# Tag Info

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There is no (widely accepted) theory that describes the structure of spacetime down to the quantum scale. You mention loop quantum gravity, but as far as I know the removal of singularities has been addressed only in the simplified form of loop quantum cosmology. However as far back as the 60s there have been suggestions that quantum effects would cause the ...

3

This is too long for a comment so I'll post it as an answer, even though this question is years old. If Alcubierre warp bubbles are physically possible, which is exceedingly unlikely, and if the equivalence principle is correct, you could definitely escape from a black hole in one, because there's nothing locally special about the event horizon. In a large ...

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The drive works by warping normal space, creating a bubble that kind of surfs through space time. I don't know what would determine the speed such a ship could achieve so not sure if a natural law would limit the ability to travel beyond visible space. Black holes exist because their extreme mass has warped space beyond to point where light can escape. It ...

0

Maybe a qualitative answer motivated from thermodynamics: If you let your black hole rotate, you reduce the number of symmetries of your system, this will decrease your entropy $S$ which is proportional to the surface area. The surface area however is for sure monotonic increasing with your Schwarzschild radius, therefore, if your break symmetries, $r$ will ...

1

Let me add something to the second part of the question. The evidence for the existence of a black hole starts with the observation that there is a very compact, massive object that is not emitting as much light as a "normal" star of that mass. This in itself does not rule out a neutron star, because it may well be as the OP proposes that the neutron star ...

4

Dark matter as far as gravitational forces go , has the same behavior as normal matter. That is how it was discovered and defined. By balancing gravitational forces in the motion of galaxies etc, it was found that more matter was needed than the matter estimated from the luminosity of the bodies. It was observed that the trajectories would not fit the ...

0

It is a supermassive black hole but its mass is nothing compared to the mass of the whole galaxy. So if you remove the black hole, we will not feel any difference because we are so far away. It is not the central black hole that keeps the galaxy together, our sun is orbiting around the center of mass of the galaxy and the supermassive black hole just happens ...

2

1. Hawking-Bekenstein entropy of a black hole is given by $S_{\text{BH}} = \frac{kAc^3}{4\hbar G}$ where $A$ is the area of the event horizon. Assuming a non-rotating black hole, there holds $r_s=\frac{2GM}{c^2}$ for the Schwartzschild radius, and therefore $A=4\pi r_s^2=\frac{16\pi G^2M^2}{c^4}$, which results in $$S_{BH}=\frac{4kGM^2}{\hbar c}$$ For ...

0

See L. Rodriguez and T. Yildirim, Class. Quantum Grav. 27, 155003 (2010), arXiv:1003.0026. Section 2.3 has the Schwarzschild-dS calculation. Lets define $f(r)=1-\frac{2M}{r}-\frac{r^2}{L^2}$ Radius of the horizon is given by the largest real root of f(r)=0 But of course $L\rightarrow \infty$ is still important. Once you obtain the energy momentum ...

21

A typical giant galaxy, such as the one you've provided a picture of, has a radius of something like $10\;\rm kpc$ (kiloparsec - $1\;\rm pc \approx 3.2\;ly$). A supermassive black hole hosted in such a galaxy has a mass of something like $10^6-10^9\;\rm M_\odot$ (solar mass, $1\;\rm M_\odot \approx 2\times10^{30}\; kg$). The monstrous billion solar mass ...

1

Astrophysically no, because the star has to go through supernova to form a blackhole. Some of the mass will be pushed away by the explosion, so the black hole will have a smaller mass than the star before supernova. If we ignore this and assume that mass stays the same then the object will feel the same gravity if it is at a safe distance, for the following ...

5

There is theory that light cone shape does not depend on the reference frame in which it is viewed. So why we draw light cones near black hole differently? In general relativity, frames of reference are local, not global. Each of the light cones in your diagram corresponds to a certain local frame of reference. An observer using that frame of reference ...

1

Light travels along paths with a metric interval of zero. In flat spacetime this would be drawn as a light cone with a 45 degree opening angle in a standard Minkowski space time diagram.Things get a bit weirder in GR when spacetime is curved by mass/energy. In GR, the concept of an invariant speed of light only applies locally in non-accelerating frames of ...

5

This isn't exactly an answer to your question, because as it stands your question can't be answered, but I thought I'd post this because the answer really surprised me. Firstly, the reason your question can't be answered is that you can never get your rope below the event horizon. From the perspective of an observer stationary with respect to the black hole ...

6

What would happen if I were to allow one end of a rope to fall past the event horizon of a black hole while I held the other end? As usual, this is in the context of a Schwarzschild black hole. First, outside the horizon, a object with constant radial coordinate 'feels' a constant proper acceleration, i.e., an accelerometer (think of a weight scale) ...

1

In order to not fall straight in, you would have to be orbiting the black hole very quickly, in fact near the speed of light. By definition event horizon is when not even light can escape as it orbits. (Edit: as John Rennie commented, hovering in a rocket is also an option) So imagine you are whizzing around at nearly the speed of light. You lower Your ...

4

When talking about black holes, you need to take into account time dilation. As you lower a rope into an event horizon, you will see time for the end of the rope slow down. You will not be able to say at some point: "Now the rope has crossed the event horizon", because you would need to wait indefinitely. The rope, on the other hand (or some observer you ...

11

Why is there no curvature outside this spherically symmetric, non-rotating, uncharged body that still has mass? I suspect you're getting confused by the fact that the Ricci tensor $R_{\mu\nu} = 0$ and therefore the scalar curvature $g^{\mu\nu}R_{\mu\nu} = 0$. This is always the case in regions of space where the stress-energy tensor is zero. The ...

1

If space-time of a black hole is infinitely curved, how can new volume be created for these particles to occupy? The spacetime of a black hole isn't infinitely curved. Only at the spacetime singularity within a black hole is the curvature infinite. The spacetime near, at, and within the horizon is highly curved but not infinitely so. I recommend ...

2

On paper, a black hole already has infinite density. Two coalescing holes would combine to another object of infinite density. Realistically, we would need quantum gravity to prevent a true singularity from forming,a nd there, we could address, more concretely, what happens when the "masses" in the center of the black holes merge. But until we ...

2

From a distance, black holes are no different from any other matter. Their gravitational attraction is the same as that of any other body with the same mass. If you were to suddenly convert a star into a black hole of the same mass, any planets around that star will keep moving in exactly the same orbits as before. Observers on the planets would not notice ...

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Why does time stop in black holes? Time according to whom? The fact is that, in special and general relativity, there is no universal time. Indeed, time is a coordinate in relativity so one must be careful to specify the coordinate system when asking questions like this. Now, every entity also has an associated proper time which is not a coordinate ...

1

If someone is falling in a black hole, the nearer he/she gets to black hole the slower time will pass and when he/she reaches the edge of event horison, time it would take for an observer to see him/her to cross event horison will be infinite (in other words if their friend was watching him/her he would never see him/her crossing the event horison). ...

5

Short answer: It doesn't stop. Slightly longer answer: The case of a non-rotating, non-charged black hole is described by the Schwarzschild solution. It is now the case that, if you draw the worldline of a particle falling into a black hole, you will find that the coordinate time in the Schwarzschild metric grows infinite as the particle approaches the ...

0

Some Googling later I have an answer for you. The relevant papers are: M. A.Markov, “Elementary particles of maximally large masses (Quarks and Maximons),” Soviet Physics (Journal of Experimental and Theoretical Physics), vol. 24, p. 584, 1967. V. I. Man’ko and M. A. Markov, “Properties of fridmons and the early stage of evolution of the universe,” ...

0

I think the idea is that the interior universe is not infinite -- it is just the finite size of the visible universe. This is not dissimiar to the "Hubble bubble" idea in cosmology: https://en.wikipedia.org/wiki/Hubble_Bubble_(astronomy)

0

Christoph Schiller argues that the log term arises from the different orientations that a black hole can have in space. His proposal also fixes the value of the constant K, as he explains, to 3/2 times the Boltzmann constant k_B. I found this in the section "Entropy of horizons" on page 269 and 270 of "The strand model - a speculation on unification". I ...

3

$r=1.5r_s$ for the Schwarzschild solution corresponds to the unstable maximum of the effective potential for a photon, therefore you won't be able to see much in practice, since practically every photon on this orbit will either fall in the black hole or escape to infinity.

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The distance where light has a circular orbit is actually $1.5r_s$ not the event horizon. This distance is known as the photon sphere. In principle a shell observer hovering at this distance could indeed see their own back. The proper distance is indeed just $2\pi r$, however the object would look bigger than expected because the curvature of spacetime has ...

1

This is essentially the same effect that you get in special relativity as the velocity approaches the speed of light. If you take a clock and accelerate it towards the speed of light then it will run slowly. If you could get the clock to the speed of light (which you can't of course) then it would stop completely. To use your words for any infinitesimally ...

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We do not know that "normal matter can not exist within an event horizon". For all we know, aliens may be sitting around and drinking coffee watching us.* We have theories on what could happen. different theories have different views. Nothing has been tested, scientifically proven as yet. Some calculations (the status thereof) on some theories (Hawking' ...

2

As a non-physicist, this is how I understand it: As a massive object collapses, you get more and more matter packed into less and less space. When you do this, the temperature rises and pressure increases. As the temperature rises, the particles get more energetic. As they gain energy, the equilibrium of various things changes... First, the lower-energy ...

4

Let me attempt a slightly different approach to the other two answers. The geometry of spacetime around a static black hole is described by the Schwarzschild metric. The metric can be written down in various ways, but I think the one that best illustrates what is going on is to use the Gullstrand-Painlevé coordinates, and in particular in the form known as ...

4

We intuitively understand why it is inevitable for a particle to progress forward in time. Relativity tells us, among other things, how objects can affect others, cause other stuff to happen and this is connected with the direction forward in time. We call this structure of "what can cause what" the causal structure. In general relativity, this causal ...

1

Aside from the escape velocity at the surface reaching c, what actually changed? The radial direction becomes time-like with the future direction in the direction of decreasing radius. In other words, the time and radial directions swap character at the horizon. See, for example, these lecture notes where we find: Outside the horizon r is a ...

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Basically, the reason is that if the matter is to keep a positive mass, the amount of force required to keep the matter distribution stable tends to infinity once the matter is fit within the schwarzschild radius for that given mass. This was proven in a very strong sense, without much in terms of assumptions about the particular form of the metric, by ...

0

It seems logical to me that a black hole has no temperature, as temperature is the moving around of molecules, and there is no movement in a black hole, as it's all compressed so densely, in fact it's meant to be infinitely dense. It also seems logical that a black hole with the mass of 100,000 stars would never allow anything to escape from it, ever. If ...

4

I discuss this in my answer to Black holes and positive/negative-energy particles. The idea of Hawking radiation being caused by virtual particles is just a metaphor. The radiation is actually due to the fact that in curved spacetime there is no unambiguous choice of the vacuum state. What appears to be the vacuum state for an observer close to the event ...

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