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69 votes

If two black hole event horizons overlap (touch) can they ever separate again?

You have already got some good answers, but I'll just try to provide one more intuitive solution on why the event horizons will never separate again if overlapping each other: First, imagine a speck ...
Mads Aggerholm's user avatar
56 votes

If two black hole event horizons overlap (touch) can they ever separate again?

If the event horizons ever touch and become one continuous surface, their fate is sealed - the two black holes will merge all the way in. They can never separate again, no matter what. There are ...
Florin Andrei's user avatar
28 votes

Can an entire star pass through the event horizon of a black hole unharmed?

In order to survive, the star's self-gravitation must be larger than the tidal stretching forces provided by the black hole. If not, then the star will get spaghettified before it crosses the event ...
ProfRob's user avatar
  • 153k
23 votes

From an outsider's perspective, how can a black hole grow if nothing ever crosses the event horizon?

First off, there's no real time dilation effect. In coordinate systems that cover the event horizon (Kruskal-Szekeres, Eddington-Finkelstein, Gullstrand-Painlevé, Kerr-Schild), objects fall through it ...
benrg's user avatar
  • 3,862
14 votes
Accepted

What is a "Gentle Giant" Black Hole?

There's no "authoritative" definition, this is just the common meaning of the word "gentle" used in a popular science and science fiction context. A black hole might be gentle ...
James K's user avatar
  • 123k
13 votes
Accepted

Is there a closest free-return trajectory to a black hole?

The trajectory of a ballistic body, whether in Newtonian or Relativistic physics depends on the initial energy and angular momentum. The difference is that in Newtonian physics, if the mass is compact ...
ProfRob's user avatar
  • 153k
12 votes
Accepted

Is the event horizon the defining characteristic of a black hole?

All black holes have an event horizon, but not all event horizons are associated with a black hole. If you fly on a rocket that accelerates at a constant rate, your path follows a hyperbola that never ...
Nullius in Verba's user avatar
12 votes

From an outsider's perspective, how can a black hole grow if nothing ever crosses the event horizon?

Due to time dilation, an outside observer never sees a falling object actually cross the event horizon. This is correct, but perhaps not quite in the way that you visualize it. It's trivially true, ...
Ilmari Karonen's user avatar
10 votes

Would a future hole in the Cosmic Microwave Background show us the edge of the universe?

First, let me clear up a misunderstanding: Particle horizon The "edge" of the observable Universe is called the particle horizon, and lies roughly 47 Gly (billion lightyears) away. It is ...
pela's user avatar
  • 38.5k
10 votes
Accepted

Whats the deal with black holes and "no information from inside the event horizon can leave"?

You are right in that, if you could send a message of any kind from inside a black hole then there is no magic required rebuild you from the information about you. If you could send even a single bit ...
James K's user avatar
  • 123k
8 votes

How does time work beyond the cosmic event horizon?

First, let's clear up a few misconceptions: The Hubble sphere The speed of light as an upper limit is valid in special relativity (SR). In general relativity (GR), which must be used to describe the ...
pela's user avatar
  • 38.5k
8 votes

Is the event horizon the defining characteristic of a black hole?

A black hole is a region of spacetime that nothing can escape from. There is no direction that you can travel in that leads from inside the black hole to a point outside the black hole. The event ...
James K's user avatar
  • 123k
7 votes

Shouldn't we not be able to see some black holes?

Well it doesn't exactly work the way you describe it. Matter doesn't just fall into a black hole. A black hole still has a finite mass, which means other matter orbits its just like it would a ...
Jeroen 's user avatar
7 votes
Accepted

Is it possible to throw a camera attached to a rope within the event-horizon of a black hole and then pull it back in?

According to the observer holding the rope, the camera would never make it past the event horizon. It would appear to slow down to a standstill just above the horizon (though would effectively ...
ProfRob's user avatar
  • 153k
7 votes

Is the event horizon the defining characteristic of a black hole?

Is the event horizon the defining characteristic of a black hole? Yes, it's the only widely accepted definition. This definition has the disadvantage that it's nonlocal. That is, it defines a black ...
user42660's user avatar
6 votes
Accepted

Why do "they" portray colliding black holes like that?

The event horizon is not a physical structure. Rather it is the boundary of a region of spacetime from which no information can escape. If you were (unfortunate enough to be) between two black holes, ...
James K's user avatar
  • 123k
6 votes

How do we know that Singularity is not an object with reasonable radius?

We don't know, but general relativity implies that no force can stop the object from shrinking to infinitesimally small. Quantum Mechanics is more uncertain. I just want to point out that 10 solar ...
userLTK's user avatar
  • 24.1k
6 votes
Accepted

Could You Escape an Event Horizon by increasing a Black Hole's velocity?

That is not how velocity works. Even without getting into general relativity, if you move away from a black hole at velocity $v_1$ and at the same time the black hole moves away at velocity $v_2$ (...
James K's user avatar
  • 123k
5 votes
Accepted

Reshaping black holes to "push" the event horizon

The only places around one black hole where gravity would be strong enough to pull an object out from inside the horizon of another black hole are all inside the horizon of the first hole. Remember ...
Asher's user avatar
  • 436
4 votes

If a black hole has a mass similar to a star, why does it have a different gravitational pull?

Actually, it doesn't. If a black hole and a star have the same mass, their gravity is the same. As the formula for the attraction (gravity) between two objects is given as $F=\dfrac{GMm}{r^2}$, where $...
WarpPrime's user avatar
  • 6,673
4 votes
Accepted

Is a black hole's gravity strongest at its event horizon or at least near it?

The strength of the gravity of the black hole, that is the curvature of spacetime increases beyond the event horizon and tends to infinite when you approach the singularity. It is very hard to ...
James K's user avatar
  • 123k
4 votes

Is the event horizon the defining characteristic of a black hole?

I was wondering about the characteristics of a black hole, and I know they have mass, spin, and an insanely strong gravitational force. But, as far as I know, basically all black holes possess the ...
Daddy Kropotkin's user avatar
3 votes

How does Hawking Radiation work exactly?

... but how does the other know how to take energy from the black hole? In order to understand this, you need to be familiar with the essence of this picture$^1$ including negative energy states ...
Ad Astra's user avatar
  • 258
3 votes
Accepted

Can the Hubble Sphere ever extend over the cosmic event horizon?

No, the Hubble sphere can never extend over the cosmic event horizon; it can only asymptotically approach it. The Hubble sphere is the region within which galaxies (and other stuff) recede from us ...
pela's user avatar
  • 38.5k
3 votes

Can an event horizon form with the observer already inside?

The event horizon is the boundary in spacetime between the black hole interior and exterior. If you exist before the black hole forms and you end up at the singularity, then you necessarily passed ...
benrg's user avatar
  • 3,862
3 votes
Accepted

Theoretical measure of distance to black hole?

Yes. If you could spatially resolve a black hole and measure the redshift of radiation at a given angular radius, then that would yield the distance to the black hole. This works because the amount ...
ProfRob's user avatar
  • 153k
3 votes

Where do the photons come from at the edge of an Event Horizon?

From the distant observer's perspective the person falling in becomes more and more time dilated and redshifted. As a result you see fewer and fewer photons from them and eventually none at all (...
Steve Linton's user avatar
  • 10.3k
3 votes
Accepted

Why are event horizons described in Schwarzschild radii rather than diameters, and either way can they change equator to pole?

We use the Schwarzschild radius $r_s$ (rather than a diameter) because it's convenient. We're want to describe what happens in the vicinity of a black hole, so it's natural to talk about the distance ...
PM 2Ring's user avatar
  • 14.8k
3 votes

Black Hole growth

Good question. I am reminded of Einstein's 1939 paper on a stationary system with spherical symmetry consisting of many gravitating masses. He said “it is easy to show that both light rays and ...
John Duffield's user avatar
3 votes
Accepted

If a photon were 1 Planck length away from the event horizon of a blackhole, would it be able to escape the blackhole's gravitational pull?

It depends on the direction the photon travels. If the photon is directed straight away from the black hole, it will escape (but will be very redshifted). Obviously all directions towards the ...
Anders Sandberg's user avatar

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