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It seems as if we've hit a new black hole craze. Normally these types of discoveries (with the exception of Hubble images and planetary spacecraft) are more or less confined to the scientific community. However, the Event Horizon Telescope discovery made headlines, was widely shared, and trended on the internet. Yes this is largely due to the connectivity social media provides us. But I think this image spread because people LOVE black holes! Their very nature inspires and confuses us, they spark curiosity, wonderment, and perplexity. Their paradoxical and inconceivable existence defies the tiny world we are confined too and tells us that there is so much more out there then we could ever hope to understand. Congratulations to the EHT team and everyone else who who helped make the image possible. This is a huge leap for science and humanity. I invite you to share any black hole thoughts in this thread. Here's one, if Saggitarious A* had relativistic matter jets like M87 shooting from it's poles would we be able to see them and if so what would they look like? |
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Here is my modest contribution to the black hole craze: M87 relativistic jet as seen in a 102 mm APO refractor |
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Great capture, that's on my to-do list of targets. Part of the reason the jet is so prominent is because it is pointed towards us, the light approaching us is intensified because of relativistic beaming, the doppler effect essentially. There are actually two equally powerful jets, one emitting from each side but the the one pointed away from Earth is visually absent. You could say your image supports Einstein's theory of relativity  |
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| Here is my all time question about Black Holes: when you approach a Black Hole at the Event Horizon the progress seen from an outside observer should be (is) infinite. I am ok with the fact that there is a Black Hole possible, but we should never see anything being swallowed in the rest of the universes lifetime. Or even more interesting: if you could survive the entry, you would be instantly at the end of time (cannot even imagine what that could be). So, where is the error in my thoughts? Has to be one since everyone (almost everyone) believes that there are Black Holes in our observable universe. |
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Uwe Deutermann: It is right, we do not see the final swalloing. But the object approaching the event horizon will fade due to gravitational redshift (its getting darker the nearer it comes to the EH because the light loses energy by escaping the gravitational potential). So even if we could wait till infinity to see the object entering the EH, there would be nothing to see :-) Uwe Deutermann: I am not sure about this one, but i dont think you would be at the end of time - your personal timeline would not change, time would not run faster there than here for you. But if you could turn around and watch the universe while approaching the EH you would see something like a fast forward of the universes progress - if your eyes were capable of watching gamma ray, because all light would be blue shifted to a very high energy level :-) Well, that gravitational monsters are mysterious. My mind is always blown away when i think about them - the same questions that you asked are often on my mind. In my second life i would be an astrophysicist to understand all these things  |
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Uwe Deutermann: There is an excellent book on this subject by Leonard Susskind "Black Hole War: My Battle with Stephen Hawking to Make the World Safe for Quantum Mechanics". Briefly, when we are observing something being swallowed by the BH, we see it asymptotically approaching the event horizon, but never crossing it. The object will be seen hanging at the horizon, slowly dimming with time, since fewer and fewer photons reflected from it are reaching us. When we are beeing swallowed by a super-massive BH, and the BH is large enough so that the tidal forces are relatively low, we will not even notice when we cross the event horizon. Once inside the horizon, we can live there until the rest of our life, circling around and slowly approaching the central dense object. If the BH is small, like star-size, then the tidal forces will simply make a spaghetti out of us and rip us apart. My questions regarding the BH concern their entropy and space-time fabric. According to the Bekenstein-Hawking formula, BH's entropy is proportional to the quarter of the area of its event horizon. Now, let's take the smallest possible BH with Planck's mass and Planck's length scale. It's entropy will be about 4*Pi ~ 12 natural units of entropy. Does it mean that the geometry of the smallest BH is dodecahedral? Does it also mean that the smallest voxel of the space-time, occupied by the smallest BH is also dodecahedral? Interestingly, rhombic dodecahedron can tesselate 3D space. Is there any anisotropy in the microwave background that could be an evidence of the geometry of the smallest BH? |
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Hi Michael! I believe that we see it the same way (btw, I studied Astrophysics, I am a life ahead of you ;-)). With the end of time I referred to the rest of the universe, looking back when you enter the Event Horizon. This is the "funny" part, you as the one who travels into the Black Hole will experience no change, while the rest of the universe is infinitely old. Infinite is a traumatic word in physics, not just time wise, also dense wise (Singularity). It usually points to a lack of understanding about the subject, that the models that we created are not sufficient. Everybody longs for the Grand Unified Theory (GUT) where those problems are eliminated (especially the Singularity). I see it way more fundamental: if infinite is such a bad thing, then the speed of light barrier and the Event Horizon are much "easier" targets to solve. And understanding that a person flying into a Black Hole will reach it "PAST" an infinite time does not make even sense. Kskostik, I agree that Suesskind if one of the physicists who is on the right track, I would like to hear from him though that a collision/merge of 2 Black Holes should never be possible in the universes life time, hence the gravitational ripple effect to prove gravity waves senseless. Since I am pretty sure that all the physicists are smarter than I am I must miss again something, still wondering though what is missing. Your entropy question is really interesting, and I would not be surprised if there is a dodecahedral shape involved, if there IS a Black Hole at all that we can observe (see Saturns hex shaped storm at the North Pole!). But ... is there even one visible at a stage that we can say it IS a Black Hole? Or are we just seeing always the close edge of an Event Horizon, never reaching the status of a Black Hole? This would very much look like a Black Hole, but still is not one (I am in favor of that one ;-)). Uwe |
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I wish I had an hard disk like a Black Hole: able to store an infinite amount of data and never get full  |
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Uwe,Uwe Deutermann: Why is it so? Sorry, I am not getting it why 2 BH should not collide. Once I asked an astrophisicist a question: will we ever register a collision of 2 BH, since they are supposed to approach each other asymptotically? The answer was that BH merging equals merging of their horizons. Since horizons are not physical bodies, they should merge just fine without the asymptotic effect. The gravity waves we are registering are produced before the horizons merge. I guess, after the horizons merge, the BHs will continue to emit GW, which just don't leave the horizon. There is a couple of interesting and controversial papers on super-massive BH, and their role in oscillating universe. Authors are claiming that both dark matter and observable acceleration of the universe expansion can be explained by general relativity without bringing in dark energy and exotic particles. I am very good in GR, and cannot say if there are any errors in their calculations. Perhaps, you can clarify. https://arxiv.org/pdf/1608.01541.pdf http://inspirehep.net/record/1666865/files/1804.02988.pdf Uwe Deutermann: I guess, all we can say is that object's density has become high enough to form an event horizon. What exactly is happening to the collapsing object we cannot know, due to the horizon. Unless we learn to extract this information using something like quantum entanglement, which, apparently, preceeds space-time. |
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Uwe Deutermann: Lucky you :-) Uwe Deutermann: Brrr, that sentence gives me a headache  Andrea Alessandrelli: It's called "google" i think ;) |
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First of all apologies to John Michael, I did not want to move the topic to a more theoretical subject!Here's one, if Saggitarious A* had relativistic matter jets like M87 shooting from it's poles would we be able to see them and if so what would they look like? My gut feeling for this is that we would be able to see it in high energy wavelength, but not necessarily visually. I might be wrong on this one though. kskostik, Very interesting articles you refer to, I read the first one and beside the fact that it is very difficult to verify the equations (I remember them, but that does not mean I still can work with them, long time ago!) the conclusions are very interesting. Michael Timm: I like your question, since it is expressing my dilemma :-). Michael Timm: This looks like the answer, but isn't the horizon the point of (the asymptotic) problem? That gravitational waves can exist without a merge sounds reasonable, so I am ok with that. The merge itself is still a riddle to me (even the papers do not answer it). Maybe I need a simple answer which is not possible. I like simple and straight forward ;-), and nature usually is behaving that way. Thank you all for an inspiring conversation!! |
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Uwe Deutermann: Now we can directly measure the radius of the photon sphere and, therefore, calculate the Schwarzschild radius. We could do it before, of course, by calculating the mass of the BH, but now we can do it directly with a ruler and an image of the BH. Let's use this in a gedankenexperiment. When a body crosses the EH, the area of the EH increases by 4 times body's entropy, and its radius increases by 2 times sqrt of body's entropy. The EH expands after swallowing the body. So does the photon sphere. Even though we cannot visualize the body crossing the EH, we still can detect the time of the event by measuring the change in photon sphere's radius. The effect will be even more dramatic when two BHs merge. A couple of questions come up: 1. When the EH expands after swallowing the body, will it also consume the image of the body hanging at it? Will the image disappear completely because of the EH expansion? 2. Will the Planck units of body's entropy integrate into the EH so that they will be permanently detectable? 3. If a body falling into the BH has a flashlight pointed outward, how long will we see its light? Body's image is supposed to hang there infinitely, but the flashlight has a battery with finite charge. Will we see flashlight emit more photons than the energy stored in its battery allows, thus breaking energy conservation law? We cannot see the event of flashlight going off, because its battery drained on the other side of the EH. |
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Uwe Deutermann: Hi there! The black hole in Sgr A* is not feeding very much. The vicinity of the BH is gas and dust-starved. There are stars whipping around it, but they don't go inside the roche limit. And there is that one gas streamer that was all a buzz in the news a few years ago. I haven't heard much on it since then. But I'm an astrophysicist. My dissertation was, in part, on BHs. They go through changes of state when they are accreting and the jets typically appear only in the low/hard state. This is when the accretion rate is low (but not so low that it's essentially zero). It needs to be accreting enough to have a prominent disk. It will have a truncation radius in the low/hard state and the jets result from magnetohydrodynamic phenomena occurring in the inner disk with the magnetic fields and strong BH rotation. These jets will produce copious amounts of synchrotron radiation peaking in the radio portion of the spectrum. This is such a significant portion of EM radiation from the jets that sometimes the low/hard state is even called the "radio loud" state! I have a paper that deals with synchrotron radiation, but in the gamma-rays and in the context of pulsars...specifically the one powering the pulsar wind nebula associated with the Crab nebula. |
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A black hole is simply where your taxes are going into  |
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A black hole is simply where your taxes are going into Yeah, if they are used well I actually do not mind  .John Kroon: As far as I can remember from my student times this should be not necessarily visible in optical wavelengths. On the other hand, it might excite some particles along the jet, and this would be definitely visible. |
