by APOD Robot » Tue Jul 02, 2013 4:09 am
Circling a Black Hole at its Photon Sphere
Explanation: What would it look like to go right up to a black hole? One particularly interesting place near a black hole is its
photon sphere, where photons can orbit in circles, a sphere 50 percent further out than the
event horizon. Were you to look out from the
photon sphere of a
black hole, half of the sky would appear completely black, half of the sky would appear unusually bright, and the back of your head would appear across the middle. The
above computer-animated video depicts
this view from the photon sphere. The reason that the lower region, as shown,
appears black is because all light paths from this dark region comes up from the
black hole -- which classically emits no light. The upper half of the sky now appears unusually bright,
blueshifted, and shows increasingly many complete sky images increasingly close to the dark-light divide across the middle. That dark-light divide is the
photon sphere -- your location -- and since photons can do circles there, light from the back of your head can circle the
black hole and come to your eye. No place on
the sky is hidden from you -- stars that would normally pass behind the black hole
now appear to zip quickly around an
Einstein ring, a ring that appears above as a horizontal line about a quarter of the way down from the video top. The
above video is part of a
sequence of videos visually
exploring the space near a black hole's
event horizon.
(Disclosure: Video creator Robert Nemiroff is an editor for APOD.)
[/b]
[url=http://apod.nasa.gov/apod/ap130702.html][img]http://apod.nasa.gov/apod/calendar/S_130702.jpg[/img] [size=150]Circling a Black Hole at its Photon Sphere[/size][/url]
[b] Explanation: [/b] What would it look like to go right up to a black hole? One particularly interesting place near a black hole is its [url=https://en.wikipedia.org/wiki/Photon_sphere]photon sphere[/url], where photons can orbit in circles, a sphere 50 percent further out than the [url=https://en.wikipedia.org/wiki/Event_horizon]event horizon[/url]. Were you to look out from the [url=http://hyperphysics.phy-astr.gsu.edu/hbase/astro/blkhol.html#c3]photon sphere[/url] of a [url=http://apod.nasa.gov/apod/ap130701.html]black hole[/url], half of the sky would appear completely black, half of the sky would appear unusually bright, and the back of your head would appear across the middle. The [url=http://www.youtube.com/watch?v=n_SqoQ60-c8]above computer-animated video[/url] depicts [url=http://adsabs.harvard.edu/abs/1993AmJPh..61..619N]this view[/url] from the photon sphere. The reason that the lower region, as shown, [url=http://asterisk.apod.com/viewtopic.php?f=39&t=21664]appears black[/url] is because all light paths from this dark region comes up from the [url=http://en.wikipedia.org/wiki/Black_hole]black hole[/url] -- which classically emits no light. The upper half of the sky now appears unusually bright, [url=https://en.wikipedia.org/wiki/Blueshift]blueshifted[/url], and shows increasingly many complete sky images increasingly close to the dark-light divide across the middle. That dark-light divide is the [url=http://casa.colorado.edu/~ajsh/singularity.html#photon]photon sphere[/url] -- your location -- and since photons can do circles there, light from the back of your head can circle the [url=http://cosmology.berkeley.edu/Education/BHfaq.html]black hole[/url] and come to your eye. No place on [url=http://apod.nasa.gov/htmltest/gifcity/nslens_bh.html]the sky[/url] is hidden from you -- stars that would normally pass behind the black hole [url=http://apod.nasa.gov/htmltest/gifcity/pscirc.html]now appear[/url] to zip quickly around an [url=http://apod.nasa.gov/apod/ap080728.html]Einstein ring[/url], a ring that appears above as a horizontal line about a quarter of the way down from the video top. The [url=http://www.youtube.com/watch?v=n_SqoQ60-c8]above video[/url] is part of a [url=http://apod.nasa.gov/htmltest/rjn_bht.html]sequence of videos[/url] visually [url=http://adsabs.harvard.edu/abs/1999ascl.soft10006N]exploring[/url] the space near a black hole's [url=http://archive.ncsa.illinois.edu/Cyberia/NumRel/BlackHoleAnat.html]event horizon[/url]. [i](Disclosure: Video creator Robert Nemiroff is an editor for APOD.)[/i]
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