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What are Black Holes? – Sixty Symbols


Sometimes people ask me how do I feel about black holes, whether I find them scary or intimidating Well they, they are a bit, but they can be cuddly too These are two cuddly toy black holes that I got as a present for my sister-in-law. I just I just like them They’re just little cuddly. There’s a little baby one, and there’s a slightly bigger, you know, mummy black hole, if you like Oh, well, a black hole is a [um] region of space in which light can’t escape from Material comes together under the force of gravity tries to fight that [uh] if you get too much material you can’t overcome that Force and eventually mathematics and the laws of physics take us into this strange regime. Well, it’s an object which which forms when a star dies. It’s something hat’s got many masses of the sun, has collapsed, and its own gravitational pull It’s got a lot of mass packed in a very small region Eventually it collapses to the point where not even light can escape from it and it forms something called an event Horizon But it’s like a shell, if you like and that event Horizon is the, within that event Horizon, light can’t escape, so that’s why it’s black At the center it has this mathematical impossibility called a singularity So they say black holes can’t have hair well these ones do, and eyes. Brady: Do you know what I’d like–to make the earth into a black hole you’d actually have to make it about that size Professor: Yeah yeah, that’s right Yeah
so if that really was a black hole it would be probably more massive than the Earth contained within a Little tiny region Here on Earth if you throw something up in the air It comes back down again, okay, and if you throw it a bit harder It goes a bit higher, but it will tend to come back down again. If you throw something hard enough It’ll actually escape from the Earth entirely [Sounds of a rocket engine firing] A rocket [uh] doesn’t have to be I mean, you know you could fire a projectile up anything as long as it’s traveling at this escape velocity, escape speed, it will escape from the Earth’s Gravity keep going and going and going off to infinity Rather than eventually turning around and coming back. If you were to make the earth a bit more Compressed then the gravitational pull would get a bit more intense And so actually you’d have to throw something a bit harder to get it to escape from the Earth’s pull of gravity And if you kept squishing the Earth and squishing the Earth eventually you’d reach a point where the gravitational pull was so strong that that escape velocity became equal to the speed of light and Because we can’t get anything to go at the speed of light apart from light, and in particular we definitely can’t get anything to go faster than light, that means if you then squish it a tiny bit more then nothing can escape not even light can escape nothing can escape from it because suddenly in order to escape from it you have to make something travel faster than light in order to do so But a black hole in general relativity is actually more sinister than that It’s not just that light can’t escape, a black hole in G.R., there is literally no way to get out of it [um] If it was just a matter of the escape velocity being high you could always build a ladder Climb out of it and with a sufficiently powerful rocket get away [uh] But there’s actually more to that space and time Collapses in on itself as you enter the event Horizon of a black hole in G.R. And there’s literally no way to escape from it Brady: Now, we all know black holes involve these things called singularities, but from what you’re saying it sounds like you don’t have to get as far as a singularity to get to a point where light can’t escape. It could be quite a lot bigger than a singularity. Professor: Indeed and in fact, so these the kind of the thought experiment I’ve just gone through predates general relativity by a long time I think probably laplace or someone was the first people who–person–who actually thought this through and realized there was a point where you would create a thing called a black hole and in, you know, if you, just dealing with Newtonian Gravity you still have a black, you can create black holes It’s only when you then say okay So what, when you try and solve the set of equations as to what’s going on within general relativity When you’ve gone through this process that’s when you realize that you actually create a singularity by doing that. In G.R. space and time becomes compressed such that any movement you make in time or space Takes you towards that center of the thing and actually that’s what led [uh] Hawking and penrose(?) in the late 60s to suggest that you have to have a singularity at the center of a black hole essentially. And then eventually your equations inevitably break down and so you have to repeat, the very existence of black holes and the singularity that you’ve seemed It looks like you have at the center of them, tells you that Einstein’s theory is not a complete theory It’s at best an effective theory that needs to be replaced by something else when you start talking about, you know, high energies, strong gravitational fields, and so on and so forth Even when you get to photons, things with no mass at all, they still get pulled by gravity, and we can see that directly through this phenomenon called gravitational lensing That way you’ve got like light from something passing a massive body, like a galaxy or something like that, you can actually see that the path of light has been bent by gravity and so actually gravity is affecting even light its path is being diverted by it so it’s being pulled by the Gravitational field so even lights not immune to gravity. Now the thing that’s different is that light doesn’t slow down because light always travels at the speed of light, so if you think about, you know, you’re on one of these incredibly compact objects and you’re shining a light upwards and so, what happens? It’s not like throwing a rock in the air that it comes back down again, with light it will keep traveling away from you at the speed of light, but it’s using up energy. Might be another way to think about these things is the energy that you’re turning the kinetic energy of something If it’s a rock into its potential energy as it goes away from the surface is the same with light, but the way that light loses its energy isn’t by slowing down, the way that light loses energy is by changing color. It would lose its energy by changing color by, you know, starting at the the blue end of the spectrum and ending up at the red end of the spectrum and then beyond red it would turn into infrared, radio waves, and actually, before it escaped, it would have used up all its energy and kind of red shifted to nothingness, there’ll be no energy left to escape So little lone black holes, which we call stellar-Mass black holes, can be one of the several end points to the evolution of a star They’re by no means the most common end point so most stars like our own sun Will turn into a white dwarf at the end of their life, and then fade into basically nothingness. If you have a star that’s massive enough, and the mass of the star governs pretty much its entire life, it will either turn into a neutron star through a Supernova, or if it’s even more massive, it’ll turn into an even denser object. which is the black hole, and so, you know, there are a lot of stars out there so there are a lot of black holes out there [um] but they’re by no means sort of ubiquitous and and littering the Galaxy we know there’s always gravity, so there’s always something pulling things together so the sort of sometimes the more salient question is what stops everything collapsing into black holes and so, for example, in something like the sun, you’ve got the nuclear fusion in the middle which is producing the energy, which is heating things up, which is kind of keeping it puffed up, but if you were to turn off that nuclear fusion, then the sun would start to collapse down and gravity would pull it and pull it and pull it and then you say, okay, so what else could stop it? I’m no longer got my source of fusion, and it turns out there are few things that might help out. [um] So this thing called Electron degeneracy pressure [um] Which is, you know, which is a very electrons basically don’t like being squashed close together And so Electron Degeneracy pressure will stop something like the sun from collapsing all the way to a black hole It’ll turn it into a thing called a white dwarf, which is held up by Electron degeneracy pressure. Turns out if it was a bit more massive than even electron degeneracy pressure wouldn’t do it, gravity would win again, so it would say ‘to hell with this’ squash all the electrons together And then it will collapse down further, to kind of nuclear density, at that point you create a thing called a neutron star And there’s a thing called neutron degeneracy pressure, which is that neutrons don’t like being squashed very close together either [um] And so that will halt the collapsing and end up with a neutron star, but then it’s a little bit more massive than that, so maybe about twice the mass of the sun, bit more than that, then even electron, even the neutron degeneracy pressure won’t stop it and then it will keep collapsing, and then, as far as we know, there’s nothing that will stop that collapse. Now maybe we’re missing some physics. Maybe there is some process we don’t know about which would actually halt that collapse, but we don’t know what it is but they’re the boring ones, let me tell you about the interesting ones, the interesting ones are the ones that live in the middle of Galaxies which are enormous they’re very very massive [the three professor’s voices are overlaid and indistinguishable]

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Comments
  • Can gravity escape a black hole? If yes, then that would mean that it is in fact not a particle, but yet a force.

  • PBS spacetime video titled "Do events inside black holes happen?" gives a more rigorous definition of a black hole. Check it out if you are interested! 🙂

  • They were made up when people thought that gravity was the main driving force behind everything in the universe.. .  .But its not.. .  .How can a force escape if nothing can escape.. .  .Sucking from within are spooky Black holes pulling from a distance? And are they surrounded by halo's of pushing dark matter from without ?.. .  .Black holes and dark matter only exist within dark regions of grey matter.. .  .

  • So if light can't escape the gravity well because it gets red shifted to nothing before it escapes, what's to stop someone putting a relay orbiting the black hole at a distance such that, for example, a gamma photon emitted deeper in the well has been red-shifted to say microwave wavelength and then re-transmitting this received signal at a lower wavelength with enough energy to make it the rest of the way out?

  • What would happen if two quantumly entangled photons flew past a black hole and one of them fell in?

  • Could you do a video on Multiple Time Dimensions or Retrocausality? Find that stuff wicked awesome and there's serious physics work done using those concepts!

  • Mike Merrifield explains the best. Crisp, clear, quick, sound, salient information. I'd be OK with interviews with just him. Discard the other profs…

  • Great video! Well… See you next MONTH for the next part. We will pray for the day, preach brother, m'kay.

  • They're just celestial vacuum cleaners which are actually gates into other universes <—– * he says so – whimsically expecting harsh criticism and ridicule *

  • A rocket lifting off has nothing to do with escape velocity. It just uses rockets to counteract earth's gravity.

  • Like black holes, I can't get enough!!! Could there be such a thing a quark degenerative pressure? Why does there need to be the nonsensical singularity?

  • At 2:14 the symbol is inverted. it should read "< 1 cm". Unless for radius less than 1 cm it could exist an even more dramatic state of matter and space time and thus, Brady is exempting him from generalizing the concept =). Either way, another great video as usual and thank you again!

  • Here is a piece I don't understand from prof Merrifield's description of forming of the black hole. As he explains, gravity has to first overcome electron degeneracy pressure then neutron degeneracy pressure to create the black hole. If gravity is so much weaker compared to electromagnetic interaction (according to wiki it's 10^-41 on the scale of quarks), how is it possible for the gravity to overpower these forces? Or am I entirely wrong?

  • I have a question: does the transition from neutron star density to black hole density happen as a quantum jump, or are there intermediate states between those two? (I imagine every black hole was once a neutron star, if only for the briefest of moments, during its creation).

  • Interesting video. I have only a layman's understanding of physics, especially quantum mechanics, so here is what I was wondering. You mentioned electron degeneracy and neutron degeneracy as the only things known to stop a collapse before a black hole forms. Where does Pauly's exclusion principle fit in? Or is that what degeneracy is referring to?

  • But doesn't F = (G*M1*M2*)/r^2 ? How can there be a gravitational five acting on a photon if its mass is 0?

  • Another question, or point of clarification. Hawking radiation is not the black hole evaporating right? It's the black hole losing mass as its energy is used by escaping virtual particles at the event horizon. The particles which escape were never from the black hole.

  • saying something is "pulled" by gravity is a bit misleading – the light in gravitational lensing is "bending" because the space through which the light is travelling is being "bent" – I know referring to "pull" is a way of explaining gravity in layman's terms but I think it's so misleading to put it that way.

  • Black holes are regions of spacetime that are causally disconnected from the observable universe and from which there is no information flow.

  • So if light goes through the whole visible spectrum when trying to escape from a black hole, as long as you have a flash light and a boombox you can have a rave party for a few minutes.

  • Um but can't a neutron star then easily turn into a black hole itself? Isn't that actually the ordinary life cicle of a massive star? Supernova->neutron star->neutron star gains more mass and becomes even smaller and denser untill it reaches the soon to be event horizon and collapses on itself to form a black hole?

  • The small ones are also interesting, they may point to particle physic processes at threshold of states of matter

  • so as the object enters these stages of degeracy what happens to the orbits of the electrons. Are there collisions and slower rates of orbit? I seems like at one point a hydrogen atom would stop being a hydrogen atom and become something new and unique.

  • if extraterrestrial life truly is visiting us, they have to meet these people! imagine the very knowledge they could provide. the gaps between theories and equations. these people are truly doing the work that is worthy of recognition in comparison to politicians and world leaders

  • Perhaps I am completely unimaginative, but black holes never interested me in physics. They're just too simple. It seems as though there is no much one could say about them. I suppose I am more interested in biophysics and the complexity of massive systems of particles.

  • Supermassive solar thorium roadways!
    Supermassive solar thorium roadways!
    Supermassive solar thorium roadways!
    Why bother driving on your old boring roadway when you can drive on a supermassive solar thorium roadways?!
    See Thunderf00t's excellent videos on these!

  • What would be the temperature of black hole. Will it be something near to absolute zero as all the matters are very close to each other without any vibrations or it will be very high because of high friction as all matters are very close

  • gravitation can never overpower neutron degeneracy pressure. you have to have a spawn point to a higher density produced by uncertainty principle within the neutron star.

  • doesn't water bend light? put a straw in a glass of water and you can see waters strong light bending effects. Light seems easily bent and not much of a trick. 4:44. And do black holes suggest the existence of Quantum gravity?
    wouldn't it be easier to Theorize the existence of dark mater stars and scrap the idea of black holes?

  • Whoa there. Light does not get "pulled" by gravity. Light travels through space, if that space happens to be curved, it follows that curved path.

  • Photons, (electromagnetic radiation energy) interact with the magnetic and electric forces of the electrons and protons in atoms and molecules, that is why light beams "bend". Gravity itself is probably a derivative of EM force interactions.

  • I expected to be complaining about the video-maker slipping into Imperial measurements on a channel run by Americans, but not one made by a fellow Aussie and a bunch of Brits. What the hell, Brady?

  • Here's a question: if all black holes collapse down to a single point of infinite mass, how is it some black holes are larger and some are smaller? wouldnt they all be the same size at that point?

  • They never seem to discuss time or space dilation as you approach the event horizon. I'd love to find a video that really explains how space and time are distorted by them.

  • If nothing can escape a black hole, then how can a black hole have magnetic fields that propagate outward? The size and shape of the spinning magnetic field is information about the black hole. How can information escape a black hole?

  • BH? ..where the curvature of observable space approaches minus infinity and reflects, as opposed to the reciprocal, – linear expansion limit, where the line appears to be fully extended relative to the first state, and reflects. …collectivly, nodes of frequencies integrating at relative probability one and zero.

  • The beginning is a description of black holes, spoken sentence by sentence by all of these scientists.
    Can you hear the symphony of Science?

  • 4:58

    Uh, maybe I'm wrong, but isn't it inaccurate to say light doesn't slow down… And that it always travels at the speed of light?
    Information cannot travel faster than speed of light, or light cannot travel faster than c but it can sure travel slower.

  • I was interested in ,,degeneracy,, and was wondering if blackholes could have formed in the very early universe not as the result of dying stars but maybe as a result of thier being hotter denser materials in this early time, could it be that infact blackholes appeared very early on and with expansion seperated draging materials with them that would end up forming galaxies.maybe the universe has to expand at a certain rate to overcome the creation of these blackholes or U wudve a failed universe

  • Hmm haven’t watched the vyd yet but I’ll just guess that a black hole is a region of space where the curvature is infinite, and where escape velocity is faster than light. Amiright? Not as if we haven’t heard “HUH DERR WHER NOT EVEN LAGHT CAN EXCAPE” 1000 Times from astronomers.

  • Build a ladder and hop off quick = not gonna work for black hole? Never mind. It's earth anyway. Take away his money and space rockets and ambition and everything……I knew I was as smart as musk.,.,.

  • A black hole is an object with such high gravity that even light can't get out! Black holes consist of at least two main parts, the event horizon and the singularity. The event horizon is the black hole's border, from this point, nothing can escape, not even light. There could be an inner horizon as well as an outer event horizon. The singularity is the black hole's core, this usually has the mass of several sun like stars crushed into a space no bigger than a nanometer, sometimes as tiny as a planck length in diameter. For a supermassive black hole, the singularity could be the size of a planet with the mass of millions or billions of sun like stars. In a rotating black hole, the singularity is likely a ring. It's thought spinning black holes may actually be wormholes, theoretical bridges connecting distant parts of the universe, different eras in time, or different universes entirely!
    We have no idea how supermassive black holes are formed, but every known galaxy has at least one.

  • escape velocity applies to non self propelled objects. a rocket may overpower earths gravitational pull at any speed.

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