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Assuming dark matter is a WIMP, NASA made a computer simulation on dark matter particle annihilation to see what would happen when dark matter fell into a black hole. According to the article, it may produce gamma ray as the collisions of dark matter particles increase closer to the black hole. Here is the preprint for the NASA simulation: ...


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There are two issues. One issue is that some people try to oversell the "no hair" theorem to areas beyond where it applies, they apply a long term analysis of final states as if it applies in the short term. The second issue is that in the shirt term we have two objects, the black hole and the incoming object. We have to think about both. I'll go into ...


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The curvature of spacetime is determined by the stress-energy tensor, and in the stress-energy tensor we do not distinguish between matter and energy. The two are treated as equivalent and interconverted using Einstein's famous equation $E = mc^2$. The Schwarzschild radius of a mass is conventionally written as: $$ r_s = \frac{2Gm}{c^2} \tag{1} $$ where ...


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Is a black hole's mass uniformly distributed? One hears conflicting answers to this. One article I rather like is the mathspages Formation and Growth of Black Holes. See this bit: "Historically the two most common conceptual models for general relativity have been the 'geometric interpretation' (as originally conceived by Einstein) and the 'field ...


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In the mathematical models of black holes that we use, there is a parameter M which is related to the mass-energy of the black hole. There is NO hint of distribution of mass-energy inside the black hole. In particular: a non-rotanting BH is spherically symmetric (these BH probably do not exist in nature, since all astronomical objects rotate and pick up ...


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A non-rotating black hole can be treated as spherically symmetric using the Schwarzschild metric. A rotating black hole has an axis of symmetry and can be represented with the Kerr metric. Treatments of black holes using either of these would make the assumption that the "test particle" you are considering does not influence the metric (is much less ...


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So lets start with the first part of your question: Black holes radiate away by the famous Hawking process. Hawking radiation has been interpreted in many ways i.e. as pair creation near the black hole, tunnelling from the black hole and almost every other physicist will have a nice way of explaining this. What is the temperature of a black hole? It ...


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Particles may be able to escape a black hole through quantum tunneling. This is possible if a particle is confronted with a barrier it doesn't have sufficient energy to surmount, but nevertheless passes by tunneling through the barrier. It's a quantum mechanical effect that depends on the particle's probability function extending through the barrier, ...


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This is a complicated question that opens up a lot of avenues for further discussion. I'm not sure the question as you posed it has an answer that will be helpful to you, so I'll answer a related question that might satisfy your curiosity - "Under what circumstances is it possible to remove energy from a black hole?" Remember, everything is energy, so this ...


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As far as I know, no value in the observable physical universe is infinite, but we don't know: If the universe itself is infinite or not We don't know what is inside a black hole, so I don't think there is a definite answer to your question.


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Those objects are orbiting closely to SgrA${}^{*}$, certainly, but they are not orbiting closely enough to exhibit significant time dilation effects. In particular, consider the Schwarzschild spacetime. The inner most stable circular orbit around the central obect is at $r = 6M$, three Schwarzschild radii away. This makes the time dilation factor: ...


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If I'm interpreting your post correctly, you may be misunderstanding time dilation. Time dilation will not cause the stars to seem to move more slowly. The apparent velocity of a star in your frame of reference is the apparent velocity, and relativity will not change it. What time dilation would change is the apparent rate at which a clock moving with the ...


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About evidence supporting the existence of Event Horizons in these very compact objects, here are some news from the well known Cygnus X-1, one of the most studied compact objects and the most promising candidate for a stellar collapse black hole: ... evidence of just such an event horizon may have been detected in 1992 using ultraviolet (UV) ...


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...why do we trust black hole physics? ... (physics which is derived by combining quantum mechanics and GR such as Hawking Radiation, things relating to the Information Paradox, etc. ) Formally, there isn't quite a reason to because we've not observed these things yet. But that's also perfectly okay as well because that is how science sometimes works: ...


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General relativity (GR) turned out to be a great mathematically beautiful theory with amazingly accurate experimental predictions/observations (e.g, bending of light, precession of Mercury, etc). This theory naturally provides some simple solutions which are called black holes. In that sense one should take them seriously as they come from a firmly ...


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At first many people didn't care much for black holes. But later people showed that they were pretty unavoidable features of the theory of general relativity and that theory made other quite precise predictions that were tested and found good. So when you are told that black holes are required if you have GR and GR looks like the best game in town then it ...


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If you correct plank units by using Einstein's Appendix 2 of "Relativity" relation meters = i c seconds, and carry the i through all equations, converting any seconds units to meters, you get better insight. For example, this gives E= -mc^2 instead of E=mc^2, which cosmology agrees with. So you also convert all mass units to negative energy units. You then ...


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To answer your last 2 questions: "Are there bounds on optimal encoding, whose purpose would be to delay a release of a finite fraction of the information?" and "Can there be encoding which releases a finite fraction of the information only at the very last stages of the decoding?" It's not a potential purpose of optimal encoding, but a required ...


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Nobody knows for sure. If you take a look at the mathspages Formation and Growth of Black Holes you can read about two different interpretations of general relativity: "Historically the two most common conceptual models for general relativity have been the "geometric interpretation" (as originially conceived by Einstein) and the "field interpretation" ...


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Although General Relativity is used to calculate effects of strong gravity fields, and has passed all tests we can do so far, we don't know about black holes as GR cannot deal with quantum level events, which a black hole may turn out to be. We have lots of theories based on GR: (wormholes, other dimensions in this universe, other universes, etc...) but no ...


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A quick answer, based partially on John's comments above: Do black holes grow in size? As Rob says in his comment below, they do grow in mass, but that, in my opinion only, does not necessarily imply that they grow in size, because of the reason listed below. We don't know for sure, as we lack experimental evidence and , theoretically we are dealing with ...


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Actually in most cases you don't see the event horizon, but instead the photon sphere. For example if you are looking from some distance, if light emitted from some star goes inside the photon sphere (where light can travel theoretically in circular orbit, though the orbit is unstable), which is located outside the event horizon, it is more or less doomed to ...


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Calculating what you would see as you fell into a black hole is straightforward but tedious. Fortunately there are lots of sites that have done this for you. Actually, if you've been to the cinema recently the film Interstellar does a pretty good job of it. Less spectacularly, have a look at this site that has videos of what the journey would look like. ...


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Lots of questions here, I can answer a few of them. The question might sound very easy - Hawking radiation, however I was pondering that as you get closer and closer to a black hole, time dilates exponentially where the surface of the black hole is "timeless". So how can anything even anti-matter destroy a black hole if it cannot touch the ...


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The popular description of Hawking radiation with one a particle from a virtual pair falling in and the other escaping is just a pretty picture, "in another model, the process is a quantum tunneling effect, whereby particle-antiparticle pairs will form from the vacuum, and one will tunnel outside the event horizon." The problem is that proper description ...


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The event horizon or 'point of no return' is the point where the escape velocity of an object falling into the black hole has to be greater than the speed of light (about 299,792,458 m/s). That concludes that light can't escape it, therefore there is a sharp, clear distinction between the event horizon itself and the background. If you moved towards the ...


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Black holes contain so much energy in so small a space that indeed many particles of tremendous variety can be created and destroyed as long as certain conservation laws aren't being violated. Although I did crack a smile at the physician comments people made, I would like to let you know that a physicist works with physics and a physician works with ...


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If my understanding is correct, an electron is an elementary particle which means that it is just a point in space, ... The electron spin is a special case of the general concept of angular momentum, which is a physical quantity generated by rotations. This is completely analogous to energy being generated by time translations and momentum by spatial ...


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We don't know if the electron is an infinitely small object. It may have size and if it does maybe that will make you feel better about the fact that it creates a magnetic field due to its intrinsic spin.


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Black holes are extremely heavy, black bodies, all of which have a singularity in the centre. They are not anti-matter, they are a sort of 'deep well' of space-time, however, their strong gravitational force attracts matter into themselves. When matter and anti-matter annihilate each other, energy is conserved in the form of photons. Ps: A physician has ...


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As the comment above says, the word "spin" should not be taken literally, as in the spin of a beachball. The word spin came about as an attempt to physically understand the differing energy levels an electron can have, due to the magnetic field associated with it. The idea behind it goes back to when the electron was discovered experimentally to have a ...


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An electron does not spin! Its intrinsic angular momentum (the so called spin), should not be confused with the point-like electron rotating in configuration space (then the gyromagnetic factor would be one which is in a way related to charge spinning in configuration space. Actually the gyromagnetic factor of the electron spin is approximately 2.) A black ...


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Have a look at the paper Experimental Evidence of Black Holes, which describes the experimental evidence that would prove the existance of black holes. There is little point in me reproducing the contents of the paper here, but it's worth giving a couple of examples: One method is to measure the mass and radius of an object. For example we can measure the ...


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Evidence for black holes are indirect. The absence of light in a certain region is not a proof of the presence of black hole, since other objects can produce the same effect. Also, a black hole does not emit radiation, apart from the hypothetical Hawking radiation, which is nevertheless very weak. One way to detect a black hole is by detecting its ...


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I just want to add to Lumo's answer: The paper by vafa and Strominger instigated a lot of work in determining the statistical formulation of entropy in black holes. Although it must be pointed out that most of these are for cases with supersymmtry and (near) extremal conditions at small couplings. There has also been work in trying to address the microscopic ...


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This is a very profound question in physics. Given that a black hole has an entropy which scales as $$S_{BH} \sim \frac{A}{4}, $$ the question is how does this relate to $S_{Boltzmann} = K_B \ln W$. As in, what are the microstates of the theory which hold the information in the black hole. This was answered in part by a series of papers by Vafa, Strominger, ...


0

In general relativity, an event horizon is a boundary in spacetime beyond which events cannot affect an outside observer, i.e. any events separated by an event horizon are space-like. An other way to say this is that any worldline with a start within the event horizon will never cross the boundary of the event horizon. If you are familiar with the light ...


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What you're missing is the second paragraph here: That's Einstein saying light curves because the speed of light is not constant$^*$. The force of gravity at some location is related to the gradient in the speed of light at that location. See Baez: "This difference in speeds is precisely that referred to above by ceiling and floor observers". Raise your ...


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It is a truly unfortunate coincidence that the "escape velocity" at the Schwarzschild radius $R_\mathrm{S}$ is $c$. This leads people to think Newtonian mechanics can predict black holes. It can't. Note that a Newtonian escape velocity is the minimum velocity needed to coast to infinity against gravity. If you have less speed, you still travel away from the ...


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I will try and answer this in a very simple way. I think you are missing a crucial piece here. It is true that from the perspective of a remote "outside" observer (Bob) the person falling into the black hole (Alice) will never cross the horizon. However from Alice's perspective, nothing unusual will happen (in terms of laws of physics not biology, ...


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If 0celo7 didn't make sense to you: Nothing special happens to the observer that crosses the horizon (according to classical GR), he just falls in and gets crushed. It appears like someone is "stuck" to someone outside the black hole, but in a sense that are just photons that were massively delayed by the gravitational field. And this processes is very ...


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Look at the paragraph "gravity and the photon" in this link: In the relativistic framework, i.e. large velocities, any energy is also a relativistic mass: For the photon this means the following equation: m is the relativistic mass of a photon with energy h*nu. Gravity attracts relativistic mass, and the photon has one. Read the link further to ...


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You misunderstand. All objects have some escape velocity, which is the velocity needed for anything (photons or matter) to escape from that object's gravitational field. And that's not the velocity it needs to maintain under some sort of constant thrust, but the initial velocity it needs to, shall we say, coast away from the object. For a black hole that ...


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A particle cannot survive in region II indefinitely. Indeed, there does exist an upper bound on the survival time of the particle. As soon as the particle crosses the horizon into region II, its remaining proper time until it reaches $r=0$ is bounded by $$\tau_\mathrm{max}=\frac{\pi GM}{c^3}$$ We shall now show this. Since Schwarzschild spacetime possesses ...


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It doesn't just rotate around the accretion disk. As to what actually happens to it isn't clear-cut. Most people will say the infalling matter goes right through the event horizon all the way to the central point-singularity in finite proper time. But check out The Formation and Growth of Black Holes on mathspages: "Historically the two most common ...


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Matter in the accretion disk is not (yet) inside the black hole. It is orbiting, and it can even escape. In fact, some of it must escape for accretion to happen at all: The disk has too much angular momentum to be accreted, and so some material must be ejected, carrying off excess angular momentum, in order for the rest to fall into the hole. The general ...


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If black holes have mass but no size, does that imply zero uncertainty in position? If so, what does that imply for uncertainty in momentum? I mean to say that the particles which were originally separate have theoretically come to occupy the same point in space. Does the uncertainty principle apply to this phenomenon? Zero size doesn't ...


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General relativity is a classical theory. The Heisenberg uncertainty principle does not apply to it. The research frontier in physics now exists in quantizing gravity and unifying it with the other three forces (strong , weak, electromagnetic). Once that is done the solution for the black hole will become a probability distribution and the Heisenberg ...


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What constitutes a blackhole firewall? I think it's rather different to what you usually read about. I say this as something of a "relativist". I consider General relativity to be one of the best tested theories we've got, see Clifford M Will's paper http://arxiv.org/abs/1403.7377. However I find the given explanation of Hawking radiation unconvincing. ...


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Indeed, Newtonian mechanics does not apply here, but it can still be used to help your understanding. As a body grows smaller, its surface gravity increases. This is just a direct application of the $\frac{1}{r^2}$ law. As the pressure that was keeping the star large dies away, it shrinks in volume and its surface gravity increases. If the gravity at a ...



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