Let's say I decide to jump into a large black hole. A distant observer never sees me enter the black hole, but he does see the black hole evaporate. According to this reasoning, I would then keep going, having never gone into the black hole.  This doesn't seem to be what happens, though. What is the flaw in my reasoning (unless I'm correct!)?
32721
I am not a professional physicist, so I may say something rubbish in here, but this question has always popped in my mind every time I read or hear anyone speak of particles hitting singularities and "weird things happen".   Now to the question at hand, please follow my slow reasoning... As far as I've learned, to reach a black hole singularity, you must first cross an event horizon. The event horizon has this particular property of putting the external universe on an infinite speed to the falling observer. Now due to the Hawking radiation, and knowing that the cosmic background radiation is slowly dimming, sooner or later every black hole in this particular instance of inflation we are living in will evaporate, according to an external observer of said black holes.  This means that every black hole has a finite timespan, as long as this universe survives that long. Now if we go back to the falling observer, we had already established that such an observer would see the outside universe "speed up" infinitely. This means that when the falling observer "hits" the event horizon, he will (or it if we speak about particles, which is clearer in this case), be immediately transported in time towards the final evaporation moment of the black hole. Either this or the particle gets some weird treatment. My point is, such a particle never gets to the singularity, because it has no time to get to it. The moment it crosses the event horizon, the black hole itself evaporates.  Where am I wrong here?
53693
I've just answered , and while I'm confident my answer is correct I'm less certain about the exact consequences. To simplify the situation consider a diatomic molecule falling into a Schwarzschild black hole with its long axis in a radial direction.    The inner atom cannot interact with the outer atom because no radially outwards motion is possible, not even for electromagnetic fields travelling at the speed of light. However I find I'm uncertain exactly what the outer atom would experience.  We know that the outer atom feels the gravitational force of the black hole even though gravitational waves cannot propagate outwards from the singularity. That's because it's experiencing the curvature left behind as the black hole collapsed. Would the same be true for the interaction of the outer atom with the inner atom? Would it still feel an electromagnetic interaction with the inner atom because it's interacting with the field (or virtual photons if that's a better description) left behind by the inner atom? Or would the inner atom effectively have disappeared?  If the latter, presumably the fanciful accounts of observers falling into the black hole (large enough to avoid tidal destruction) are indeed fancy since it's hard to see how any large scale organisation could persist inside the event horizon.  Later:  I realise I didn't ask what I originally intended to. In the above question my molecule is freely falling and the question arose from a situation where the object within the event horizon is attempting to resist the inwards motion. I'll have to go away and re-think my question, but thanks to Dmitry for answering what I asked even if it wasn't what I meant :-)
55819
Let's say we have one black hole that formed through the collapse of hydrogen gas and another that formed through the collapse of anti-hydrogen gas. What happens when they collide? Do they (1) coalesce into a single black hole or do they (2) "annihilate" into radiation?  One would expect (1) to be the case if the No Hair Theorem were to hold. So I guess what I'm really asking for is a modern understanding of this theorem and its applicability given what we know today.
13592
I am a layman in physics and just read about black holes on the internet. I read that matter encounters geodesic incompleteness in the singularity in an uncharged black hole.  I heard an analogy of geodesic incompleteness as a straight line on a paper reach a hole on the paper, so it cannot continue. But in this analogy, isn't the straight line possible to continue into 3D (continue down the paper)? So, if matter reaches the singularity, is it possible too (to reach another dimension)?   I also heard that the matter is annihilated when reaching the singularity, does it mean it disappear from this world, and violates conservation of energy?
49057
I understand that the event horizon of a black hole forms at the radius from the singularity where the escape velocity is $c$. But it's also true that you don't have to go escape velocity to escape an object if you can maintain some kind of thrust. You could escape the earth at 1 km/h if you could maintain the proper amount of thrust for enough time. So if you pass just beneath the event horizon, shouldn't you be able to thrust your way back out, despite the $&gt;c$ escape velocity? Or does this restriction have to do solely with relativistic effects (time stopping for outside observers at the event horizon)?
85966
A black hole and white hole experience a direct collision.   What happens? What shall be the result of such a collision?
50637
If an answer does exist, I'd love to hear it. I'm trying to incorporate a doomed earth story in something I'm writing, and the end of the world I'm going for is a black hole.  Let's say the black hole was created in the Philippines (I chose it because it's on the equator). Here's  for reference.  If I was standing 90 degrees of that, in Kenya, or if I was standing 180 degrees of that, in Mexico; what would I see in those two separate instances? What if I was standing in Japan above it, or in Australia below it?  Let's also say I was riding a boat towards the source of the black hole. Unlike most imagined scenarios where I am orbiting the black hole and slowly falling into it, this time I'm no longer orbiting it - I'm getting sucked into it instead. I'm imagining that the black hole will "half" be inside Earth and "half" will be in the atmosphere. But as it pulls me in, what will it look like to me (if I was massless, so no tidal forces)? It's probably not going to be a black hemisphere sticking out of the ground. Light will be lensed around it in some way so as to appear like a hole in the air.  Also, if the black hole was very small, it probably won't have a strong gravitational pull, and assuming it doesn't disappear, you'd be able to look at it (or at least, its effect on the light in its surroundings). Of course, as you approach it, it would still have an event horizon, so spacetime would still be warped if you're near enough and the surroundings will still experience tidal forces.  As the black hole gains mass, it will get larger, and so its event horizon will increase. If I was standing in Kenya and the event horizon "washes over me," (as in a water wave) what will I see?  I've so far imagined that, at least when seen from outer space, there will be a hole on the ground and light will be lensed around it. When you are on the ground, though, it becomes a bit harder to imagine.  I do hope someone can lend me a hand!  Edit: Someone had previously answered this question, but I'm not sure why it's been deleted. It was pointed out to me, though, that a black hole cannot stay in place. So an additional question for me is, does that mean it is impossible to be standing still, and the event horizon "washes over you"?  Edit2: Here's a guiding thought experiment that could answer the question. Imagine that you are in outer space directly above the black hole, looking at the black hole that is on the earth's surface. Can you imagine what it would look like? It would simply be a hole with the surrounding light gravitationally lensed around it (possibly lensing the entire earth if it's big enough - but let's say it's not).  Now imagine that you are on the direct opposite of where you were above - you're right behind the black hole this time, looking directly at the black hole but unable to see it because the earth is blocking your view.  Now try to imagine, from the second image, rotate the earth into the first image. I find it a bit hard to do. Just as the black hole is appearing as you rotate the earth, how would the light be lensed?
41356
I'm not very knowledgeable about physics generally, but know that nothing can escape a black hole's gravitational pull, not even light (making them nearly invisible?).   My question is: What has been obtained from direct observation of a black hole to prove their existence? Please note that I'm not questioning the existence of black holes, but am just curious as to how we have come to realize it.
84313
An object of mass m is slowly lowered into a black hole of mass 1000 m. Is the amount of braking energy larger than $0.6 mc^2$?   Now what if, after lowering the mass close to the event horizon, we wait the black hole to shrink to mass 100 m, by emitting Hawking radiation, and then we lower the mass some more? Is the braking energy of the second lowering process larger than $0.5 mc^2$?   I wonder can we extract more than $E=mc^2$ energy from mass m?  (In this thought experiment we use a wire and a winch, or anything that works)
55245
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?
33873
Given the idea that gravity is always a factor (even though greater distances are excessively tiny), would everything eventually end up forming a single black hole?  My thinking is that, with infinite time, all mass will eventually be pulled together.
16595
This question was prompted by . Notoriously, if you calculate the Schwarzschild coordinate time for anything, matter or light, to reach the event horizon the result is infinite. This implies that the universe ages by an infinite time before someone falling into the black hole reaches the event horizon, so could that person see the universe age by an infinite time?  To be more precise, suppose the observer starts falling from rest at time $t = 0$ and some initial distance $r &gt; r_s$. If we wait for some time $T$ then shine a light ray at the falling observer. Will the light ray always reach the falling observer before they cross the event horizon? If not, what is the formula for the longest time $T$ that we can wait and still be sure the ray will catch the observer? If $T$ is not bounded it implies that observer could indeed see the end of the universe.  I can think of a qualitative argument for an upper limit on $T$, but I'm not sure how sound my argument is. The proper time for the observer to fall to the event horizon is finite - call this $\tau$. The proper time for the light ray to reach the horizon is zero, therefore the light ray will reach the observer before they cross the event horizon only if $T &lt; \tau$. Hence $T$ is bounded and the observer won't see the end of the universe.  I think a more rigorous approach would be to determine the equations of motion (in the Schwarzschild coordinates) for the falling observer and the light ray, and then find the condition for the light to reach the falling observer at some distance $\epsilon$ from the event horizon. Then take the limit as $\epsilon \rightarrow 0$. In principle this seems straightforward, but in practice the algebra rapidly defeated me. Even for a light ray the radial distance:time equation isn't closed form (Wolfram claims it needs the $W$ function) and for the falling observer the calculation is even harder.
64333
Let's do this thought experiment. You have an insanely powerful rocket and it can accelerate to 0.999999c. Now you fly to a supermassive black hole and hover just above its event horizon, where the inner gravitational pull towards the singularity matches the outward thrust of your rocket.  Consider: 1. The black hole is non spinning, and doesn't distort space-time into swirling around it.   The black hole is so massive that above the event horizon, you are not sphagettified. The rocket can shield the energy of extreme blueshifted photons that bombard you.    At this point over the event horizon, you would see like 1 million or even 1 billion years of the universe in 1 second of your life, right? Can you witness the end of the universe if you hung around for long enough?  And what if the black hole evaporates by Hawking Radiation before that? Will you immediately return to real time?  PS: This is quite different from falling into a black hole. In this thought experiment, we are not crossing the Event Horizon of the black hole in a split second, but we are physically hanging in there for a long time by Counter Force.  Edit: I'm not sure why this question has been marked as a duplicate, but this differs vastly from an observer falling into a black hole, and nowhere else has it been answered so perfectly and succinctly, as done by Mr. Bob. Many thanks to him.
84917
I am wondering whether a black hole can absorb photons whose wavelengths are greater than the horizon size of the black hole. I have seen different contradicting opinions on the answer. For instance , the accepted answer claims that the answer is no, without giving any justification whatsoever. While on , I think all the answers are pointing that yes, a black hole should be able to absorb photons whose wavelengths are greater than the size of the horizon of the black hole.  If the answer is yes, then black holes are perfect blackbodies, right? Or is it more complicated, because, like some answer on physicsforums claim, a black hole might have only a small probability to absorb photons whose wavelengths are greater than the horizon size (though no mathematical justification is given), thus even though it might be possible for a BH to absorb such photons, it might still not be a blackbody.  Edit: Here's what I've been told by a QFT specialist:      I think your question is not answerable. "A single photon" requires   knowing how to quantize the EM field near a black hole. That is an   open problem. And also, "single" particles don't objectively exist in   accelerated frames. See the Unruh effect.   I am not satisfied with the current two answers. They do not deal with the above comment at all. Furthermore they seem to contradict each other and they aren't crystal clear.
104400
Can two particles remain entangled even if one is past the event horizon of a black hole? If both particles are in the black hole?  What changes occur when the particle(s) crosses(cross) the event horizon?  I have basic Physics knowledge, so I request that answers not assume an in-depth understanding of the field. Thank you all in advance!
55353
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.
103749
If the event horizon of a black hole is the distance from the center from within which light cannot escape, imagine a person with a flashlight falls into the black hole.  He points his flashlight in a precisely radial direction and turns it on. Now there is a light ray moving outward at the speed of light. If it now cannot move in an arc, but rather is constrained to radial motion, it must, at some point before the horizon, switch directions and fall back into the black hole.  If the speed of light is constant, how does it suddenly change directions, without either decelerating, or requiring an infinite amount of energy?