Black Holes and the Event Horizon?

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Discussion Overview

The discussion revolves around the nature of black holes and their event horizons, exploring questions about detection, the relationship between the event horizon and the accretion disk, and the implications of approaching a black hole from different angles. Participants also seek resources for further reading on these topics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Homework-related

Main Points Raised

  • Some participants question whether a black hole is detected by its event horizon and how the event horizon is positioned relative to a star's equatorial plane.
  • There is a description of the event horizon as a spherical boundary surrounding a black hole, separating regions from which photons can escape and those from which they cannot.
  • Some participants express confusion about the distinction between the event horizon and the accretion disk, with clarification that they are indeed different entities, with the accretion disk lying outside the event horizon.
  • One participant raises a question about whether the accretion disk is spherical or exists along the equatorial plane of the black hole, leading to a clarification that the accretion disk is a disk, not a sphere.
  • There is a discussion about the possibility of observing a black hole while remaining outside the accretion disk, with some participants noting that the accretion disk is observable from a safe distance.
  • Participants share recommendations for books and resources related to black holes and Hawking radiation.

Areas of Agreement / Disagreement

Participants generally agree on the distinction between the event horizon and the accretion disk, but there are unresolved questions regarding the detection of black holes and the implications of approaching them from different angles. The discussion remains open with multiple viewpoints expressed.

Contextual Notes

Some claims about the nature of black holes and their components depend on specific definitions and assumptions that are not fully explored in the discussion. There are also unresolved questions about the observational capabilities related to black holes and their event horizons.

Who May Find This Useful

This discussion may be useful for individuals interested in astrophysics, particularly those seeking to understand black holes, their properties, and the theoretical frameworks surrounding them.

TDS
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My questions are these: When a Black Hole is detected, Is it found by its Event Horizon? Is the Event Horizon located in relation to the stars equitorial plane? If I were to approach a Black Hole from its "North Pole" or "South Pole", would I be affected by its Event Horizon?
 
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TDS said:
My questions are these: When a Black Hole is detected, Is it found by its Event Horizon? Is the Event Horizon located in relation to the stars equitorial plane? If I were to approach a Black Hole from its "North Pole" or "South Pole", would I be affected by its Event Horizon?
The event horizon is a sphere surrounding the center of a black hole.
 
Consider the region of spacetime from which it possible for a photon to escape to (future null) infinity. Now consider he complement of this region, i.e., a region of spacetime from which it is not possible for a photon to escape to infinity. This latter region is the black hole region.

The boundary between the two regions is the event horizon.

Unfortunately, there are no warning makers that tell you when you cross an event horizon.
 
I had not though about the Event Horizon as being spherical! What a concept!

Would you happen to know of any books or links where I could read more about this!


MeJennifer said:
The event horizon is a sphere surrounding the center of a black hole.
 
So is it safe to say that the Accretion Disk and the Event Horizon are two different things? I had thought that they were the same thing.


George Jones said:
Consider the region of spacetime from which it possible for a photon to escape to (future null) infinity. Now consider he complement of this region, i.e., a region of spacetime from which it is not possible for a photon to escape to infinity. This latter region is the black hole region.

The boundary between the two regions is the event horizon.

Unfortunately, there are no warning makers that tell you when you cross an event horizon.
 
TDS said:
IWould you happen to know of any books or links where I could read more about this!

In my opinion, https://www.amazon.com/dp/0316483419/?tag=pfamazon01-20 has the best popular-level treatrment of black holes - I can't recommend it highly enough. Great diagrams.
 
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TDS said:
So is it safe to say that the Accretion Disk and the Event Horizon are two different things? I had thought that they were the same thing.

Yes, they are different. The accretion disk lies outside the event horizon.

Sorry, got to run! I'll try and add some more details tonight.

Are asking about evidence for an event horizon that can be seen from afar, or evidence as you cross an event horizon?
 
George Jones,

Thanks for this link! My home library has just gained mass!:biggrin:
If you have more links or suggestions, feel free to send them to me. The more that I learn - the better I will be!


George Jones said:
In my opinion, https://www.amazon.com/dp/0316483419/?tag=pfamazon01-20 has the best popular-level treatrment of black holes - I can't recommend it highly enough. Great diagrams.
 
Last edited by a moderator:
Any information that you can provide on both will be more than I have at the moment. Basically what I am trying to understand is this; everytime you see a depiction of a Black Hole, you see a ring of "light" that signifies the event horizon around the Black Hole. And in the same breath the narrator starts talking about the accretion disk. I would like to understand the difference. As for your questions - both.



George Jones said:
Yes, they are different. The accretion disk lies outside the event horizon.

Sorry, got to run! I'll try and add some more details tonight.

Are asking about evidence for an event horizon that can be seen from afar, or evidence as you cross an event horizon?
 
  • #11
TDS said:
Any information that you can provide on both will be more than I have at the moment. Basically what I am trying to understand is this; everytime you see a depiction of a Black Hole, you see a ring of "light" that signifies the event horizon around the Black Hole. And in the same breath the narrator starts talking about the accretion disk. I would like to understand the difference. As for your questions - both.

Accretion disk are swirling matters being pulled into the black hole. They glow as the result of being heated up by the friction. The event horizon is a spherical boundary which you can't escape once you get passed. You can't see the event horizon, at least not directly. If you have a black hole sitting around a region with not much matter, there will be no accretion disk. But the event horizon is always there.
 
  • #12
@pervect,

Thanks for the book link! There goes the mass of my bookshelf again!

@yenchin,

Thanks for clearing up my questions about the Event Horizon and the Accretion Disk!

Now for the really big question. Since the Event Horizon is spherical, is the Accretion Disk also spherical or does it exhist along the equitorial plane of the Black Hole?
 
  • #13
TDS said:
Now for the really big question. Since the Event Horizon is spherical, is the Accretion Disk also spherical or does it exhist along the equitorial plane of the Black Hole?

No, the acretion disk is a disk, i.e, it lies in a plane, for roughly the same reason that the orbits of all the planets lie in a plane.

For accretions disks, I can do no better than to quote from Gravity: An Introduction to Einstein's General Relativity, by James Hartle.

Hartle said:
Consider for example, a black hole or neutron star in mutual orbit with a more normal companion star - one like the Sun for instance. The binary pair can lose orbit energy - by gravitational radiation among among other mechanisms - decreasing the size of the orbit. The orbit can bcome small enought that the outermost layer of the companion is more strongly attracted to the compact object than to its own center. In that case the more normal star will shed mass, which will fall (accrete) onto the compact object. Conservation of its initial orbital angular momentum means that the accreting material does not fall directly onto the compact object but rather forms a disk around it called an accretion disk. Various dissipative mechanisms associated with interactions between the particles in the disk cause them to slowly lose energy and angular momentum and gradually spiral towards the object. They spiral slowly inward on nearly circular orbits until they reach the innermost stable circular orbit, after which they fall rapidly into the compact object. The energy they lose leaves the disk as radiation - characteristically at X-ray wavelengths for compact objects around a solar mass.
 
  • #14
First let me say THANKS to all that has posted answers to my questions on this thread!:biggrin:

I have received more answers in two days on this site than I did in my first semester of Astronomy! This is a good thing for you folks, a sad thing for the school that I went to.

Now for another question.

Would it be possible to observe a Black Hole so long as the probe stayed outside of the Accretion Disk?

I know that the only thing that could actually be observed would be the Accretion Disk unless the probe had a device to detect Hawking Radiation.
 
  • #15
Yes. If you stay sufficiently far away at a safe distance, you can observe the accretion disk. Hawking radiation is too weak to be detected as it is many times weaker than the microwave background radiation.
 
  • #17
@chronon,

My thanks goes out to you as well! If this keeps up I shall need to get another bookshelf!:biggrin:


chronon said:
I've a list of books related to black holes (and in particular Hawking Radiation) at Black Hole Evaporation - useful books
 

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