Black holes and the event horizon

In summary: From that, we can infer the presence of the black hole and its mass. In summary, the image of an object that passes the event horizon remains on the horizon, but becomes more redshifted due to time dilation and the inability to discern details smaller than the black hole itself. Black holes are too small to be directly observed and are generally detected through the presence of an accretion disk or the orbit of surrounding stars.
  • #1
thedude36
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I'm curious - once an object passes the event horizon the image of that object remains on the event horizon only to become more redshifted rather than dissipating. two questions: 1) why does the image remain if the light stops traveling? if the light cannot travel to the observer, than there would be no 'image' in the classical sense. and 2) if the image is seen, why can we not spot black holes merely by looking for clusters of objects that have ceased moving relative to one another?
 
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  • #2
As an object approaches the event horizon it also becomes [to an external observer] time dilated, hence, appears to take literally forever to actually reach the event horizon. Its light also approaches infinite redshift.
 
  • #3
Chronos said:
As an object approaches the event horizon it also becomes [to an external observer] time dilated, hence, appears to take literally forever to actually reach the event horizon. Its light also approaches infinite redshift.

Note: to avoid confusion, I am not the OP. I've been bothered about this for a while. Shouldn't Black Holes fail to form, since they'll take literally forever to collapse to the Event Horizon? (To an external observer, of course.)
 
  • #4
A couple of points in addition to what Chronos said.
thedude36 said:
I'm curious - once an object passes the event horizon the image of that object remains on the event horizon only to become more redshifted rather than dissipating. two questions: 1) why does the image remain if the light stops traveling? if the light cannot travel to the observer, than there would be no 'image' in the classical sense.
Allow me to explain something from an completely idealized, theoretical case. Consider something falling into a large, non-rotating black hole, and the black hole is far more massive than the object. Assume that we, the observers, are far away (such as here on Earth). From our frame of reference (not moving at relativistic speeds, relative to the black hole), the thing that we observe hanging around near the horizon --approaching the orizon but never quite getting there -- is the object itself. It's not just an 'image'. It really is the object. And this is based on theoretical grounds which can be shown with the mathematics of relativity.

By the same mathematics of relativity, in the frame of reference of the falling object, it passes through the horizon in a very uneventful way (this assumes that the black hole is very large, so we can ignore tidal forces). If a person fell through the event horizon, they wouldn't notice anything particularly special at the moment they crossed the horizon.

This might seem like a paradox, but it's really not. "But which scenario is really the 'true' scenario?" isn't the question that's worthwhile asking. Both are correct. It just depends on one's frame of reference.

-------------

Moving on, that incredible red-shift, that we see from our frame of reference, is more important than what it might seem. The effective resolution of an object that can be observed is dependent on the wavelength of light involved. It's not possible to observe any details on/of an object smaller than around 1 wavelength. This incredible red-shift caused by the black hole will eventually cause the wavelength of the light emitted by the falling object to become as large as the black hole itself. At that point, no more detail can be obtained (from our position/frame of reference) about the object than the fact that the object is somewhere near the black hole's horizon. But nothing else; all other details smaller than the black hole itself (that we can "see" e.g. with a radio telescope) are gone. (Edit: rather than say "gone," maybe I should just say "hidden from our detection.") The wavelength of light is just to large to discern details smaller than a black hole.

That by the way is still described from a theoretical point of view. I'll talk about some practical aspects in a moment.

2) if the image is seen, why can we not spot black holes merely by looking for clusters of objects that have ceased moving relative to one another?
Black holes are really pretty small objects. If (hypothetically speaking) our sun formed a black hole, it would only be about 6 km wide. A super-massive black hole at the center of the galaxy is still just a tiny, tiny speck compared to the overall central galactic bulge. So even large black holes are far too small to "see" directly with telescopes (even if they weren't 'black').

Now to more practical aspects. When matter falls into a black hole, it is likely to get compressed and scattered with other infalling matter well before it approaches the horizon. Together the matter forms what's called an accretion disk, which orbits the black hole. Due to the high pressure and high kinetic energy, such accretion disks can give off x-rays and high energy particles. When the accretion disk is giving off detectable amounts of energy, we call the black hole "active."

Another way to detect a black hole, particularly the super-massive black hole at the center of our galaxy, it to track the path of the stars that orbit it. We can't see the black hole, but we can see the light from the orbiting stars.
 
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  • #5


I can provide some insight on these questions about black holes and the event horizon. First, it is important to understand that the event horizon is the point of no return for objects entering a black hole. Once an object passes this point, the gravitational pull of the black hole becomes so strong that even light cannot escape. This means that any light emitted from the object beyond the event horizon cannot reach an outside observer, which is why the image of the object appears to remain on the event horizon.

To answer your first question, the image of the object remains on the event horizon because of the phenomenon of gravitational lensing. This is when the intense gravitational pull of the black hole bends the path of light, causing it to curve around the black hole and create a distorted image of the object. This image will become more and more redshifted as the object gets closer to the event horizon, but it will not dissipate because the light is still being bent and trapped by the strong gravitational pull.

As for your second question, it is not possible to spot black holes merely by looking for clusters of objects that have ceased moving relative to one another. This is because not all objects that have ceased moving relative to one another are black holes. There are many other factors that could cause objects to appear stationary, such as gravitational interactions with other objects or being in a stable orbit. Additionally, black holes are not always surrounded by visible matter that we can observe, making them difficult to detect using traditional methods.

In conclusion, the image of an object appears to remain on the event horizon of a black hole due to gravitational lensing, and looking for clusters of stationary objects is not a reliable method for detecting black holes. The study of black holes is a complex and ongoing area of research, and there is still much to learn about these mysterious and fascinating objects in our universe.
 

1. What is a black hole?

A black hole is a region in space where the gravitational pull is so strong that nothing, including light, can escape from it. This occurs when a massive star runs out of fuel and collapses under its own gravity.

2. How is the event horizon related to black holes?

The event horizon is the point of no return for anything that gets too close to a black hole. It is the boundary where the gravitational pull becomes so intense that even light cannot escape from it. Beyond the event horizon, the escape velocity exceeds the speed of light.

3. Can we see a black hole?

No, we cannot see a black hole directly because it absorbs all light that enters it. However, we can indirectly observe black holes by detecting their effects on nearby matter and light.

4. What happens if something falls into a black hole?

If something falls into a black hole, it is stretched and compressed by the intense gravitational forces until it reaches the singularity at the center of the black hole. At this point, the object's mass is added to the black hole's mass, making it even more massive.

5. Can a black hole destroy the entire universe?

No, a black hole cannot destroy the entire universe. While it has a strong gravitational pull, it only affects objects in its immediate vicinity. The universe is vast, and there are many other forces at play that prevent a black hole from destroying everything.

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