11 votes
Accepted

How massive objects affect distance measurements, or "But how far away is a black hole singularity, really?"

The trouble with using a measuring tape is that it only shows the correct length if it's neither stretched nor compressed. You can't just hold it taut at both ends, because the collapse time of a ...
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8 votes

How massive objects affect distance measurements, or "But how far away is a black hole singularity, really?"

This question already has a really nice answer on the difficulties of measuring lengths, so I'll focus on something else. The question Is the singularity of a black hole infinitely far away? is way ...
6 votes
Accepted

In pure GR can a black hole spontaneously appear?

Q1: Can incoming gravitational waves form a black hole? A: Yes. Source: https://arxiv.org/abs/0805.3880 Q2: Can a black hole evaporate in classical GR? A: No, such a process would violate the area ...
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4 votes

Can we measure gravitational constant at high accuracy using a black hole?

No. To measure G, you must have independent measurements of mass and gravitational field. The mass of the black hole can only be inferred from its gravity.
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3 votes

Can we measure gravitational constant at high accuracy using a black hole?

This is fundamentally impossible. In the metric for the black hole, $G$ only ever appears in the combination $GM$. Consequently, there is no way of interacting with a black hole that will allow you to ...
  • 8,466
3 votes

4-velocity in Schwarzschild metric

Given a set of coordinate $q^i$ to describe a point in the 4-dimensional spacetime, $\mathbf{X}(q^i)$, you can define the 4-velocity of a point as $\mathbf{U} := \dfrac{d\mathbf{X}}{d \tau}(q^i(\tau)) ...
  • 1,808
3 votes

4-velocity in Schwarzschild metric

Remember that the norm of the four velocity is given by: $$ V^2 = g_{\mu\nu} v^\mu v^\nu $$ If we have a stationary object at a distance $r$ then $dr = d\theta = d\phi = 0$ so the only non-zero ...
3 votes

If I jump into a black hole one million years after another person, how do we end up relative to one another in space and time?

From the way you word your question, I gather you are thinking of a black hole in terms of the 'frozen star' picture common in early theory. Things fall 'onto' the event horizon, but as time slows ...
2 votes
Accepted

Destroying a black hole

The standard definition of a black hole in classical GR is that it has an event horizon. By that definition, there is no way to convert the stuff that has fallen into the hole to other stuff that can ...
2 votes

How Existence Of Matter Is Possible Inside The Black Hole?

Big bang occurs in the black hole at the center of the galaxy You appear to be confusing the Big Bang, which is the origin of the whole universe, with supermassive black holes which lie, we believe, ...
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1 vote
Accepted

Why are inner horizons Cauchy horizons?

Any event beyond the inner horizon, will have part of the timelike singularity in its causal past. Consequently, starting from the inner horizon the spacetime cannot be obtained as a Cauchy evolution ...
  • 8,466
1 vote

What happens to the matter already at the very center of the star when it turns to a black hole?

This is just a long comment. In all theories of physics, classical mechanics, classical electrodynamics, Newtonian gravity, all potentials lead to singularities, i.e. infinities in the value , since ...
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1 vote

What happens to the matter already at the very center of the star when it turns to a black hole?

According Roger Penrose's single paper that netted him a Nobel Prize, the singularity forms after the event horizon: Any light cone that has no part pointing away from the singularity is inside the ...
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1 vote

Escaping photons from a forming black hole

Event horizons are defined, formally, by mathematically going to the final future of all geodesics in the spacetime, finding all of the geodesics that went into the black hole, and then taking the ...
1 vote

Escaping photons from a forming black hole

The event horizon and Schwartzchild radius are exactly the same. There is no "in between." Perhaps you are asking, "When the event horizon forms in what used to be the core of the star, ...
  • 3,685
1 vote

Will a black hole disappear?

We cannot be sure yet if a Black Hole fully evaporates into "nothing" or leaves a stable quantum remnant. Hawking Radiation is hard to observe because it is an incredibly slow process for ...
1 vote

How can the connections of the Schwarzchild Metric remain torsion free?

If the question is only about the consequences of the first case be different from zero, while the second one is zero, it is possible to use a simpler example. In the polar coordinate system in the ...
1 vote
Accepted

What prevents two particles that made a black hole to unmake it?

If the laws of physics are time reversible, then I could start my problem with these two same particles with their momentums reversed, and the solution should be a black hole that splits into the two ...
1 vote

Can I really see what is on the opposite side of a black hole?

Árpád Szendrei asked: "Can I really see what is on the opposite side of a black hole?" Yes. Let's take this 360°x180° full panorama: Now we zoom in one direction: and place a black hole ...
  • 8,743
1 vote

Can I really see what is on the opposite side of a black hole?

Yes. You may have seen the visualization of a black hole in the film Interstellar. The following figure comes from Figure 13 of a companion paper (Ref. 1 -- note the author list includes Nobel Prize ...
  • 36.1k
1 vote

Invariance of binary black hole gravitational waves

The Einstein equations are scale invariant, there is no intrinsic length scale set by the theory itself. As a consequence, all solutions to the (vacuum) Einstein equations have an overall scale ...
  • 8,466

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