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Supermassive black holes: most powerful objects in the universe | Martin Gaskell | TEDxMeritAcademy

[Applause]

in the year 1783 a famous British

scientist called the Reverend John

Mitchell the rector of this church in

fawn Hill Yorkshire presented a paper to

the Royal Society in London in his paper

he argued that there could be in space

possibly objects whose gravity was so

strong that you could not escape from

them even if you went as fast as the

speed of light and of course if light

would not escape then someone on the

outside was not going to be able to see

the object the Reverend Mitchell had had

become the first person to postulate the

existence of what we now call black

holes now John Mitchell had used Isaac

Newton's theory of gravity a theory

which we now know is not accurate when

gravity is very strong but in 1958 and a

American physicist called David

Finklestein used Albert Einstein's

general theory of relativity a much more

accurate theory of gravity to prove that

black holes could indeed exist even

though light can't escape from the black

hole the black hole has still got

gravity so it can be detected by its

gravitational influence on things around

it now currency dentally back in the

1780s while John Mitchell was

speculating about the existence of black

holes a friend of his a composer called

William Herschel was using very large

homemade backyard telescopes to sweep

the sky and discover objects that we now

recognize as being distant galaxies and

in some of these in the very center we

call the nucleus he detected bright

points of light here's a picture of one

of those ones we call it NGC 55 48

this is a Hubble Space Telescope picture

of it and it turns very bright in the

middle in what we call the nucleus the

central galaxies there well and Herschel

had no way of knowing this but he was

seeing light being emitted not by stars

in the galaxy there but by gas spiraling

into a supermassive black hole it was

not until the beginning of the 20th

century that astronomers got more

indications that something bizarre was

happening in the nuclei of galaxies like

NGC 55:48 astronomers at Lick

Observatory analyzed the light coming

out of nucleus of his galaxy and

discovered that there was lots of hot

glowing gas there and remarkably this

gas was moving very quickly it was being

much faster than the stars were in the

galaxy in fact it was even going up to a

few percent of the speed of light over

the next couple of decades astronomers

began to detect radio emission from

galaxies such as this one here called

sin a and that radio mission was very

powerful in the mid-1950s the

husband-wife team of Jeffrey and

Margaret Burbidge calculated that the

energy being put out by an active

galactic nucleus in something like cen a

was the equivalent of 100 million

supernova explosions going off as a star

exploding as a supernova it was a

tremendous amount of energy

in 1963 two of their collaborators Fred

Hoyle and really Fowler published a

paper in which they they argued that

such a huge source of energy could only

be a supermassive black hole some kind

of hypothetical super star where the

nuclear reactions going on inside it

simply could not provide enough energy

most of you are probably familiar with a

concept of energy particularly if you

pay

and electric bill well where is the

electric company get that energy from

well one possibility is something like

this this is the itaipu hydroelectric

dam it's very tall it's about 40 stories

from bottom to top there that's like a

big skyscraper at the top of the dam up

here upstream is the water and we say

that this water has gravitational

potential energy that means it has the

potential to convert and produce other

forms of energy so the dam the water

falls over the dam here falls down 118

meters and as it does that it is

accelerated by the Earth's gravitational

speed and as it as it gains speed the

gravitational potential energy is

changed into energy of motion and that

rushing water the bottom of the dam

drives turbines and generators to make

the electric current which is then

shipped off and heats and lights

people's houses and so on the

hydroelectric dam like this is a very

good analog of how a supermassive black

hole produces energy here the water is

only falling down 118 meters matter

falling into a black hole will fall

trillions of kilometers that's further

than the outer regions of the solar

system a good fraction the way to the

nearest star and it's being serrated not

by the gentle gravitational pull which

is holding us down on our seats right

now it's being accelerated by the

tremendous gravitational pull of the

black hole if we calculate how much

energy is being produced there for let's

say one kilogram of material falling

into a black hole it's enormous

it's more than a nuclear explosion of

this material would generate well a mere

five weeks after

Hoyle and Fowler had published their

paper the same Journal carried some

papers that identified this object

scored 3c273 looks like a bright star

and indeed that was what people had

previously thought it was but

astronomers at Caltech showed that this

is in fact a distant active galactic

nucleus and it's very bright and it's

very far away and the brightness of this

is more than hundreds of times greater

than the brightness of an entire bright

galaxy of a trillion stars we can

appreciate that in this Hubble Space

Telescope picture up here these little

blurry patches which you might not be

able to see over here

those are entire galaxies like our own

but it's look how bright 3c273 is it's

brilliantly bright here compared to

these ordinary galaxies over here it's

so blindingly bright that we can't even

tell that it's located in a galaxy there

that of course caused enormous

excitement among astronomers and pretty

quickly a whole lot more objects like

this were being found in 1969 the

English astrophysicist Donald Lyndon

Bell argued a very important point he

said well if objects like this by 3c273

faraway in the distant universe in the

past were being powered by supermassive

black holes then in nearby galaxies

there must be boremont black holes

because a supermassive black hole is

simply not going to go anywhere

overnight and a large body of research

continuing down to this present day by

many astronomers as shown that this is

indeed the case we now think that every

massive galaxy has a supermassive black

hole in its center the nearest and best

example of this is actually our own

galaxy the Milky Way the center of the

Milky Way the nucleus the Milky Way is

about 25,000 light-years away in

direction the constellation of

Sagittarius but you will not be able to

see it with your eye but because there's

a lot of dust in space in the way

however very large telescopes using

infrared cameras can penetrate the dust

and get us pictures of what's going on

there and I want to show you a

time-lapse movie which is based on

images taken by the European Southern

Observatory's Very Large Telescope or

VLT in Chile notice there's nothing at

the cross there notice that and each one

of those blobs there's a star and I

click to start the movie then the date

of observation if you're wondering when

it was will appear up there ok so you

can see that the stars are moving around

this is from year to year here and

they're moving fastest around this cross

in the middle here let's zoom in on the

cross and watch these stars are going

around in elliptical orbits there and

let's watch again to this star number 2

up there

astronomers have real exciting names for

stars here it comes to comes around here

zips around that cross at high speed

there well we can use Newton's laws of

motion use law of gravity to calculate

what force you need to make that star

swing round like that and how much mass

is needed if it's done by gravity which

it is and that comes out to be over 4

million times the mass of the Sun well

notice the obvious thing there there

isn't anything at the location of the

Cross there if there's 4 million solar

masses were in some kind of hyper super

star powered by nuclear reactions as our

Sun is that would be a blindingly bright

star so bright that that picture we

completely washed out and white and you

wouldn't be able to see anything up

there instead the stars are orbiting

around 4 million solar masses of

darkness over there these observations

are our clearest proof that supermassive

black holes do indeed exist there's now

no reasonable doubt that in the nuclei

of galaxies there are supermassive black

holes well how do you feed a black hole

let me tell you something about black

holes black holes do not suck black

holes are not some kind of cosmic vacuum

cleaner going around sweeping up sucking

up everything around them it is in fact

rather hard to go into a black hole the

reason for this is that except when

you're extremely close to the black hole

gravity is a black hole is pretty normal

it's just like any other gravity in the

universe the earth is orbiting the Sun

if the Sun were suddenly to turn into a

black hole and don't worry it won't

happen but if it did if the Sun suddenly

became a black hole well the earth would

just keep on orbiting around the Sun

it'll be dark that's all but the earth

will keep going around in the same orbit

let's watch go back to our little movie

of the center of our galaxy and zoom in

again and again let's watch this star -

at the top here go slowly first a little

bit like Halley's Comet clear around the

Sun it speeds in and zips around there

the important point here is star 2 there

watch it again is not getting sucked

into the black hole to go into a black

hole that star would have been on a

direct collision course with the black

hole and that's probably not going to

happen because the black hole is really

small so it's gonna miss and it's gonna

go flying off this matter going the

black holes is mostly gas or is entirely

gas and the thing about gas is gas

doesn't easily pass through gas

if you've seen a couple of fires burning

with columns of smoke when the smoke

comes together they doesn't pass through

like this and keep on going it comes

together and it swirls together in one

column of smoke

well Donald Lyndon Bell recognized that

gas that was falling in towards a

supermassive black hole was going to end

up swirling together and end up

spiraling around it and settling down

into we caught an accretion disk now

disks are quite common in astronomy this

is a the best-known nearby example the

rings of Saturn the rings of Saturn are

particles of dust and ice orbiting

together around Saturn here in a very

thin plane the material into black hole

is actually not dust and ice like

Saturn's rings they're it's gas gases

and liquids have a special property that

we call viscosity viscosity means that

one part of a gas is pulling another

part of one part of liquid it's pulling

on another part and I've got a little

demonstration to show this over here so

here I have a jar and inside this jar is

an apple I just thought if we can do an

experiment with gravity than Apple was

the right fruit to use if I take this

Apple here in the jar I turn the jar

upside down you all know what's going to

happen it's going to fall down on the

ground so here we have a fine Apple who

would like an apple in my other jar over

here I have something different this is

a jar of honey

organizers won't mind if I tip this

honey upside down upward on the carpet

right oh it's not down very quickly is

it that's because honey is viscous and

the viscosity of the honey means that

there's a blob in the middle which would

like to fall down on the floor make a

mess on this nice red carpet here

but it's being held back by the

viscosity of the honey which is around

it and that honey in turn is being held

back by the edge of the jar there well a

similar thing happens in the accretion

disk the viscosity in the gas is not

quite like the viscosity in the honey

it's actually provided by tangled up

magnetic fields but let's have a look I

decide on rings again what you can

probably tell from this picture here is

that the inner part of the Rings is

going faster than the outside part you

can see sort of directly dragged off

here this is like the planets going

around the Sun mercury the closest

planet a Sun goes faster than Venus and

explanatory it goes fast in the earth

and so on the gas on the faster orbits

is being held back by the gas further

out and that's going to slow it down

it's gonna make you go into lower orbit

and thus spiral into the black hole so

Donald Lyndon Bell proposed that that is

how you go inside a black hole well

viscosity doesn't just make stuff go

into a black hole does something else as

well and let's do a little demonstration

of the other thing that it does if you

can could you take your two hands and

squeeze them together and then I want to

push hard and then rubbed up really hard

and really vigorously okay what's

happening they're getting hot and you're

moving your hands pretty slowly here

inside the accretion disk the gas inside

is going faster than the gas outside we

call this a differential motion the

differential motion between your hands

here and the friction between them

generates heat that you fairly quickly

feel the viscosity in the accretion disk

is like the friction in your hands and

this is going to heat up that gas

enormously it's going to heat it up so

much that it's going to become white-hot

and that is the cause of the brilliant

light we're seeing from 3c273 there and

that is also how those Dark Star

postulated by the Reverend John Mitchell

over two centuries ago turn out to be

the most powerful sources of energy in

the universe

thank you

[Applause]