# Tag Info

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A sudden singularity is a singularity that forms in the universe in a finite time. This may see like a strange definition. After all, don't the singularities in black holes form in a finite time, so shouldn't they, and indeed all singularities be sudden? Actually, no! For observers like us, floating around the universe, in our frame of reference it takes an ...

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You're assuming that the Kruskal–Szekeres (U,V) coordinates have to be defined in terms of the Schwarzschild (r,t) coordinates, but there is nothing special or fundamental about the Schwarzschild coordinates. General covariance says that we can use any coordinates we like. If the K-S coordinates had been the ones originally chosen by Schwarzschild, then ...

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Physicists are tasked to construct models of reality that are internally consistent and that have predictive power. Working on black holes and Hawking radiation fits this bill. The growing theoretical framework on black hole thermodynamics is absolutely essential to eliminate any internal inconsistencies that seemed to affect thermodynamics when applied to ...

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There are so many ways we can either directly or indirectly detect black holes that this answer will be necessarily incomplete. Although no light can escape a black hole, the effects of black holes on the space, matter, and activity around them is often very dramatic. One of the most common ways is via an accretion disk of matter spiraling around the ...

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Entropy isn't a force that causes things to happen. But anyway, the answer is no. Not all matter in the universe is expected to eventually collapse into black holes. See Adams and Laughlin, http://arxiv.org/abs/astro-ph/9701131 , section VD. Note also that black holes eventually evaporate, so when matter collapses into black holes, the result is that it ...

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There is nothing wrong with your calculations. From the Wikipedia article on supermassive black holes: "the average density of a supermassive black hole (defined as the mass of the black hole divided by the volume within its Schwarzschild radius) can be less than the density of water in the case of some supermassive black holes" Given that black hole ...

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I remember attending a seminar by Unruh a few months ago and the same question arised. As far as I remember, he enfasized that in these hydrodynamic analogs of black holes, the flow is not quantized, it is a classical fluid, and everything is classical and that the dumb hole behaves like a quantum amplifier emitting quantum noise from the Horizon. ...

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Our current understanding suggests that black holes can have electric charge, and that in addition to mass and angular momentum, these are the only ways that black holes can have distinguishable physical properties. This is a result of the famous no-hair theorem, although keep in mind that no definitive proof of this theorem yet exists. As this wikipedia ...

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The entropy of a black hole is given by the area $A$ of its event horizon according to the formula $S=\frac{kA}{4l_P}$ where $k$ is Boltzmann's constant and $l_P$ is the Planck length. For a rotating black hole with mass $M$ and a Kerr parameter $a$ the area is $A=\frac{8\pi G^2}{c^4}M(M+\sqrt{M^2-a^2})$. This is largest when $a=0$ corresponding to the ...

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Nice question! I'll try to put together a few ideas into what might be a valid answer, but this could be wrong. The event horizon is a lightlike surface. Therefore no proper time passes at the horizon, and any photon emitted exactly at the horizon would remain exactly at the horizon until the moment when evaporation was complete, after which it would fly ...

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In your diagram where the black hole is just behind the observer on the line between the observer and the object being observed the answer is "yes", the apparent size of the object will shrink. The color of the object will be blue-shifted and the whole universe will appear to contract. There is a great demo of the effect at Journey into a Schwarzschild ...

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The conceptual hang-up you have with Hawking radiation falling back into a black hole is a misunderstanding. You're right that classically "nothing can escape a black hole". The trick to Hawking radiation though is that it forms on the horizon of the black hole and saps energy from it. The radiation is not escaping from the black hole, it is stealing ...

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where does it go and what happens when it clogs up? In the context of an ideal, static black hole in general relativity, world lines end at the singularity. Consider the diagram below: This is the Schwarzschild geometry in Kruskal–Szekeres coordinates. In these coordinates, it is clear that there is no place or time that the entities falling into ...

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Why would you assume they do not ? Of course they do. But as you probably know, the gravitational pull decreases with distance (inverse square law). From a safe enough distance any other object (star, galaxy) would feel the normal gravitational pull of an object of the black-hole's mass at that distance; it makes no difference to the stars if the source ...

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This is a great question, although unfortunately it turns out to be very difficult to interpret it in a way that allows a definite answer. The question is ambiguous because of the way mass is defined in relativity. From the way the question is posed, I assume the OP doesn't have a lot of technical background in relativity. However, there is no way to resolve ...

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You can take a Reissner-Nordström solution for the charged non-rotating black hole, and put its mass $m=0$. Then it would become a so-called naked singularity. More precisely, singularity is a point where some value ends at infinity, while density of mass being just one option. For more thorough consideration of Reissner-Nordstrom and Kerr-Newman solutions, ...

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It's easy to forget that, in the context of relativity, there is no universal time. You write: For an outside observer the time seems to stop at the event horizon. My intuition suggests, that if it stops there, then it must go backwards inside. Is this the case? But your intuition doesn't seem to take into account that, for an observer falling ...

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Your intuition is misguided, time does not run backwards inside a black hole. For an observer inside a black hole, time passes in a perfectly "normal" way, such as it does at the horizon. The stopping of time of time at the horizon is, as you mentioned, a phenomenon that only an outside observer experiences. It can for example be measured by noticing change ...

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People have known for a long time about the existence of cosmological models that include a Big Crunch singularity. In these models, the density of matter in the universe is great enough to cause it to recontract. These models are no longer of interest as descriptions of the actual universe, since we now know that the expansion of the universe is ...

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The idea that every prediction should be falsifiable is good as a first approximation. But the science is complex, and in it everything is linked to everything, almost literally. So actually we never know if some unfalsifiable prediction would be of any use. It can become falsifiable later. It can lead to some ideas which would lead to some new falsifiable ...

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These are two separate questions. It's better if you don't try to combine two questions into one. In answer to the first question, yes, a black hole can have a measurable charge. You measure it the same way you'd measure any other charge. This is all purely theoretical, however. Many real-life black holes have been observed and characterized by their ...

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I will have a go at an answer. Many good theoretical physicists attempt to create the Theory Of Everything (TOE): a mathematical theory that will model anything we observe in physical reality. Very many of them are concentrating their effort on some model using string theory. Suppose that we find a TOE that fits all known elementary particle data and ...

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Your question is essentially asking for opinions and will therefore likely be closed, but I think what you're missing is the fact that in any discipline where theoretical questions are inextricably linked with experimental questions it's impossible to judge in advance which theoretical questions are going to have practical ramifications. Physics and science ...

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This is an extremely common misconception so I will try to make my answer as simple as possible. A black hole is black and light can't escape because of mass for a given volume (density), not because of mass alone. Take our solar system for example. Our sun is very massive and it pulls on all of the planets hard enough that they orbit the sun. As long as ...

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This answer is kind of parallel to Brandon's, because I want to emphasise the point underlying these types of observations. We will never be able to observe a black hole, because for external observers the formation of an event horizon takes an infinite time. This may seem a bit pedantic, but it's an important point because our aim is not to directly ...

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Your question touches on the quantum behaviour of black holes, a subject with fundamental, unsolved problems. Classically, black holes cannot emit light, because light cannot escape their strong gravitational fields. The gravity is so strong that any matter that fell into a black hole would be torn into fundamental particles, and become part of the black ...

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You question is as same as asking what happens after a spaceship accelerates to faster than the speed of light. First I would say you have to reach c and the same goes for Rs. It may only take your one second to reach Rs in your own time but in that second the whole universe would change infinitely. Why? Because according to the scharzschild metrics ...

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They see nothing special. From the point of view of an infalling observer, the event horizon is just an invisible "point of no return". If Alice fails to start accelerating before her half of the ship crosses the line then she will be doomed along with Bob, but there is no physical measurement that either of them can make that would tell them where this ...

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In order to escape Alice's section of the space ship has to accelerate. The required acceleration increases rapidly as you approach the horizon (to infinity right at the horizon, meaning escape from there is impossible). So what they both see initially is Alice's section of spaceship accelerating, placing the spacecraft under increasing tension until is ...

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The metric is spherically symmetric. This means that angular momentum of the system is conserved (you can show this directly using the metric by computing the three killing vectors associated with spatial rotation and their corresponding conserved quantities) and therefore that the motion is contained to lie in a plane. If the motion is in a given plane, ...

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