New answers tagged black-holes
0
The "No hair theorem" is only a theorem when you consider only gravity and the electromagnetic forces (i.e. it is a statement about vacuum solutions of the Einstein-Maxwell equations.) In the 1970s and 1980s many theorists expected that an generalized no-hair conjecture would also hold: a black hole solutions including weak and strong forces would ...
2
"When an object approaches the speed of light, its mass increases" is NOT true. If it were, you could do something like put that person on a weighing machine which triggers a bomb if its reading were above a fixed value (e.g. $100 kg$). This would be contradictory to what that person sees, since it is equally valid for them to consider themselves ...
1
Unruh/Hawking radiation only affects you if you move noninertially. Gravitational acceleration doesn't count. If you (free)fall into a black hole then you objectively aren't affected by the radiation, from anyone's perspective. If you try to avoid falling in by accelerating away then you are.
If A is in an inertial frame and observes B in an accelerated ...
1
Unlike in electromagnetism, the Einstein's gravitational equations are non-linear. This non-linearity becomes apparent when the field is strong and causes effects of the orbit precession, time dialtion, frame dragging, gravitational lensing and others. The field becomes especially strong near neutron stars, but reaches the extreme in black holes. For this ...
2
I would like to answer with a quote by Subrahmanyan Chandrasekhar, a Nobel laureate notable for his contributions to the study of black holes:
Macroscopic objects, as we see them all around us, are governed by a variety of forces, derived from a variety of approximations to a variety of physical theories. In contrast, the only elements in the construction ...
1
But black holes are just very dense...
They actually do not need to be dense if they are very massive. There is quantity called Schwarzschild radius, which tells you for the given mass what is the radius of created black hole:
$$r_s=\frac{2G}{c^2}M,$$
with standard meaning of the symbols. You can rewrite this using average density instead of mass and you ...
5
why do we see black holes as [...]
Well that's the first issue. We cannot see black holes. Light cannot escape the event horizon, so talking about anything that goes on inside is hypothesis (more or less compatible with available experimental data depending on which theory) or speculation.
black holes are just very dense stars which bend space-time like ...
0
here is a quick written description.
Let's say we throw into a black hole a clock tied to a flashlight. as the pair comes closer to the event horizon, the hands on the clock appear to us to slow down and the light from the flashlight reddens in color and grows dim. the closer they get to the EH, the slower the hands move, and the redder and dimmer the ...
-1
The Schwarzchild-radius of a black hole is $$r_s=\sqrt{\frac{2GM}{c^2}}$$
everything inside the Schwarzchild-radius is invisible because nothing, not even light, can escape the Schwarzchild-radius of a black hole due to it's strong gravity.
1
It has been said that falling into a black hole and crossing the event horizon would appear as "nothing out the ordinary" for the in-faller (especially for a super-massive BH [...]).
That's correct. In the case of a large black hole, even fairly large regions in the vicinity of the event horizon are roughly flat (i.e. roughly just look like ...
1
You cannot apply special relativity in curved spacetime, except locally, within the immediate neighbourhood of an observer.
From the point of view of an exterior observer, one is not accelerated to the vacuum speed of light, but actually one stops before crossing the event horizon. It happens that light stops too, but this is rather different from being ...
1
I'm not an expert and I may deprecate or delete this answer if someone writes a better one.
De Sitter space in de Sitter's original coordinates resembles a Schwarzschild black hole turned inside out. It looks a lot like a maximum entropy solution, and indeed it seems to be commonly believed that if the cosmological constant is positive then the number of ...
1
A typical method is to make the ansatz that the spherically-symmetric metric has the form
$$ds^2=-A(r)dt^2+B(r)dr^2+r^2(d\theta^2+\sin^2{\theta}d\phi^2)$$
and determine which functions $A(r)$ and $B(r)$ make the metric satisfy the Einstein field equations, which in vacuum are $R_{\mu\nu}=0$ everywhere (except perhaps at some singularity).
This ansatz lets ...
4
Yes, black holes can precess.
In binary consisting of two black holes, both spins and the orbital angular momentum will precess about the total angular momentum.
In April, LIGO and Virgo announced the first detection of a binary black hole merger where the black hole spins were precessing: GW190412
0
Quotes from a review of a theoretical paper dealing with :
What Would Happen if a Small Black Hole Hit the Earth?
But what if the black hole is small, perhaps a left over remnant from the Big Bang, passing unnoticed through our neighborhood, having no observable impact on local space? What if this small singularity falls in the path of Earths orbit and ...
3
You can find the Kerr metric in pseudo-Cartesian coordinates for instance in "The Kerr Spacetime", by Wiltshire et al, as
\begin{eqnarray}
ds^2 = &&-dt^2 + dx^2 + dy^2 + dz^2 \\
&+& \frac{2mr^3}{r^4 + a^2 z^2} \left[ dt + \frac{r(x dx + y dy)}{a^2 + r^2} + \frac{a(y dx - x dy)}{a^2 + r^2} + \frac{z}{r} dz \right]^2
\end{eqnarray}
...
3
Probably much less than its present rate.
Moon has a lot of surface to "catch" meteorites and convert their kinetic energy into heat (so they won't bounce back into space). If made a black hole, most of these will pass in a parabolic/hyperbolic orbit around the black hole and leave for good.
In order to get something into the black hole (less than ...
9
About 32 femtograms per day
As others have explained, a black hole doesn't really suck in matter. It uses gravity to pull matter in, but that is not any different from how the moon acts now. Only matter that is on a direct collision course with the black hole will actually get eaten up, any other matter will just pass by and fly off into space again.
A black ...
15
Right now, it already eats a fair amount of mass, as tiny meteors (and occasional big ones) rain down on it every day. Once it became a black hole, it would eat far less. Gravitationally, meteors would behave exactly the same while they're outside the moon's former radius. This is because the gravitational field of a spherical shell is exactly the same as ...
0
It has everything to do with distance as you can see in the formula g=G*M/R2 . If the moon became a black hole it would still have the same mass. Right now the moon has a radius of about 1000 miles which equates to an acceleration due to gravity of 1.6 m/s squared at the surface. If the moon were a black hole its radius would be tiny and you would be able to ...
22
It would suck in basically nothing. If the mass of the moon were concentrated in a black hole, you could draw a sphere around that black hole the size of the moon, and for everything outside that sphere, nothing would have changed. The gravity field would be just the same as it is now.
Now, black holes are tiny. At the mass of the moon, the radius is on the ...
42
It'd suck in very little mass.
One thing to understand about black holes is that they have super-strong gravity, but only when you are very close to their event horizon. Otherwise they're just normal objects. If the Moon became a black hole, it would have a radius of about 0.1 millimeters. You need to get pretty close to this distance before you notice ...
1
As far as I can tell, the context for BH scattering is actually not relevant, so I will answer within the context of a scalar field theory.
Question 2 is actually fairly easy to answer. You should think of $\hbar$ in this context as a formal expansion parameter, and not a constant of nature. You may recall, that in ordinary perturbation theory, a common ...
4
LIGO and VIRGO detect a very specific subsample of black holes in the universe - those with masses below about $100 M_{\odot}$ and which are in close, merging binary systems with other compact objects (another black hole or neutron star). There is a strong bias towards detecting the most massive of these systems because they emit more powerful gravitational ...
3
As follows from energy conservation, the gravitational time dilation near a Schwarzschild object is equal to the velocity time dilation at the escape speed:
$$\dfrac{\tau}{t}=\sqrt{1-\dfrac{r_s}{r}}=\sqrt{1-\dfrac{v^2}{c^2}}$$
$$v=c\sqrt{\dfrac{r_s}{r}}$$
$$v=\sqrt{\dfrac{2GM}{r}}$$
Where $c$ is the speed of light, $G$ is the gravitational constant, $M$ is ...
2
Black hole will absorb the momentum and start to move with a constant speed.
A black hole solution can be transformed to any other inertial frame via a global Lorentz transformation, so it can move with any speed through space.
1
I completely agree with Rob Jeffries and just want add another aspect.
Imagine marbles are released one shortly after the other towards a mass, a planet, a star or a black hole, e.g. During their free fall the distance between these marbles will increase, because the closer to the mass the more gravitational attraction they feel.
So, the radial chain of ...
4
Spaghettification is not just a General Relativistic effect and not just confined to strong gravity regimes. It is a consequence of different parts of an extended body feeling different gravitational forces and this produces a differential force acting across that body - a.k.a. a tidal force.
In the specific case of an object of finite size falling towards a ...
2
Tidal forces: gravitational forces on the matter that is closer to the black hole, is way higher than gravitational forces on matter just a little distance away from the black hole. this happens on Earth too, but it is at strong gravitational fields that this effect becomes highly noticable
0
A black hole (or alternative compact object) has infinitely many quasi normal modes (QNMs), all of which generically get some level of excitation. So from a physical point of view, you should include infinitely many terms.
However, in practice this neither feasible nor desirable. First of all one should note that we are dealing with exponentially decaying ...
2
The quantity $C = g_{\alpha\beta}\dot{x}^\alpha K^\beta$ is a scalar invariant and is therefore independent of the coordinate system used.
Since coordinate systems exist that are not singular at the event horizon (for example Kruskal Szekeres) you could calculate $C$ at the horizon using these coordinates. The quantity is not affected by a change of ...
2
There have been serious suggestions about how to detect a 5-10 earth mass black hole in the outer solar system (as was mentioned in the comments, the horizon would be very small, and thus conventional searches would fail). This recent paper (by Witten!) suggests using hundreds of small spacecraft to probe the gravitational field via timing signals sent to ...
1
The simplest explanation is that a star above a certain mass collapses to a singularity because there are no known forces that can exert sufficient pressure to stop the collapse.
For example, above the Tolman-Oppenheimer-Volkoff mass, a neutron star collapses because neither the repulsive interaction between neutrons due to QCD nor the quantum degeneracy ...
0
I looked at Page’s 1976 paper on emission of massless particles from a Kerr hole. He numerically solves two differential equations for the hole’s changing mass and angular momentum. I didn’t find a simple formula for the resulting lifetime.
0
If there were mechanisms for the production of negative-mass particles, then we would have instability of all forms of matter, not just instability of a positive-mass Schwarzschild black hole. Inconsistency with the laws of black hole thermodynamics would prove that such a process cannot exist, but consistency with them does not prove that it can.
If a ...
2
at the moment after the big bang, how much gravity would a person 'standing', say a metre away experience
The Big Bang didn’t happen at a point, so you can’t be one meter away from it. The Big Bang model is one of a homogeneous and isotropic universe in which spacetime curvature invariants are the same at every spatial location. The curvature gets weaker ...
1
The radiation, as a property of the curvature of space is called Unruh effect.
We study the framework of quantum field theory in curved spaces. The Unruh effect and Hawking radiation are described for both Minkowski and anti-de Sitter spaces,and grey body factors are approximated for asymptotically flat black holes.
Hawking radiation is one of ...
4
Hawking radiation is emitted only if an event horizon is present i.e. only if the object is a black hole. So masses like the Earth, or my coffee mug, are not going to evaporate by emitting Hawking radiation.
As you would expect from such a click-bait title Siegel's article is misleading, which is a shame since Siegel is a well respected physicist. Hawking ...
4
Trapped surface is a quasilocal notion — a closed two-surface which has the property that the expansions in each of the two forward-in-time pointing, normal-to-the-surface, null directions are everywhere negative. One does not need to know the metric outside of immediate vicinity of a surface to determine whether it is trapped.
Event horizon, on the other ...
Top 50 recent answers are included
