What Is a Black Hole? A Calm Bedtime Story to Fall Asleep To | SleepWise

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Good evening and welcome back to
sleep.

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Wise to night.
We drift towards some of the

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strangest and most quietly
astonishing objects in the known

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universe, places where gravity
has gathered itself so

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completely that even Light, the
swiftest traveller we know,

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leans inward and stays, places
The old imagination called

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impossible that early scientists
doubted could ever be real, and

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that modern instruments have
only recently begun to glimpse

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faintly against the glow of
distant skies to night.

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We drift toward black holes.
The phrase itself carries a

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certain weight for many people.
It conjures images of cosmic

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monsters, of ravenous voids of
danger lurking somewhere in the

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dark between stars.
But the truth, as so often in

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nature, is gentler and more
interesting than the legend.

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Black holes are not holes
punched through the fabric of

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reality, nor drains, pulling the
universe down into nothingness.

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They are, in the quiet language
of physics, regions where the

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geometry of space and time has
folded so deeply that no path

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leads back outward.
They are not absences, They are

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extremes.
They are what happens when one

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of the simplest forces in the
universe, the patient pull of

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gravity, is allowed to follow
its own logic to the very end.

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And so tonight, we will set
aside the theatrics.

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We will not hurry through
equations or cling to

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frightening images.
Instead, we will move slowly,

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the way one moves through an
unfamiliar room in the dark,

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letting the eyes adjust, letting
the shapes reveal themselves at

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their own pace.
There is no need to understand

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everything at once.
There is no need to remember

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each name or number.
The story will carry us, and the

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night will hold us as it does to
approach a black hole gently.

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It helps to begin far from one.
It helps to begin with the sky

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as humans once knew it, long
before telescopes and equations,

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when the heavens were a quiet
ceiling of light and the stars

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seemed pinned in place.
From that older world, we will

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follow a patient thread through
whispered ideas in the 18th

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century, through Einstein's
reimagining of gravity, through

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decades of doubt and discovery,
all the way to the first faint

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image of a shadow cast against
distant fire.

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Along the way, we will meet
event horizons and

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singularities, spinning
geometries and silent giants at

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the centers of galaxies.
We will think briefly about what

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happens to time near such
places, and how light behaves

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when geometry itself begins to
lean.

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And we will end, as all sleep
Wise journeys do, somewhere

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closer to home, somewhere softer
and more familiar than where we

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began.
If these quiet stories help you

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rest, you can follow Sleep Wise
and share it with someone who

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might enjoy drifting off while
learning something new.

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And now, with the night around
us and the stars patient

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overhead, let us begin.
For most of human history, the

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sky was a place of quiet
certainty.

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The sun rose and set with great
regularity, the moon passed

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through its familiar phases, the
stars wheeled overhead and

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patterns so dependable that
travellers could find their way

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by them and farmers could
measure the year by their slow

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procession.
Whatever the heavens were, they

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seemed to be governed by a kind
of patient order.

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In 1687, that order found a
precise voice.

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In a long Latin work titled
Philosophii Naturalis Principia

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Mathematica.
Isaac Newton set down three laws

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of motion and a single sweeping
law of gravitation.

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Everybody in the universe, he
wrote, attracts every other body

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with a force proportional to the
product of their masses and

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inversely proportional to the
square of the distance between

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them.
Double the distance and the pull

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becomes four times weaker.
It was a simple rule written in

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the cool language of
mathematics, and it described an

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astonishing range of things at
once.

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The same rule that explained why
an apple falls from a tree also

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explained why the moon does not.
The moon is, in a sense, falling

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toward Earth all the time.
It simply moves sideways quickly

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enough that it keeps missing.
The same rule explained the

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tides, which rise and fall as
the moon and sun pull gently on

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the oceans.
It explained the slow, looping

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return of comets, including the
one the astronomer Edmund Halley

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used Newton's equations to
predict, correctly forecasting

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its return in 175816 years after
his own death.

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Newton's framework rested on 2
quiet assumptions, so deeply

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held that they were rarely
questioned.

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The first was that space existed
as a kind of fixed and absolute

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container, the same for
everyone, everywhere.

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The second was that time flowed
evenly throughout the universe

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like a single great river.
A second on Earth was a second

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on Mars, on the Moon, in the
depths of empty space.

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Within this framework, gravity
was strong where mass was great

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and where distance was small.
Light was treated as something

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else entirely, a swift and
luminous traveller whose nature

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was still being debated, but
which was not generally thought

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to feel gravity in any
noticeable way.

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Stars shone, planets moved, and
the stage of space and time was

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assumed to remain perfectly
still beneath them.

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Newton himself was uneasy about
one part of his own picture.

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How could one body act upon
another across vast empty

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distances with no visible thread
between them?

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In his private writings he
confessed that this puzzled him

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deeply.
But the equations worked so

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well, predicting eclipses,
orbits and tides to remarkable

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precision, that the deeper
question was set aside.

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For nearly two centuries the
universe seemed to behave like

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an exquisitely well made clock.
And in such a universe, the idea

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of a star whose own light could
not escape would have to wait

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for someone willing to follow
Newton's logic into stranger

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territory.
In the late 18th century, in a

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quiet rectory in the village of
Thornhill in Yorkshire, an

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English clergyman and natural
philosopher named John Michel

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sat down to think carefully
about gravity and light.

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Michelle was unusual.
He had been a professor of

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geology at Cambridge before
settling into parish life, and

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he was the first person to
suggest that earthquakes

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travelled as waves through the
earth.

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He also designed the apparatus
later used by Henry Cavendish to

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weigh the world.
Michelle was, in short, a man

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comfortable with strange ideas.
In November 1783, he sent a

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letter to the Royal Society in
London, addressed to Cavendish

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himself, in which he asked a
quietly daring question.

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Newton had taught that an object
thrown upward from the Earth

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must reach a certain speed, the
escape velocity, in order to

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leave the planet entirely.
For Earth, that speed is about

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11 kilometers per second.
For larger and denser bodies,

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the required speed is greater.
Michelle wondered what would

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happen if a star was so massive
that its escape velocity

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exceeded the speed of light
itself.

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He worked the numbers using
Newton's framework alone.

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He calculated that a body with
the same average density as the

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Sun, but with a diameter 500
times larger, would have an

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escape velocity greater than the
speed of light.

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Such a star, he reasoned, would
still pull on its surroundings,

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but its own light would never
quite leave its surface.

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From a great distance, it would
appear entirely dark.

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Michelle called these
hypothetical objects dark stars,

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and he even proposed how
astronomers might detect them by

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watching how their gravity
disturbed the motion of any

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visible companion stars in orbit
around them.

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A few years later, in 1796, the
French mathematician Pierre

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Simon Laplace arrived at a
similar conclusion through his

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own calculations.
Laplace was one of the great

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minds of his era, a careful and
confident thinker who had

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reformulated much of celestial
mechanics.

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He included the idea of an
invisible gravitational body in

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the first two editions of his
book Exposition de sistem de

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Monde, suggesting that the
largest bodies in the universe

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might be dark for precisely this
reason.

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By the 3rd edition, however, he
quietly removed the passage,

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perhaps uncertain about how
seriously to take such a strange

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suggestion in a work meant for a
wide audience.

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For more than a century, the
dark stars of Michelle and

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Laplace remained a curiosity, a
footnote in the margins of

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natural philosophy.
The instruments of the time

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could not test the idea.
Light was not yet fully

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understood as a wave or a
particle, and the very notion of

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a star whose light could not
escape sat somewhere between

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mathematics and dream.
But it had been spoken aloud,

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the thread had been laid down,
and nature, as she so often

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does, was patient.
She would wait until the

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language of gravity itself was
rewritten before that thread was

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picked up again.
A century passed before the dark

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stars of Michel and Laplace
returned, transformed almost

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beyond recognition.
The change came not from new

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telescopes, but from a new way
of thinking about gravity.

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Altogether, in 19 O 7, while
working as a patent clerk in

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Baron Albert Einstein had what
he later called the happiest

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thought of his life.
He realized that a person

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falling freely from a roof would
not feel their own weight.

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Gravity, and the absence of
gravity in that moment would be

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locally indistinguishable.
From this small insight grew the

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equivalence principle, and from
the equivalence principle, over

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the next eight years came
general relativity.

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Einstein worked on the theory
slowly and with great

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difficulty.
He needed a mathematics.

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He had not yet learned the
geometry of curved spaces

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developed in the 19th century by
Bernhard Riemann.

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With help from the mathematician
Marcel Grossman, he eventually

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found his way through In
November 1915, he presented the

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field equations of general
relativity to the Prussian

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Academy of Sciences in Berlin.
They were 10 interlocking

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equations describing how matter
and energy shape the geometry of

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space-time and how that geometry
in turn guides the motion of

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matter and energy.
The physicist John Wheeler later

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summarized the idea in a single
sentence.

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Mass tells space-time how to
curve, and curved space-time

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tells mass how to move.
In Einstein's account, a massive

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body does not reach across empty
space to tug on its neighbors.

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Instead, it gently shapes the
geometry around it, and other

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objects follow the natural paths
available within that shaped

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geometry.
A planet orbiting the sun is not

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being pulled by an invisible
string.

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It is following the straightest
path it can find through the

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gentle valley the sun has carved
into space-time.

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Even light obeys this geometry
because there is no perfectly

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straight line for it to follow.
Einstein's theory made

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predictions that could be
tested.

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It explained a small drift in
the orbit of Mercury that had

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puzzled astronomers since the
1850s.

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Mercury's elliptical path slowly
rotated by about 43 arc seconds

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per century, more than Newton's
equations could account for.

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Einstein's equations gave that
exact extra amount naturally,

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with no adjustment.
The theory also predicted that

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light passing close to the sun
would be deflected by about 1.75

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arcseconds, twice what a simple
Newtonian calculation suggested.

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In May 1919, an expedition led
by the British astronomer Arthur

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Eddington travelled to the
island of Principe, off the

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coast of West Africa, to
photograph stars near the sun

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during a total solar eclipse.
The measured deflection matched

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Einstein's prediction.
The news made him famous

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overnight.
And once gravity was understood

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as curvature, an old question
could be asked again with new

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tools.
What happens when curvature

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becomes severe?
The answer would arrive sooner

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than anyone expected from the
unlikeliest of places.

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Karl Schwarzschild was a German
astronomer and physicist of

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remarkable gifts.
By the time the First World War

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began, he had already directed
an important observatory and

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contributed to several branches
of his science.

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He was respected, productive,
and deeply curious.

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When the war came, he could
easily have remained at home,

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but he chose instead to join the
German army, and by 1915 he was

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serving on the Eastern Front,
where he calculated artillery

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trajectories and endured the
cold and hardship of the field.

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It was there, in those difficult
conditions, that Schwarzschild

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read Einstein's newly published
equations of general relativity.

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They were dense and complex, and
Einstein himself had not yet

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found exact solutions to them.
Most physicists assume that

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exact solutions might be a long
time coming.

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But Schwarzschild, working with
paper and pencil between his

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military duties, found one.
Within a few weeks of seeing

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Einstein's paper, he had solved
the equations for the simplest

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possible case, the gravitational
field outside a single perfectly

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spherical, non rotating mass.
He sent his work to Einstein,

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00:19:15,440 --> 00:19:20,240
who was startled and delighted.
Einstein presented

231
00:19:20,280 --> 00:19:24,840
Schwarzschild's solution to the
Prussian Academy of Sciences in

232
00:19:24,840 --> 00:19:29,560
early 1916.
It was the very first exact

233
00:19:29,560 --> 00:19:34,400
solution to the field equations
of general relativity, and it

234
00:19:34,400 --> 00:19:38,200
described the geometry of
space-time around any spherical

235
00:19:38,200 --> 00:19:45,680
body, from a planet to a * for
everyday distances and ordinary

236
00:19:45,680 --> 00:19:49,280
masses.
It agreed beautifully with what

237
00:19:49,280 --> 00:19:53,080
was already known.
It explained the motion of

238
00:19:53,080 --> 00:19:57,880
planets, the bending of light
near the sun, and the small

239
00:19:57,880 --> 00:20:03,160
drift in Mercury's orbit that
had puzzled astronomers for

240
00:20:03,160 --> 00:20:06,080
decades.
But hidden inside

241
00:20:06,080 --> 00:20:10,400
Schwarzschild's solution was
something else, something

242
00:20:10,400 --> 00:20:14,120
stranger.
The mathematics contained a

243
00:20:14,120 --> 00:20:18,920
particular distance from the
center of the mass, a radius at

244
00:20:18,920 --> 00:20:22,240
which the equations seemed to
misbehave.

245
00:20:23,320 --> 00:20:27,640
At this radius, certain
quantities grew without limit,

246
00:20:28,040 --> 00:20:31,960
as if the geometry itself were
folding into something the

247
00:20:31,960 --> 00:20:34,800
formulas could not easily
handle.

248
00:20:35,440 --> 00:20:40,000
We now call this the
Schwarzschild radius, and the

249
00:20:40,000 --> 00:20:45,360
surface it defines is the event
horizon of a black hole.

250
00:20:46,040 --> 00:20:50,520
For a body the size of the Sun,
this radius would be only a few

251
00:20:50,520 --> 00:20:55,080
kilometers across.
For Earth, it would be smaller

252
00:20:55,080 --> 00:20:59,120
than a marble.
To reach it, ordinary matter

253
00:20:59,120 --> 00:21:04,480
would have to be compressed to
almost unimaginable density.

254
00:21:05,320 --> 00:21:09,640
At the time, no one quite knew
what to make of this strange

255
00:21:09,640 --> 00:21:13,160
feature.
Schwarzschild himself did not

256
00:21:13,160 --> 00:21:16,080
live long enough to explore it
fully.

257
00:21:16,800 --> 00:21:23,720
He fell I'll on the front and
died in May 1916, only a few

258
00:21:23,720 --> 00:21:27,600
months after sending his
solution to Einstein.

259
00:21:28,120 --> 00:21:34,920
He was 42.
He left behind a quiet, lasting

260
00:21:34,920 --> 00:21:41,560
gift, a piece of mathematics
that would decades later become

261
00:21:41,560 --> 00:21:45,280
the doorway through which
physics finally walked into the

262
00:21:45,280 --> 00:21:49,480
country of black holes.
When Schwarzschild's solution

263
00:21:49,480 --> 00:21:55,440
first appeared in 1916, most
physicists treated the strange

264
00:21:55,440 --> 00:22:00,880
radius at its heart as a
mathematical curiosity rather

265
00:22:00,880 --> 00:22:06,400
than a physical reality.
The equations misbehaved there,

266
00:22:06,840 --> 00:22:11,880
but surely people thought, no
actual object in the universe

267
00:22:12,160 --> 00:22:16,600
would ever be compressed tightly
enough for it to matter.

268
00:22:17,320 --> 00:22:22,640
A star like the Sun would need
to be squeezed inside a sphere

269
00:22:23,000 --> 00:22:28,440
only about 3 kilometers across
for its Schwarzschild radius to

270
00:22:28,440 --> 00:22:32,280
reach its surface.
The Earth would need to be

271
00:22:32,280 --> 00:22:38,440
crushed to the size of a peanut.
Such densities seemed

272
00:22:38,440 --> 00:22:43,240
impossible, the kind of thing
equations could describe but

273
00:22:43,240 --> 00:22:49,080
nature would never permit.
Einstein himself shared this

274
00:22:49,080 --> 00:22:54,200
doubt.
In 1939, more than two decades

275
00:22:54,200 --> 00:22:59,880
after Schwarzschild's work, he
published a paper arguing that a

276
00:22:59,880 --> 00:23:05,240
true gravitational collapse to
such a point could not actually

277
00:23:05,240 --> 00:23:09,880
occur in nature.
He believed some other effect,

278
00:23:10,040 --> 00:23:15,120
some unknown physical process,
would always intervene to

279
00:23:15,120 --> 00:23:18,080
prevent matter from crossing
that threshold.

280
00:23:18,720 --> 00:23:25,120
He was, in this instance,
mistaken, but his reluctance was

281
00:23:25,360 --> 00:23:28,080
understandable.
Nothing.

282
00:23:28,080 --> 00:23:32,120
An ordinary experience prepared
the human imagination for a

283
00:23:32,120 --> 00:23:37,960
region of space from which not
even light could return, and

284
00:23:37,960 --> 00:23:42,720
Einstein was cautious about
pushing his own theory past the

285
00:23:42,720 --> 00:23:47,120
point where its predictions
could be tested.

286
00:23:47,840 --> 00:23:52,160
The mathematician and astronomer
Arthur Eddington, who had

287
00:23:52,160 --> 00:23:57,240
confirmed Einstein's light
bending prediction in 1919, was

288
00:23:57,240 --> 00:24:02,720
another skeptic.
In 1935, he famously clashed

289
00:24:02,720 --> 00:24:07,320
with a young Indian physicist
named Subramanyam Chandra

290
00:24:07,320 --> 00:24:12,600
Sarkar, who had calculated that
sufficiently massive stars at

291
00:24:12,600 --> 00:24:17,320
the end of their lives could not
hold themselves up against their

292
00:24:17,320 --> 00:24:21,120
own gravity and would collapse
without limit.

293
00:24:21,640 --> 00:24:27,320
Eddington could not accept this.
There should be, he insisted, a

294
00:24:27,320 --> 00:24:31,760
law of nature to prevent a star
from behaving in so absurd A

295
00:24:31,760 --> 00:24:35,840
manner.
The confrontation was painful

296
00:24:35,840 --> 00:24:41,160
for Chandrasekar, who was young
and relatively unknown, and it

297
00:24:41,160 --> 00:24:45,600
delayed the full acceptance of
his ideas by years.

298
00:24:46,080 --> 00:24:51,160
What seems strange now is how
slowly the community moved

299
00:24:51,160 --> 00:24:54,640
toward what the mathematics had
been quietly saying.

300
00:24:55,600 --> 00:25:01,440
The term black hole itself did
not appear in common use until

301
00:25:01,440 --> 00:25:06,640
the late 1960s, when the
American physicist John Wheeler

302
00:25:07,240 --> 00:25:12,120
popularized it during a lecture
in New York in 1967.

303
00:25:12,640 --> 00:25:17,880
Before then, such objects were
often called frozen stars or

304
00:25:18,240 --> 00:25:23,040
collapsed stars, or simply
referred to by the unwieldy

305
00:25:23,040 --> 00:25:26,800
phrase Schwarzschild
singularity.

306
00:25:27,560 --> 00:25:31,040
The new name was short,
memorable, and a little

307
00:25:31,040 --> 00:25:34,560
haunting.
It helped the idea settle into

308
00:25:34,560 --> 00:25:39,560
the public imagination.
But acceptance was not merely a

309
00:25:39,560 --> 00:25:43,480
matter of language.
It took decades of patient

310
00:25:43,480 --> 00:25:49,440
theoretical work and eventually
observational evidence before

311
00:25:49,440 --> 00:25:54,400
most physicists agreed that the
universe really did contain such

312
00:25:54,400 --> 00:25:59,160
objects.
The resistance was not foolish.

313
00:25:59,640 --> 00:26:04,000
It was the careful hesitation of
people who had been trained to

314
00:26:04,000 --> 00:26:09,080
trust their intuitions about
what nature would and would not

315
00:26:09,120 --> 00:26:13,280
allow.
The universe, as it turned out,

316
00:26:13,520 --> 00:26:18,920
was more daring than they were.
To understand why some stars

317
00:26:18,920 --> 00:26:24,680
have no choice but to collapse,
it helps to remember what a star

318
00:26:24,720 --> 00:26:29,240
actually is.
A star is a vast sphere of hot

319
00:26:29,240 --> 00:26:34,480
gas, held together by its own
gravity and prevented from

320
00:26:34,480 --> 00:26:38,920
collapsing by the pressure
generated in its core, where

321
00:26:38,920 --> 00:26:44,120
hydrogen atoms fuse into helium
and release enormous energy.

322
00:26:44,480 --> 00:26:50,320
For most of a star's life, these
two forces remain in gentle

323
00:26:50,320 --> 00:26:54,200
balance.
Gravity pulls inward.

324
00:26:54,960 --> 00:27:01,160
Pressure pushes outward.
The star glows steady and

325
00:27:01,160 --> 00:27:05,320
patient, for millions or
billions of years.

326
00:27:05,680 --> 00:27:07,920
But the fuel does not last
forever.

327
00:27:08,520 --> 00:27:13,400
When the hydrogen in the core is
spent, the star begins to burn

328
00:27:13,400 --> 00:27:18,560
heavier elements, each stage
shorter and more frantic than

329
00:27:18,560 --> 00:27:22,920
the last.
Eventually, even these sources

330
00:27:22,920 --> 00:27:26,400
run out.
Without the heat of fusion, the

331
00:27:26,440 --> 00:27:30,760
outward pressure falters and
gravity begins to win.

332
00:27:31,560 --> 00:27:36,960
What happens next depends on how
much mass the star began with.

333
00:27:37,320 --> 00:27:42,400
In 1930, the young Indian
physicist Subrahmanyan

334
00:27:42,400 --> 00:27:48,360
Chandrasakhar was travelling by
ship from Madras to England on

335
00:27:48,360 --> 00:27:52,760
his way to study at Cambridge.
During the long voyage, he

336
00:27:52,760 --> 00:27:57,280
worked out a calculation that
would later carry his name.

337
00:27:58,120 --> 00:28:04,240
He showed that a white dwarf,
the dense remnant left behind by

338
00:28:04,240 --> 00:28:11,040
a moderate star, cannot exceed a
mass of about 1.4 times the mass

339
00:28:11,040 --> 00:28:14,720
of the Sun.
Above that limit, the quantum

340
00:28:14,720 --> 00:28:18,840
pressure that normally holds a
white dwarf up becomes

341
00:28:19,120 --> 00:28:23,080
insufficient and the star must
collapse further.

342
00:28:24,000 --> 00:28:29,880
This boundary is now called the
Chandra Sekhar limit, and it

343
00:28:29,880 --> 00:28:34,720
marks the first quiet line
beyond which ordinary stellar

344
00:28:34,720 --> 00:28:40,800
endings become impossible.
For a while, it was thought that

345
00:28:40,800 --> 00:28:47,040
collapsed beyond a white dwarf
would simply produce a neutron *

346
00:28:47,680 --> 00:28:52,200
an even denser object in which
electrons and protons have

347
00:28:52,200 --> 00:28:58,600
merged into neutrons, packed so
tightly that a single teaspoon

348
00:28:58,600 --> 00:29:02,160
of the material would weigh
billions of tons.

349
00:29:02,360 --> 00:29:08,520
Neutron stars are real,
discovered in 1967 as pulsars by

350
00:29:08,520 --> 00:29:12,440
the young astronomer Jocelyn
Bell Burnell.

351
00:29:13,360 --> 00:29:19,240
But neutron stars also have a
limit somewhere between two and

352
00:29:19,240 --> 00:29:24,440
three solar masses, beyond which
no known pressure can hold them

353
00:29:24,440 --> 00:29:29,200
up.
In 1939, the American physicist

354
00:29:29,520 --> 00:29:34,320
J Robert Oppenheimer and his
student Heartland Snyder

355
00:29:35,000 --> 00:29:37,080
followed the mathematics
further.

356
00:29:37,520 --> 00:29:42,040
They published a paper titled On
Continued Gravitational

357
00:29:42,040 --> 00:29:47,320
Contraction in which they showed
that a sufficiently massive

358
00:29:47,320 --> 00:29:52,480
collapsing star must, according
to general relativity, fall

359
00:29:52,480 --> 00:29:58,320
inward indefinitely.
Nothing in the equations stopped

360
00:29:58,440 --> 00:30:02,440
the contraction.
The outer layers of the star

361
00:30:02,440 --> 00:30:07,160
would pass through their own
Schwarzschild radius, and to a

362
00:30:07,160 --> 00:30:12,200
distant observer the collapse
would appear to slow and freeze

363
00:30:12,200 --> 00:30:17,720
at that boundary, while from the
star's own point of view, the

364
00:30:17,720 --> 00:30:21,640
inward fall would continue
toward a central region of

365
00:30:22,040 --> 00:30:26,160
unimaginable density.
Oppenheimer's paper was

366
00:30:26,160 --> 00:30:31,160
published on the same day that
Germany invaded Poland, and it

367
00:30:31,160 --> 00:30:36,240
received little attention at the
time, but it had quietly

368
00:30:36,240 --> 00:30:41,320
answered Einstein's doubt.
Collapse was not forbidden.

369
00:30:41,960 --> 00:30:46,320
It was, for the heaviest stars,
inevitable.

370
00:30:46,640 --> 00:30:51,200
Of all the strange features of a
black hole, the event horizon

371
00:30:51,600 --> 00:30:54,600
may be the most often
misunderstood.

372
00:30:55,320 --> 00:30:59,280
It is not a wall.
It is not a surface you could

373
00:30:59,280 --> 00:31:03,000
touch or lean against.
There is no shimmer in the

374
00:31:03,000 --> 00:31:06,800
vacuum, no sudden boundary
visible to the eye.

375
00:31:07,840 --> 00:31:13,000
If you were to approach one in a
spaceship carefully and from a

376
00:31:13,000 --> 00:31:18,520
safe distance, you would see no
obvious marker at the edge.

377
00:31:19,160 --> 00:31:22,840
The horizon is something subtler
than that.

378
00:31:23,560 --> 00:31:29,160
It is a boundary defined not by
anything material, but by the

379
00:31:29,160 --> 00:31:34,600
shape of the paths that light
and matter are able to follow.

380
00:31:34,960 --> 00:31:38,760
The word horizon is well chosen.
On Earth.

381
00:31:39,120 --> 00:31:43,720
The horizon is not a place, it
is the line beyond which you

382
00:31:43,720 --> 00:31:48,160
cannot see, determined by the
curvature of the planet and the

383
00:31:48,160 --> 00:31:52,920
position of the observer.
Walk forward and the horizon

384
00:31:52,920 --> 00:31:57,040
walks with you.
The event horizon of a black

385
00:31:57,040 --> 00:32:01,080
hole is similar in spirit.
It is the boundary beyond which

386
00:32:01,080 --> 00:32:07,720
no signal, no particle and no
beam of light can ever reach an

387
00:32:07,760 --> 00:32:12,520
outside observer.
Events that happen inside it are

388
00:32:12,520 --> 00:32:16,040
forever hidden from the universe
beyond.

389
00:32:16,760 --> 00:32:21,240
Hence the name for a non
rotating black hole.

390
00:32:21,680 --> 00:32:26,000
The event horizon sits at
exactly the Schwarzschild radius

391
00:32:26,000 --> 00:32:29,720
Karl Schwarzschild had
calculated in 1916.

392
00:32:30,520 --> 00:32:33,800
The formula is unexpectedly
simple.

393
00:32:34,360 --> 00:32:37,240
The radius is proportional to
the mass.

394
00:32:38,000 --> 00:32:42,720
A black hole with the mass of
the Sun has an event horizon

395
00:32:43,120 --> 00:32:48,480
about 3 kilometers in radius.
A black hole with the mass of

396
00:32:48,480 --> 00:32:55,480
Earth would have a horizon about
9mm across, roughly the size of

397
00:32:55,480 --> 00:32:58,920
a marble.
And a supermassive black hole

398
00:32:59,160 --> 00:33:03,760
with 4 million solar masses like
the one at the center of our

399
00:33:03,760 --> 00:33:10,080
Galaxy has a horizon roughly 17
times the radius of the sun.

400
00:33:10,560 --> 00:33:16,000
Still small on cosmic scales,
but enormous compared to

401
00:33:16,000 --> 00:33:20,600
anything close to home.
What makes the horizon strange

402
00:33:21,040 --> 00:33:26,680
is not its size, but its nature.
In ordinary physics, any

403
00:33:26,680 --> 00:33:29,400
boundary can be crossed in
either direction.

404
00:33:30,040 --> 00:33:32,960
You walk into a room, you walk
out of it.

405
00:33:33,560 --> 00:33:36,800
A photon enters a window, a
photon leaves.

406
00:33:37,400 --> 00:33:43,280
But the geometry of space-time
near a black hole is arranged in

407
00:33:43,280 --> 00:33:49,480
such a way that once you cross
the horizon, every direction you

408
00:33:49,480 --> 00:33:53,920
could possibly move leads inward
toward the center.

409
00:33:54,280 --> 00:33:59,760
The outward directions simply do
not exist anymore, not because

410
00:33:59,760 --> 00:34:05,120
something is pushing you, but
because the geometry itself has

411
00:34:05,120 --> 00:34:08,360
tilted.
It is a little like walking into

412
00:34:08,360 --> 00:34:12,280
a valley, so deep that every
step you take from a certain

413
00:34:12,280 --> 00:34:17,480
point onward in any direction is
a step further down.

414
00:34:18,239 --> 00:34:22,600
You have not done anything
wrong, You have not been

415
00:34:22,600 --> 00:34:27,199
captured, You are simply
following the shape of the land.

416
00:34:28,040 --> 00:34:33,120
The universe does not punish you
for crossing a horizon, it only

417
00:34:33,120 --> 00:34:36,520
quietly rearranges the options
available to you.

418
00:34:37,360 --> 00:34:43,280
And from the outside, no news of
you can ever come back, because

419
00:34:43,280 --> 00:34:48,920
no light can climb out.
The event Horizon in this sense

420
00:34:49,199 --> 00:34:52,520
is not a door, it is a
direction.

421
00:34:52,880 --> 00:34:58,200
Imagine very gently that a
distant observer has sent a

422
00:34:58,200 --> 00:35:02,160
small probe toward a large,
isolated black hole.

423
00:35:02,800 --> 00:35:07,600
The probe is equipped with a
simple clock and a light that

424
00:35:07,600 --> 00:35:13,000
flashes once every second.
For the purposes of this story,

425
00:35:13,240 --> 00:35:17,520
we will ignore the violent
conditions near a realistic

426
00:35:17,520 --> 00:35:22,280
black hole and simply follow
what relativity says would

427
00:35:22,280 --> 00:35:27,240
happen to the light of the probe
as it approaches the horizon.

428
00:35:27,640 --> 00:35:30,640
At first, everything looks
ordinary.

429
00:35:31,280 --> 00:35:36,200
The probe moves inward, the
clock ticks, the light flashes

430
00:35:36,200 --> 00:35:40,720
once per second, and each flash
travels back to the distant

431
00:35:40,720 --> 00:35:46,200
observer at the speed of light.
But as the probe draws closer to

432
00:35:46,200 --> 00:35:51,000
the event horizon, something
strange begins to unfold.

433
00:35:51,680 --> 00:35:57,840
The observer far away sees the
flashes arriving slightly slower

434
00:35:58,040 --> 00:36:01,800
than once per second, then
slower still.

435
00:36:02,680 --> 00:36:09,360
The time between flashes grows
longer and longer, as though the

436
00:36:09,360 --> 00:36:14,760
probe's clock were winding down.
This is not an illusion produced

437
00:36:14,760 --> 00:36:18,200
by distance.
It is a real effect, a

438
00:36:18,200 --> 00:36:22,600
consequence of what Einstein's
equations say about time.

439
00:36:23,000 --> 00:36:29,160
In a strong gravitational field,
clocks closer to a massive body

440
00:36:29,440 --> 00:36:32,800
tick more slowly than clocks
farther away.

441
00:36:33,600 --> 00:36:37,800
This is called gravitational
time dilation, and it has been

442
00:36:37,800 --> 00:36:42,880
measured in laboratories and
with atomic clocks flown on

443
00:36:42,880 --> 00:36:46,760
aircraft.
Near a black hole, the effect

444
00:36:46,760 --> 00:36:50,880
becomes dramatic.
As the probe approaches the

445
00:36:50,880 --> 00:36:56,040
horizon, the distant observer
sees its clock slow toward a

446
00:36:56,040 --> 00:36:59,000
crawl.
The flash is becoming more and

447
00:36:59,000 --> 00:37:04,120
more widely spaced, the light of
each flash growing redder as it

448
00:37:04,120 --> 00:37:07,640
climbs out of the deepening
gravitational well.

449
00:37:07,960 --> 00:37:12,840
From outside, it looks as though
the probe is slowly freezing at

450
00:37:12,840 --> 00:37:15,760
the edge.
It never quite reaches the

451
00:37:15,760 --> 00:37:19,600
horizon.
The image of the probe lingers

452
00:37:19,600 --> 00:37:25,160
there, fainter and fainter, its
final flashes stretched out

453
00:37:25,160 --> 00:37:29,840
across eternity.
Older physicists sometimes

454
00:37:29,840 --> 00:37:35,160
called such objects frozen stars
for exactly this reason.

455
00:37:35,920 --> 00:37:40,760
To a patient watcher far away,
the last moments of the fall

456
00:37:40,760 --> 00:37:44,640
seem to last forever.
But from the probe's own point

457
00:37:44,640 --> 00:37:47,600
of view, nothing so dramatic
happens.

458
00:37:48,200 --> 00:37:52,720
The clock on board continues to
tick at its usual rate.

459
00:37:53,600 --> 00:37:56,520
The lights continue to flash
once per second.

460
00:37:57,280 --> 00:38:00,800
The probe does not feel itself
slowing down.

461
00:38:01,560 --> 00:38:07,080
It simply falls inward, crosses
the event horizon without any

462
00:38:07,080 --> 00:38:11,200
particular fanfare, and
continues its descent toward the

463
00:38:11,200 --> 00:38:14,640
center.
Relativity does not grant one of

464
00:38:14,640 --> 00:38:17,600
these two perspectives the final
word.

465
00:38:18,200 --> 00:38:22,200
Both are real, both are
consistent.

466
00:38:22,880 --> 00:38:26,480
The difference lies entirely in
the geometry of space.

467
00:38:26,480 --> 00:38:31,360
Time which has arranged matters
so that time itself is not

468
00:38:31,360 --> 00:38:34,320
universal.
There is something quietly

469
00:38:34,320 --> 00:38:37,080
moving in that near a black
hole.

470
00:38:37,480 --> 00:38:41,200
The universe keeps two different
accounts of a single journey,

471
00:38:41,760 --> 00:38:47,400
and neither is wrong.
The traveller falls, the distant

472
00:38:47,400 --> 00:38:53,760
observer watches a figure grow
still, and in between them

473
00:38:54,000 --> 00:38:58,960
stretches the patient geometry
of a world in which time near

474
00:38:58,960 --> 00:39:03,200
great gravity is not one river
but many.

475
00:39:03,560 --> 00:39:08,800
The slowing of clocks near a
black hole is not a trick of

476
00:39:08,800 --> 00:39:12,280
imagination or a feature of
science fiction.

477
00:39:13,120 --> 00:39:17,680
It is the same effect that
physicists must account for

478
00:39:17,680 --> 00:39:21,000
every day in ordinary
technology.

479
00:39:21,360 --> 00:39:26,320
The satellites of the Global
Positioning System carry atomic

480
00:39:26,320 --> 00:39:31,160
clocks in orbit about 20,000
kilometers above Earth's

481
00:39:31,160 --> 00:39:35,000
surface.
Because they are farther from

482
00:39:35,000 --> 00:39:40,520
Earth's mass than clocks on the
ground, gravity is weaker where

483
00:39:40,520 --> 00:39:45,520
they fly, and their clocks tick
slightly faster than ours, by

484
00:39:45,520 --> 00:39:51,280
about 45 microseconds per day.
Special relativity adds a

485
00:39:51,280 --> 00:39:54,040
smaller correction in the
opposite direction.

486
00:39:54,560 --> 00:40:00,720
Because the satellites are also
moving swiftly, engineers must

487
00:40:00,720 --> 00:40:06,120
subtract 1 effect from the other
every day, or the navigation

488
00:40:06,120 --> 00:40:10,000
system would drift by kilometers
within hours.

489
00:40:10,360 --> 00:40:14,480
A difference of 45 microseconds
per day is almost nothing in

490
00:40:14,480 --> 00:40:20,960
daily life, but it shows that
gravitational time dilation is

491
00:40:20,960 --> 00:40:25,680
real measurable.
And woven into the ordinary

492
00:40:25,680 --> 00:40:30,520
workings of the modern world
near the surface of Earth, the

493
00:40:30,520 --> 00:40:35,640
effect is gentle.
Near a neutron star it becomes

494
00:40:35,880 --> 00:40:41,000
significant.
Near a black hole it becomes

495
00:40:41,680 --> 00:40:45,800
extraordinary.
Just outside the event horizon

496
00:40:46,120 --> 00:40:50,520
of a non rotating black hole
with the mass of the Sun, a

497
00:40:50,520 --> 00:40:54,680
clock would tick at roughly half
the rate of a clock far away.

498
00:40:55,520 --> 00:41:01,920
Closer still, the slowdown
becomes steeper as the horizon

499
00:41:01,920 --> 00:41:05,160
is approached.
The ratio grows without limit,

500
00:41:05,440 --> 00:41:09,960
so that a single second on the
infalling clock corresponds to

501
00:41:09,960 --> 00:41:13,120
longer and longer intervals
outside.

502
00:41:13,480 --> 00:41:18,640
This is why light emitted from
near the horizon reaches distant

503
00:41:18,640 --> 00:41:24,800
observers not only delayed but
shifted toward the red end of

504
00:41:24,800 --> 00:41:29,000
the spectrum.
Its waves have been stretched by

505
00:41:29,000 --> 00:41:34,680
the steepening gradient of time,
itself a phenomenon known as

506
00:41:35,040 --> 00:41:39,800
gravitational redshift.
This redshift has been measured

507
00:41:39,800 --> 00:41:43,560
in more modest circumstances
with great precision.

508
00:41:44,320 --> 00:41:50,840
In 1959, 2 physicists at
Harvard, Robert Pound and Glenn

509
00:41:50,840 --> 00:41:57,360
Rebka, sent gamma rays upward
through a 22 meter tower and

510
00:41:57,360 --> 00:42:02,280
measured the tiny shift in their
frequency as they climbed out of

511
00:42:02,280 --> 00:42:08,920
Earth's gravitational well.
The shift was extraordinaire at

512
00:42:08,920 --> 00:42:11,760
the very heart of a non rotating
black hole.

513
00:42:12,120 --> 00:42:17,160
The mathematics of general
relativity points to a place

514
00:42:17,160 --> 00:42:22,400
called the singularity.
It is perhaps the strangest

515
00:42:22,400 --> 00:42:27,320
feature in all of modern
physics, and one of the most

516
00:42:27,560 --> 00:42:31,920
honestly humbling.
The equations say that matter

517
00:42:31,920 --> 00:42:36,800
falling through the event
horizon continues inward, and

518
00:42:36,800 --> 00:42:41,040
that the inward journey cannot
be stopped by any force we

519
00:42:41,040 --> 00:42:45,160
currently know.
Everything converges toward a

520
00:42:45,160 --> 00:42:50,000
central region where density
grows without limit and the

521
00:42:50,000 --> 00:42:54,040
curvature of space-time becomes
infinite.

522
00:42:55,080 --> 00:43:01,280
The word singularity is used
because the equations themselves

523
00:43:01,280 --> 00:43:05,520
at that point cease to give
finite answers.

524
00:43:06,520 --> 00:43:10,280
They do not calmly describe
something strange.

525
00:43:10,800 --> 00:43:17,120
They stopped describing at all.
For a long time, physicists were

526
00:43:17,120 --> 00:43:22,360
unsure whether singularities
were a real feature of nature or

527
00:43:22,360 --> 00:43:26,280
an artifact of particular
perfectly symmetric

528
00:43:26,280 --> 00:43:30,360
calculations.
Perhaps they thought the

529
00:43:30,360 --> 00:43:35,320
infinities appeared only because
Schwarzschild had assumed a

530
00:43:35,320 --> 00:43:41,560
perfectly spherical collapse.
In the real universe, stars are

531
00:43:41,560 --> 00:43:47,200
never perfectly spherical.
Perhaps imperfections would save

532
00:43:47,200 --> 00:43:51,720
the mathematics.
This hope was gently closed off

533
00:43:51,720 --> 00:43:57,800
in 1965 when the British
mathematician Roger Penrose

534
00:43:58,240 --> 00:44:04,400
published a short but powerful
paper showing that singularities

535
00:44:04,560 --> 00:44:08,560
are not merely the result of
special symmetries.

536
00:44:08,960 --> 00:44:12,960
Using the methods of what came
to be called global geometry,

537
00:44:13,600 --> 00:44:18,320
Penrose proved that under very
general conditions, once

538
00:44:18,320 --> 00:44:22,920
gravitational collapse has
proceeded past a certain stage,

539
00:44:23,480 --> 00:44:26,840
a singularity of some kind must
form.

540
00:44:27,560 --> 00:44:33,720
It was a theorem, not a guess.
Nature, or at least general

541
00:44:33,720 --> 00:44:38,480
relativity, does not seem to
offer an escape.

542
00:44:39,600 --> 00:44:44,520
For this work, along with
related contributions, Penrose

543
00:44:44,520 --> 00:44:49,440
shared the Nobel Prize in
Physics in Tuan Tuani.

544
00:44:49,800 --> 00:44:53,880
Stephen Hawking, then a young
graduate student at Cambridge,

545
00:44:54,400 --> 00:44:58,680
quickly extended Penrose's
methods and applied them to the

546
00:44:58,680 --> 00:45:03,640
universe as a whole.
Together they proved a series of

547
00:45:03,880 --> 00:45:09,520
singularity theorems which
suggested that under general

548
00:45:09,520 --> 00:45:14,040
relativity, singularities are
not a rare accident.

549
00:45:14,600 --> 00:45:21,200
They are woven into the theory.
And yet most physicists do not

550
00:45:21,200 --> 00:45:26,960
believe that true infinities
really exist inside black holes.

551
00:45:27,800 --> 00:45:33,480
The appearance of a singularity
is more often read as a warning.

552
00:45:34,360 --> 00:45:40,200
It is a sign that general
relativity, beautiful and well

553
00:45:40,200 --> 00:45:43,880
tested as it is, is not the
final word.

554
00:45:44,120 --> 00:45:49,480
At such extreme densities, the
effects of quantum mechanics

555
00:45:49,600 --> 00:45:53,800
must become important, and
general relativity and quantum

556
00:45:53,800 --> 00:45:57,160
mechanics have never been fully
reconciled.

557
00:45:58,040 --> 00:46:02,880
The singularity, in this view,
is not a thing, It is a

558
00:46:02,880 --> 00:46:06,240
question.
It is the place where our

559
00:46:06,240 --> 00:46:11,840
current description of reality
softly admits that it is

560
00:46:11,840 --> 00:46:15,640
incomplete.
There is a kind of dignity in

561
00:46:15,640 --> 00:46:20,200
that admission.
Science is not obliged to know

562
00:46:20,200 --> 00:46:24,400
everything.
It is obliged to be honest about

563
00:46:24,400 --> 00:46:29,480
where its knowledge ends.
And at the center of every black

564
00:46:29,480 --> 00:46:35,200
hole, written into the geometry
itself, is that quiet

565
00:46:35,200 --> 00:46:38,080
confession.
Schwarzschild's original

566
00:46:38,080 --> 00:46:43,960
solution described the simplest
kind of black hole imaginable,

567
00:46:44,560 --> 00:46:49,320
one that was perfectly spherical
and did not rotate.

568
00:46:50,080 --> 00:46:56,200
But real stars spin.
They rotate on their axes for

569
00:46:56,200 --> 00:47:01,000
the same reason skaters rotate
when they pull in their arms.

570
00:47:01,320 --> 00:47:07,040
As a massive star collapses, its
rotation speeds up, often

571
00:47:07,040 --> 00:47:11,480
dramatically.
A slowly turning star can become

572
00:47:11,720 --> 00:47:18,040
a rapidly spinning neutron star,
and a truly massive collapse can

573
00:47:18,040 --> 00:47:22,560
leave behind a black hole that
carries most of that original

574
00:47:22,560 --> 00:47:27,120
rotation with it.
For nearly fifty years after

575
00:47:27,160 --> 00:47:32,120
Schwarzschild, the mathematics
of such a rotating black hole

576
00:47:32,480 --> 00:47:38,240
remained unsolved.
Einstein's field equations, so

577
00:47:38,240 --> 00:47:44,720
elegant in simple cases, became
fiercely difficult when rotation

578
00:47:44,720 --> 00:47:49,440
was added.
The breakthrough came in 1963

579
00:47:49,840 --> 00:47:54,880
from a New Zealand mathematician
named Roy Kerr.

580
00:47:55,800 --> 00:47:59,400
Kerr was working at the
University of Texas at Austin,

581
00:48:00,000 --> 00:48:04,280
and he was trying to find exact
solutions to Einstein's

582
00:48:04,280 --> 00:48:10,480
equations that would describe
the space-time around a spinning

583
00:48:10,480 --> 00:48:14,000
mass.
After months of careful work, he

584
00:48:14,000 --> 00:48:18,480
found one Its structure was far
more intricate than

585
00:48:18,480 --> 00:48:23,320
Schwarzschild's.
It contained not one but two

586
00:48:23,320 --> 00:48:29,000
relevant surfaces, and it
described a geometry in which

587
00:48:29,000 --> 00:48:34,680
the rotation of the central mass
actually dragged the surrounding

588
00:48:34,680 --> 00:48:39,800
space-time along with it, like a
slow Whirlpool pulling water

589
00:48:39,800 --> 00:48:43,720
around a drain.
This effect is called frame

590
00:48:43,720 --> 00:48:47,240
dragging near a rotating black
hole.

591
00:48:47,560 --> 00:48:52,880
Space-time itself is set gently
in motion by the spin of the

592
00:48:52,880 --> 00:48:56,760
mass.
An observer close enough would

593
00:48:56,760 --> 00:49:01,280
find that they could not remain
still even in principle.

594
00:49:02,040 --> 00:49:06,880
To stand perfectly motionless
relative to distant stars, they

595
00:49:06,880 --> 00:49:10,800
would have to move against the
flow of space-time faster than

596
00:49:10,800 --> 00:49:16,040
light, which is impossible.
Instead, they would be carried

597
00:49:16,040 --> 00:49:21,200
along, swept quietly around the
black hole by the geometry

598
00:49:21,200 --> 00:49:24,200
itself.
Outside the event horizon of a

599
00:49:24,200 --> 00:49:29,640
curb black hole lies a region
called the ergosphere, from the

600
00:49:29,640 --> 00:49:35,280
Greek word for work.
Inside this region, an object

601
00:49:35,280 --> 00:49:40,000
must rotate along with the black
hole, but it can still escape to

602
00:49:40,000 --> 00:49:45,400
Infinity if it chooses.
The ergosphere is one of the

603
00:49:45,400 --> 00:49:48,560
most remarkable features in
relativity.

604
00:49:48,880 --> 00:49:56,000
In 1969, Roger Penrose showed
that it is in principle possible

605
00:49:56,000 --> 00:50:00,760
to extract energy from a
spinning black hole by sending

606
00:50:00,760 --> 00:50:05,760
an object into the ergosphere
and splitting it, letting one

607
00:50:05,760 --> 00:50:10,520
piece fall in while the other
escapes with more energy than

608
00:50:10,520 --> 00:50:14,760
was brought in.
This process, now called the

609
00:50:14,760 --> 00:50:20,000
Penrose process, does not
violate conservation laws.

610
00:50:20,680 --> 00:50:24,440
The energy comes from the
rotation of the black hole

611
00:50:24,440 --> 00:50:29,360
itself, which gradually slows as
energy is removed.

612
00:50:29,760 --> 00:50:34,800
Real black holes in the universe
are almost certainly CUR black

613
00:50:34,800 --> 00:50:38,040
holes rather than Schwarzschild
ones.

614
00:50:39,200 --> 00:50:42,840
Rotation is the rule, not the
exception.

615
00:50:43,680 --> 00:50:49,280
The silent giants of the cosmos
are, in their own hidden way,

616
00:50:49,880 --> 00:50:55,040
turning, and the very fabric of
space-time turned softly with

617
00:50:55,040 --> 00:50:58,360
them.
Black holes come in a surprising

618
00:50:58,360 --> 00:51:03,960
range of sizes, and astronomers
today recognize several broad

619
00:51:03,960 --> 00:51:08,160
categories.
The smallest kind we know of for

620
00:51:08,160 --> 00:51:12,240
certain are called stellar mass
black holes.

621
00:51:12,840 --> 00:51:18,800
They form when a single massive
star, roughly 20 times the mass

622
00:51:18,800 --> 00:51:23,880
of the sun or more, reaches the
end of its fuel and collapses.

623
00:51:24,160 --> 00:51:29,520
What remains is a black hole,
typically between about 3 and

624
00:51:29,520 --> 00:51:35,080
several dozen solar masses,
compressed into a region only a

625
00:51:35,080 --> 00:51:40,760
few 10s of kilometers across.
The Milky Way alone is thought

626
00:51:40,760 --> 00:51:46,000
to contain hundreds of millions
of such objects, most of them

627
00:51:46,000 --> 00:51:50,200
invisible, drifting quietly
between the stars.

628
00:51:50,520 --> 00:51:54,480
At the other end of the scale
lie the supermassive black

629
00:51:54,480 --> 00:51:58,160
holes.
These are enormous beyond

630
00:51:58,160 --> 00:52:03,360
ordinary comparison, with masses
ranging from hundreds of

631
00:52:03,360 --> 00:52:07,280
thousands to billions of times
that of the Sun.

632
00:52:08,160 --> 00:52:13,120
They sit at the centers of
nearly every large Galaxy that

633
00:52:13,120 --> 00:52:16,600
has been carefully studied,
including our own.

634
00:52:17,080 --> 00:52:23,240
The very largest known examples
exceed 10 billion solar masses.

635
00:52:24,080 --> 00:52:27,480
Their event horizons can be
larger than the entire solar

636
00:52:27,480 --> 00:52:32,000
system.
How such giants grew so massive

637
00:52:32,000 --> 00:52:37,000
so early in the history of the
universe remains one of the most

638
00:52:37,160 --> 00:52:42,640
interesting open questions in
astrophysics, and several

639
00:52:42,640 --> 00:52:49,760
theories compete to explain it.
Between these two extremes lies

640
00:52:49,760 --> 00:52:56,560
a more elusive third category
called intermediate mass black

641
00:52:56,560 --> 00:53:01,000
holes.
These would have masses between

642
00:53:01,000 --> 00:53:06,880
roughly 100 and 100,000 times
that of the sun, and for many

643
00:53:06,880 --> 00:53:10,360
years they were mostly
theoretical.

644
00:53:10,640 --> 00:53:16,920
In recent decades, observations
have begun to reveal candidates

645
00:53:16,920 --> 00:53:21,240
for such objects, including some
detected through the

646
00:53:21,240 --> 00:53:23,640
gravitational waves they
produce.

647
00:53:24,000 --> 00:53:28,760
When they collide and merge,
they may turn out to be the

648
00:53:28,760 --> 00:53:33,760
missing link that explains how
small black holes grow into

649
00:53:33,760 --> 00:53:39,400
giant ones over cosmic time.
There is also a speculative 4th

650
00:53:39,400 --> 00:53:42,520
category that has never been
confirmed.

651
00:53:43,440 --> 00:53:49,640
In the 1970s, Stephen Hawking
proposed that very small black

652
00:53:49,640 --> 00:53:54,360
holes might have formed in the
first fractions of a second

653
00:53:54,920 --> 00:53:59,480
after the birth of the universe,
when pressures were so extreme

654
00:53:59,760 --> 00:54:04,560
that even modest concentrations
of matter could have collapsed.

655
00:54:04,920 --> 00:54:08,640
These are called primordial
black holes.

656
00:54:09,480 --> 00:54:13,200
Some might have the mass of a
mountain compressed into a

657
00:54:13,200 --> 00:54:19,160
region smaller than an atom.
Whether any exist, and whether

658
00:54:19,160 --> 00:54:23,200
they could account for some of
the unseen mass in the universe

659
00:54:23,800 --> 00:54:29,840
is still an open question.
What unites this whole family is

660
00:54:29,840 --> 00:54:35,240
simpler than it first sounds.
In general relativity, a black

661
00:54:35,240 --> 00:54:40,760
hole is fully described by only
a few numbers, its mass, its

662
00:54:40,760 --> 00:54:43,880
rotation, and its electric
charge.

663
00:54:44,680 --> 00:54:49,160
Everything else about the matter
that formed it is forgotten at

664
00:54:49,160 --> 00:54:52,840
the horizon.
This is sometimes called the No

665
00:54:52,840 --> 00:54:57,440
Hair Theorem, a name given by
John Wheeler in a moment of

666
00:54:57,440 --> 00:55:02,440
gentle wit.
A black hole, in this sense is

667
00:55:02,440 --> 00:55:08,280
nature at its most austere.
Almost everything is set aside

668
00:55:08,840 --> 00:55:13,600
and only the essentials remain.
At the center of our own Galaxy,

669
00:55:13,840 --> 00:55:19,560
about 26,000 light years from
Earth, sits a very dark and very

670
00:55:19,560 --> 00:55:25,040
quiet object known as
Sagittarius, a star.

671
00:55:26,000 --> 00:55:30,240
It is the supermassive black
hole at the heart of the Milky

672
00:55:30,240 --> 00:55:34,600
Way, and its existence was
established not by seeing it,

673
00:55:34,920 --> 00:55:40,640
which for many years was
impossible, but by watching what

674
00:55:40,640 --> 00:55:46,160
happened to the things nearby.
Beginning in the early 1990s,

675
00:55:46,680 --> 00:55:51,800
two groups of astronomers, one
led by Reinhard Genzel in

676
00:55:51,800 --> 00:55:57,560
Germany and the other by Andrea
Gaez in the United States, began

677
00:55:57,560 --> 00:56:02,400
a patient, long term study of
the central region of our

678
00:56:02,400 --> 00:56:06,680
Galaxy.
Using infrared telescopes that

679
00:56:06,680 --> 00:56:10,880
could peer through the thick
dust of the galactic disk, they

680
00:56:10,880 --> 00:56:14,960
tracked the positions of stars
very close to the galactic

681
00:56:14,960 --> 00:56:18,960
center.
Over years and then decades,

682
00:56:19,480 --> 00:56:25,040
they watched these stars move.
What they saw was extraordinary.

683
00:56:25,480 --> 00:56:29,600
The stars were tracing tight,
elliptical orbits around

684
00:56:29,600 --> 00:56:35,280
something they could not see.
One of them, called S2, was

685
00:56:35,280 --> 00:56:41,560
eventually found to complete a
full orbit in just 16 years,

686
00:56:42,080 --> 00:56:47,960
whipping past the central object
at velocities of thousands of

687
00:56:47,960 --> 00:56:51,880
kilometers per second.
From the shapes and speeds of

688
00:56:51,880 --> 00:56:57,240
these orbits, the mass of the
unseen central object could be

689
00:56:57,240 --> 00:56:59,960
calculated with growing
precision.

690
00:57:00,880 --> 00:57:04,720
The ants are settled near
4,000,000 times the mass of the

691
00:57:04,720 --> 00:57:10,840
Sun, all packed into a region no
larger than the orbit of

692
00:57:10,840 --> 00:57:14,800
Mercury.
Nothing known to physics could

693
00:57:14,800 --> 00:57:19,960
account for such a mass so
tightly confined, except a

694
00:57:19,960 --> 00:57:27,760
supermassive black hole.
In 2020, Genzel and Gez shared

695
00:57:27,760 --> 00:57:32,680
the Nobel Prize in Physics for
this work, along with Roger

696
00:57:32,680 --> 00:57:35,840
Penrose.
There is something tender about

697
00:57:35,840 --> 00:57:40,280
this discovery.
For decades, astronomers watched

698
00:57:40,280 --> 00:57:45,680
the silent choreography of stars
at the galactic centre and

699
00:57:45,680 --> 00:57:49,080
quietly inferred the shape of
something darker.

700
00:57:49,160 --> 00:57:54,200
At its heart, the black hole did
not announce itself.

701
00:57:54,760 --> 00:58:01,480
It left only a signature written
in the paths of its neighbors.

702
00:58:02,600 --> 00:58:07,440
By reading that signature
carefully, patient observers

703
00:58:07,880 --> 00:58:14,280
slowly pieced together a picture
of an object that refused to be

704
00:58:14,280 --> 00:58:18,960
seen directly.
Sagittarius, a star, is, by the

705
00:58:18,960 --> 00:58:23,080
standards of supermassive black
holes, rather modest.

706
00:58:24,000 --> 00:58:29,840
Some galaxies host central black
holes, 1000 times more massive.

707
00:58:30,880 --> 00:58:35,920
It is also relatively calm at
the moment, consuming only small

708
00:58:35,920 --> 00:58:40,640
amounts of matter and emitting
little light compared with the

709
00:58:40,640 --> 00:58:43,840
brilliant active nuclei of other
galaxies.

710
00:58:44,840 --> 00:58:49,800
Our own galactic center is not a
place of cosmic violence.

711
00:58:50,560 --> 00:58:55,320
It is a place of quiet gravity,
where one of the most extreme

712
00:58:55,320 --> 00:59:02,640
objects in physics sits in the
dark, holding its cord of stars.

713
00:59:02,920 --> 00:59:08,280
Every night when we look toward
the constellation Sagittarius in

714
00:59:08,280 --> 00:59:13,600
a dark sky, we are looking,
without quite realizing it,

715
00:59:14,160 --> 00:59:18,800
toward that hidden heart and
toward the stars that have been

716
00:59:18,800 --> 00:59:23,560
teaching us for years what must
live there.

717
00:59:23,960 --> 00:59:27,760
There is a quiet paradox in the
study of black holes.

718
00:59:28,200 --> 00:59:32,720
The darkest objects in the
universe are often found at the

719
00:59:32,720 --> 00:59:35,560
centers of the brightest things
in the sky.

720
00:59:36,480 --> 00:59:41,600
This is not a contradiction, but
a consequence of how matter

721
00:59:41,600 --> 00:59:46,160
behaves when it falls into a
deep gravitational well.

722
00:59:46,480 --> 00:59:51,760
When gas, dust, or even the torn
remains of a star drift too

723
00:59:51,760 --> 00:59:57,160
close to a black hole, they do
not simply fall straight in.

724
00:59:58,040 --> 01:00:02,280
Almost always, the in falling
material carries some sideways

725
01:00:02,280 --> 01:00:09,600
motion, and as gravity draws it
inward, it begins to spiral.

726
01:00:09,800 --> 01:00:15,160
Over time, the material settles
into a flattened rotating disk

727
01:00:15,560 --> 01:00:20,720
called an accretion disk, in
which particles orbit the black

728
01:00:20,720 --> 01:00:26,120
hole at enormous speeds.
Friction between layers of the

729
01:00:26,120 --> 01:00:31,920
disk transfers angular momentum
outward, allowing matter to

730
01:00:31,920 --> 01:00:35,400
gradually drift inward toward
the event horizon.

731
01:00:36,400 --> 01:00:42,280
As it does so, it is squeezed
and heated to extraordinary

732
01:00:42,280 --> 01:00:45,360
temperatures.
The temperatures reached in the

733
01:00:45,360 --> 01:00:50,400
innermost parts of an accretion
disk can be millions of degrees.

734
01:00:51,200 --> 01:00:56,880
At such heat, the gas radiates
fiercely, producing not only

735
01:00:56,880 --> 01:01:03,400
visible light but X-rays and
ultraviolet radiation that can

736
01:01:03,400 --> 01:01:06,400
outshine the stars of an entire
Galaxy.

737
01:01:07,560 --> 01:01:12,400
The first strong evidence for
stellar mass black holes came

738
01:01:12,400 --> 01:01:15,200
from precisely this kind of
glow.

739
01:01:16,240 --> 01:01:22,880
In 1964, astronomers detected a
powerful X-ray source in the

740
01:01:22,880 --> 01:01:30,000
constellation Cygnus, which they
named Cygnus X1.

741
01:01:30,360 --> 01:01:35,560
Careful observation showed it to
be a binary system in which a

742
01:01:35,560 --> 01:01:40,760
normal supergiant star was
apparently losing matter to an

743
01:01:40,760 --> 01:01:46,200
unseen companion about 15 times
the mass of the Sun.

744
01:01:47,200 --> 01:01:51,560
The companion was too massive to
be a neutron star and too

745
01:01:51,560 --> 01:01:56,960
compact to be anything else.
It was almost certainly a black

746
01:01:56,960 --> 01:02:02,800
hole, and it became the first
widely accepted observational

747
01:02:02,800 --> 01:02:05,760
case.
At the largest scales,

748
01:02:06,320 --> 01:02:12,800
supermassive black holes at the
centers of some galaxies devour

749
01:02:12,800 --> 01:02:18,560
matter so efficiently that their
accretion disks become the most

750
01:02:18,560 --> 01:02:23,760
luminous sustained sources in
the universe.

751
01:02:24,120 --> 01:02:29,120
These are called active galactic
nuclei, and their brightest

752
01:02:29,120 --> 01:02:34,960
versions are known as quasars.
A single quasar can shine with

753
01:02:34,960 --> 01:02:39,040
the light of a trillion Suns,
yet its light comes from a

754
01:02:39,040 --> 01:02:42,240
region no larger than our solar
system.

755
01:02:43,200 --> 01:02:47,400
For a long time, quasars seemed
impossible.

756
01:02:48,040 --> 01:02:53,160
Now they are understood as the
glow of matter falling into

757
01:02:53,160 --> 01:02:58,640
supermassive black holes in the
distant, younger universe.

758
01:02:58,960 --> 01:03:04,920
In some cases, a black hole also
launches narrow jets of matter

759
01:03:04,960 --> 01:03:10,200
and radiation, shooting outward
at nearly the speed of light,

760
01:03:10,680 --> 01:03:14,360
stretching across 10s of
thousands of light years.

761
01:03:15,440 --> 01:03:19,800
How exactly these jets are
focused and accelerated is still

762
01:03:19,800 --> 01:03:25,080
being studied, but the outline
of the story is now clear.

763
01:03:25,360 --> 01:03:30,800
Black holes are not simply dark.
They are often surrounded by

764
01:03:30,800 --> 01:03:34,720
some of the brightest, hottest,
most restless places in the

765
01:03:34,720 --> 01:03:38,720
cosmos.
And much of that fire is the

766
01:03:38,720 --> 01:03:44,240
long, slow descent of matter
reaching the final edge.

767
01:03:44,600 --> 01:03:48,880
For a long time, black holes
seemed to be the most final

768
01:03:48,880 --> 01:03:53,160
objects in the universe.
Matter and light could fall in.

769
01:03:53,720 --> 01:03:59,200
Nothing could ever come out.
The event horizon was a one way

770
01:03:59,200 --> 01:04:04,640
boundary, absolute and
uncompromising, and whatever

771
01:04:04,640 --> 01:04:09,520
crossed it was assumed to be
lost to the rest of the cosmos

772
01:04:09,800 --> 01:04:14,400
forever.
Then, in the early 1970s, a

773
01:04:14,400 --> 01:04:19,200
young physicist at Cambridge
began to think more carefully

774
01:04:19,920 --> 01:04:25,200
about what happens very close to
the horizon, and the picture

775
01:04:25,480 --> 01:04:29,800
gently shifted.
Stephen Hawking had been working

776
01:04:29,800 --> 01:04:33,880
on problems in general
relativity for years, including

777
01:04:33,880 --> 01:04:37,600
the singularity theorems he had
developed with Roger Penrose.

778
01:04:37,960 --> 01:04:44,800
But in 1974 he published a short
startling paper with the

779
01:04:44,800 --> 01:04:48,960
deceptively simple title Black
Hole Explosions.

780
01:04:49,440 --> 01:04:53,520
In it, he argued that black
holes are not perfectly black

781
01:04:53,520 --> 01:04:56,400
after all.
When the rules of quantum

782
01:04:56,400 --> 01:05:00,120
mechanics are applied to the
empty space just outside an

783
01:05:00,120 --> 01:05:05,160
event horizon, the result is
that black holes should slowly

784
01:05:05,160 --> 01:05:09,480
emit A faint thermal glow of
particles and radiation.

785
01:05:10,440 --> 01:05:15,440
The effect is now called Hawking
radiation, and it came as a

786
01:05:15,440 --> 01:05:19,720
genuine surprise even to Hawking
himself.

787
01:05:20,080 --> 01:05:27,000
The underlying idea is subtle.
In quantum physics, the vacuum

788
01:05:27,000 --> 01:05:29,800
of empty space is not truly
empty.

789
01:05:30,520 --> 01:05:34,960
Tiny fluctuations constantly
produce pairs of particles that

790
01:05:35,320 --> 01:05:38,840
briefly appear and then
annihilate one another.

791
01:05:39,760 --> 01:05:44,760
Near the horizon of a black
hole, Hawking showed the

792
01:05:44,760 --> 01:05:49,600
geometry of space-time can
separate such pairs before they

793
01:05:49,600 --> 01:05:55,240
have time to vanish. 1 member of
the pair falls inward while the

794
01:05:55,280 --> 01:05:59,840
other escapes outward, carrying
a small amount of energy with

795
01:05:59,840 --> 01:06:03,680
it.
From far away, it looks as

796
01:06:03,680 --> 01:06:09,280
though the black hole is quietly
radiating, and because that

797
01:06:09,280 --> 01:06:15,200
radiation carries energy, the
black hole must slowly lose mass

798
01:06:15,200 --> 01:06:18,640
over time.
The effect is extraordinarily

799
01:06:18,640 --> 01:06:21,200
small for any ordinary black
hole.

800
01:06:22,040 --> 01:06:27,160
A stellar mass black hole emits
Hawking radiation at a

801
01:06:27,160 --> 01:06:31,360
temperature of less than one
millionth of a degree above

802
01:06:31,440 --> 01:06:37,760
absolute 0, far colder than the
cosmic background radiation that

803
01:06:37,760 --> 01:06:42,840
already bathes it.
In practice, such a black hole

804
01:06:43,040 --> 01:06:48,320
is not shrinking at all, it is
steadily gaining mass from its

805
01:06:48,320 --> 01:06:51,600
surroundings.
Only very small black holes

806
01:06:51,600 --> 01:06:55,760
would radiate quickly enough to
evaporate within the age of the

807
01:06:55,760 --> 01:06:59,480
universe.
For a primordial black hole of

808
01:06:59,480 --> 01:07:04,200
the right initial mass, the
final stages of evaporation

809
01:07:04,520 --> 01:07:10,440
would release a bright burst of
energy, a kind of quiet flash at

810
01:07:10,440 --> 01:07:15,720
the end of a very long life.
Hawking's result has never been

811
01:07:15,720 --> 01:07:20,480
directly observed, since the
glow is too faint to detect near

812
01:07:20,480 --> 01:07:25,120
any known black hole.
But it is considered one of the

813
01:07:25,120 --> 01:07:29,400
most important theoretical
discoveries in modern physics

814
01:07:30,160 --> 01:07:35,080
because it is one of the few
places where general relativity

815
01:07:35,080 --> 01:07:38,080
and quantum mechanics clearly
meet.

816
01:07:38,960 --> 01:07:43,680
Black holes, it seems, are not
the final word after all.

817
01:07:44,480 --> 01:07:50,280
They too can very slowly give
something back to the universe

818
01:07:50,440 --> 01:07:54,320
that made them.
Einstein had predicted as early

819
01:07:54,320 --> 01:08:00,720
as 1916 that violent motions of
massive objects should send

820
01:08:00,720 --> 01:08:04,880
ripples outward through the
fabric of space-time itself.

821
01:08:05,920 --> 01:08:09,440
He called these gravitational
waves.

822
01:08:10,000 --> 01:08:14,160
Unlike the ripples on a pond,
they would not move through

823
01:08:14,160 --> 01:08:17,520
space.
They would be ripples in space,

824
01:08:17,520 --> 01:08:20,560
gentle stretchings and
compressions of distance

825
01:08:20,920 --> 01:08:23,920
travelling outward at the speed
of light.

826
01:08:24,680 --> 01:08:29,120
For decades, even Einstein
wondered whether they could ever

827
01:08:29,120 --> 01:08:33,520
be detected, or whether they
were so faint as to remain

828
01:08:33,520 --> 01:08:38,120
forever beyond reach.
The challenge is hard to

829
01:08:38,120 --> 01:08:42,000
overstate.
A typical gravitational wave

830
01:08:42,000 --> 01:08:46,520
passing through Earth changes
distances by an amount smaller

831
01:08:46,640 --> 01:08:49,760
than 110 thousandth the width of
a proton.

832
01:08:50,640 --> 01:08:56,080
To measure something so small,
scientists built extraordinarily

833
01:08:56,080 --> 01:09:01,000
sensitive instruments called
laser interferometers.

834
01:09:01,359 --> 01:09:06,319
The Laser Interferometer
Gravitational Wave Observatory,

835
01:09:06,560 --> 01:09:13,439
known as LIGO, consists of two
facilities, one in Louisiana and

836
01:09:13,439 --> 01:09:18,200
one in Washington state, each
with two long perpendicular

837
01:09:18,200 --> 01:09:23,840
tunnels 4 kilometers in length.
Laser beams are sent down each

838
01:09:23,840 --> 01:09:29,560
tunnel, reflected off mirrors at
the far ends, and recombined.

839
01:09:30,439 --> 01:09:34,520
Any tiny change in the relative
lengths of the tunnels caused by

840
01:09:34,520 --> 01:09:40,279
a passing gravitational wave
would shift the recombined light

841
01:09:40,640 --> 01:09:45,359
in a measurable way.
LIGO operated for years without

842
01:09:45,359 --> 01:09:48,680
detecting anything.
The instruments were carefully

843
01:09:48,680 --> 01:09:55,560
upgraded, and in September 2015,
just as the new advanced version

844
01:09:55,560 --> 01:09:59,200
began its first science run, a
signal arrived.

845
01:09:59,880 --> 01:10:03,120
It lasted only about 1/5 of a
second.

846
01:10:03,760 --> 01:10:09,440
It was a faint rising chirp,
exactly the pattern theorists

847
01:10:09,440 --> 01:10:14,800
had predicted for two black
holes spiraling inward and

848
01:10:14,800 --> 01:10:19,640
merging into one.
Detailed analysis showed that

849
01:10:19,640 --> 01:10:24,720
the signal had come from 2 black
holes, each about 30 times the

850
01:10:24,720 --> 01:10:29,920
mass of the sun, that had
collided in a distant Galaxy

851
01:10:30,480 --> 01:10:36,160
roughly 1.3 billion years ago.
In the final moments of their

852
01:10:36,160 --> 01:10:40,800
merger, they had converted about
3 solar masses of material

853
01:10:40,800 --> 01:10:44,280
entirely into the energy of
gravitational waves.

854
01:10:44,920 --> 01:10:49,800
That energy had then travelled
across the universe and reached

855
01:10:49,800 --> 01:10:55,320
Earth as a tiny tremor in the
geometry of space itself.

856
01:10:55,720 --> 01:11:01,600
The discovery was announced in
February 2016, and in 2000

857
01:11:01,600 --> 01:11:06,400
Tebahan, the Nobel Prize in
Physics, was awarded to three of

858
01:11:06,400 --> 01:11:13,080
the leading figures behind Ligo
Rayner Weiss, Barry Barish and

859
01:11:13,080 --> 01:11:17,160
Kip Thorne.
Since then, dozens of additional

860
01:11:17,160 --> 01:11:20,840
detections have followed,
including mergers of neutron

861
01:11:20,840 --> 01:11:23,280
stars and other black hole
pairs.

862
01:11:23,840 --> 01:11:28,240
A new branch of astronomy has
been born, one that listens

863
01:11:28,320 --> 01:11:32,000
rather than looks.
There is something deeply sleep

864
01:11:32,000 --> 01:11:38,480
wise about that idea. 2 enormous
invisible objects met in the

865
01:11:38,480 --> 01:11:44,160
dark long before the Earth had
life or oceans, and the universe

866
01:11:44,520 --> 01:11:48,520
carried the news of their
meeting outward in the form of a

867
01:11:48,520 --> 01:11:52,560
faint quiver.
Patient instruments listened,

868
01:11:53,000 --> 01:11:56,120
and at last the universe was
heard.

869
01:11:56,480 --> 01:12:01,720
For most of the history of
physics, no one expected ever to

870
01:12:01,720 --> 01:12:07,840
see a black hole directly.
By definition, the object itself

871
01:12:08,000 --> 01:12:12,680
emits no light.
Even the matter falling toward

872
01:12:12,680 --> 01:12:18,680
it, however bright, was thought
too small and too distant to be

873
01:12:18,680 --> 01:12:22,440
resolved against the glow of the
rest of the cosmos.

874
01:12:22,880 --> 01:12:28,440
To picture a black hole,
scientists relied on diagrams,

875
01:12:28,440 --> 01:12:33,520
simulations, and patient
inference from the motion of

876
01:12:33,800 --> 01:12:39,920
nearby stars.
The idea of an actual photograph

877
01:12:40,600 --> 01:12:44,120
belonged to a future no one
could quite imagine.

878
01:12:44,480 --> 01:12:48,240
That future arrived in April
2019.

879
01:12:49,000 --> 01:12:53,440
After years of preparation, An
international collaboration

880
01:12:53,440 --> 01:12:57,920
called the Event Horizon
Telescope released the first

881
01:12:57,920 --> 01:13:03,840
image of a black hole, or rather
of the glowing matter and shadow

882
01:13:03,840 --> 01:13:08,440
surrounding one.
The target was the supermassive

883
01:13:08,440 --> 01:13:12,160
black hole at the center of the
elliptical Galaxy known as

884
01:13:12,600 --> 01:13:20,560
Messier 87, or M87, about 55
million light years from Earth.

885
01:13:20,960 --> 01:13:25,280
It is one of the largest black
holes known, with a mass of

886
01:13:25,280 --> 01:13:31,800
about 6.5 billion times that of
the Sun and an event horizon

887
01:13:32,200 --> 01:13:34,640
larger than our entire solar
system.

888
01:13:35,600 --> 01:13:40,360
From that great distance, it
appears as a tiny dark speck

889
01:13:40,360 --> 01:13:44,360
against the sky.
But the Event Horizon Telescope

890
01:13:44,360 --> 01:13:47,920
was built to resolve exactly
such specks.

891
01:13:48,280 --> 01:13:53,600
The trick was simple in concept
and astonishingly difficult in

892
01:13:53,600 --> 01:13:59,240
practice. 8 Radio telescopes
scattered across the world, from

893
01:13:59,240 --> 01:14:04,960
Hawaii and Arizona to Mexico,
Chile, Spain and Antarctica were

894
01:14:04,960 --> 01:14:10,520
linked together using precise
atomic clocks and a technique

895
01:14:10,520 --> 01:14:14,240
called very long baseline
interferometry.

896
01:14:14,560 --> 01:14:19,280
By combining the signals from
all of them, the collaboration

897
01:14:19,280 --> 01:14:25,880
created what was in effect a
single virtual telescope as wide

898
01:14:26,080 --> 01:14:31,400
as the Earth itself.
Only such a telescope could

899
01:14:31,400 --> 01:14:36,720
possibly resolve a feature as
small as a black hole's shadow

900
01:14:37,320 --> 01:14:42,320
at intergalactic distances.
The resulting image showed a

901
01:14:42,320 --> 01:14:47,840
bright, asymmetric ring of
glowing gas surrounding a

902
01:14:47,840 --> 01:14:52,120
central darkness.
The asymmetry came from the

903
01:14:52,120 --> 01:14:56,480
rotation of the disk.
Material moving toward us

904
01:14:56,480 --> 01:15:00,080
appeared brighter, while
material moving away appeared

905
01:15:00,080 --> 01:15:04,280
dimmer, exactly as relativity
predicted.

906
01:15:04,640 --> 01:15:09,800
The size of the dark central
region matched the predicted

907
01:15:09,800 --> 01:15:15,360
shadow of a black hole of M87's
measured mass.

908
01:15:16,320 --> 01:15:21,960
It was not a literal photograph
of the black hole, but rather of

909
01:15:21,960 --> 01:15:25,400
the silhouette it cast against
the light around it.

910
01:15:25,760 --> 01:15:31,560
In 2022, the same collaboration
released a second image, this

911
01:15:31,560 --> 01:15:37,680
time of Sagittarius AR, the much
smaller supermassive black hole

912
01:15:37,800 --> 01:15:40,200
at the center of our own Milky
Way.

913
01:15:40,800 --> 01:15:45,920
Despite its smaller size, it
sits much closer to us, so the

914
01:15:45,920 --> 01:15:50,720
apparent dimensions on the sky
were comparable to those of M87.

915
01:15:51,480 --> 01:15:56,640
Once again, the predicted shadow
appeared ringed by light.

916
01:15:57,000 --> 01:16:02,360
After a century of theoretical
work, 2 of nature's quietest

917
01:16:02,360 --> 01:16:09,040
giants had finally allowed
themselves to be seen in outline

918
01:16:09,600 --> 01:16:15,080
through the patient cooperation
of telescopes across the world.

919
01:16:15,440 --> 01:16:20,360
After all this, it can be
tempting to think of black holes

920
01:16:20,800 --> 01:16:27,640
as a strange corner of physics,
a place where ordinary rules

921
01:16:27,640 --> 01:16:31,480
give way and reality grows
alarming.

922
01:16:32,680 --> 01:16:34,960
But there is another way to see
them.

923
01:16:35,520 --> 01:16:40,480
Black holes are not exceptions
to the laws of nature.

924
01:16:40,920 --> 01:16:46,000
They are the laws of nature,
followed all the way through to

925
01:16:46,000 --> 01:16:51,840
their natural conclusion.
Take gravity seriously enough,

926
01:16:52,440 --> 01:16:58,160
allow it to gather and gather
without interruption, and black

927
01:16:58,160 --> 01:17:03,880
holes are what you get.
They are not intrusions on the

928
01:17:03,880 --> 01:17:08,760
universe, they are part of how
the universe works.

929
01:17:09,120 --> 01:17:13,680
Almost every theme in this
episode meets inside them.

930
01:17:14,320 --> 01:17:17,840
The patient pull of Newtonian
gravity.

931
01:17:18,440 --> 01:17:22,600
The curvature of space-time
introduced by Einstein.

932
01:17:23,200 --> 01:17:27,480
The slowing of clocks measured
to within parts in a

933
01:17:27,480 --> 01:17:31,480
quadrillion.
The bending of light first

934
01:17:31,480 --> 01:17:38,320
confirmed during a 1919 eclipse.
The spin of stars carried into

935
01:17:38,320 --> 01:17:43,000
the curved solutions.
The trimmer of merging black

936
01:17:43,000 --> 01:17:51,080
holes detected by LIGO in 2015.
The silhouette imaged by the

937
01:17:51,080 --> 01:17:58,040
Event Horizon Telescope in 2019.
Each of these threads leads

938
01:17:58,040 --> 01:18:01,440
sooner or later, to the same
place.

939
01:18:02,320 --> 01:18:08,000
The black hole is where general
relativity becomes most fully

940
01:18:08,000 --> 01:18:14,120
visible, where geometry becomes
most fully geometry.

941
01:18:14,560 --> 01:18:18,000
There is a humility in studying
them.

942
01:18:18,800 --> 01:18:23,720
The very center of every black
hole reminds us that even our

943
01:18:23,720 --> 01:18:29,960
best theories have edges, places
where they hand the question

944
01:18:29,960 --> 01:18:35,480
over to a deeper physics that
has not yet been written.

945
01:18:36,520 --> 01:18:40,400
The strange marriage of general
relativity and quantum

946
01:18:40,400 --> 01:18:45,400
mechanics, glimpsed in Hawking's
calculation of black hole

947
01:18:45,400 --> 01:18:51,280
radiation remains one of the
great unfinished projects of

948
01:18:51,280 --> 01:18:56,360
modern science.
Black holes mark the

949
01:18:56,360 --> 01:19:01,080
Borderlands.
They are where the map becomes

950
01:19:01,120 --> 01:19:06,960
uncertain and where future
generations may find new

951
01:19:07,080 --> 01:19:10,840
continents.
And yet, for all their

952
01:19:10,840 --> 01:19:16,160
strangeness, they have also
become surprisingly familiar.

953
01:19:16,880 --> 01:19:21,920
Astronomers now find them by the
dozens in stellar binary

954
01:19:21,920 --> 01:19:27,600
systems, by the hundreds in the
centers of galaxies, by the

955
01:19:27,600 --> 01:19:33,680
dozens of merger events recorded
in gravitational wave detectors

956
01:19:33,680 --> 01:19:37,360
each year.
The universe is dotted with

957
01:19:37,360 --> 01:19:42,000
them, quietly, the way a forest
is dotted with hidden hollows.

958
01:19:42,800 --> 01:19:50,160
Most of them do not threaten us.
They sit in the dark, holding

959
01:19:50,160 --> 01:19:55,920
cord over their nearby stars,
occasionally swallowing a stray

960
01:19:55,920 --> 01:20:02,960
cloud of gas, occasionally
meeting another of their kind in

961
01:20:02,960 --> 01:20:08,200
a final spiral.
That sends a faint ripple across

962
01:20:08,200 --> 01:20:11,400
the cosmos.
When you see a picture of a

963
01:20:11,400 --> 01:20:18,560
black hole, the soft orange ring
of 87 with a smaller glow of

964
01:20:18,560 --> 01:20:24,120
Sagittarius, a star, what you're
really seeing is a story.

965
01:20:24,960 --> 01:20:30,960
A story that began with John
Michelle's quiet Letter in 1783,

966
01:20:31,600 --> 01:20:36,520
passed through Einstein's
reimagining of gravity, survived

967
01:20:36,520 --> 01:20:42,760
decades of doubt, and ended for
now in patient images and

968
01:20:42,760 --> 01:20:48,520
patient signals received here on
a small blue planet.

969
01:20:49,000 --> 01:20:54,800
Black holes are reminders that
even the strangest things in the

970
01:20:54,800 --> 01:21:01,400
universe are still part of the
universe and still, in the end,

971
01:21:01,880 --> 01:21:05,960
knowable.
So now the journey can grow

972
01:21:06,160 --> 01:21:10,800
quiet.
We began far from any black hole

973
01:21:11,160 --> 01:21:15,840
in an old sky that seemed steady
and unquestioned.

974
01:21:16,760 --> 01:21:22,360
Newton had gathered the patient
logic of gravity into a few

975
01:21:22,360 --> 01:21:28,840
elegant rules, and the heavens
turned overhead, as they always

976
01:21:28,840 --> 01:21:32,680
had.
From there, we followed a long

977
01:21:32,680 --> 01:21:37,160
thread.
A country clergyman in 1783

978
01:21:37,560 --> 01:21:43,120
imagined dark stars.
A French mathematician quietly

979
01:21:43,120 --> 01:21:47,680
reached the same conclusion a
few years later, then withdrew

980
01:21:47,680 --> 01:21:50,720
the thought.
A century passed.

981
01:21:51,400 --> 01:21:58,440
Einstein reimagined gravity as
the curvature of space-time, and

982
01:21:58,440 --> 01:22:04,160
almost immediately a soldier on
the Eastern Front named Karl

983
01:22:04,160 --> 01:22:11,200
Schwarzschild found a hidden
radius inside the new equations.

984
01:22:11,560 --> 01:22:17,800
For decades, even the greatest
physicists hesitated to believe

985
01:22:17,800 --> 01:22:24,680
such a thing could be real.
Then Chandrasekhar, Oppenheimer,

986
01:22:24,680 --> 01:22:28,960
Penrose, Hawking, Wheeler, and
many others followed the

987
01:22:28,960 --> 01:22:34,480
mathematics through, and the
universe slowly revealed that

988
01:22:34,480 --> 01:22:38,920
the heaviest stars truly had
nowhere else to go.

989
01:22:39,320 --> 01:22:44,600
The dark stars of imagination
became the black holes of

990
01:22:44,600 --> 01:22:48,400
physics.
Telescopes traced them by the

991
01:22:48,400 --> 01:22:53,880
orbits of nearby stars,
gravitational wave detectors

992
01:22:53,880 --> 01:22:59,760
heard them collide, The Event
Horizon Telescope photographed

993
01:22:59,760 --> 01:23:05,800
their shadows and 1 quiet object
at the center of our own Galaxy,

994
01:23:06,160 --> 01:23:13,040
4,000,000 times the mass of the
Sun was finally given its name

995
01:23:13,440 --> 01:23:17,640
and its place.
You do not need to remember any

996
01:23:17,640 --> 01:23:21,560
of this.
The names will fade, the dates

997
01:23:21,560 --> 01:23:26,080
will soften, the technical
details will drift away.

998
01:23:26,080 --> 01:23:32,040
By morning, what matters is the
mood that remains.

999
01:23:32,760 --> 01:23:38,640
The universe is a place where
gravity, given enough mass,

1000
01:23:39,200 --> 01:23:46,160
becomes geometry, where time can
slow near great weight, where

1001
01:23:46,160 --> 01:23:53,200
light can be bent, swallowed,
and sometimes returned in faint

1002
01:23:53,840 --> 01:23:58,680
predicted ways.
Where even the darkest objects

1003
01:23:59,000 --> 01:24:03,920
leave signatures in the orbits
of their neighbors, in the

1004
01:24:03,920 --> 01:24:09,640
ripples of space-time, in the
silhouettes they cast against

1005
01:24:09,640 --> 01:24:14,480
distant fire outside the room,
the Earth turns through

1006
01:24:14,480 --> 01:24:17,440
space-time without your
noticing.

1007
01:24:18,640 --> 01:24:23,440
The Milky Way wheels slowly
around its hidden center.

1008
01:24:23,920 --> 01:24:29,040
Where Sagittarius, a star, sits
in the dark with its court of

1009
01:24:29,040 --> 01:24:35,280
stars far beyond it.
In galaxies whose light has

1010
01:24:35,280 --> 01:24:40,360
travelled millions of years to
reach us, supermassive black

1011
01:24:40,360 --> 01:24:44,680
holes glow at the centers of
their own quiet kingdoms.

1012
01:24:46,080 --> 01:24:53,280
And somewhere, perhaps tonight,
2 distant black holes are

1013
01:24:53,280 --> 01:24:58,720
spiraling toward one another,
sending a faint tremor outward

1014
01:24:58,720 --> 01:25:04,480
through the geometry of the
cosmos, a tremor that may pass

1015
01:25:04,480 --> 01:25:10,240
through the earth long after
this room has gone still.

1016
01:25:10,680 --> 01:25:16,600
Yet here, in the small shelter
of a night routine, gravity is

1017
01:25:16,600 --> 01:25:19,920
gentle.
The bed holds you.

1018
01:25:20,880 --> 01:25:26,880
The lamp glows softly.
The clock ticks at its own quiet

1019
01:25:26,880 --> 01:25:30,200
rate.
You do not need to solve the

1020
01:25:30,200 --> 01:25:34,200
universe tonight.
You only need to drift within

1021
01:25:34,200 --> 01:25:39,880
it, held for a little while by a
force that, in its most extreme

1022
01:25:39,880 --> 01:25:43,280
form can fold the very fabric of
space.

1023
01:25:43,800 --> 01:25:49,640
But that here, now simply keeps
you in place against the turning

1024
01:25:49,640 --> 01:25:54,760
of the world.
The world slows, and so do we.

1025
01:25:55,040 --> 01:25:55,800
Good night.