# Confused about the event horizon

• Paul_Bartosik
In summary, the conversation discussed the behavior of time at the event horizon of a black hole, with the question of whether objects actually fall into the black hole or just accrete at the event horizon. It was mentioned that the coordinate system breaks down at the event horizon and that the Eddington-Finkelstein, Painleve-Gullstrand, and Kruskal-Szekeres coordinate systems cover the event horizon. It was also suggested to read "Gravitation" by Misner Thorne and Wheeler or "General relativity from A to B" by Geroch for a basic introduction to General Relativity.

#### Paul_Bartosik

I have recently finished "Gravity from the Ground Up: An Introductory Guide to Gravity and General Relativity".

Great book, but I am confused about the behavior of time at the event horizon of a black hole. I spent some time looking at existing threads on this site, but was unable to find clarification.

I read repeatedly, that from the perspective of an outside observer, time will stop at the event horizon. So if I am watching something fall into a black hole, it will seem that it stops at the brink of the precipice and I will not actually see it cross into the abyss.

I understand that this statement is not fully true; I would not see the something frozen in time. Instead, I would see the red shift of any light to longer and longer wavelengths.

My question: Time will stop at the event horizon, so from the perspective of the outside observer, nothing actually ever falls into the black hole. Does it all just accrete at the event horizon?

Welcome to PF!

Hi Paul! Welcome to PF!
Paul_Bartosik said:
I have recently finished "Gravity from the Ground Up: An Introductory Guide to Gravity and General Relativity".

Great book, but I am confused about the behavior of time at the event horizon of a black hole.

My question: Time will stop at the event horizon, so from the perspective of the outside observer, nothing actually ever falls into the black hole. Does it all just accrete at the event horizon?

Time doesn't stop at the event horizon, it's just that our coordinate system breaks down there.

Yes, in our coordinate system, the speed of an infalling object gets slower and slower.

(btw, this has nothing to do with the accretion disc of a black hole , which is at radius 6M, three times as far out as the event horizon).

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Great. Thanks for the quick response.

If "our coordinate system breaks down" at the event horizon, what coordinate system does apply?

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Paul_Bartosik said:
Great. Thanks for the quick response.

If "our coordinate system breaks down" at the event horizon, what coordinate system does apply?

The Eddington-Finkelstein, Painleve-Gullstrand, and Kruskal-Szekeres coordinate systems all cover the event horizon of a Schwarzschild black hole.

I'm going to go out on a limb here and suggest perhaps tiny-tim meant that the coordinate time (time measured by an observer at infinity) is what gives this whole "time stopping at the horizon" business. For an infalling observer, he will experience nothing of the sort.

George Jones said:
The Eddington-Finkelstein, Painleve-Gullstrand, and Kruskal-Szekeres coordinate systems all cover the event horizon of a Schwarzschild black hole.

Great. That is what I was afraid of. I am not so good at calculus, and tensor calculus is way beyond my meager skills.

Thanks for the responses.

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tiny-tim said:
You might like to try "Gravitation" by Misner Thorne and Wheeler, or the limited http://books.google.com/books?id=Uk...roch+"General+Relativity"&client=safari&cd=1" of "General relativity from A to B‎" by Geroch.

The second book looks great. I will buy a copy from Barnes and Noble. It looks like it will give a basic introduction of why General Relativity was needed to give gravity a consistant behavior across multiple frames of reference.

Thank you for the guidance.

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## 1. What is the event horizon?

The event horizon is the boundary around a black hole where the gravitational pull is so strong that nothing, not even light, can escape from it.

## 2. How does the event horizon form?

The event horizon is formed when a massive star dies and collapses in on itself, creating a singularity with an extremely strong gravitational pull.

## 3. Why is the event horizon significant?

The event horizon is significant because it marks the point of no return for anything that gets too close to a black hole. Once something passes the event horizon, it is impossible for it to escape.

## 4. Can anything escape from the event horizon?

No, once something passes the event horizon, it is trapped by the black hole's intense gravity and cannot escape. However, some particles can escape through quantum processes called Hawking radiation.

## 5. How is the event horizon related to the size of a black hole?

The event horizon is directly related to the size of a black hole. The more massive a black hole is, the larger its event horizon will be. A smaller black hole will have a smaller event horizon, but it will still have the same intense gravitational pull.