<p>My understanding is that any object with non-zero angular momentum which undergoes gravitational collapse to a black hole will become a rotating black hole. In reality all black holes will be rotating black holes because all objects undergoing gravitational collapse will have <strong>some</strong> angular momentum (even if it is relatively small).
I understand that the inner structure of a rotating back hole contains:its boundary i.e the event horizon, the “inner horizon” and the singularity which is ring-shaped. Once an object crosses the event horizon it has passed the point of no return and the singularity is a one-dimensional ring of infinite density. However,  I’ve been unable to locate  an article explaining exactly what the <strong>inner horizon</strong> is which is understandable to someone who has some mathematical background, but without a degree-level knowledge. For example, entering “Inner Horizon” into Wikipedia redirects to the following:
<a href="https://en.wikipedia.org/wiki/Cauchy_surface#Cauchy_horizon" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/Cauchy_surface#Cauchy_horizon</a></p>
<p>Which with my lack of detailed mathematical background is incomprehensible!</p>
<p>I have found that many of the popular science articles about black holes make statements which are unsatisfactory and  and raise a lot of other questions .
E.g. <a href="https://www.discovermagazine.com/the-sciences/peering-inside-realistic-black-holes" rel="nofollow noreferrer">https://www.discovermagazine.com/the-sciences/peering-inside-realistic-black-holes</a> states</p>
<p><em>“To visualize the inner horizon, consider this thought experiment: Imagine you could enter a spinning black hole and cross the event horizon while holding a flashlight that’s pointed outward. That light would eventually freeze along, or within, a spherical surface. That’s the inner horizon, ….. the point where the black hole’s gravity is strong enough to keep light from leaving, but not strong enough to drag it all the way into the center. The trapped light becomes so energized that it turns into a firewall of radiation, incinerating anything that passes through it.”</em></p>
<p>Does anyone know of an article explaining exactly what is the inner horizon is to the educated layman? Or would be able to explain themselves  by answering this question? I would like to know what happens when an object crosses the inner horizon and WHY?</p>
<p><a href="https://i.sstatic.net/jeSlS.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/jeSlS.png" alt="Image from Wikipedia" /></a></p>

<black-hole>
What is the Inner Horizon of a rotating black hole?
John Davies
# What is the Inner Horizon of a rotating black hole?

(Asked by: username_0 on 2021-04-30)

My understanding is that any object with non-zero angular momentum which undergoes gravitational collapse to a black hole will become a rotating black hole. In reality all black holes will be rotating black holes because all objects undergoing gravitational collapse will have **some** angular momentum (even if it is relatively small).
I understand that the inner structure of a rotating back hole contains:its boundary i.e the event horizon, the “inner horizon” and the singularity which is ring-shaped. Once an object crosses the event horizon it has passed the point of no return and the singularity is a one-dimensional ring of infinite density. However, I’ve been unable to locate an article explaining exactly what the **inner horizon** is which is understandable to someone who has some mathematical background, but without a degree-level knowledge. For example, entering “Inner Horizon” into Wikipedia redirects to the following:
<https://en.wikipedia.org/wiki/Cauchy_surface#Cauchy_horizon>

Which with my lack of detailed mathematical background is incomprehensible!

I have found that many of the popular science articles about black holes make statements which are unsatisfactory and and raise a lot of other questions .
E.g. <https://www.discovermagazine.com/the-sciences/peering-inside-realistic-black-holes> states

*“To visualize the inner horizon, consider this thought experiment: Imagine you could enter a spinning black hole and cross the event horizon while holding a flashlight that’s pointed outward. That light would eventually freeze along, or within, a spherical surface. That’s the inner horizon, ….. the point where the black hole’s gravity is strong enough to keep light from leaving, but not strong enough to drag it all the way into the center. The trapped light becomes so energized that it turns into a firewall of radiation, incinerating anything that passes through it.”*

Does anyone know of an article explaining exactly what is the inner horizon is to the educated layman? Or would be able to explain themselves by answering this question? I would like to know what happens when an object crosses the inner horizon and WHY?

[![Image from Wikipedia](https://i.sstatic.net/jeSlS.png)](https://i.sstatic.net/jeSlS.png)

**Tags:** black-hole


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## 0 answers

