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00:00Why are we here? Where do we come from? These are the most enduring of questions and it's
00:21part of human nature to want to find the answers. We can trace our ancestry back hundreds of
00:31thousands of years to the dawn of humankind, but our story extends far further back in
00:38time. Our story starts with the beginning of the universe. 13.7 billion years old filled
00:56with countless stars and galaxies. It's a universe of unimaginable wonders, yet each
01:07is connected to us. Ultimately, we are part of the universe, so its story is our
01:17story. The force at the heart of the story is gravity, the architect of the universe.
01:27This fundamental force of nature builds everything we see. It creates shape and
01:34order and initiates patterns that repeat across the heavens. Gravity also forges some of the
01:45most alien worlds in the cosmos. Worlds that defy belief. The quest to understand this fundamental
01:56force of nature has unleashed a golden age of creativity, innovation and discovery. And
02:06it's led to a far greater understanding of our place in the universe.
02:11There is one force that we are all familiar with. We cannot see it or touch it, but the ever-dependable
02:27presence of gravity keeps our feet firmly on the ground. Yet what we experience every day is but one small
02:40facet of the true story of gravity of the true story of gravity. Because gravity is more subtle and far stranger than any of us realise.
02:55To experience just how strange, you have to be willing to go to extreme lengths, which is exactly what this flight is about to do.
03:10As they climb steeply, the pilots on this flight do something no other crew would.
03:16They reduce power to the engines. The plane begins to fall back to earth. At that moment, something astonishing happens.
03:37For the passengers and everything else on the flight, gravity vanishes.
04:00By simply falling at the same rate as the plane, the passengers have escaped gravity's clutches.
04:17We are only now beginning to unravel some of the mysteries that surround this fundamental force.
04:22And the harder we look, the more we realise just how extraordinary and crucial gravity is.
04:31Far from simply keeping our feet on the ground, gravity is part of the bedrock of the universe.
04:37From the very first soup of interstellar gas and cosmic dust, this invisible force forged the first stars.
05:06It sculpted the planets and moons and set them in orbit around the newly formed suns.
05:21Gravity connects these star systems together in vast galaxies and steers them on their journey through unbounded space.
05:29Gravity has also shaped our knowledge of the cosmos.
05:44Over the centuries, our quest to understand gravity has allowed us to explain some of the true wonders of the universe.
05:51But at a deeper level, that quest has also allowed us to ask questions about the origin and evolution of the universe itself.
06:03To begin to understand gravity, we need to look no further than beneath our feet.
06:21At its simplest, gravity is a force of attraction generated between objects.
06:28The amount of attraction, the strength of gravity generated, depends on the mass of the objects.
06:37For a tiny rock, that force is infinitesimally small.
06:43But for our vast planet, that force of attraction is strong enough to shape the entire surface of our world.
06:52For the last 20 years, Paul van der Pleur has been guiding people through the wilds of Namibia, an arid landscape where the effects of gravity are laid bare.
07:11Well, this is it, the Fish River Canyon, the deepest canyon in Africa, one of the most amazing geological features here in Namibia.
07:26The magnificent scale of the canyon belies the simple process that created it, the influence of gravity on water.
07:45The water has created the fish river canyon.
07:48And over many, many millions of years, the water has cut its way through this canyon, through this rock, the fault line, down to the sea.
07:58The irresistible pull of Earth's gravity creates the flow of water.
08:07And it's this invisible force that lies at the heart of the other agents at work, shaping our planet.
08:15The process that we see here in the Fish River Canyon is not unique just to this particular area.
08:20This is the same process that's happening everywhere else around the world, creating the mountains, creating the valleys, creating the river canyons.
08:27So basically behind all of these processes is gravity.
08:39Gravity drives the large scale cycles of water, rock and ice that create the epic landscapes of our world.
08:50But Earth's gravity stretches far beyond the bounds of our planet, reaching out past our atmosphere.
08:57The skies are always changing.
09:02The constellations rise and fall in different places every night.
09:06But thanks to Earth's gravity, throughout human history, there has been one constant up there in the night sky.
09:13Every human that's ever lived has gazed up at the moon and seen the same face shining back down.
09:26But it hasn't always been this way.
09:31Millions of years ago, the view would have been very different.
09:38Back then, the moon spun much faster on its axis, showing a fresh face to the Earth every day.
09:45The Earth's gravity changed that forever, through a mechanism that's very familiar to us.
10:01As the Earth spins below the moon, the moon's gravity pulls at the Earth, creating the ocean tides.
10:07But in the past, as it spun above the Earth, the gravity of our planet had an extraordinary effect on the moon.
10:18It created a tide in the solid rock of the moon's surface.
10:29The gravitational pull of our planet generated a wave several meters high that swept across the landscape.
10:37As the wave was dragged through the solid rock in the crust, the resistance generated enormous amounts of friction.
10:49As a result, the tide acted as a powerful break, slowing down the moon's spin,
10:56until the time it takes for the moon to spin once on its axis,
11:00became the same as the time it takes to travel once around the Earth.
11:07That brought an end to the lunar tide,
11:14and provided us with our unchanging view of the moon.
11:18The bond that gravity creates between the Earth and the moon repeats across the cosmos.
11:43It's the glue that holds the planets in orbit around the sun.
11:49And gravity binds our solar system, and countless other solar systems together,
11:56to form some of the most beautiful and varied wonders in our universe.
12:05The galaxies.
12:06For years, the dynamic processes at work shaping these glittering gems were hidden from our view.
12:26But now the true lives of the galaxies are beginning to be revealed to us.
12:30We're able to actually see gravity at work, shaping and controlling the development and ultimate fate of these beautiful structures.
12:43To do so, we have to look at the universe through different eyes.
12:54This is the very large array in the US state of New Mexico.
13:01With its help, astrophysicists like Chris Corelli are gaining a new insight in the role that gravity plays in shaping the lives of galaxies.
13:14The last decade has seen an absolute revolution in our understanding of galaxy formation.
13:21And what drives these processes is effectively the nature of gravity.
13:27Each of these vast dishes collects radio waves, very long wavelengths of light invisible to the human eye.
13:41From this light, the VLA creates truly illuminating images.
13:52In other wavelengths of light, this group of galaxies seem to exist in perfect isolation.
13:59But the VLA reveals a very different reality.
14:02If you make the same image in the radio, you see a very different picture.
14:13You see gas that's been pulled out of these galaxies due to the strong gravitational interaction between the galaxies.
14:20This image shows gravity at work.
14:27The glowing blue trails reveal gas being torn out of one galaxy and drawn into the other as they pass nearby,
14:35in a kind of intergalactic cannibalism.
14:44The influx of gas combined with the huge tidal forces sweeping through the galaxies
14:48generate bursts of star formation, changing the shape and structure of both.
15:00Beyond these near misses, there is another even more dramatic and direct way that gravity influences galaxies.
15:08It is this fate that gravity has in store for our home galaxy, the Milky Way.
15:14And it is one that will forever change our galactic neighbourhood.
15:21The Milky Way has a very interesting future in store.
15:24Given that we have a very massive companion galaxy, the Andromeda galaxy.
15:29It could be fairly dramatic over the next few billion years.
15:32Andromeda is a spiral galaxy roughly the same size and mass of the Milky Way.
15:43This island of around a trillion stars sits over two and a half million light years away.
15:51But every hour that gap shrinks by half a million kilometres.
16:03As this computer simulation shows, gravity is inexorably bringing our two vast galaxies together in a cosmic sized collision.
16:13As the Milky Way and Andromeda plough into each other, planets, gas and dust are ripped out of the galaxies and thrown off into interstellar space.
16:28Eventually, the two galaxies merge, destroying the beautiful spiral arms and forming one single giant ball of randomised stars that's called an elliptical galaxy.
16:43The universe is littered with these huge balls of stars.
16:47Many of the results are past cosmic collisions.
16:55The two different types of galaxies we have in the nearby universe.
16:59Elliptical galaxies, which have no ordered motion, completely random motion of stars, and they look like big spherical balls of stars.
17:07And spiral galaxies, which are uniformly rotating, very ordered motion, very beautiful things to look at actually.
17:13And the fundamental difference is the way gravity has affected them.
17:21Through the interplay of cannibalism and collisions, gravity engineers vast variety in the galaxies across the universe.
17:32What we see are snapshots in their ongoing evolution.
17:43And gravity's embrace stretches far further than these individual islands of stars.
17:56Its influence can bind countless galaxies together, in doing so creating the most magnificent structures.
18:03Our own Milky Way is part of one of these grand structures, the Virgo Cluster.
18:14Every point of light in this image is not a star, but a galaxy.
18:19There are 2,000 galaxies in this cluster, and they are all bound together by gravity, making it the largest structure in our intergalactic neighbourhood.
18:33Contained within these vast hosts are billions upon billions of stars and planets, every single one of which has its own, unique gravity.
18:48Gravity exists across the universe in almost unlimited variety, and in many places it occurs at strengths that dwarf the gravity of our own home planet.
19:07It's possible to gain a taste of what it would actually be like to visit some of these worlds without having to leave Earth.
19:14To do so, you have to go for a spin.
19:29This is the human centrifuge at the heart of the Aerospace Physiology Department in Sosterburg, Netherlands.
19:34For 50 years, pilots and astronauts from around the world have trained here, learning to cope with the gruelling conditions encountered in flight.
19:47To do so, you have to go for a spin.
19:57Roy Fincan is one such fighter pilot, and today he's volunteered to be a human guinea pig, exploring the effects of gravity as found on other worlds.
20:06nurlautnant Hans Wittenberg ist��u
20:16Cortthe Gumpatt
20:26Jupiter is the first stop on our tour, the planet with the strongest gravity in the solar system.
20:35So we slowly are increasing g at this moment up to 2g.
20:392g means twice gravity, that means that the pilot who is inside at the moment is twice his weight.
20:47Jupiter is over 1,300 times bigger than the Earth.
20:51But because it's a gas giant and not very dense, its gravity is just over twice as strong.
20:59What we're going to do now is bring him up to 4g to, well, increase g again.
21:07Roy has left our solar system behind.
21:11This is gravity as found on OGLE-2TR L9b,
21:15a planet recently discovered orbiting a star nearly 3,000 light years from the Earth,
21:22in the Carina constellation.
21:25Roy can now feel the effects across his whole body.
21:31At this moment, his legs fill up with blood.
21:33His eyelids are getting heavier, his cheeks are getting heavier, it's hard to breathe in.
21:39The centrifuge rapidly climbs to above 8g.
21:43Gravity near to that found on planet Korot-10b.
21:51Even a trained fighter pilot can barely cope.
21:57Roy's entire physiology fights to keep gravity at bay.
22:03His heart has to work furiously,
22:06simply in order to get blood the short distance from his heart to his head.
22:10My eyelids are really heavy right now.
22:14His brain and eyes are slowly being starved of oxygen.
22:18Without oxygen, his vision begins to falter,
22:21the colour literally draining from his sight.
22:29Now 8.5gs, and now I've got a greyout.
22:32OK, 8.5g, he has a greyout, and now he will stop the centrifuge.
22:35OK, now I've got to stop it.
22:43Exposure to this level of gravity for any length of time
22:46would lead to loss of consciousness
22:47and eventually death.
22:51We humans are really only designed for gravity here on Earth.
22:55And 8g is only the beginning.
23:06Gravity can create worlds of true wonder,
23:10where the strength of gravity is unimaginably savage.
23:14So strong, it would crush us in an instant.
23:18Although these worlds were detected less than 50 years ago,
23:25their story begins 1,000 years earlier.
23:28This is Chaco Canyon in New Mexico,
23:43in the southwest of the United States.
23:47It was once home to the Chacoan civilisation.
23:53The Chacoans were stargazers.
23:55They first came to the valley over 1,000 years ago
24:05and built magnificent structures
24:09aligned with the stars
24:10and the cycle of the moon and the sun.
24:15The Chacoans' dedication to the skies
24:17has carried down through the ages.
24:19But it is not their magnificent buildings
24:25that fascinate modern-day astronomer
24:27Maura McLaughlin.
24:30Rather, a drawing they made,
24:32a recording of an incredibly rare event,
24:35one that created a world of supergravity.
24:40Oh, wow, there it is.
24:48It is amazing to actually see this in person.
24:51I have been showing a picture of this pictograph
24:54for about 10 years in every talk that I've given,
24:58and it's just so cool to actually see it.
25:00The swirling red disc is thought to represent
25:06a bright new star that appeared in the night sky
25:09near to the crescent moon.
25:13The star outshone every other star in the night sky
25:16for over three weeks
25:17and was so bright that it was visible
25:20even during the daytime.
25:21The new star that first blazed in the skies in 1054 AD
25:35was, in fact, the explosive death of an old star,
25:41a supernova explosion,
25:44a star literally blowing itself apart
25:46at the end of its life
25:48with the brightness of a billion suns.
26:01Never in our lifetime
26:02has there been a supernova explosion
26:05that's been remotely powerful enough
26:06for us to actually see with our naked eye,
26:10much less during the day when the sun is out,
26:12and to think that these people
26:13could observe this explosion with our naked eye
26:16for 20 days or something
26:18is absolutely phenomenal,
26:20and I can only put myself back in their mindset,
26:23not knowing anything about astronomy
26:24and thinking what they must have thought.
26:28What the Chicoans could never know
26:30is that the stars above them were not immortal.
26:37Throughout a star's life,
26:39there is a constant battle
26:40between energy pushing out
26:42and gravity pushing in.
26:45As long as the star burns,
26:47the two forces balance each other out.
26:50But when it runs out of fuel,
26:53gravity wins.
26:54The star collapses and explodes.
26:56Although we can no longer see the supernova
27:13the Chicoans saw,
27:15we can still marvel at what it left behind.
27:22Where the star once blazed,
27:24there now exists the Crab Nebula,
27:27an intricate web of gas and dust
27:29formed from the shattered outer layers of the star.
27:35And at the heart of these beautiful filaments
27:37is the true treasure of the nebula,
27:42the last remnant of the exploding star.
27:49It was revealed to us through the detection
28:00of a strange and perplexing signal
28:02emanating from the very centre of the nebula.
28:07If we could actually take the signal
28:09from that nebula
28:10and digitise it and listen
28:11to what we're seeing,
28:13we would hear a rattling,
28:15a periodic rattling,
28:16coming out of the nebula.
28:21The source of the rattle
28:23is the heart of the star,
28:25its core,
28:26crushed by the force of gravity
28:27and reborn as a neutron star.
28:30In this remarkable image
28:39captured by the Chandra telescope,
28:42the point of light at the centre
28:43is the neutron star,
28:45an object with a mass
28:47greater than that of our sun,
28:49crammed into a sphere
28:50only 20 kilometres across.
28:53The star is the star,
28:53the star is the star,
28:53the star is the star.
29:00The neutron star is spinning
29:03at a rate of 30 times a second,
29:061,800 revolutions a minute.
29:11As it spins,
29:13beams stream out.
29:20When the beams sweep across the earth,
29:22they can be heard as regular pulses,
29:25and so we call them pulsars.
29:27But this fantastic pulsing signal
29:32is not the true wonder of this world.
29:35Rather,
29:36it is the phenomenal strength
29:37of the gravity found there,
29:39100,000 million times
29:42that of earth.
29:45It is so strong
29:46that if you dropped an object
29:48from just one metre above the star,
29:51it would be travelling
29:51at over 6.5 million kilometres per hour
29:55by the time it struck the surface.
29:59This ferocious gravity
30:00comes from so much mass
30:02crammed into such a small space.
30:05During the supernova,
30:07the gravitational collapse
30:09was so extreme
30:10that it created a world
30:12made of matter so dense
30:13that it defies imagination.
30:16If we could go to a neutron star
30:26and take a teaspoonful of material,
30:29just one teaspoonful,
30:31it would weigh as much
30:32as an ocean-going supertanker.
30:34If we brought that teaspoon
30:35back to earth
30:36and we dropped it right here,
30:37it would be so massive
30:39and so dense,
30:40it would just plummet
30:41straight through the earth.
30:41This fantastic world
30:48has been revealed to us
30:49through our knowledge of gravity,
30:52a knowledge born
30:52from a constant fascination
30:54with the heavens
30:55that stretches down
30:57through history.
30:59Our understanding of gravity,
31:02hard won by some
31:03of humanity's greatest minds,
31:06has become one
31:06of the most powerful tools
31:08with which to explore
31:09and explain the universe.
31:19For over 200 years,
31:22Isaac Newton's law of gravity
31:23was the pinnacle
31:24of our understanding.
31:30His simple law,
31:31that gravity was a force
31:32of attraction between two bodies,
31:35was an incredibly powerful tool.
31:37We could now derive
31:39the motion of the heavens
31:40as if it were driven
31:42by clockwork,
31:43predicting the position
31:44of the planets
31:45in our solar system
31:46with astonishing precision.
31:51All that is bar one.
31:57The predictions for the position
31:59of Mercury
32:00were always fractionally out.
32:02The explanation
32:04for this slight inaccuracy
32:06came from the mind
32:07of a genius
32:08and revolutionised
32:10our understanding
32:11of gravity.
32:22It took to the turn
32:24of the 20th century
32:25and the extraordinary insight
32:27of Albert Einstein
32:28to formulate this new theory.
32:30Since then,
32:34physicists like Brian Cox
32:36have used that understanding
32:37as the bedrock
32:38on which to build
32:39our knowledge
32:40of the universe.
32:47It's without doubt
32:48one of the greatest achievements
32:49in the history of physics
32:50because not only
32:51does it explain
32:52with absolute precision
32:54the details
32:55of the orbit
32:56of the planet Mercury,
32:57but it also explains
32:58everything we see
32:59in the universe
33:00to this day
33:01that has anything at all
33:02to do with gravity.
33:05But I think
33:05most importantly of all,
33:07it's not only
33:08the precision
33:09with which the theory works
33:10that's impressive.
33:11The theory actually explains
33:13how gravity works.
33:15To understand,
33:24it's important
33:25to look at the universe
33:26as Einstein did.
33:28He realised
33:29that gravity
33:30is the effect
33:31that the stars,
33:32planets and galaxies
33:33have
33:34on the very space
33:36which surrounds them.
33:37according to Einstein,
33:44space is not just
33:45an empty stage.
33:47Instead,
33:47it's like a fabric
33:48that he called
33:49space-time.
33:51This fabric
33:53can be warped,
33:54bent and curved
33:55by the enormous mass
33:57of the planets,
33:58stars and galaxies.
34:01Anything with mass
34:03will warp
34:03and bend this fabric,
34:05space and time.
34:08And the more mass,
34:09the more pronounced
34:10that warping.
34:21Now, this idea
34:23of curved space
34:24is difficult to imagine,
34:26but if you could only
34:27step out of it,
34:29if you could only
34:29somehow float above space
34:32and look at it,
34:33then this is what
34:34it would look like,
34:35the fabric of our universe,
34:37would look like this.
34:42You would see
34:42the mountains
34:44and the valleys.
34:45You would see
34:46the little peaks
34:47and troughs
34:48created by planets
34:49and moons,
34:51and you would see
34:51these vast,
34:53deep valleys
34:54created by the galaxies.
34:56Einstein's key idea
35:04was that everything
35:05moves in straight lines
35:06over this curved fabric,
35:09the curved space-time
35:10of the universe.
35:18So what we see
35:19as a planet's orbit
35:20is simply it falling
35:22into the curved space-time
35:24created by the huge mass
35:26of a star.
35:29So one way to think
35:30about gravity
35:31is that everything
35:32in the universe
35:33is falling.
35:34So the moon
35:35is falling
35:37into the valley
35:38created by the mass
35:39of the earth.
35:40The earth
35:41is falling
35:42into the valley
35:43created by the sun.
35:44The solar system
35:45is falling
35:46into the valley
35:47in space-time
35:48created by
35:49our galaxy.
35:59It also explains
36:01Mercury's stubbornly
36:03odd orbit.
36:05Closest to our star,
36:06Mercury is the planet
36:08deepest in the curved space
36:10created by the sun.
36:13Mercury's movement,
36:15its orbit,
36:16is influenced
36:16by this extra curvature
36:18in a way
36:19unknown to Newton.
36:26Einstein's theory
36:27of general relativity
36:28is so profound,
36:31so beautiful,
36:32that it can describe
36:33the structure
36:34and shape
36:35of the universe itself.
36:36The theory
36:48also reveals
36:49another side
36:49to gravity,
36:51one infinitely
36:52more savage.
36:55It predicts
36:56the existence
36:57of objects
36:58so dense
36:59with so much mass
37:01that they warp
37:03and bend
37:04the fabric
37:05of the universe
37:05to such an extent
37:07that they can stop time
37:09and swallow light.
37:12These are objects
37:13that can tear
37:14all the other wonders
37:15of the universe apart.
37:17since the dawn
37:26of civilization,
37:28we've peered
37:28at the stars
37:29in the night sky
37:30and tracked
37:30the movements
37:31of the planets.
37:32we can see patterns
37:38repeating across
37:39the whole universe.
37:45But when we train
37:46our telescopes
37:47to the stars
37:48that orbit
37:48around the center
37:49of our galaxy,
37:50we see something
37:52very unusual.
37:53something that astonished
37:59physicists
37:59like Carlos Frenk.
38:04A few years ago,
38:05astronomers discovered
38:06something quite remarkable
38:08going on
38:09in the center
38:09of our own
38:10Milky Way galaxy.
38:12They discovered
38:12a population
38:13of stars
38:14that are whizzing
38:15around
38:16an invisible object
38:17at amazing speeds,
38:19speeds of thousands
38:20of kilometers
38:21per second.
38:23In comparison,
38:27our sun
38:28is traveling
38:29around 200
38:30to 300 kilometers
38:31per second
38:32as it orbits
38:33the center
38:34of our galaxy,
38:35taking a staggering
38:37250 million years
38:39to make
38:40one complete circuit.
38:46The orbit
38:47of the stars
38:48discovered
38:48at the center
38:49of our galaxy,
38:50known as the S-stars,
38:52takes a fraction
38:53of that time.
38:55And because astronomers
38:56have been able
38:56to chart
38:57the full orbit
38:58and speed
38:58of some of these stars,
39:00they are able
39:01to work out
39:01the mass
39:02of the object
39:03driving the breakneck
39:04pace
39:05of their orbits.
39:08And this one,
39:08the white star,
39:09known as S-2,
39:10is particularly special.
39:13It goes
39:13around the center
39:14every 15 years.
39:16and from this,
39:18astronomers are able
39:19to figure out
39:20how massive
39:21the object has to be,
39:23that invisible object,
39:24to cause S-2
39:26to go around
39:27in this very small orbit.
39:29And the answer is
39:304 million times
39:33the mass of the sun.
39:36What's even more remarkable
39:37is that all this mass
39:39is contained
39:39within an object
39:40no bigger
39:41than our solar system.
39:44So imagine
39:454 million suns
39:48squashed
39:49into this tiny volume,
39:52the volume
39:53of the solar system.
39:55That is a phenomenon
39:58that stretches
40:00one's imagination.
40:02It's a phenomenon
40:03that physicists
40:03refer to
40:04as a supermassive
40:06black hole.
40:10Black holes
40:11are the ultimate
40:12expression of gravity.
40:15Some form
40:16when a star,
40:17which is at least
40:1815 times
40:19the mass of our sun,
40:20collapses
40:20at the end
40:21of its life.
40:30Nothing can stop
40:31the gravitational implosion.
40:33All the matter
40:35in the heart
40:35of the star
40:36is crushed
40:36into an infinite
40:37void of darkness
40:38known as a black hole.
40:56Black holes
40:57have such enormous mass
40:58crammed into
40:59such a tiny space
41:00that they warp
41:02space-time
41:03more than any
41:03other object
41:04in the universe.
41:20The immense
41:22gravitational pull
41:23of these monsters
41:24can rip a star
41:25apart
41:26and drag
41:27its remains
41:27into orbit.
41:28This superheated
41:35matter spins
41:36around the mouth
41:37of the black hole
41:37and great jets
41:39of radiation
41:40fire out
41:41across the cosmos.
41:45As the matter
41:46is consumed,
41:47the black hole
41:48grows,
41:49over time
41:49increasing
41:50from an object
41:51a few times
41:52the mass
41:52of our sun
41:53to one-hundreds,
41:55millions,
41:56even billions
41:57of times
41:57the mass
41:58becoming a super-massive
42:00black hole.
42:09The ultimate fate
42:11of the matter
42:11feeding the black hole
42:12is hidden from our view.
42:16Black holes bend
42:17and warp space
42:18to such an extent
42:19that they can even
42:21trap light
42:22that comes too close.
42:24Because no light
42:25can escape,
42:27black holes
42:27and everything in them
42:28are invisible to us.
42:31And the boundary
42:32beyond which
42:33not even light
42:34can escape
42:34is known
42:35as the event horizon.
42:39The event horizon
42:41is what I like to think
42:42of as a point
42:43of no return.
42:44Once inside
42:45the event horizon
42:46there's no way out.
42:47there's only one direction
42:49you can go
42:50into the guts
42:51of the black hole.
43:02It may be invisible
43:04to us
43:04but through our knowledge
43:06of gravity
43:07we can picture
43:08what is happening there.
43:12The event horizon
43:13of a black hole
43:15is similar
43:15to a waterfall.
43:17Just as water
43:18in a river
43:18flows over the waterfall
43:20space itself
43:21can be thought of
43:22as flowing
43:23over the event horizon
43:24before disappearing
43:26into the black hole.
43:30Now imagine a swimmer
43:31who is upstream
43:33of the waterfall
43:34if the swimmer
43:35is strong enough
43:35it might be able
43:36to swim
43:37against the current.
43:38But as the swimmer
43:39approaches
43:39the waterfall
43:41it would have to
43:42swim faster
43:42and faster
43:43because the water
43:44is falling faster
43:45and faster
43:45just like space
43:46is falling faster
43:47and faster
43:47into the black hole.
43:52Inevitably
43:52once the swimmer
43:53reaches the waterfall
43:54the current
43:55will be too swift
43:56becoming impossible
43:58to defeat.
43:59The swimmer
44:00will be carried
44:01over the edge.
44:02at an event horizon
44:12space is flowing
44:14into the black hole
44:15at the speed of light.
44:17The only way
44:18to escape
44:18being dragged in
44:19would be to travel
44:21faster than the speed
44:22of light
44:22and nothing
44:24in the universe
44:25can travel
44:26that fast.
44:26then you're doomed
44:30because
44:31once you're falling
44:33on the black hole
44:34once you've fallen
44:35down the waterfall
44:35there's only one fate
44:37that awaits you
44:37you will fall
44:39into the guts
44:40of the black hole.
44:44Somewhere beyond
44:45the event horizon
44:46lurks the very center
44:48of the black hole
44:49a point of infinite mass
44:51of infinite warping
44:53of space
44:54and time.
44:56A place that is
44:56forever hidden
44:57from the universe
44:58and currently
44:59beyond our ability
45:01to understand.
45:04Inside a black hole
45:06space and time
45:07go crazy.
45:09Inside a black hole
45:11our understanding
45:12of physics
45:12goes equally crazy.
45:14We don't yet
45:15have a theory
45:17that tells us
45:18what happens
45:20in a black hole
45:21because conditions
45:22that are so extreme
45:23that the equations
45:24of physics blow up.
45:29Through gravity
45:30we've been able
45:30to explore the universe
45:32as never before.
45:34Yet some of its
45:35perhaps greatest secrets
45:37still remain hidden
45:38to us.
45:47Gravity
45:47is the great creator
45:49the constructor
45:52of worlds.
45:55It's the only force
45:56in the universe
45:57that can reach out
45:58across the vast
45:59expanses of space
46:01and pull matter
46:02together
46:03to make the planets
46:04the moons
46:05the stars
46:07and the galaxies.
46:10But gravity
46:12is also
46:13the destroyer
46:14because it's relentless.
46:16for the most
46:18massive objects
46:19in the universe
46:20for the most
46:22enormous stars
46:23gravity
46:24will eventually
46:25crush matter
46:26to form black holes.
46:34This dual nature
46:36has produced
46:37some of the
46:38true wonders
46:38of the universe.
46:42Wonders that
46:43have been revealed
46:44to us through
46:45our understanding
46:46of gravity
46:46and shaped
46:50by some
46:51of our greatest
46:52minds.
46:56I think gravity
46:58holds the key
46:59to the most
47:00profound questions
47:01that human can ask
47:02like how did
47:03the universe begin?
47:05How does the universe
47:06work?
47:07How did it become
47:08the way it is today?
47:10These most profound
47:12questions drive us
47:13onward, inspire us
47:15to keep pushing
47:15at the boundaries
47:16of our understanding
47:17to keep reaching
47:19for the next
47:20great leap forward.
47:22I think the most
47:23fascinating thing
47:24about gravity
47:25is that we know
47:27that our theory
47:28of it is not
47:30complete
47:30but it's not
47:31perfect
47:32it's not able
47:32to describe
47:33everything
47:33it's not able
47:34to describe
47:35what happens
47:36inside a black hole
47:38it's not able
47:38to describe
47:39what happened
47:39right back
47:41at the start
47:41of the universe
47:42and that is
47:44perhaps the most
47:45fascinating thing
47:46of all
47:47because that's
47:47where scientists
47:48want to be
47:49they want to be
47:50on the edge
47:51of our understanding
47:51peering into
47:53the unknown
47:53and that's what
47:54gravity is.
48:03Often people wonder
48:04what's the point
48:06of doing research
48:07like this
48:07figuring out
48:08what's going on
48:09in the center
48:09of a neutron star
48:10or in the center
48:11of a black hole
48:12isn't going to
48:13save any lives
48:13but I think
48:15there's not much
48:15point in living
48:16if you can't
48:17get excited
48:17about how
48:19the universe works.
48:27In that
48:28gravity encompasses
48:30the true wonder
48:31of the universe.
48:33We are only
48:34at the start
48:34of our journey
48:35there is so much
48:37more left
48:38to explore.
48:39the world.