Path of light@ various distance from Black Hole

  • #1

Main Question or Discussion Point

Have some doubt regarding the behaviour of the light near the Black hole.....What happens to the path of light at various distances from the Black hole??........If we send a small bot, with sensor attached to it , radially towards the Blackhole, what would be the last thing that the sensor records before it perceives the blackness of the black hole?

to avoid complexities, lets assume that there is no dust or any obstacle kind of thing around the BH :)
 

Answers and Replies

  • #2
tiny-tim
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welcome to pf!

hi pranathi! welcome to pf! :smile:
Have some doubt regarding the behaviour of the light near the Black hole.....What happens to the path of light at various distances from the Black hole??
from any distance, there is a cone in front of you from which no light comes

as you approach the black hole, the angle of that cone gets larger

at the photon sphere (r = 3M, where the event horizon is at r =2M), the cone has a solid angle of 2π … ie it's flat

between the photon sphere and the event horizon, the solid angle is more than 2π, and loosely speaking you're now "inside" the cone: instead of a small black cone in front you, and everything else is light, there's a small light cone behind you, and everything else is black!

when you reach the event horizon, the light cone has reduced to zero :redface:

(i should add that on the edge of the cone, there could be a "caustic" of bright light, caused by light from far away circling the black hole many times close to the photon sphere before finally escaping again)​
 
  • #3
Thank you ... Sir :)

I got some idea with what you have posted and regarding this ????

"If we send a small bot, with sensor attached to it , radially towards the Blackhole, what would be the last thing that the sensor records before it perceives the blackness of the black hole?"
 
  • #4
tiny-tim
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hi pranathi! :wink:
"If we send a small bot, with sensor attached to it , radially towards the Blackhole, what would be the last thing that the sensor records before it perceives the blackness of the black hole?"
the last thing will be a photo of a point of light in the direction it came from (rather like the disappearing dot we used to have in really old television sets!)

oh, and don't call me Sir! … i'm only a little goldfish! :smile:
 
  • #5
Ok Tim ..... Thanks for it anyways and am new to pf ...... and this is my homework question but unable to relate your answer to my homework :( So can I post my question, here?
 
  • #6
An alien civilization has sent a small Spherical bot, made of Sterling Silver,
fitted with a light emitter and sensor as depicted below, towards a black hole
to explore the region around it. The bot moves slowly towards the black hole,
radially into it. The light source emits light in such a way that it
is tangential (at
the pt. of emission) to a circle with ‘centre of black hole’ as its
centre, and the
distance between the black hole and the centre of the bot as its radius. The
region around the black hole is free of dust or any other obstacle, and there
are no other light sources anywhere nearby which can be perceived by the
sensor.
Due to some malfunction, the sensor of the bot receives only those signals,
which fall onto it radially.
What will be the last thing that the sensor records before it perceives the
blackness of the black hole?
NOTE : The effect of gravity of any other bodies other than the black hole on
the bot is negligible
 
  • #7
My doubt is....when we dont have any obstacle or dust kind of thing around the black hole, wat can the sensor record?

The only light source in this question is the so called "light emitting area" ..... So what i thought is the last thing would be the multiple image of surrounding stars as the light bends around the Black hole....

Is it alright or am I going wrong anywhere??
 
  • #8
tiny-tim
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My doubt is....when we dont have any obstacle or dust kind of thing around the black hole, wat can the sensor record?
i agree with you … the question seems to be saying that the sensor can only record light from the probe itself …

and since there's no dust to reflect the light back, that presumably means that it doesn't see anything at all for most of the journey (except the distant stars)
The only light source in this question is the so called "light emitting area" ..... So what i thought is the last thing would be the multiple image of surrounding stars as the light bends around the Black hole....
what about the light from the probe bending?

(btw, in future please put your homework questions in the https://www.physicsforums.com/forumdisplay.php?f=152")
 
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  • #9
that presumably means that it doesn't see anything at all for most of the journey (except the distant stars)

Then is the answer multiple images of stars???


what about the light from the probe bending?


Dint get you tim.....what is probe bending?
n yeah i will post them in homework thread :)
 
  • #10
tiny-tim
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Dint get you tim.....what is probe bending?
the light bends!
 
  • #11
Yeah am talking about the light bending only....The multiple images of surrounding stars is due to the bending of light only !!
 
  • #12
tiny-tim
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i am talking about the light from the probe bending!
 
  • #13
aint getting you tim.....why would the probe bend? unable to understand such a scientific language !! :cry:
 
  • #14
tiny-tim
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the light bends …

the light (from the probe) bends :rolleyes:
 
  • #15
:( the light from the probe only should cause the multiple images of stars :O
 
  • #16
:( the light from the probe only should cause the multiple images of stars :O
Why do you believe that?
 
  • #17
Nismarath.....because there is no other source of light except that emitted by the bot in our present situation and so, the light from the bot should bend around the black hole, thus giving us multiple images of surrounding stars...... :(
 
  • #18
Nismarath.....because there is no other source of light except that emitted by the bot in our present situation and so, the light from the bot should bend around the black hole, thus giving us multiple images of surrounding stars...... :(
Would it be MULTIPLE stars, or just a kind of "light-smear" around the BH? I think it would be like seeing the light mapped on a torus... not actually that, but a similar effect.
 
  • #19
Um....not convinced.....:frown:. but still thanks for d replies tim n nismarath :smile:
 
  • #20
Um....not convinced.....:frown:. but still thanks for d replies tim n nismarath :smile:
I'm not sure either, so I don't blame you! :smile:
 

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