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  • 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]

Sometimes people ask me how do I feel about black holes, whether I find them scary or intimidating

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What are Black Holes? - Sixty Symbols

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    林宜悉 posted on 2020/03/30
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