# Tag Info

## New answers tagged black-holes

0

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 ...

0

Since in GR equations can get very complicated very quickly it's common practice to simplify then by using a system of units where pretty much every constant is set to unity. These are generally referred to as geometrised units. The trouble is that people tend to get a bit causal with the units. For example since $E = mc^2$ and in geometrised units $c = 1$ ...

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) ...

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 ...

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 ...

0

I think your guess is correct for mass-less "rockets" (a photon), but for real rockets (with mass m), the pertinent equations are the same one would use to calculate a satellite's orbit around the earth, at an altitude = to the radius of the earth, and making the velocity a little larger. Substitute the mass of the rocket and the equivalent mass of the ...

-1

I believe the issue is that external information from infinity will have difficulty catching up to the infalling object, ie, the light cones will not intersect. I have seen this answered in detail based on an infalling object having passed the event horizon, but since that can't happen, the person answering the question has oversimplified the problem. One ...

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 ...

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 ...

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 ...

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 ...

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 ...

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There were no "pre big bang" conditions. Not physically and not even theoretically. Time itself started with the big bang. How can there be a "time" before the start of time itself?

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The Big Bang was not like a black hole. See the canonical question Did the Big Bang happen at a point? to see why. The evolution of the universe from the Big Bang onwards is determined by the FLRW metric and the initial conditions. The FLRW metric is given to us by Einstein's equation, but we have no theory to explain why the initial conditions were what ...

1

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

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 ...

0

The simple answer is that general relativity does not, and indeed cannot, tell us what happens to the matter when it is all compressed into the singularity. We commonly describe black holes using the Schwarzschild metric because it's a relatively simple metric. However the Schwarzschild metric only describes the end result and doesn't tell us anything about ...

1

There are frameworks in physics, dimensional and energetic frameworks. There is the classical framework which has classical theories of mechanics and electrodynamics etc, where the dimensions are compatible with the meters/seconds/kilograms measures. There is the quantum framework which has quantum mechanics, quantum electrodynamics and quantum field ...

1

I'm just learning this myself, but for the the first one, the thermal state I think just means that if you throw any field in the resulting space-time, it will immediately acquire the specified temperature. In Hawking's original calculation, he shows that this radiation will be dominated by the massless, lowest-spin particles available, which in our universe ...

0

Have a look at the Wikipedia article on Binary Black Holes. Essentially when two black holes come in close proximity they are believed to merge into a bigger black hole so you won't get one black hole sitting at the event horizon of another for infinity. Super massive black hole binaries are believed to form during galaxy mergers Regarding the answer ...

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 ...

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 ...

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.

1

In short: the laws of conservation (angular momentum, charge, mass-energy, etc.) still work during the process of creation of a black hole. So if a star had some angular momentum/charge before it collapsed, the resulting black hole will also have some (assuming the angular momentum/charge was not radiated away during the collapse). Also, the claim that ...

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, ...

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 ...

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 ...

0

Does the photon add to the mass of the black hole an amount of mass m = e/c^2, where e is photon energy? Yes. If so, is it the energy the photon had at the beginning, or the energy the blue shifted photon has as it crosses the horizon? The energy the photon had in the beginning. As such, it will reach the horizon with no energy at all. He will ...

-2

Black holes do explode after their life is over, thus vomiting out all the matter they ingested. Apologies for not providing credible explanation for my claim (in the form of mathematical equations). I read that (that black holes do explode) in Stephen Hawking's book titled Black Holes And Baby Universes. He stated that there is an inverse relation between ...

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 ...

0

I would imagine that, the energy that pulls the quark pair into the black hole is intrinsic energy (ie it comes from the black hole), and the quarks created through 'spaghettification' would only be proportional to that energy pulling on them. Therefore, seeing as energy is equal to mass and vice versa this new mass would not be created, rather transformed ...

0

Your scenario can't physically exist. You can't separate areas of space within bubbles and adjust time within them arbitrarily then compare them. Time dilation occurs due to speed or gravitational potential. This means you can't instantly step from one place where time is running normally to one where it is running faster. This is because you need to ...

0

It appears the main confusion here lies in the concept of a particle. So to simplify things, first consider flat space-time which is sufficient to discuss most of the issues here. An accelerating observer in flat space-time will see an event horizon of a finite temperature, emitting particles. This is called Unruh radiation. This is purely a coordinate ...

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 ...

1

A pair of virtual particles are formed on the event horizon of a black hole--this is a particle anti-particle pair. These form all the time, but they usually just annihilate each other. However, if they form just on the black hole's event horizon, then one will be trapped in the gravitational field of the black hole, while the other can escape freely. Thus, ...

2

Nobody knows. That's it, in a nutshell. However, there are some various ideas floating about. Here's a (long) list of some: From this page: Collapse of massive gas clouds Merger of lots of stellar-mass black holes Growth of a stellar-mass black hole to astronomical (pun intended) proportions From Wikipedia: Core collapse of a cluster of stars ...

1

There is a relatively new theory (2012) called the firewall theory, that says that at the event horizon there is a huge "wall of fire" as such. This is because quantum entangled particles that cross the horizon (or one half of a pair of entangled particles) becomes tricky and starts breaking laws like the monogamy of entanglement. So a group of physicists ...

2

You are correct that physicists think a black hole is a link to another universe. No, not really, that's just an attention grabbing headline. Physicists (most of them) don't believe black holes are links to other universes, apart from some fringe theories that I'll come back to. But let me explain where this misconception comes from. This absolutely ...

4

I think the following image sums up why your model, at least for our galaxy, is wrong rather nicely: These are the orbits for 6 stars in the inner region of the galaxy. The orbital period for S2, for instance, is 15 years for an orbit that is roughly twice the size of Sedna's orbit--which takes it 12 thousand years to complete its orbit. Using Kepler's ...

1

There is no basis for such speculation. General relativity defines the event horizon of a black hole as the boundary from which nothing -- no matter, no light, no information -- can ever escape. Thus the question of what is inside a black hole becomes rather moot. The answer cannot even in principle affect us outside the event horizon. Now black holes are ...

8

There are many problems with this line of reasoning. The most common galaxy types are elliptical galaxies and spiral galaxies, and there might be a parallel with star systems, where the most common types are systems with a single star, and binary systems with two stars in the middle. There is simply no justification for this. The dynamics of stellar ...

2

You're focusing too much on the stuff at the center of the hole. The thing about general relativity is that lots of mass in any form will alter the space and time around it. If you bend things enough, the direction we used to unambiguously call "forward in time" bends into "toward the singularity at the 'center' of the black hole." So there is a region -- ...

0

Because the gravitational field is so strong, everything (ie all matter and light), if it gets close enough, "falls" into them, just like a hole.

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This is explained thoroughly in Thorne's book "The Science of Interstellar". There were two scientific papers based on the simulations: One in physics and one in computer rendering. The two circles are caused by gravitational lensing by a very rapidly spinning black hole. The radius of this black hole is 150 million kilometers with a mass of 100 million ...

20

First note that this is a fictional movie and the image is an artist's impression, not a detailed simulation. The public seems to think the movie is some sort of fictionalized documentary, which it never claimed to be. That said, the image is qualitatively conveying some of what happens near a black hole. The diagonal disk is the accretion disk -- this is ...

4

The horizontal circle is probably the accretion disc of the black hole. The vertical circle might depict the effect of gravitational lensing (although I am not sure this depiction is accurate).

1

Black holes are not immortal and are theorised (by Stephen Hawking) to radiate Hawking radiation, which is a quantum mechanical effect. The section "Emission Process" in the Wiki article has quite a good summary of how this happens: by the equivalence principle, one can compute the physics for an observer hovering near the black hole's Schwarzschild horizon ...

1

I'm no expert but there is a mechanism that causes black holes to lose mass called Hawking radiation. To understand it, there is a thing in quantum mechanics called the uncertainty principle that basically means that if you know the exact (or nearly exact) position of a particle then you cant know the momentum of that particle to the same degree of accuracy, ...

2

One thing to note is that this horizon would only be present in an idealized eternal black hole, for a realistic non-rotating black hole formed by a collapsing star the Kruskal-Szekeres diagram would look more like the right-hand diagram below (from Gravitation by Misner, Thorne and Wheeler), where the gray area represents the interior of the star and the ...

0

There are a couple of reasons why your question isn't answerable. Firstly if you're sitting outside the horizon trying to measure the colour of the particle then you'll never see it reach the horizon, let alone cross it, so you would just measure a white particle as usual. If you were sitting alongside the particle as it fell then there would be no horizon ...

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