# Questions tagged [black-holes]

A black hole is a volume from which matter cannot escape. More formally, the coordinate speed of light at the event horizon - the boundary of a black hole - is zero, as measured by a sufficiently separated observer.

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### What is inside the Schwarzschild radius of the Sun?

According to https://www.britannica.com/science/Schwarzschild-radius, the Schwarzschild radius for a black hole with the mass of the Sun is about 2 miles. My question is what is inside of it? Is ...
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### Change in area of a disk near black hole?

I know that the length contraction can be determined by the formula: $L\sqrt{1-\frac{2GM}{Rc^2}}$ But how can we calculate the decrement in area due to the black hole?? Suppose we have a disc of area ...
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### What does Schwarzschild refer to as Einstein's approximation regarding his exact solution?

In 1916, Schwarzschild published his $R$-metric solution that differs from the $r$-metric solution we are all familiar with. The relation between $R$ and $r$ is $R^3=r^3 + α^3$ with $r$ been the ...
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### Do “almost black holes” exist?

The only things I read about so far in astrophysics are either black holes, developing black holes or not black holes at all. So I am wondering, is it physically possible to have an object that is ...
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### How do astronomers detect the spin of the black holes?

There are various theoretical models for accretion disks and relativistic jets from black holes that considers the black hole to be either rotating (Kerr geometry) or non-rotating (Schwarzschild ...
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### How can we see black holes form and gain mass if nothing can cross a horizon (when viewed from our reference frame)? [duplicate]

As things approach the horizon of a black hole, their time dilation approaches infinity. How then can we observe black holes gaining mass if, according to our reference frame, nothing can pass the ...
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### Charged Black Hole in 1+2 dimensions without cosmological constant

Does a charged black hole in 2+1D exist? I am interested in a theory of the form: $$S = \int d^3 x \sqrt{-g}\big(\cfrac{R}{2} - \cfrac{1}{2}F^2\big)$$ where $F^2 = F_{ab}F^{ab}$. The field equations ...
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### Are the original Schwarzschild solution and Hilbert's solution exactly the same?

Schwarzschild original metric solution can be found here in equation (14): http://old.phys.huji.ac.il/~barak_kol/Courses/Black-holes/reading-papers/SchwarzschildTranslated.pdf Hilbert's metric ...
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### Charged Black Hole: Self-Discharge

Palti writes in his introduction to the swampland (page 39) that a charged black hole (BH) could discharge itself, when the Hawking temperature $$T_H=\frac{R_+-R_-}{4\pi R_+^2}$$ ($R_+$ and $R_-$ ...
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### How do black holes get their magnetic fields?

I imagine the magnetic field comes from the accretion disk. But what causes it to "spring" upward?
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### What would happen to a particle caught exactly in the middle of two merging black holes both of which are also at the ends of their lives?

If the two tiny, about-to-vanish black holes are of the same mass then such a particle should not go anywhere and remain stationary in the middle even as it gets enveloped inside the new unified event ...
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### What happens to a hydrogen atom under an intense gravitational field (or intense acceleration)?

I'm curious how the observed quantum-mechanical properties of a hydrogen atom would change under very high gravitational fields. Do the energy levels change? Do the chemical bonding properties ...
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### Is it possible that a black hole has a universe in itself? [duplicate]

Every black hole has a singularity. But the singularity is covered by a thing called 'Event horizon'or ' Schwarzchild Radius'. Our universe is originated from a naked singularity. By naked singularity,...
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### Child Universes

I was watching a program on the future of the Universe and it got into the eventual heat death resulting from Hawking Radiation as the last hurdle till nothingness (assuming the unproven theory of ...