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Scientists May Get Best View Yet of a Black Hole in Action
- deleted 13y ago[deleted]
- sillysaurus3 13y agoA question for the physicists on HN: I've heard that black holes emit radiation. But if nothing can go faster than the speed of light, and a black hole's gravity is so strong that not even light can escape, then how can a black hole emit anything? Is there a kind of black hole which is so massive that not even that radiation can escape, or do all black holes emit some kind of radiation? (In fact, do they emit radiation proportional to their size?)
- IanCal 13y agoArmchair Physicist here, not a real one. There are two reasons why you'd say black holes emit radiation. An interesting one and a very interesting one (warning, other peoples scales may be calibrated differently to mine). 1) Black holes accelerate things massively, so they're travelling at an astonishing speed before they "enter". This can result in huge amounts of x-rays being emitted due to heating things to millions of degrees (well beyond white hot!). It's not the black hole itself, but the black hole is certainly to blame. http://hyperphysics.phy-astr.gsu.edu/hbase/astro/blkbin.html http://hyperphysics.phy-astr.gsu.edu/hbase/astro/blkbin.html 2) The weirder one. Hawking radiation: http://en.wikipedia.org/wiki/Hawking_radiation http://en.wikipedia.org/wiki/Hawking_radiation Hawking radiation happens when a pair of virtual particles "pop" into existence near the event horizon. Normally these pairs annihilate quickly, but if it happens near the event horizon it's possible for one of the particles to fall in and the other to escape. This results in a loss of mass of the black hole (told you it was weird) so could be considered to be the black hole emitting radiation.
- Ygg2 13y agoAlso some particles might travel faster than speed of light by tunneling or similar quantum mechanisms.
- jjoonathan 13y ago> one of the particles to fall in and the other to escape. This results in a loss of mass of the black hole (told you it was weird) How does the energy to create the virtual particles come from the black hole (which it has to in order for the accounting to work: -2+1=-1)? Is it a "Quantum Field Theory doesn't care about the event horizon" type thing?
- pdonis 13y agoIt doesn't take any energy to create virtual particles; virtual particle pairs are constantly being created and destroyed everywhere, according to Quantum Field Theory, but when they're created, on average, they have zero net energy: one has positive energy and one has negative energy. (Note that this is a heuristic description and not every quantum field theorist would agree with it. The only really unambiguous way to describe the process is using math; but translating math into everyday language is often difficult because our intuitions don't really match up with what the math is telling us. I'm doing the best I can.) However, if a virtual particle pair happens to be created just outside a black hole's horizon, the hole's tidal gravity can pull the negative energy particle inside the horizon before it can be annihilated by the positive energy particle. The positive energy particle can then escape. Effectively, this means the positive energy particle's energy is taken from the hole's mass, so the hole's mass decreases slightly.
- blaze33 13y agoYou may be referencing two different kind of radiations: - the one emitted by the heated matter falling in / spinning around the black hole (the quasar referenced in the article [1]) - the Hawking radiation. Basically a pair of particle/antiparticle randomly appears near the black hole event horizon, one of the particle falls into the black hole while the other one escapes. The black hole would eventually evaporate if you waited long enough. [2] [1] http://en.wikipedia.org/wiki/Quasar http://en.wikipedia.org/wiki/Quasar [2] http://en.wikipedia.org/wiki/Hawking_radiation http://en.wikipedia.org/wiki/Hawking_radiation
- deleted 13y ago[deleted]
- pdonis 13y agoIanCal and blaze33 both gave good answers, but just to clarify one thing: the radiation we see coming from regions where there are black holes is of the first type: radiation emitted by objects like gas clouds that are falling into the holes, before those objects cross the event horizon. If we leave out quantum effects like Hawking radiation (see below), it's impossible for light, or any kind of radiation, or indeed anything at all, to escape from inside the event horizon of a black hole. Hawking radiation is a quantum effect, which nobody has ever observed; the reasons for thinking that it exists are purely theoretical.
- jjoonathan 13y agoIsn't the nonexistence of large black holes from cosmic ray collisions proof of black hole evaporation? Or have we not been able to measure that / have reason to doubt micro-black-holes are created by cosmic ray collisions in the first place?
- pdonis 13y agoI would say we don't have accurate enough measurements or an accurate enough theoretical understanding to know how many micro-black-holes we should expect to see from cosmic ray collisions, on the assumption that none of them ever evaporate, or to be able to measure how many there actually are, so as to be able to compare the two numbers to see if there's a significant difference. In principle this would certainly be a good experimental test for the existence of black hole evaporation; I just don't think it's a test we can make with any confidence now or in the near future.
- fargolime 13y agoThe simplest answer: it's the region immediately above the black hole that these reports refer to, not the black hole itself (its event horizon and below there).
- Oculus 13y agoIs there a kind of black hole which is so massive that not even that radiation can escape, or do all black holes emit some kind of radiation? (In fact, do they emit radiation proportional to their size?) Also armchair physicist here - my understanding is that since information can never be destroyed, Blackholes must emit something. http://en.wikipedia.org/wiki/Black_hole_information_paradox http://en.wikipedia.org/wiki/Black_hole_information_paradox
- pdonis 13y agoI think there's pretty general agreement among physicists that we will eventually confirm that black holes emit Hawking radiation. However, that by itself doesn't help us to choose between all the various proposed resolutions of the information paradox.
- fargolime 13y agoIf only black holes weren't illogical! [1] Any decent software developer (i.e. highly logical thinker) who knows basic info about black holes can look at the picture there to quickly see that black holes are inconsistent with the core postulate of general relativity, the equivalence principle. They're a bug! (which Einstein spent a decade trying to find, or some other way that nature could prevent black holes he thought didn't "smell right", and no that's not my blog) [2] is previous discussion showing no bona fide flaw in the argument. I've not seen anyone refute that simple picture, which should be easy if it's incorrect. That black holes are a bug doesn't mean we can't explain observations. Gravitational redshift still exists to explain massive objects emitting no discernible light (might be only one photon every year, away from Earth). [1] http://finbot.wordpress.com/2008/03/05/no-black-holes/ http://finbot.wordpress.com/2008/03/05/no-black-holes/ [2] https://news.ycombinator.com/item?id=4926603 https://news.ycombinator.com/item?id=4926603
- pdonis 13y agoHere's another relevant discussion on HN that you conveniently forgot to link to, which does explain in detail the flaw in your argument (the discussion you did link to does too, but not in detail): https://news.ycombinator.com/item?id=5233894 https://news.ycombinator.com/item?id=5233894 Click on "parent" to see the post of yours that I was responding to.
- fargolime 13y agoYou've linked to that before, but there's nothing there that refutes the blog (not my argument). If there's a flaw it should be a sentence or two of logic, not multiple paragraphs that summarize as "it's not that simple". The blog shows that inertial frames falling through a horizon are that simple (after all, the equivalence principle demands that), and Taylor and Wheeler agree there. Extraordinary evidence is not a requirement per the scientific method. "Extraordinary" is not a scientific concept.
- pdonis 13y agothere's nothing there that refutes the blog I understand that you don't agree that what I posted refutes the blog, but I'm not trying to convince you, and I'm not going to rehash the argument here. I'm just linking to that discussion for the record, so others reading this thread will understand that your claims and the blog's claims about black holes are not undisputed. (not my argument). You may not have written the blog post, but you are claiming it's correct, so it is "your" argument in the only sense that matters here. If you're not willing to own the argument when it's challenged, then you shouldn't be linking to it. If there's a flaw it should be a sentence or two of logic, not multiple paragraphs that summarize as "it's not that simple" That's not the summary of what I said. The summary of what I said is this: the blog post's claims about what the theory of relativity actually says are not correct. So the blog post is not refuting the actual theory of relativity; it's refuting a straw man version of the theory that the blog post's author has constructed in his own head. Taylor and Wheeler agree there Agree with what? That there can be an inertial frame that falls through the horizon? Yes, of course. But that does not mean Taylor and Wheeler agree with the blog post's claims about black holes. The fact that there can be an inertial frame that falls through the horizon does not mean the blog post's claims about the details of how such frames work are correct. I went into detail about what's wrong with them in the thread I linked to.
- pdonis 13y agoOne small nitpick about a statement in the article; it says: Though we think of them as cosmic vacuum cleaners, black holes are actually just like any other massive body, such as a star. This means other objects can safely orbit them, until they get within a particular distance and pass what’s known as the event horizon, after which there is no escaping being sucked in. Technically, you can't "safely orbit" a black hole if you're any closer to it than three times the horizon radius. That is, there are no stable free-falling orbits, like orbits around a planet or star, closer than that. If you want to get closer than that to the event horizon without falling in, you will need to continuously fire your rockets to hold yourself at altitude against the hole's gravity.
- higherpurpose 13y agoTechnically there may be no "safe distance" at all, depending on how much time you're talking about. The whole galaxy seems to be caught in the spiral of a super massive blackhole, and it's just a matter of time before everything falls into it, although most stars will probably die out before they do. So I guess you could consider that "safe", since the time it takes to fall into the blackhole is longer than the time the object itself exists. Whole galaxies are probably drawn to each other, too, like we are to Andromeda and Andromeda to us.
- pdonis 13y agoThe whole galaxy seems to be caught in the spiral of a super massive blackhole, and it's just a matter of time before everything falls into it That's not necessarily true. From far away, a black hole is just like any other object with the same mass; there's no extra ingredient to a black hole's gravity that makes things more likely to fall into it from far away.
- millstone 13y agoThe central supermassive black hole does consume the galaxy, but in fact the rate of consumption is so excruciatingly slow that the galaxy will have "evaporated" long before then. Random processes like collisions with other galaxies will eventually eject nearly all stellar remnants into intergalactic space. The central black hole will only end up consuming about 1% of the galaxy's stellar remnants (as you say, all stars will have died trillions of years before the galaxy evaporates). The Milky Way is fated for just such a collision with Andromeda in a few billion years, and there's a chance our solar system will be ejected!
- th3iedkid 13y agowell yesterday some were talking about tracking a plane lost in earth and today i read about tracking a gas cloud in space!And supposedly the latter seems more like a soccer penalty shot!
- maxjones1 13y agomumble, mumble, something....Oprah...
- andrewflnr 13y agoAstronomers know that as recently as a few hundred years ago, the Milky Way’s central supermassive black hole was producing much more radiation. Wow, what a near miss. Would that have been visible at all? To think that if human technology had kicked off a couple centuries earlier we might have observed a black hole eating something already, that the answers to our questions about black holes could be common knowledge... wow.
- shire 13y agothis might sound like a stupid question but what happens when you get sucked up by a black hole? what's in the other side of it.
- ChuckMcM 13y agoIf you read Stephen Hawking's book "A Brief History of Time" you will see that a variant on this question is what started him on the path of reasoning about black holes in the first place. The question does not currently have a definitive answer. Although current mathematical analysis has in falling matter being dismantled at the sub-atomic level as it undergoes the tidal stresses associated with gravity. Basically if you were standing at the event horizon the pull on your feet would be several billion times the pull on your head. The confounding factor is that if you're falling into a black hole the acceleration can get your velocity to nearly light speed, and at that velocity your perception of time slows, to the point of nearly stopping. So while people watching you fall in might see a burst of xrays as your physical being converted into energy, "you" might perceive nothing at all, simply that time stopped (which is really not something you can perceive) followed by your non-existence (which depending on your theology either has you returning you energy to the entropy of the universe or a visit with your deity and/or anti-deity if there a judgement step.) Most theories do not currently postulate a 'far side' of a black hole, mostly because "hole" is a metaphor rather than a physical description of the object. In theory its really just a point where the numbers go out of whack because the equations have a divide by zero error there. This too is what fascinates a lot of people, the universe sets up this problem where it gets to divide by zero. A bit of calculus, a bit of fudging with infinities of both the positive and negative variety, and your guess is as good as anyone else's at this point. Fun to think about though.
- sigterm 13y ago> Basically if you were standing at the event horizon the pull on your feet would be several billion times the pull on your head. This is not always the case. If the black hole is massive enough there won't be strong tidal force at the event horizon to disintegrate objects. I don't know GR enough to judge your other statements. However, when discussing the time slowing down close by a black hole I suspect one must take into account of the strong gravity field and not just the velocity of the falling object.
- deleted 13y ago[deleted]