images
• Actually, an infinite number of images exist
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
o Appear reddero Clocks appear to run slower
TRIP TO A BLACK HOLE: ORBITING AT THE PHOTON SPHERE
TRIP TO A BLACK HOLE: ORBITING AT THE PHOTON SPHERE
• Everything below you is blacko because those light paths fall into the BH
• The whole sky appear above youo 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
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
o its own infinite set of Einstein ringso its own redshift (or blueshift)
GRAVITATIONAL LENSING: EINSTEIN RING
• Einstein rings near a black hole
top related