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Hot answers tagged black-holes

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… if an object could stay in a locked position encasing a black hole without collapsing in on itself. Yes. Solutions for a thin static spherical shells around a nonrotaing black hole have been considered in the following paper: Frauendiener, J., Hoenselaers, C., & Konrad, W. (1990). A shell around a black hole, Classical and Quantum Gravity, 7(4), 585,...

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When a Black Hole is present in a bulk, it gives the background some temperature due to Hawking radiation. Thus a stable spacetime in presence of a generic BH has a fixed tempertature $T$. The dual CFT theory also has thermal states. It has been recently shown that these thermal states have one-to-one correspondence with the Black Hole backgrounds of AdS ...

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No, you can’t explain accelerating expansion in this way. There are three separate problems with your proposed explanation. First of all, mass isn’t relevant. In General Relativity, gravity and the expansion of the universe depends on energy (and momentum), not on mass. The massless photons carry away energy, and the energy of the hole decreases by the same ...

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The BEC is held in a focused laser beam acting as a dipole trap (this is a useful search term if you want to look for more information on the scheme). Atoms in the beam feel a space-dependent potential which is proportional to the laser beam's intensity. If the laser is blue-detuned this potential will be repulsive, but for the set-up described in the ...

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I don't know if it is the best way to look for such coordinate (probably not), but for general metric $$ds^2=g_{tt}dt^2+2g_{tr}dtdr+g_{rr}dr^2$$ on given timelike subspace you can look for coordinate transformations $t\rightarrow t'(t,r), r\rightarrow r'(t,r)$ that takes metric into the form $$ds^2=2g_{t'r'}dt'dr'+g_{r'r'}dr'^2.$$ The coordinate $t'$ will ...

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Firstly, there is a considerable amount not known about Black Holes. As a physicist, when people say, that it's a singularity, what they mean is that there is no known physics that explains what goes on there. Our understanding of both Gravity and Quantum fields are valid upto an energy scale(that means upto some small length scale). Beyond that we'll ...

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Schwarzschild coordinates work Most astrophysical calculations involve things that are outside the event horizon. In these cases Schwarzschild coordinates work just fine. Observed phenomena like gravitational lensing, orbital changes relative to Newtonian gravity, and gravitational redshift can all be understood using Schwarzschild coordinates. ...

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This setup is impossible in general relativity. The Einstein field equations require that the energy-momentum acting as a source be divergence free, i.e. its energy and momentum must be conserved. Clearly, "deleting" a mass at some instant violates energy-conservation, and there simply will be no valid solution to the GR equations. For black holes, there ...

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The black hole is defined by its event horizon. This is the point at which the escape velocity reaches $c$. But the accretion disc forms outside the event horizon, so stuff can still escape from it. It is this outer stuff that finds its way into the jets, super-accelerated beyond escape velocity by magnetic fields being dragged round the hole. If the escape ...

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To calculate the proper acceleration, you need to calculate all the components of the four vector, especially the radial one: (be careful to make a good covariant derivative) $$a^r = u^t(\partial_t u^r + \Gamma^r_{t\alpha}u^\alpha)$$ The proper radial velocity of this observer is 0. The time component can be calculated by making use of the fact that $u_\mu ... 1 A black hole bomb draws its power from the rotation of the black hole (through a process known as superradiance). The effect of the "bomb"* on the black hole would therefore be to spin it down, converting part of its rotational energy (and therefore mass) to energy in the "bomb". The end result is a black hole with a lower spin than the one you began with, ... 1 No. When a black hole absorbs a graviton, it simply becomes more massive because the graviton has a tiny amount of energy. There is no limit to how many gravitons it can “eat”. 1 You can find the Schwarzschild mass perturbation in synchronous gauge by starting from the mass perturbation in any other gauge, applying an arbitrary gauge transformation, and imposing the synchronous gauge condition. Doing this (quickly, you should check for yourself) I get $$h = c\left(\frac{r}{r-2m} dr^2+r^2d\Omega^2\right)$$ 1 Schwarzschild (Droste) coordinates have a number of advantages over other coordinates systems for the Schwarzschild metric such as Eddington-Finkelstein (EF) or Gullstand-Painlevé (GP) coordinates. 1. Symmetry One is that in Schwarzschild coordinates the time translation and axial symmetries are explicitly manifest. In these coordinates the vector fields$\...

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To calculate its apparent position, would we have to calculate the path of a beam of light from the projectile to an observer? Wouldn't we also have to do that in Schwarzschild coordinates? This seems like a non-trivial calculation mathematically. This is exactly what you have to do to show this. Fortunately, this is not a very hard calculation. Outward ...

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If black holes are mass of the star concentrated on a single point Black holes are a solution of General Relativity equations, which describes very large masses and the distortion they bring to space and time. It is a classical theory, and the mathematical form has a discontinuity at radius equal zero. It is expected that quantization of gravity will ...

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Yes, as shown by Oppenheimer and Snyder gravitational collapse takes place in finite proper time for infalling matter, but infinite coordinate time as measured by an exterior observer, such as ourselves. From our point of view, the singularity never actually forms. A "black hole" is really a "frozen star". On the other hand, infalling matter does hit the ...

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The radiation mechanism is quite different. The thermal radiation temperature of a hot "black body" is governed by the surface temperature of the body. The radiation temperature of Hawking radiation from a black hole is governed by the curvature of spacetime at its event horizon. The smaller the black hole, the tighter the curvature and the more energetic ...

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In general there is no easy way to construct initial data that pre-dominantly excites a particular quasi-normal mode. Certainly not without essentially inputting what the quasinormal mode is from the beginning. Note that the code in the paper you cited, certainly does not do anything like this. In general their initial data while excite most quasinormal ...

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In addition to the other answers given, I should add that the energy we observe from the GC is that of either some very exotic mass like a boson star, or of a supermassive black hole. We know mathematically a black hole is much more likely to form than these sorts of objects. Here is a diagram showing the progress in nailing this down over time with ...

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