# Tag Info

### Event horizon is a null surface - what about the angular coordinates?

Two wrongs apparently do make a right. First, you’re using bad coordinates for the event horizon, and second, as a result, you divided by $0$ by plugging in $r=2m$. But, somehow you got that the ...
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### Event horizon is a null surface - what about the angular coordinates?

Before anything else, I should point out that Schwarzschild coordinates $t,r,\theta,\phi$ are not good to discuss the event horizon, as they are ill-defined there. You might prefer advanced Eddington–...
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### How do we know that a black hole radius is not significantly contracted for a stationary outside observer?

In this answer, I am ignoring optical effects and illusions of how a body would appear to look from a distance and focussing on actual measurements of length. Starting with the Schwarzschild metric, ...
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### Does an object approaching a black hole ever cross the combined event horizon of the black hole and itself?

I'm not completely certain of this but I think there's a fundamental flaw here in that real world objects always have size. No object can fly close enough to the black hole to lift the event horizon ...
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1 vote

### How do we know that a black hole radius is not significantly contracted for a stationary outside observer?

It is rather inaccurate to say that gravity contracts length in the same way as special relativity. Gravity is the curvature of spacetime as a whole which is a different concept from special ...
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### Does an object approaching a black hole ever cross the combined event horizon of the black hole and itself?

All of the following statements can be simultaneously true: An observer outside the event horizon will never observe a photon (or any other signal) emitted from inside the event horizon. As an ...

### What happens to the ring singularities when two Kerr black holes merge?

Roy Kerr's interest in black holes goes beyond the ringlike singularity which is presented in his mathematical model only as a placeholder for a progenitor physical object like a dead star or ...
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### Does an object approaching a black hole ever cross the combined event horizon of the black hole and itself?

In the animation below, every colored Kugelblitz shell has an energy equivalent of 1Mc², so together they have a total of 5Mc². The innermost shell will asymptotically, but never quite reach r→2GM/c², ...
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### What happens to the ring singularities when two Kerr black holes merge?

The ring singularity in Kerr is something that exists inside the inner horizon. Therefore the ring singularity exists only in the future of anyhting that happens outside of the black holes, including ...
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### Can you exit the event horizon with a rocket?

Absolutely not, mass can not exceed the speed of light, so if a rocket(made of mass) goes into the black holes event horizon it is too late, in fact at around 2r no object with mass can escape. Also, ...

### Does an object approaching a black hole ever cross the combined event horizon of the black hole and itself?

Once you start studying black holes, one of the first things you'll probably hear is that from an outsider's perspective objects falling into the black hole take an infinite time to do so due to time ...
• 11.9k
Accepted

### Does an object approaching a black hole ever cross the combined event horizon of the black hole and itself?

No, in the coordinate system of an external observer the infalling object never crosses the event horizon even allowing for the horizon growing outwards to meet the object. This is a rather arm waving ...
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### Understanding Time Dilation at the Event Horizon

Absolutely good question. And the answer is not complicated, and it's been written about in many places, but I'll summarise it here in very simple terms. First, what does infinite time dilation mean? ...
• 21
1 vote

### Singularity of a black hole: point or solid sphere?

Why do most people think of a black hole as having an infinitely dense point (the singularity) Einstein field equations describes gravitation of every body in the universe. Canonical solution to ...
• 13.9k
Accepted

### Singularity of a black hole: point or solid sphere?

It's all about time. The singularity (of a Schwarzschild black hole) is not a point in space, it is a spacelike surface in the future. Within the event horizon, spacetime has been warped in such a ...
1 vote

### Singularity of a black hole: point or solid sphere?

Why do most people think of a black hole as having an infinitely dense point (the singularity) when there is a clear possibility that the black hole could instead be caused by a highly dense sphere ...
• 21.3k

### Singularity of a black hole: point or solid sphere?

If the hypothetical compact object you're imagining isn't a point, but instead a sphere of radius $R < R_S$, then there must be some force counteracting gravity exactly when $r=R$, where $r$ is the ...
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### Singularity of a black hole: point or solid sphere?

Hritik RC asked: "Why do most people think of a black hole as having an infinitely dense point (the singularity) when there is a clear possibility that the black hole could instead be caused by a ...
• 11.9k
1 vote

### Singularity of a black hole: point or solid sphere?

You are mixing up the singularity with the black hole horizon. For simplicity, let's focus only on the nonrotating uncharged Black Hole. The standard coordinates (Scharzschild) break down at the event ...

### Is there anything actually inside a black hole, do they actually exist?

Conjecture 1: Black holes are empty. Argument, from the perspective of a distant observer: Black holes form in at least one way: Increasing mass or decreasing temperature causes increasing density at ...

### Could I peel a black hole or at least sneak a peek beyond the event horizon with compact matter?

could you peer beyond the theoretical event horizon [...]? No. You don't even need to check the details of the spacetime to answer this question, in fact. The definition of a black hole is (roughly) &...
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### Do black holes evaporate prior to crossing their event horizon?

Let's now take into consideration that at some point in the future, the black hole evaporates due to Hawking radiation. From Alice's point of view, Bob never passes the event horizon. This last ...
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### Electrons keeping dynamical quantum fluctuations?

There is no assumption of proton decay in that article. Moreover, the time scale for proton decay (if it is finite at all) is known to be much longer than the time scales discussed in that article. So ...
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1 vote

### Is there any observational evidence for the existence of Schwarzschild black holes?

We haven't even observed a nonspinning star or planet, and since there are much more of those than black holes the likelyhood for a nonspinning black hole is even smaller. You could create one ...
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### Can any object pass into the event horizon of a black hole and then escape?

Hawking radiation is not escaping the black hole. Instead, the effect of this apparent "escaping" is due to two random particles appearing, one extremely close to the event horizon of the ...

### Does someone falling into a spinning black hole see the end of the universe?

I don't think this can be the case. Here is some reasoning why: You and a friend agree to rocket yourselves into a black hole one year apart. You both travel the same trajectory at the same speed from ...
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