Today a friend of mine posed me a paradox involving black-holes, one that I couldn't solve.  Suppose we have planet, with a density such that it is almost to the point of turning into a black hole, armed with a powerful laser on the surface (kind of like Star War's Death Star) capable of shooting down any star ship.  Now I, on a really fast spaceship (for the sake of discussion let's say around 10% of the speed of light) approach this planet.  Now, because of relativistic effects, from my point of view the planet will shrink, keeping mass constant, therefore increasing its density and finally turning in a black hole. Because of that the laser will never be able to escape the planet, and I'll be fine.  Instead, from the planet's point of view, nothing as really changed, only a puny space-ship is flying into its direction, the laser will shoot and take me down.  How is this possible?
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Possible Duplicate:          Imagine, a rod of length L is moving with velocity approaching the speed of light with respect to a human observer on Earth. Due to Lorentz contraction, the rod will observed to be very short. And since all laws of physic hold true in every frame of references, the gravitation law which state the force acting is inversely proportional to square of distance between them, will be acting between various part of the rod. Now, as velocity approach c , L will approach 0. This should cause an enormous gravitational force enough to form a black hole. Isn't this suggesting a black hole can be formed when the object velocity comes near the speed of light?   According to the rod's frame of reference, it will see itself as stationary and a man who observe it is moving, so according to the rod, the human must be a black hole. Isn't this a paradox?
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From watching cosmology lectures, it seems that the space between galaxies is expanding at an accelerating rate, my question is since it is the space that is (acceleratingly  expanding), the special relativity does not apply in this case? in other words since it is not anything that is accelerating with relative distance, would at some point galaxies that are far enough from each other separate faster than speed of light?  Please note, I am not asking if the galaxies would move faster than speed of light, but whether the rate of expansion of space be faster than speed of light so in effect each galaxy would be as if it is behind an event horizon where even light can not escape?    If yes, is there a name for such an event horizon that is cause by expanding space outside rather than the conventional black hole event horizon caused by stretching of space within the event horizon?   How would one be able to differentiate between a black hole even horizon and a galactic even horizon caused by accelerating expansion of space outside? (not sure if the concepts of inside and outside are still meaningful in such cases)
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Pedestrian question from a non-physicist:  I read on Wikipedia that larger black holes emit less net Hawking radiation than smaller black holes.  This seems counterintuitive to me. If black holes are essentially sucking in mass and converting it to Hawking radiation, why wouldn't a more massive black hole emit more Hawking radiation?  Moreover, how is it that Hawking radiation can escape black holes in the first place when visible light (which I know to be another form of radiation) cannot?
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My question is in the title: Do black holes have a moment of inertia?  I would say that it is: $$I ~\propto~ M R_S^2,$$ where $R_S$ is the Schwarzschild radius, but I cannot find anything in the literature.
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The event horizon of a black hole is where gravity is such that not even light can escape. This is also the point I understand that according to Einstein time dilation will be infinite for a far-away-observer.  If this is the case how can anything ever fall into a black hole.  In my thought experiment I am in a spaceship with a powerful telescope that can detect light at a wide range of wavelengths.  I have it focused on the black hole and watch as a large rock approaches the event horizon.  Am I correct in saying that from my far-away-position the rock would freeze outside the event horizon and would never pass it? If this is the case how can a black hole ever consume any material, let alone grow to millions of solar masses.  If I was able to train the telescope onto the black hole for millions of years would I still see the rock at the edge of the event horizon?  I am getting ready for the response of the object would slowly fade.  Why would it slowly fade and if it would how long would this fading take?  If it is going to red shift at some point would the red shifting not slow down to a standstill? This question has been bugging me for years!  OK - just an edit based on responses so far.  Again, please keep thinking from an observers point of view.  If observers see objects slowly fade and slowly disappear as they approach the event horizon would that mean that over time the event horizon would be "lumpy" with objects invisible, but not passed through?  We should be able to detect the "lumpiness" should we not through?
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So lately i heard of this theory that planet 9 might be a Primordial Black Hole (PBH) with 5 - 10 times the mass of Earth. I was thinking to myself, what would happen, if such a PBH (if it even exists) were to fall into our sun? Would it slowly consume the sun, even tough its mass is just a fraction of the star? Would the sun eventually become a black hole itself?  PS: Sorry for any typos or any wrong wording (I´m not a native english speaker)  PPS: I´m not a physicist or anything, i´m just interested in the universe and how it works.
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I have an intuitive scenario. Consider we have a spaceship just below the event horizon of a BH, which is merging with another black hole.  Finally, the singularities merge and we have a single black hole again.  But, in the transient stage, it is unclear to me if a timelike world-line would exist to leave the system.  I suspect, the metric is probably far too complex for an analytical solution, but in the worst case, it could be maybe solved numerically.  As far I know, black hole merges are examined mainly in an inspiral scenario. I suspect, maybe the escape is possible only if they have a hyperbolic-like orbit (i.e. there is no inspiral, but they simply collide).  Is it possible?
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Imagine, if you will, there exists somewhere out there a supermassive planet. For some reason, this planet is only a shell of its former existence and all that is left is a crust of substantial size to keep it from collapsing in on itself (let's imagine it is stable at 5000' thick). Now, this planet was HUGE! How large, you ask? Large enough that the remaining shell is still able to maintain a gravity well strong enough to keep you anchored to the surface.  Pretty neat, huh?  Well, since this is a sight to be seen by everyone, you stroll along and, not looking where you are going, you tumble into a chasm and straight down into a hole that permeates the thickness of the shell. As you are falling down and down and down, you think:  "At what point do I stop falling down and I start falling up? And would I be able to retain the velocity, since gravity is a conservative force, and fall back through another hole on the surface on the opposite side? Where would the center of gravity even exist? I mean, aaaaahhhhhhh!"  What say you, browsers and scientists? A stretch of the imagination, sure, but is it physically possible? Could there exists a configuration similar to this where gravity could be concentrated to a point but still allow free travel up to the focus?
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Theoretically, What is the difference between a black hole and a point particle of certain nonzero mass. Of-course the former exists while its not clear whether the later exists or not, but both have infinite density.
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Yesterday we started relativity on our physics class, and my professor taught us a few concepts. We did some examples on how things changed by looking them from different reference systems, and a paradox came to my mind from an example he did on the blackboard:  Suppose that we have a bug inside a hole (and suppose that the height of the bug is $0$). This hole is $L$ meters deep. There's a man outside the hole that wants to kill the bug with a nail of length $l$, with $l&lt;L$. Obviously, it's not possible for him to kill the bug by trying to push the nail in the hole. However, if the nail moves really fast, from it's reference system the hole will have a height $L_n&lt;L$, and if it moves fast enough then it will happen that $L_n&lt;l$, so he finally could kill the bug. On the other hand, the bug knows that from his reference system, the nail will have a lenght $l_b&lt;l$, so it would never reach him.  Is it possible to quantify the problem? How is it possible that the nail reach the bug from it's reference system and from the bug's reference system it's even smaller than before?
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So I understand the concept of gravity, in that it's not actually a force, but more of a displacement in the spacetime grid. An object with a big enough mass will bend the spacetime, causing smaller objects to "attract" to it (not really, but good enough for what I am trying to figure out)   Ok so that makes sense, but what I am not understanding is, how come planets don't just get engulfed by the Sun? There must be something acting against the gravity for the planet to be a certain distance away from the sun. Am I not understanding this concept correctly?
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Last week, I went to Switzerland and visited the LHC. I took a tour, and the guide told us that it is possible for black holes to appear in the LHC, but they will be so small, that they would evaporate away due to Hawking Radiation and there would be virtually no time for the black hole to "suck up" anything. I suddenly remembered something I read on  that said a "a 100 tonne black hole would evaporate in 8.4×10−2 s, emitting approximately E=Mc2=9×1021 joules of energy as it does so – equivalent to more than a million megatons of TNT. I guess you could call this an explosion!"  If Mount Everest was condensed into a black hole, it would be smaller than a hydrogen atom. Mount Everest is 357 trillion pounds. 357 trillion pounds converted into energy would be a mind-boggling amount.  If a black hole formed in the LHC, wouldn't it completely annihilate it due to the spontaneous energy release due to Hawking Radiation? I'm new to the topic of Hawking Radiation, so I do not know if I have been misinformed in any way.
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Assuming for a second that there were a pocket of anti matter somewhere sufficiently large to form all the type of object we can see forming from normal matter - then one of these objects would be a black hole.  Question is, would there be a difference between an anti matter black hole and a normal matter black hole - in terms of how would the matter/anti-matter make the black hole different, or would they be the same?  I would expect the answer to be that the black hole formed from anti matter would retain the anti-matter properties in such a way that if it was to merge with a black hole at same size formed from normal matter that they would annihilate  each other con convert into pure energy - Would that be a correct understanding?
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So, the plausible explanation to who put the wormhole and the tesseract in the place is   Humanity survived through the plan B, that's why they mention it all the time. Then they build the worm hole to save the people of the planet.  But this explanation raises another question and yet another (similar) paradox. Question -   Why didn't they just simply give us the information from black hole that we need figuring out how to manipulate gravity?  Paradox -   Not to mention that why on Earth would the future descendants (by a margin of more than 1,000,000 years) care to change their timeline in order to save the long dead inhabitants of Earth by eradicating themselves from existence in the process? This is more than a logical question, this is a Mad Scientist Paradox.  References &amp; Sources:   ,  (on reddit).
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I'm making several assumptions, not sure if any are correct:   there is a black hole at the center of a galaxy the black hole is eating the galaxy   Eventually the galaxy will be gone, right?  Has this been observed? Do we know what happens afterwards?  Posting here since astronomy got merged into physics
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I was surprised to only recently notice that      An object of any density can be large enough to fall within its own   Schwarzschild radius.   Of course! It turns out that supermassive black holes at galactic centers can have an average density of less than water's.  Somehow I always operated under the assumption that black holes of any size had to be superdense objects by everyday standards.  Compare the Earth to collapsing into a mere 9mm marble retaining the same mass, in order for the escape velocity at the surface to finally reach that of light.  Or Mt. Everest packed into one nanometer.  Reading on about this , it increases proportionally with total mass.  Assuming matter is accumulated at a steady density into a spherical volume, the volume's radius will only "grow" at a cube root of the total volume and be quickly outpaced by its own gravitational radius.  Question: For an object the , what would have to be its diameter for it to qualify as a black hole (from an external point of view)?  Would this not imply by definition that:   The Earth, Solar system and Milky Way are conceivably inside this black hole? Black holes can be nested/be contained within larger ones? Whether something is a black hole or not is actually a matter of perspective/where the observer is, inside or outside?
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I'm a big fan of the podcast Astronomy Cast and a while back I was listening to a Q&amp;A episode they did. A listener sent in a question that I found fascinating and have been wondering about ever since.   From the show :      Arunus Gidgowdusk from Lithuania asks: "If you took a one kilogram mass and accelerated it close to the speed of light would it form into a black hole? Would it stay a black hole if you then decreased the speed?"   Dr. Gay, an astrophysicist and one of the hosts, explained that she'd asked a number of her colleagues and that none of them could provide a satisfactory answer. I asked her more recently on Facebook if anyone had come forward with one and she said they had not.  So I thought maybe this would be a good place to ask.