If I were to observe a black hole....

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what would I see?
If I was looking at a black hole head on (from outside its Schwarzschild radius), would I be able to discern it? Would the stars etc from behind the hole be gravitationally lensed as to "obscure" the hole?
Would any of this change when positioned ON, or within, its Schwarzschild radius?

Excuse the ignorance on my end, I realize it might be a stupid question for the initiated. I am really curious though :)
 

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  • #2
BvU
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Hello PG, :welcome: !

There's a ton of stuff waiting to be looked at if you google 'picture of a black hole'

I think we had a thread on this one in PF. It's beautiful, but the explanation isn't for the faint-hearted ...
 
  • #3
ChemAir
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Veritasium had a pretty good description of the images from earlier this year and why they look that way.

 
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  • #4
PG1
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Thanks both! I have seen the image of that hole and I sort of get why it looks like that.

But let's imagine an ancient bh without a disk and me being an observer outside its radius, would the gravitational lensing cause the stars etc behind it to "appear in front of the hole"(Effectively making it indiscernable)? Or would there be some "artifacts" from the lensing that would make it "visible"?

Iam sorry if I make it confusing, having a hard time posing it properly (english isn't my 1st or 2nd language)
 
  • #5
BvU
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Google is your friend. Go out and explore !
This one continues with the one we've already seen
This one shows you don't see the hole, but the artefacts are enormous
 
  • #6
Orodruin
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This one shows you don't see the hole, but the artefacts are enormous
You would in fact see a black disk in the center. The optical size of this disk would have a radius of a few times the Schwarzschild radius (it is an exact multiple of the Schwarzschild radius that I do not remember by heart, but it is relatively easily computable).
 
  • #7
Ibix
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You would in fact see a black disk in the center. The optical size of this disk would have a radius of a few times the Schwarzschild radius (it is an exact multiple of the Schwarzschild radius that I do not remember by heart, but it is relatively easily computable).
Are you sure? Light grazing 3/2 the Schwarzschild radius must fall in (all free-fall orbits crossing that radius fall in), so this sphere is black. But the angle subtended by rays grazing that sphere seems unlikely to depend trivially on distance - although I'd need a computer to confirm that.
 
  • #8
PG1
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Google is your friend. Go out and explore !..
I am sorry, and I don't want to be cross with anyone but.. This is a forum about these kinds of discussions is it not? I realize the question might be childish or something to you, but just pointing to google defeats the entire purpose of having this platform imho.
Tnx Orodruin and Ibix, that seems to be what I was trying to ask, albeit somewhat clumsely
 
  • #9
phinds
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I am sorry, and I don't want to be cross with anyone but.. This is a forum about these kinds of discussions is it not? I realize the question might be childish or something to you, but just pointing to google defeats the entire purpose of having this platform imho.
This is not a Q&A forum where you just ask a question and get an answer. We expect people to have done at least some basic research before asking questions. Our goal is not to spoon feed answers but to help people learn how to GET answers on their own and then come here when something in their research is not clear.
 
  • #10
PG1
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The answer that you find so obvious has two other gurus of these forum in a discussion of what is the case (Orodruin and Ibix) so asserting its "basic knowledge" seems moot. I won't bother you anymore though, you want me out, iam out.
 
  • #11
Ibix
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Just to add, outside the black disc you will see gravitational lensing effects, extreme enough that you will be able to see the same star on both sides of the black hole.
 
  • #12
BvU
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You would in fact see a black disk in the center. The optical size of this disk would have a radius of a few times the Schwarzschild radius (it is an exact multiple of the Schwarzschild radius that I do not remember by heart, but it is relatively easily computable).
Yes. It's mentioned in the video in #3.
The answer that you find so obvious has two other gurus of these forum in a discussion of what is the case (Orodruin and Ibix) so asserting its "basic knowledge" seems moot. I won't bother you anymore though, you want me out, iam out.
Even if I only had typed the first sentence in #5 it should be clear to you that I am trying to help: I looked at the videos and found them helpful. Instead you choose to be offended -- have it your way.
 
  • #13
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The answer that you find so obvious has two other gurus of these forum in a discussion of what is the case (Orodruin and Ibix)

They're not discussing "what is the case" as far as the answer to your question: they both agree on this:

You would in fact see a black disk in the center.

Which means the answer to your original question is "yes". They are only discussing the exact optical size of the black disk.

you want me out

Nobody has said any such thing.
 
  • #14
Ibix
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Indeed - I don't think I'm disagreeing with Orodruin's answer except for the angular size of the black area. And we probably disagree on language rather than physics.

There are a few plots of light rays near black holes near the end of this thread, which may be of interest.
 
  • #15
PG1
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There are a few plots of light rays near black holes near the end of this thread, which may be of interest.

Thank you very much, that is very helpfull indeed.

ps. (very long)
Please understand not eveyone is in a position to discuss these things on a campus, with friend or at work. I work at a chemical plant as a shift supervisor and there are plenty of engineers and chemical engineers to talk about every day physics/chem. I have a family and some friends, none of whom are even remotely interested in these things. I have followed a lot of online material from lectures to "mainstream physics" like CBSst.
Point is: There is ALOT of material out there, but unless there are people willing to explain/go into some nuance and have a discussion it is very very difficult to self-educate oneself in these topics. Sometimes all one needs is just a little nudge.
I apologize for my crossness, but you don't know half of how lucky you guys are living/studying these field in an environment of likeminded people. I am not that lucky, and hoped to find some here. That all.
 
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  • #16
BvU
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Thanks for sharing that with us -- and I work in a chemical company too :smile:
 
  • #17
phinds
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I am not that lucky, and hoped to find some here. That all.
And indeed you will. No one, myself included, intended any rudeness towards you. You are probably more accustomed to traditional social media where insults and flames are rampant and it is sometimes reasonable to infer that rudeness is intended. We work pretty hard here to NOT be rude to individuals, but ideas are another thing entirely. This next does not apply to you, I'm just making you aware: when ideas that don't make sense crop up, folks are quick to point out the flaws, but the intent is never to rude to the poster, just to lead them to understanding. Likewise, and this does apply to you, when people seem to have made little or no effort on their own, this is often pointed out. Again, no rudeness is intended.

So, I still mean what I said about doing some research on your own. You certainly do raise the good point that there is just so MUCH stuff out there that it can be difficult. Still the effort should be made.

I, like you, have absolutely no one that I talk to in person about science or math. I took college physics over 50 years ago and only a few years back decided to start studying cosmology and quantum mechanics, at the amateur level. I watched TONS of TV shows and read LOTS of pop-science books and then discovered this place and found out that a large amount of what I "knew" was totally wrong because pop-sci presentation are entertainment, not education. HERE, I actually learned a bit of physics and you will to if you stick around.
 
  • #18
PG1
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tnx both! Well I was just a little frustrated (and acted that out abit too much :) ) to be pointed back to the only place I had, and one that I can never have a discussion with. I am happy too be here

@BvU we make metal alkyls, not many of us around, pretty interesting stuff with some nice challenges safety wise.
 
  • #19
willem2
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You would in fact see a black disk in the center. The optical size of this disk would have a radius of a few times the Schwarzschild radius (it is an exact multiple of the Schwarzschild radius that I do not remember by heart, but it is relatively easily computable).
This might be valid if you are relatively far away. If you get closer, I don't even know what 'optical size'even means.
If you are at the photon sphere, the black disk will take up exactly half of the sky. Any light that started outside the photon sphere must come to you from a direction outside the photon sphere, because if it ever went into the photon sphere it would never get out again.
This means that any light that comes from a direction inside the photon sphere must have started inside the photon sphere, so you can only see light generated inside the photon sphere in this direction. And there won't be much of it, because you can't have orbits inside the photon sphere, so everything that gets inside the photon sphere will quickly end up in the black hole. The accretion disk would be further away.

If you get close to the Schwarzschild radius all the light from outside will come from a narrow cone, pointing away from the black hole.
 
  • #20
Orodruin
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This might be valid if you are relatively far away.
It is (of course) in the limit of being much further away than the Schwarzschild radius. Finding this optical size is just a matter of backtracing the lightlike geodesics and finding the impact parameter for which they start in the black hole.
 

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