[MUSIC PLAYING]
It's a great
pleasure to be here,
a building and a
room full of history,
full of wonderful
talks about science.
My accent will not tell
you, but I am a Brit.
I was born here
born in Edinburgh.
I was an undergrad
at Imperial College
and then a Ph.D.
student in Edinburgh.
And then I left because
I'm an astronomer
and it might have
escaped your attention,
but there's a lot
of clouds in the UK
and so I went to Arizona, where
the skies are clear about 300--
I don't to make you feel bad--
about 310 days in the year we
have blue skies and then
dark skies at night.
So I've been there ever
since for my whole career.
And I want to talk about
a subject obviously
of great interest
generally and topically
because it's in the news
pretty much every week
and it's been very much in the
news in the last month or so--
black holes.
And the books that
I decided I needed
to write based on black holes.
It's my eighth book.
If you care for
other topics, I've
written about cosmology
and astrobiology.
I've written a book about
teaching cosmology to Tibetan
monks in a programme
started by the Dalai Lama--
that's a slightly strange book--
and a science fiction novel
somewhere in there too.
Tonight I'm talking
about black holes.
And I guess the
message up front is
going to be that they're just
as exciting and enigmatic as you
might have thought they
were and hoped they were,
but we've learned
a lot about them.
And so it's at that wonderful
cutting edge of science.
Murray Gell-Mann, who is a
Nobel Prize winner in physics,
once said, "Research
is what I do when
I don't know what I'm doing."
And you can just
smile and say, well,
if you have a Nobel Prize, you
can say anything and get away
with it.
What he meant was that science--
and this subverts the bad
archetype or stereotype
about science and
scientists, which
nobody in this room of course
adheres to, but out there
in the polus, in
the public place,
it's held that science is
sort of cut and dried facts
and, you know, it ends
up being a little dull.
Gell-Mann was
saying that science
is the edge between what we
know and what we don't know.
And the implication
of that is that you
might be wrong at any point.
The implication is
of great uncertainty.
It's a very dynamic thing to
be at the edge of knowledge.
And that's why research is fun.
That's why I do research.
And that's why black
holes are still fun
because that cutting edge is
everywhere with black holes.
There are things we know,
things we don't know,
things we thought we
knew that turned out
to be wrong and so on.
So it's a fun
subject in that way.
We're going to meet two of
the fathers of black holes,
well known to everyone, of
course, Einstein and Hawking.
And I'm going to
start by pointing out
that the concept of
a black hole actually
predates general relativity
and Einstein by over 100 years.
John Mitchell, who was
an amateur astronomer
and physicist-- there were
no professional astronomers
in the late 18th century--
he was a clergyman.
He thought deeply about physics
and philosophy and mathematics.
And he imagined a star that was
large enough or massive enough
that the escape velocity that
every object with gravity
has equaled the speed of light.
And, logically,
that's a dark star.
Now, you can't formally
understand black holes
with Newton's theory
of gravity, which
is all he had available to him.
But it's an interesting step in
a direction of imagining dark
stars in the universe, things
that light cannot escape.
And so again, 100 years before
the theory that really let you
understand them, the
idea is out there.
So that's the very earliest
inkling of black holes.
Of course, the real
understanding of black holes
starts with relativity
and it starts
with Einstein, who is he's
owned by the popular culture.
He's become a
creature of culture,
rather than of
physics and astronomy.
In his day, clearly the most
famous scientist and still one
of the greatest
scientists in history.
Instantly recognisable.
Very elevated in public esteem.
He was offered and turned
down the presidency
of the state of Israel.
He was celebrated in many
ways and with good reason.
So we can't talk
about black holes
without talking
about relativity.
You're a sophisticated
audience, so I think you
I can hit you with the
full horror of it, which
is a 10 second order partial
couple differential equations,
which I will ask you to solve.
And we'll take answers
here before you're
allowed out of the room.
And that even for physics
students gives them the sweats,
the night sweats.
So I took a general
relativity course.
It was not that much fun.
I'm not a mathematical
physicist.
But relativity is conceptual.
So let me give you the
conceptual understanding
of relativity.
And it starts with an
awareness of Einstein
based on a coincidence.
Physicists hate coincidences.
When there's something
that's a coincidence,
either numerical or
physical, it means
we don't understand something.
The coincidence that
Einstein was struck by it
was the fact that the
inertial mass of an object--
that's its resistance to
a change in its motion.
If I pushed something, it
doesn't want to be pushed.
And we can imagine this is
a perfectly smooth surface
like ice.
It will still resist a
change in its motion.
Gravity is not
really involved here.
It's keeping the
book on the table,
but it's not resisting
the motion sideways.
That's the inertial mass.
The gravitational
mass is the mass
that leads it to fall
in the Earth's gravity.
Those are really quite
different things conceptually.
And, yet, numerically,
those masses
are identical, not known
with this precision,
but now to 1 part
in 1,000 trillion.
That begs for an explanation.
Einstein conceptualised
this with a little thought
experiment.
He said, well, if
you imagine these two
situations illustrated,
there is no way
you can distinguish
these experimentally.
Someone in a
spaceship that's being
accelerate with no windows--
they can't see their situation--
accelerated 9.8
metres per second
per second into space, and
then someone in a stationary,
in a lift, on the Earth's
surface, again they
can't see out and objects being
dropped, experiments done,
you couldn't tell
the difference.
One situation clearly
involves gravity--
the lift sitting there
on the floor held down
by the Earth's gravity--
and the other no gravity
at all, deep space.
There doesn't have to
be any object there.
It's a rocket.
He also realised that
these two situations are
indistinguishable.
One is fairly innocuous.
It's an astronaut
floating around
in deep space
inside a spaceship.
In the space station, you
would see the same thing.
And there's no
gravity involved here.
In deep space, there might
be no object anywhere near.
Zero gravity, these are
just floating things.
The other is much more ominous.
The cable on the lift is broken.
A person is plunging
to their death.
And they're floating
around in the lift.
And Einstein,
sadist that he was,
said this is the
happiest thought
of his life, this awareness
that these two situations are
indistinguishable.
So general relativity
is a statement
that there is nothing special
about acceleration due
to gravity relative to any other
force, like chemical rockets
or something like that, when
obviously those things are
totally different.
And that simple
awareness led him
down a road that led to a
mathematical theory of gravity
based on a very different
concept of gravity, the concept
of curved space-time.
Newton's gravity
theory was based
on objects that exerted
forces over a distance.
And the fact that this
happened in the vacuum of space
apparently instantaneously
was a little puzzling.
And when Newton was asked
about this, he said,
I don't frame no hypothesis.
He didn't actually know.
There was some deep
philosophical underpinning
of his theory that
were still mysterious.
It worked.
You can calculate it.
It's how we send
people to the moon
and how we launch rockets still.
Einstein got rid of the idea of
forces in action at a distance
and the vacuum of space
and how does that work.
And he said, no, mass and
energy bend space and time.
He'd already hyphenated mass and
energy with equals MC squared.
And he hyphenated space and
time in general relativity.
And he said general
relativity, those
horrid 10 partial differential
equations are just
equations that relate mass
and energy to space and time--
the density of mass and
energy to the curvature
of space and time
in three dimensions.
And it's a very different
idea of gravity.
And it works.
It's been tested and it works.
And I'll show you
some of the tests.
Some of the early
tests in the 1950s
involve very subtle experiments
because these were done
on the gravity of the Earth.
And the Earth is a
weak gravity object.
Black holes that we're going
to talk most of the time about
are intense gravity objects.
So doing these tests is really
difficult. But it was done.
These were done in
the '50s and '60s.
And general relativity implies
that a clock, a timekeeping
piece, runs slower in strong
gravity than weak gravity.
And that extends to the Earth.
And so in principle
and in theory
and in general
relativity, a clock
here runs very slightly
slower than a clock here.
Now, this was
tested in the 1950s
with atomic clocks flown
in high altitude planes
compared to an identical
atomic clock on the ground.
And the timekeeping
was slightly different.
Clocks are now so good, optical
switch clocks are so good,
that we can measure
time working differently
in one metre in the lab.
And the precision of
this experiment, you
can see it's 1 part in 10 to
the 19, incredible physics
experiment.
Another experiment
done also decades ago
shows that gravity photons
struggle as they-- you
can think of them struggling.
It's very anthropocentric to
do that, but they lose energy.
And that's a redshift.
So losing energy for light
is stretching the wavelength,
is redder.
It's called the
gravitational redshift.
And that's also been
measured many, many times
in many different ways.
So general relativity
by the 1960s
had been measured, tested
in all manner of situations
of weak gravity.
And it passed every test
with flying colours.
And that's still
a true statement.
The most dramatic demonstration
of general relativity
comes in astronomy when we take
pictures of the sky and watch
light being banned by space.
So if space-time is curved by
objects, by mass and energy,
then light essentially follows
the curvature of space-time.
And so it undulates.
It moves.
It bends.
It can be focused by an
object just like a lens.
Like an optical lens, it
could be a lens of mass.
And in the Hubble
Space Telescope,
this is just one of now
thousands of pictures
that have been taken that
show that the bright bunch
of galaxies in the middle
is a cluster about 3
billion light years.
And the little arcs of light
that you can see are arranged
roughly in concentric circles
around this centre of mass
of the cluster are much
more distant galaxies, 5, 7,
8 billion light years away.
And their light has been
sheared and amplified
actually and obviously bent
by the intervening cluster.
So this is a lens of
mass bending light--
dramatic demonstration
that needed
imaging from space
to see initially.
Now, we can do it from
the ground as well.
Techniques are better from the
ground to take sharp images.
And literally thousands of
these pictures have been taken.
So mass bends light
unequivocally.
[CLASSICAL MUSIC PLAYING]
So what are some of
the other differences
conceptually between Newtonian
gravity and Einstein's gravity?
Because if we're going to
believe general relativity,
we have to believe
these differences.
Here's a beautiful experiment
that was done, very difficult
experiment done in the '90s.
It took almost 20 years
to get the result.
But in Newton's theory, as
I've mentioned, space-time--
space and time are not coupled.
Space and time are
different things.
And really intuitively,
that's what it feels like.
Space is the stuff around
us and objects fill space.
And time is something that
flows and never goes backwards.
And we experience that they
don't not seem related.
So to hyphenate them is
audacious just in principle.
So in Newton's theory,
a spinning object
or a gyroscope on a satellite
around a spinning object,
because that's the experiment
we're going to talk about,
doesn't really care about
its larger situation.
The space and the time,
they don't care about this.
They're, in Newton's theory,
space was infinite and flat,
and time was infinite and
smooth and never changed.
But in Einstein's
theory, these things
are now coupled and
interesting in subtle ways.
And in principle
these are testable.
And in this particular
situation of the Earth,
the Earth will bend space
time, but very subtly,
because it's not a big object.
It's not a very
dense object either.
And so in this
2-dimensional analogy,
we can imagine it like
the surface of a sheet
being bent or distorted,
space-time around the Earth
by the gravity of the Earth.
But something else is going on.
The Earth is spinning.
And that actually leads
to a second effect,
which is the twisting of
the space-time contours.
Just think of it like a
vortex, like a whirlpool.
And in principle, both
of those phenomena--
the twisting of the space-time
contours and the bending
of space-time by the Earth--
are detectable.
And the probe used in this
one satellite experiment,
called Gravity Probe B,
involved a gyroscope.
And gyroscopes are always
supposed to point locked
on one direction in deep space.
In Newton's theory,
that would not
change as the satellite
orbits the Earth.
In Einstein's
theory, the gyroscope
is subtly tugged by the
curvature of space-time
and the twisting of the
space-time contours.
And you can measure
it in principle.
So what does it take
to make a black hole?
Here's a physical of it.
You would have to
crush something
like the sun, which is 1.5
million kilometres in diameter,
down to the size of a
small town, 3 kilometres.
And then it would
formerly be a black hole.
It would have an
escape velocity that
was the speed of light
and nothing could escape,
because light is the
fastest thing there is.
But the same principle
is true for any object.
Nature does not impose a
limit on the size or mass
of a black hole.
So if you could take the
Earth and crush it down
to a golf ball sized,
then formally it
would be a black hole.
You could take a rock and
crush it down to proton size.
And it would be a black hole.
So nature in theory
and in principle
can make black holes of
any mass and any size.
And the question
for astronomers,
for scientists being
empirical, is which of these
does nature make?
Which actually exist
in the universe?
The first type of black
hole to be proven to exist,
the type that was
anticipated from the theory
within a few decades
of the theory,
is the kind of object
left behind when
a massive star dies.
This is not the fate of the
sun, because when the sun dies,
having lost some of
its outer envelope,
the 2/3 of its mass that
remains will crunch down
to a white dwarf, which is a
cooling, carbon-rich ember,
which is incredibly dense,
millions of times denser
than the sun now, but
formally not the density
of a black hole.
But a star that started its life
10 times the mass of the sun
will lose some
mass along the way.
And then its core,
when all fusion stops--
and the fusion, remember,
is the only thing
that keeps a star puffed up.
So the sun is only
the size it is because
of a balance between
pressure from fusion
reactions and gravity in.
Well, when that
equation is broken,
because there's no energy
from fusion, gravity will win.
And so the inexorable victory
of gravity in a massive star,
in theory, leads
to a black hole,
because there is no force
to resist compression
to that dense state.
The theory, the calculation
of this, so the basis for it
is from general relativity
in Einstein's theory.
The actual calculation emerged
from mostly Robert Oppenheimer
and Hans Bethe in the 1930s.
And it was essentially
a byproduct
of Oppenheimer's work on
the Manhattan Project,
figuring out super
dense states of matter
that are how we generate
bombs and fusion.
And he used those
same calculations
to show that logically
a massive star should
have no force that could resist
it turning into a black hole.
So from the late 1930s, early
'40s, the prediction was there.
There should be dead stars
that are black holes.
Go look for them.
Black holes in the
theory are very simple.
They're incredibly
simple objects.
They are characterised by
an event horizon, which
is not a physical boundary.
It's an information membrane.
It marks the difference
between places we can see
in the universe and a part of
the universe that's sequestered
off that's hidden
from our view forever,
that nothing can escape from
and we cannot see inside.
No information can escape.
And that has a particular
size that scales linearly
with the mass of the object.
And in the theory the
black hole also has
something a little more
monstrous, a singularity.
So if you calculate
in general relativity,
there is a cusp of density
that's infinite at the centre.
And that's the singularity.
And that's a problem.
Any time in physics you
get an infinity coming out
of your calculation
for physical quantity,
it means you don't
understand something.
And Einstein, or actually
Hawking has said famously,
this means that
black holes contain
the seeds of their own demise.
So everyone from
Einstein through Hawking
to the present day is aware
that the theory of black holes
is incomplete, because
singularities are nonsensical.
They're also
impossible to inspect,
because they lie inside
the event horizon.
So all we can do is speculate.
But the theory has a problem
just because they're predicted.
The other property of a black
hole is spin, because the stars
that form black
holes are spinning.
As they collapse,
they spin faster--
angular momentum conserved.
So we would anticipate all
black holes in the universe
are spinning and
probably very fast.
So the black holes have mass.
They have size.
They have angular momentum.
And that's it.
Very simple objects.
And in 1969, Cygnus X-1,
the brightest X-ray source
in the sky in the
constellation of Cygnus,
was shown with very clever and
quite complicated observations
to be a binary
star system, where
one member of the
binary pair was
a giant star, super
giant star, and the other
was a black hole sucking
material off the giant star.
And that material glows
brightly, so brightly
and so hot that it emits X-rays,
enormous amount of X-rays.
So the irony of black holes
is that, yes, an isolated
black hole is black
by definition,
is invisible by definition.
So how do you find them?
Answer, you don't look for
an isolated black hole.
They could be all
over the place.
We won't find them.
Most stars are in binary
or multiple systems.
So it's anticipated
it's normal that they're
going to be binary stars
where one is massive enough
to die as a black hole and
the other is not a black hole.
And that if they're in
a tight binary orbit,
the black hole will pull
material off the normal star,
heat that material up.
And the heating of that
material will be the telltale
that there's a compact object.
And then you measure the orbital
properties of the binary.
And by Kepler's laws don't
need general relativity.
And if the mass is sufficient
and it's a dead star,
it has to be a black hole.
In the intervening half
century, we still only
have 50 examples of black
holes that are like this.
It's not many.
It's a pretty thin haul
for half a century of work.
So it's hard to
find black holes.
The nearest ones are
hundreds of light years away.
So we can rule out right
away that popular notion
in the culture that they're
going to eat everything
and they're nearby and
their danger and a threat.
They're quite rare because
only a tiny fraction of stars
are massive enough
to die that way.
So the nearest examples
are going to be far,
hundreds of light years away.
But we do know examples.
And so black holes
to most astronomers
are confirmed as real
astrophysical objects.
And then we come to the
contributions of Hawking.
So sadly the bard of
gravity and black holes
is lost to us just last year.
Being in London on
sabbatical, actually now
I'm here for six months,
I got to visit his grave,
and, of course, many other
illustrious scientists
in Westminster Abbey.
And what did Hawking add to the
conversation about black holes?
A lot.
Much of this work
was done when he
was quite young, when he was a
graduate student or a postdoc.
His singular contribution
was the prediction
that black holes have another
property beyond the three
I mentioned.
And that last property
is temperature.
How can a black hole
have a temperature?
How can anything
escape a black hole?
Hawking saw that there was
a very clever mechanism
in physics and in the lab.
It's known that spontaneously
from the vacuum,
from a pure vacuum of space,
particle antiparticle pairs
can appear and disappear.
That's allowed by Heisenberg's
uncertainty principle.
So you can steal
energy from the vacuum,
as long as you give
it back very quickly.
If that happens near the
event horizon of a black hole,
there's a finite chance
that one member of the pair
will be lost inside the black
hole, the other will escape.
And in the aggregate, that is
a net loss of either mass--
and mass and energy, as we know,
are equivalent-- or energy.
And the radiation
from the black hole
is called Hawking radiation.
Because this is a very
subtle phenomenon,
it's a very subtle temperature.
The Hawking radiation for a
black hole that's a dead star
is about a billionth
of a Kelvin.
This is completely
unmeasurable in astronomy
and may never be
measurable unfortunately.
Hawking was kind
of sad about this,
because he realised Nobel
Prizes are only awarded
for discoveries,
not for theories.
And he knew that if Hawking
radiation were ever detected,
he was a lock for a Nobel Prize.
But the truth is it's
very, very hard to detect.
And the corresponding,
the concomitant effect
that black holes are
slowly losing mass
is black hole evaporation.
And that's also an
incredibly subtle effect.
A black hole that's left
when a massive star dies
will take 10 to the power 68--
one with 68 zero--
years to fully evaporate
by Hawking radiation.
And, again, no way astronomers
can ever measure that, probably
ever.
I mean certainly not now and
maybe not even in principle.
But this is a contribution
to black hole theory
that has various implications.
And they're all important.
Obviously, the fact that black
holes evaporate is important.
It means they're not eternal.
They are going to
eventually disappear.
And a logical question is what
becomes of the information that
was lost in a black hole?
And so one of the consequences
of Hawking's theorising
was something called
the information paradox.
And there are different ways to
frame it, but it's a big issue.
And this is a totally
current issue.
There are probably 50
or 60 papers a year
written right now on information
paradox and related issues.
And it's something very
simple to describe.
A black hole is mute to all of
the things that went into it.
And so a black hole that
was made of a dead star
looks exactly the same and
has the same simple monolithic
properties as a
black hole that's
made of all the odd socks that
humans or other civilizations
ever lost or made of
cats or made of whatever.
It doesn't matter.
And so black holes
have lost information.
You could toss books.
You can make a black hole out
of books and encyclopaedias.
And you'd never get
that information out.
What happens to the information?
So this has become the
information paradox.
It's a problem because a
premise of quantum theory
is that information is preserved
in microscopic interactions.
So when you have
particle interactions
in the lab at a
subatomic scale, they
can run forward or
backward in time.
They can evolve
matter and antimatter.
And the information
and the interaction
is always preserved.
It's a premise of
quantum physics.
This premise is violated
by black holes overall.
And it's definitely violated
at the event horizon.
And this is
essentially a statement
that-- the information paradox
is a statement of the fact
that our gravity theory
and our quantum theory
don't play well together.
They are not
compatible theories.
And we've known
this for many years.
This is just a
particular example of it.
And it's led to all
sorts of speculations
of how you may preserve-- maybe
the information is preserved,
because, as we'll see, when
things fall into a black hole,
their time slows down
asymptotically, infinitely.
So maybe the information is
preserved as matter falls
slowly onto a black
hole like a hologram,
and it's coded onto the event
horizon if we could somehow
extract it, which we can't.
Maybe it's destroyed in
something called a firewall.
So there are many
theoretical ideas.
Like I said, dozens and
dozens of papers a year
written about this,
all unresolved issues
in theoretical physics.
And it signs that
black holes still
have plenty of juice left in
them as theoretical objects
of study.
Meanwhile, astronomers
were busy trying
to find out whether nature made
other types of black holes.
And nature does.
Nature makes some
amazingly big black holes.
Here's an example of
how black holes have
been found over a range of
actually a factor of a billion
in mass, incredible.
And probably the most
dramatic evidence--
and I would say the best
evidence for any black hole
in the universe, better than
any of those nearby black holes
in binary stars, the first
ones to be discovered--
the best evidence for any
black hole is the object
at the centre of our galaxy.
So what you're seeing here
now, it's not a simulation
or a cartoon-like animation,
these are real 3-dimensional
orbits of stars near the
centre of our own galaxy,
the Milky Way, 26,000
light years away.
And each of these stars is a
test particle that's probing
the mass at the centre.
And they're moving really fast.
This data spans
about two decades.
This is very hard
data to obtain.
It was possible until the early
1990s to get data this good
close to the centre.
Each star is testing
and measuring the mass
of the centre of our galaxy.
And if you do them close
enough to the centre,
they're showing that the galaxy
has 4 million times the mass
of the sun crushed or contained
in an incredibly small space,
such that it must
be a black hole.
And like I said,
because the black hole
is diagnosed by each of
these stars and their orbits,
the evidence for
this black hole is
better than any black
hole in the universe.
There is no doubt that
there is a 4 million
solar mass black hole right
in the middle of our galaxy.
Very exciting.
Meanwhile, the Hubble Space
Telescope was working.
And through the '80s and '90s
in observations that are not
as dramatic-- they don't prove
beyond the shadow of a doubt
the black hole--
you dropped the slit
of a spectrograph
on the left over a galaxy.
And on the right, the coloured
lines show the Doppler shift
of stars near the
centre of the galaxy.
And you can see the strong
blue and red deviations close
to the centre.
Those high stellar
velocities are also
probes of that central
mass of that galaxy.
And when you do the math, nearby
galaxies have black holes too.
And every galaxy
that was studied
through a period of 20
years with the Hubble
has a black hole.
So black holes are not
special to our galaxy.
Why should they be?
That would violate the
Copernican principle.
They're found in every galaxy
that we've studied so far.
But these galaxies are quiet.
Their centres are not
especially active or bright.
So if these black holes
are doing anything,
they're probably not
eating a lot of material.
They're not consuming material.
They don't have
active accretion.
So another puzzle
raised by this discovery
was if every galaxy
has a black hole
and there's tonnes of stuff
for them to be accreting,
that as we've seen in
binary star systems
when that gas is
sucked in, it gets
really bright and emits X-rays
and optical waves and radio
waves and everything, why aren't
these black holes doing that
too?
And the answer seems
to be that they're only
active about 1% of the time.
So every galaxy
has a black hole.
But each one is only
switched on or bright
or actively accreting
material 1% of the time.
And in between, it seems that
they blow out enough material
that they sort of
starve themselves.
And that material has to
accumulate in the centre
of a galaxy and trigger a
new episode of activity.
That's the guess.
We don't get to
stare at a galaxy
and watch it evolve and
see them switch on and off.
You do it with statistics.
And in these studies, it
was shown that the centre,
the central black hole in
a galaxy scales very nicely
and beautifully actually
over orders of magnitude with
the mass of the old
stars in a galaxy.
In the Milky Way, that would
be the bulge component,
the centre part of
our star distribution.
So somehow the black hole
knows about the galaxy
that it sits in.
And if you could zoom in on
these, these are radio images.
And the Galaxy, in this
case, was about the size
of the central dot.
And it's sending huge jets of
plasma, hundreds of thousands
of light years, a couple
of million light years
actually, out into space.
If we could zoom
in close enough,
we'd see there was a
spinning black hole.
It was accreting material
around its equator
in a scaled up version of
those binary star systems.
And it was emitting jets
of plasma very, very
close to the speed of
light out of the poles
and sending it deep into
intergalactic space.
And radio astronomers made these
pictures starting 40 years ago.
And now, we've
seen many examples.
And this is what the black hole
looks like when it is active,
when it's doing
something dramatic,
as opposed to the one in our
galaxy, which is pretty quiet
and pretty dark and
pretty quiescent.
The information
that's recent and very
exciting from within the last
month is, of course, the fact
that we now can
see the black hole.
So the first image
of a black hole
was made with an array
of radio telescopes,
essentially treating the Earth
as a giant radio telescope,
and so getting the angular
resolution, the sharpness
of imaging, as if you
had a telescope that
was 10,000 kilometres across
by combining information
from radio dishes
across the planet.
One of those telescopes
was at my observatory.
And some of the
lead investigators
in the event horizon project
are at my university.
So I've heard about this,
and I saw the images.
You know, I was sworn to
secrecy like a few other people
before this image was put
out just a few weeks ago.
And this is the M87
galaxy, Messier 87.
And this is a direct image
of the event horizon.
The dark circle is
the event horizon.
The glowing ring is the gas
around the black hole that's
being heated up by gravity
and energy basically.
The asymmetry is because
the black hole is rotating
and the gas on the lower
part is coming towards us
and it's Doppler boosted.
Its radiation is boosted by the
Doppler effect to be brighter.
And the radiation on
the top is moving away--
or the top part is moving
away from us and is dimmer.
And all of this is
understood by relativity.
And here's a zoom in.
So this shows the
biggest scale of M87,
which is sending jets
way out into space,
like that other
example I showed.
And then you zoom in
on the right-hand side
closer and closer
and closer until you
get to a scale that's
about a few light weeks.
And that's when you
see the black hole.
And this data has already
shown some amazing things.
There's things that it can't do.
But it's a simple image.
It doesn't look that impressive.
But it's already
been used to show
what the spin and the
orientation of the black hole
is because you can run
simulations and models
and see whether the models
fit the ring that you observed
and its asymmetry
and its brightness.
And these models have
also measured the mass
to be 6 and 1/2 billion
times the mass of the sun.
So, remember, that's
a billion times more
than the mass of those
stellar black holes
that were the first
to be predicted
and the first to be measured.
So spectacular black hole.
And because even astronomers
get blase about big numbers,
I just want to remind you
how spectacular it is.
But first, the fact
that black holes
do not overwhelm the universe.
If you make a pie
chart of the universe,
dark matter and dark
energy dominate.
That's another talk.
I mean, that's
research I work on too.
And it's the biggest
question in cosmology.
What are those two things?
But black holes, if you can
see, are 5,000th of a percent
of the universe.
So as spectacular
as they are they're
a little minor component
of the universe.
Each galaxy has a black
hole that's roughly 0.1%
of its mass.
So they don't dominate the
budget of the universe in mass.
But they are
spectacular, because when
they are accreting, when
they are chewing in material,
there are superb
gravity engines.
That's the way we
would refer to them.
They can convert mass
into radiant energy
with 40% efficiency,
by which I mean
equals MC squared,
40% efficient.
The sun and all stars are
less than 1% efficient.
So big black holes are
dozens, nearly 100 times
more efficient gravitational
energy engines than stars are.
And, remember, the sun
is 100 million times
more efficient than our
energy sources on the Earth.
So humans are pretty pathetic
in this scale of the universe
and how we get energy.
This is the way you would get
energy, incredibly efficient.
And so they're spectacular
for that reason.
And we're studying.
The M87 black hole, just
to put it in context,
is this compares it to the
size of the solar system
and our most distant
message in a bottle tossed
outside the solar system.
So this is a black hole that
has the mass of a small galaxy,
7 billion times the mass of
the sun, squashed into a region
not much bigger than
the solar system.
And the event
horizon at the edge
of the dark circle, that
event, because of the angular
momentum, that event
horizon is probably
moving at 80% of
the speed of light.
So just imagine
something that size
and that mass spinning at
80% of the speed of light.
And what does 7 billion
solar masses even mean?
Well, let's just step up the
scales of black hole mass,
because again,
astronomers, we do it too.
We talk about
billions and billions,
and we just get used
to those numbers.
But you need to
digest what that means
to put 7 billion
stars worth of stuff
into the solar system or
something not much bigger.
So here's a sort of modest
sized black hole we start with.
And then we'll scale up a couple
of times to get to the big one.
So this is what we would call
an intermediate mass black hole,
the kind we found in the edge
of the Milky Way galaxy, maybe
the size of a planet instead
of the size of a small town.
So that's about 1,000
times the mass of the sun.
Like I said, nature
makes black holes
of all these different masses.
But what about the big one?
Now, we're not using M87.
We're using a different
big black hole.
But there are a number of
billion solar mass black holes
now.
[MUSIC PLAYING]
And while you're seeing
it, how does this happen?
We think that the black holes
at the centre of galaxies must
have formed around the
time the galaxy started.
And they grew together.
So over 14 billion years,
black holes and galaxies
in the universe have grown.
It didn't all
happen this quickly.
It took 14 billion years.
So it could happen gradually.
Yeah, that's 20.
That is the record.
The record black hole is
about 20 billion times
the mass of the sun.
So the other new discovery,
not as new as that image
of the black hole you just
saw, but only a couple
of years old and
very spectacular
is, as if we needed
any more absolute proof
that black holes exist, LIGO,
the Laser Interferometer
Gravitational Observatory,
showed that they exist.
When this signal was measured
a couple of years ago--
it's called a chirp signal,
because if you translate it
into frequency, it's in the
mid-audible range basically,
like the middle part of
a piano in a crescendo.
And these signals detected
with a 7 millisecond time
difference, which reflected
the difference in time
of a gravitational wave crossing
between the two LIGO sights,
represent the merger of two
black holes that area each
a few dozen times
the mass of the sun--
a spectacular event that
opened up a new window
onto the universe.
And, again, because these
signals can really only
come from black holes
combining, there's
no other way to explain it.
So, again, unequivocally
black holes exist.
What LIGO is doing in cartoon
form was is two vacuum
cylinders, each 5 kilometres
long, very, very good vac--
almost as good as
the best lab vacuum--
lasers bouncing along each tube.
And when a gravitational wave,
which flexes space-time--
remember, space-time
is invisible.
It's nothing.
And yet in a gravitational
wave, predicted
by general relativity,
space-time flexes
in all the ways it can.
It flexes sideways
and longitudinally.
It flexes in all
three dimensions.
And that means anything in the
space, like a physical object,
is flexing too.
But even the vacuum is
flexing, space-time out
there between galaxies.
And so the instrument
is flexing.
And if you make the
two arms orthogonal,
they will each measure different
components of the flexure.
And so what LIGO is
doing by bouncing
lasers up and down
the arms and trying
to see the way the arms
are differently squeezed
by a gravitational wave,
in an interferometer
is it's measuring this
tiny space-time distortion.
By tiny, so this
is an instrument
that's miles across that's
measuring a space distortion
that is smaller than a proton.
That was the experiment.
It's amazing that it could
succeed, but it did succeed.
This is what you
would see if you
could be close to two big
black holes-- not big,
they're a few dozen times
the mass of the sun.
So these are dead
star black holes.
We can't get close
enough to black holes
to see what happens
when they merge.
But this is what
it would look like.
And all these distortions
of the surrounding stars
are that gravitational
lensing effect.
So this is a pretty
accurate simulation
of what would happen.
What LIGO is looking at is not
the visible picture of that,
which we couldn't
make anyway, LIGO
is looking at gravitational
space-time ripples.
And this is a visualisation
of those essentially invisible
waves.
As the black holes merge,
that little crescendo was
a cacophony of
gravitational waves here,
sent out into the universe,
travelling at the speed
of light for over a billion
years to reach us and be
measured by that instrument.
So it's an amazing experiment.
They've since detected another
eight binary black hole
mergers.
And when LIGO comes back
online in less than a year,
its sensitivity
should be at the level
where every week, where
it's going to get old hat,
every week they'll be talking
about a new black hole
merger or neutron stars
merging or neutron
stars merging with black holes.
When any of the two
compact types of objects
in the universe merge, they send
out a torrent of gravity waves.
And LIGO can detect them.
But what about the
big black holes?
Well, LIGO doing
its incredible job,
which resulted in the
Nobel Prize being awarded
to the architects of that
experiment just two years ago,
has given juice to some
very ambitious experiments
to measure gravity
waves in space.
So if you think of a black
hole as a physical object
that oscillates and rings
like a bell or an organ pipe,
then bigger black holes will
ring with lower frequencies
than small black holes.
And it's linear with the mass.
LIGO can only detect
the ringing and merger
and oscillation and gravity
ripples of small black holes.
The big ones at the
centres of galaxies
are millions or billions
of times bigger.
So their oscillations are
millions or billions of times
slower.
That means one of their
space-time ripples
might take months
or years or decades.
And there's no way you can
do that from the Earth.
You don't have the sensitivity.
And so you see in the middle
and to the left in this diagram,
people are conceiving
of space versions
of LIGO, satellite versions in
the super quiet, super still
environment of
space, that will be
able to detect the massive black
holes of the universe merging
and combining and
growing from the Big Bang
till now, an
incredible experiment.
And if LIGO had not
succeeded, these projects
would never have had
a chance of funding.
They're so audacious, so
ambitious, so difficult.
But LIGO having succeeded,
there is now budget.
The Europeans have
put serious money.
These are multi-billion dollar,
multi-billion euro missions.
And they are going to be funded.
So this is very exciting
science for about 10 or 15 years
from now, which we'll look
at how big black holes have
grown in the universe and
combined, because astronomers
think that galaxies
in the universe
grew by initially
being small and then
gradually merging and combining
to form bigger galaxies.
And logically if every galaxy
has a black hole, then when
the galaxies combined and
mered, the black holes
combined and merged and grew
that way by combination,
by addition.
And these space experiments will
actually show that happening.
They'll also possibly
show which came first,
the galaxy or the small
seed black hole that formed.
And we don't know
the answer to that.
That's an open question.
[VIDEO PLAYBACK]
Believe me, I've been waiting
a long time for someone
like you to record this moment.
Thank you, Doctor.
Then I'm ready.
Ready to embark on
man's greatest journey.
Certainly, his riskiest.
The risk is incidental
compared to the possibility
to possess the great
truth of the unknown.
There, long cherished laws of
nature simply do not apply.
They vanish.
And life?
Life?
Life forever.
[END PLAYBACK]
So now, at the end
of my talk, I want
to address two issues
that concern people
or are relevant to black holes.
They're just out there, and
it's nice to know about them.
But what about black
holes and mortality?
The issue of
creating black holes?
Black holes and death?
Well, it's known and
been written many times
that if we, any of us, fell
into a normal stellar mass
black hole, you'd
be spaghettified.
And that's a pretty
unpleasant fate,
because that's not like
a Stretch Armstrong doll
it gets pulled from
the feet and the head.
That's being
stretched to the level
of muscles, tendons, molecules.
Probably an excruciatingly
painful way to die.
But if you work
it out, any black
hole more than 1,000 times the
mass of the sun, of which there
are plenty, the
gravity is strong,
but the stretching
force is not as strong.
And so above that mass, you'd
probably feel a little queasy,
but you would not
be ripped apart.
So, yes, you can survive falling
into the bigger black holes
in the universe in principle.
And so it becomes a question.
Could it happen?
Could it be done?
It would be a
bizarre experiment,
because as seen
from afar, you would
appear to slow asymptotically.
Your light would be redshifted.
And you would never actually
reach the event horizon.
The people who went with
you to watch you fall in
would get bored and go home,
because your time would
be running slowly,
asymptotically
and infinitely slowly
at the event horizon.
However, to you, you
would fall in just
on a straight trajectory
through the event
horizon to an unknown fate.
Bizarre difference in perception
of the observer and the person
falling into the black hole.
Wouldn't it be
nice to test that?
Of course, you still wouldn't
be able to get the information
out, because you've passed
through the event horizon.
So this idea of surviving a
black hole, if it's big enough
is interesting.
This is what the
journey would look like.
So this is not again
a cartoon or anything.
This is a general
relativity simulation,
or general relativity
calculation.
Showing on the left, you can
see the spiral trajectory
towards the event horizon,
which is the red boundary.
And this is what it would
look like from your spaceship
as you were falling in.
Let's make it a big
black hole, so you're
going to survive this.
And all of these
distortions, all
of the hot gas near the black
hole in the accretion zone,
this is what it would look
like, all caused by gravity.
And, finally, as you
hit the event horizon,
the universe is lost to you.
You lose view of the
rest of the universe.
It's no way, no real way
to sum up the experience.
But this is what the
physics would suggest.
Wouldn't that be an
amazing experiment?
So I'm going to leave
you with a thought.
I hope I've reassured
you the world
is not going to be destroyed.
We do not have the
technology to make
black holes, large or small.
But they exist in nature.
And I hope I've tantalised
you with the possibility that
black holes are objects of
true inspection and discovery.
In some distant future
where we do have the ability
to travel through space
thousands of light years--
OK, that's not
around the corner--
then the experiment can be done.
And it does, in
some bizarre way,
offer immortality to
the person who falls in.
Because if they
do that experiment
you go with your
buddies, and you have--
we're all going to
die, so why not do it
by falling into a black hole--
you go out with all your
friends and your family.
You have an incredible
party in a safe orbit
far from the black hole.
You go into the black
hole in a nice spaceship
with a big bubble dome.
You wear your best clothes.
You make sure you're
looking pretty nice.
And you fall towards
the event horizon.
And then you probably time
your salutary final wave,
because your time
will slow down to zero
and so you'll be frozen
in your final wave
as seen by all the
people that came out.
And once they've drunk all the
beer and they've got bored,
they then go home.
And you're memorialised
on the event
horizon of the black hole.
That's pretty cool.
OK, well, what about the rest
of the time of the universe?
So rather than making
black holes objects
of fear and death,
let me offer them
as hopes for sustenance
for life and immortality
and the future of the universe.
Now, this is a
future far beyond us.
In about a trillion years,
all the stars in the universe
will be dead.
Don't get sad about that.
Don't get all sentimental about
stars, you know, whatever.
They're just stars.
You can get your energy
lots of different ways.
You don't need a fusion reactor
sitting there in the sky.
The cycle of star birth
and death that's caused
stars to form through the
history of the Milky Way
will eventually be broken,
because the stars will all
form stellar remnants--
white dwarfs,
neutron stars, black holes.
And there won't be enough
gas to make new stars.
And the lowest mass stars
will die last, the red dwarfs.
But, eventually, all
the lights will go out,
like a big rheostat turned
down, not only on our galaxy,
but on every galaxy
in the universe.
So the universe will go
dark in a trillion years.
And eventually,
according to physics,
normal matter will decay.
Solar systems will evaporate.
The planets will spin off
into interstellar space.
Galaxies will evaporate.
The stars will spin off
into intergalactic space.
And the universe will turn
into a thin, uniform gruel.
This is the victory of
entropy, the second law
of thermodynamics.
The exception is black holes.
They will be the last concrete
objects in the universe.
And so advance civilizations
of the very, very far future
will naturally use black
holes to run their lives
and their civilizations.
They will be the stars
of the far future.
And in the intermediate future,
between a trillion years,
10 to 12 and about
10 to the 50 years--
stay with me on this.
These are long timescales--
it will be easy to extract
energy from the black hole
by judiciously
dropping in probes
and retrieving their
energy or angular momentum.
So essentially what
you're doing is
tapping the rotational
energy of the black hole.
And so in this intermediate,
long-term future, civilizations
around black holes, because
that's where they'll all
congregate, that's where
the gravity power is,
will be able to run
their civilizations
and run their wireless
internet and whatever
else they do using rotational
energy of black holes.
But eventually the black
holes will spin down.
And so in the far
future, they're
left with a much more
subtle form of energy.
And that's in a universe
where everything
is a thin, uniform,
super low temperature
gruel of positrons, electrons,
and super low energy photons.
The energy remaining will
be the Hawking radiation
of the black hole.
And so an enterprising
civilization
will build a Dyson
sphere, an energy trapping
sphere to capture that
feeble Hawking radiation.
And in logical terms, the
last black hole to survive
will be the biggest black hole.
And the number on that is
about 10 to the 100 years.
That's how long it'll
will take the biggest
black hole in the universe
to finally evaporate.
And that is all she
wrote when that happens.
And so the last image
I'll leave you with
is of a super advanced, super
far in the future civilization
that's huddled around the last
black hole in the universe,
warming their hands by what's
essentially a kilowatt of power
from the Hawking radiation.
It's not much, but they'll
be efficient with it.
And they look at each
other and they say,
you know, 10 to 100 years,
we gave it a good run.
Thank you.
[APPLAUSE]
