# Tag Info

## Hot answers tagged black-holes

13

The low-entropy initial state of the universe is an open problem without a satisfactory answer. Your question is the first time I've heard the suggestion that the initial state should have been a crystal; you remind me that the quark-gluon plasma, which was the state of the universe while it was too hot for nucleons to be stable, has been shown to be a ...

9

The bright parts around the black hole are the accretion disk, which is in reality just a flat disk in the equatorial plane similar to the rings of Saturn, but is distorted visually by gravitational lensing. You can see a page here that gives some code for creating images using ray-tracing of light rays in curved spacetime, which offers a more schematic ...

8

"According to Newton's law the negative mass should be repelled" -- Nope, in both Newtonian physics and in general relativity, negative mass would be attracted gravitationally to positive mass, although negative mass would exert a repulsive gravitational effect on positive mass (but if the negative mass is small compared to the mass of the black hole this ...

8

the two paradigmatic cases that illustrate these two possibilities is a gas, for the first, and a crystal for the second. Paradigms and examples are well and good, but be careful not to assume they are the only possibilities. In particular, black holes have entropy -- a lot of entropy. In fact they saturate the Beckenstein Bound. The entropy of a black ...

7

In the case of Hawking radiation, the direct answer is "no, there is no direct test, nor can we imagine one with anything like current technology." But it is not some wild speculation made in vacuum. The extremely closely related Unruh effect can be derived from basic quantum field theory on a curved spacetime, and many QFT and GR texts have at least an ...

5

This is due to gravitational lensing which distorts the apparent visual shape of what is really just a disk in the equatorial plane. You can see a page here that gives some code for creating images using ray-tracing of light rays in curved spacetime, which offers a more schematic diagram of the visual appearance of a disk around a black hole (with a ...

5

First we need to clear up a number of fundamental problems. Always use units. You give all these numbers, but no sense of whether they are meters or nanoseconds or newtons or furlongs. You should read up on significant figures. There is no point in writing out all those digits, since past the first one or two they are all uncertain. In science, writing out ...

4

No, throwing matter out isn't contrary to a BH; there is often an accretion disc surrounding the black hole and that is what forms the jet. No, the ejected material cannot condense to form a galaxy. A galaxy requires the material to be gravitationally bound to some central point, a jet moving at $\sim c$ is moving too fast to be gravitationally bound to ...

4

Object A increases in mass, and so increases in volume I'm going to make the assumption that we are adding mass to A by providing more material of the same density $\rho_{A}$, rather than exchanging the current material with denser material or adding varying densities. I don't have to make that assumption, but it seems like it's what you're going for ...

4

It's not quite what you're looking for, but the article here from Physics World shows such a diagram for a charged Reissner-Nordström black hole. They note that Reissner-Nordström black holes were used to try to model the effects of incoming radiation being infinitely blueshifted at the inner (Cauchy) horizon, including the infinite blueshift of incoming ...

4

What I will state is speculative and based on the statistical mechanics derivation of entropy, and just the way I view it and do not consider that there exists a problem. After all thermodynamic theory emerges from the underlying statistical level of atomic and molecular interactions. where p_i is the probabability of microstate i. Setting aside quantum ...

4

The black hole initially lost the gravitational energy that was needed to create the pair. The pair-creation model is a bad description of Hawking radiation, which for macroscopic black holes is really photons. The second particle that gets created above the event horizon doesn't have nearly enough energy to escape. It does, however, produce photons above ...

3

The point of the fuzzball conjecture is that spacetime is geometrically altered at the black hole horizon. Rather than having an interior, the extra dimensions pinch off at the horizon and encode the complicated data from incoming particles in complicated geometry. This resolves 2 issues There's no singularity because the black hole effectively doesn't ...

3

It's a calculation in Quantum Field Theory in Curved (fixed) Space Time. The point is not whether the calculation is exact (on this, there is probably no doubt), but whether that semiclassical approximation is adequate to describe the black hole horizon. So yes, there are no experimental evidence (the effect is very small), and the radiation may very well be ...

3

I don't think the answer is too exciting. The Schwarzschild solution is a static solution to the Einstein field equations. The Einstein field equations alone don't take into account quantum effects. Taking quantum effects into account will give you a modification of the solution, and the result that the Schwarzschild 'solution' is no longer static (and so ...

2

Yes there is a maximum gravittational field, although of course the gravitationational force will be unbounded because there is no upper limit to the force you can putinto that hravitational field. The gravitational field outside a black hole has an upper limit: First notice that the surface gravity of a black hole is actually larger for less massive, ...

2

There is no maximum gravity. Assuming constant density, mass grows as $r^3$, while gravity attenuates as $r^{-2}$. Therefore as long as density is constant, the force of gravity between two touching spheres will grow like their radii (or the cube root of their masses), meaning there is no sweet spot. Of course, real matter does compress under enough ...

2

http://t.space.com/all/25691-dark-matter-black-hole-atoms Some people much smarter than me think that "quantum" sized black holes are a candidate for dark matter. There are problems with it, but as far as I know there are problems with every candidate so far. I'm just reporting what I have read. My understanding of "dark matter" (meaning 1) is that it is ...

2

It's a bit misleading to say that a gravitational singularity breaks the laws of physics. To see why you need to understand what the phrase laws of physics means. You'll find this discussed in various questions on this site, but in brief a physical theory is a mathematical model that gives an approximate description of reality within some boundaries. So for ...

2

The photon is not deflected by the charge. It's deflected by the spacetime curvature. The spacetime curvature is related to the stress-energy tensor (by Einstein's equation), so if you change the stress-energy tensor you change the curvature. A static black hole, described by a Schwarzschild metric, has a very simple stress-energy tensor since it's ...

2

There is no exact solution of Einstein's equation smoothly modeling the metric of a rotating star, so a diagram like this can only be a heuristic.

2

The equivalence principle tells us that we can evaluate $\nabla_u u$ in a co-moving reference frame and that for geodesics we should find no acceleration (to the occoupants of an elevator in free-fall, the contents seem to be experiencing no acceleration). Therefore, if we evaluate this when we are not along a geodesic (elevator sitting on earth), we find ...

2

The coordinates you are using are called the Schwarzschild coordinates, and they are the coordinates that match measurements made by an observer at an infinite distance from the black hole. That is, if you're an infinite distance from the black hole then the Schwarzschild $t$ coordinate matches what you'd measure on your clock and the $r$ coordinate matches ...

1

Firstly, your idea does make on important prediction, which is that the universe originated at a point in space (the location of a black hole) and we should be sucked towards that point, eventually being sucked in and the cycle happening all over again. There is plenty of observational evidence that your theory can not be correct. Firstly, the universe did ...

1

Gravity is the only force remaining to use for capture and co-acceleration. It turns out that there's a reasonable sweet spot in the design space. Mutual attraction between BH and ship balances the thrust on the ship via reflection of the radiation off the paraboloid, which is attached to the ship. A ship-BH separation measured in centimetres up to a good ...

1

According to p. 303-304 of the book Gravity from the Ground up by physicist Bernard Schutz, viewable on google books here, it's because in terms of the pair-production explanation for Hawking radiation, one member of the pair actually has negative energy and thus causes the black hole to lose mass (negative mass/energy falling into a black hole can also ...

1

OK... I can't give a definitive answer to the problem. My intuition tells me that any massive particle or macroscopic mass, boosted high enough, has to look like a black hole. Why? Because it is very hard to see why/how gravity, if we believe in the equivalence principle, should be able to distinguish between kinetic energy and other forms of internal energy ...

1

In the Schwarzschild geometry, the Schwarzschild radius breaks naive dilation symmetry. In the simple case of a radial dilation $r \to \lambda r$, the geometry is only preserved by $R_S \to \lambda R_S$. So, it naively seems like it would be difficult to find a working dilation, even just a radial dilation. I went to some effort (as an exercise for myself) ...

1

It cannot disrupt the gravity of the Earth. There are other infinitesimal possibilities of doomsday scenarios though these stories have been debunked many times. The report ruled out any doomsday scenario at the LHC, noting that the physical conditions and collision events which exist in the LHC, RHIC and other experiments occur naturally and routinely ...

1

In the early Universe, entropy is preserved (dS=0). This comes out of the equations of general relativity, but it can be also understood by thinking in terms of classical dynamics: the Universe is a closed system, no heat is exchanged when expanding, so its entropy must not variate.

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