# Tag Info

54

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 of dust that comes inside the EH of a black hole. I believe we'll agree this speck can never escape the black hole, because nothing can come back from behind the ...

52

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 several possible ways to explain it, with varying degrees of rigorousness. An intuitive explanation is that escape velocity at the event horizon equals the speed ...

9

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 always receding, both because the Universe expands and because light from increasingly large distances eventually reach us. In comoving coordinates (the coordinate ...

9

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 of information, then the black hole wouldn't be so black. You want to do this using the rotation of the black hole. This doesn't work, for various reasons. ...

8

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 expansion of the Universe, although locally (i.e. where SR is a good approximation) you cannot exceed the speed of light, there is no limit to the relative ...

6

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, you may be in a position in which there is no net force, and yet you could not travel from your location to the point distant from the two black holes, no ...

6

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 disappear because of redshift). At some point the rope would snap, but the camera would still not appear to cross the horizon. According to an observer with the ...

6

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 masses would have a radius 29.53 km at the event horizon, not 1 KM. A Neutron Star is pretty close to its event horizon already. The problem with "some ...

4

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 comparable object of the same mass, like a star. In fact the gravitational disturbance it causes in this way is usually how we determine that there are, for example, ...

3

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 through it at some point. You can't get inside without crossing the boundary. From the perspective of some foliation of the spacetime, the event horizon encloses ...

3

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 (although you never have any way of knowing that one more, very redshifted photon isn't going to arrive some time). So there are no extra photons.

3

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 from the black hole. For example, there's the photon sphere at $3r_s/2$ (for a Schwarzschild BH), and the innermost stable circular orbit or ISCO, the smallest ...

3

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 of redshift depends on the ratio of radius to Schwarzschild radius, but the latter is known if the mass is known. Thus the redshift yields the radius. The ...

3

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 event horizon will lead to capture. But since the spacetime is so curved, this also includes many directions that point away from the hole: the photon trajectory ...

3

Good question. I am reminded of Einstein's 1939 paper on a stationary system with spherical symmetry consisting of many gravitating masses. He said â€œit is easy to show that both light rays and material particles take an infinitely long time (measured in "coordinate time") in order to reach the point r = Î¼/2". That would suggest black holes can never form, ...

2

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 that at minimum an object just inside the horizon of a black hole is traveling toward the center at the speed of light (as its speed would be measured from ...

2

The particle horizon marks the region from within which we may have received light. It started out at zero, because light from nowhere had had the time to reach us, and increased as time went by because light from increasingly larger distance reached us. Due to the expansion it will always increase in physical size, but due to the accelerated expansion, ...

2

TL;DR look at the diagram at the end of the post. Following the Principle of Relativity, the laws of physics are the same everywhere and for every observer. Despite that, measurements of observers can differ, as long as they are consistent within the rules of general relativity. Furthermore, horizons limit what can be measured by some observer. In this ...

2

According to Wikipedia, the angular resolution is inversely proportional to the baseline (the separation between the telescopes). The order of magnitude of the baseline of the Event Horizon Telescope is 10,000 km; the Earth-Moon distance is about 40 times larger, and Earth-Mars at its maximum 25,000 times larger. That means we could theoretically reach 1.5 ...

2

The diameter of a black hole (the distance "through" it) or the radius (distance to the center) aren't really meaningful concepts. The singularity is in the future, not in any spatial direction. The $r$ Schwarzschild coordinate is actually a so-called "reduced circumference" â€“ the metric length of a circle around the hole, divided by $2Ï€$ ...

1

When we talk of kinetic energy you need to specify a frame of reference. In its own reference frame a body is at rest and its kinetic energy is zero. So there is no special effect as a body crosses the event horizon. From the point of view of an infalling object, there is nothing at the event horizon, and the body will just continue to fall (though not for ...

1

Smarter guys will get through the complicated, maths-heavy explanations, but in term of wrapping your mind around the idea: In that local frame of reference over the cosmic event horizon, you're not moving, no more than you're currently moving at higher speed than 3x10^8m/s from a over-the-horizon-distant point in space. Keep in mind there is no such thing ...

1

To be entirely covered by material, it'd have to be surrounded by material in the first place. It's my understanding that most matter in the universe tends to organize in a plane or some description (maybe because it's a vastly simpler orientation; or most material came from stars/supernovae, and in anything rotating in multiple directions at once those ...

1

A space based radio telescope has been tried, although I can see little evidence of it being successful. To get an image you do need baselines at multiple angles, not just a single line. This can be achieved on Earth with just two telescopes by combining measurements made at different times as the Earth rotates, and an Earth-Moon baseline could be treated ...

1

Let's have a look at the Physical_properties subsection of the Wikipedia article on Black holes: The Schwarzschild radius which is defined only for a non-rotating black hole is given there as $$r_S=\frac{2GM}{c^2}\approx 2.95 \frac{M}{M_{Sun}} \text{km}.$$ The Schwarzschild radius was named after the German astronomer Karl Schwarzschild, who calculated ...

1

How exactly can Hawking's Radiation carry the information? It does not, if you mean information about the interior of the black hole. The relevant theorem is the No Hair Theorem. Basically you can only get gross statistical properties of black holes, nothing else and no information about the interior structure (meaning any info inside stored in any way, ...

1

Within the event horizon gravity is so strong that no matter how great is the force you apply, you can't move away. For an object to remain as an object it would need to be more than infinitely strong (an obvious impossibility) This assumes that general relativity is a good model. It is possible that general relativity is incorrect at these extreme ...

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