[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]
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]
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