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00:00A journey to infinity and beyond.
00:11A place where the rules of physics collapse, where time and space twist and distort.
00:20Travel back to the past and see into the future.
00:26To the most extreme, least understood place in the universe.
00:33The heart of a supermassive black hole.
00:46A black hole, terrifying and fascinating.
00:51The universe's ultimate enigma.
00:54How can you not love a black hole?
00:55It's the end point of everything.
00:58It's distorting space, distorting time.
01:00There's some ideas that if you fall into one, you could pop out someplace else in the universe.
01:05It's a science fiction writer's dream.
01:06And now consider that these things are real, that we actually observe thousands of them
01:12routinely.
01:14Black holes are monsters, real monsters in our midst.
01:18And they will never cease to be dramatic.
01:22What would we see if we could look this monster in the eye?
01:27I would love to have a trip around a black hole, looking out the window, seeing this
01:32scary black disk in front of you and marveling at the gravitational distortions of light
01:38and experience this spectacular manifestation of all the stuff that I've just calculated
01:42and never seen.
01:45What would we discover if we could actually venture inside a black hole?
01:51We're going to find out.
01:53Because we're sending an imaginary probe to the supermassive black hole at the centre
01:58of the Milky Way.
02:01It's called Sag A Star.
02:06It's so far away that even if the probe could travel at the speed of light, it would take
02:1026,000 years to get there.
02:15On the journey, the probe would pass tens of thousands of smaller black holes.
02:20We think that there's sort of a zoo of black hole sizes.
02:23There's hypothetical, microscopic black holes.
02:26There's middleweight black holes that are formed when stars collapse and maybe another
02:30star falls onto them.
02:31Then there are the supermassive black holes.
02:33They're lurking at the centres of galaxies.
02:41Most black holes are born when a giant star dies.
02:47When the star collapses, it all gets blown out in the most violent fireworks in the universe,
02:53a supernova, the explosion of a star, which shines briefly with the brightness of ten
02:58billion stars.
03:04After the explosion, gravity seizes control of the star's core.
03:09A star is already crushing itself down under gravity, but what stops it is the pressure
03:14of the light coming out of it.
03:16So if a star can no longer produce that light, it's going to crush itself down.
03:22And it'll get smaller and smaller and smaller until finally no force can hold it up and
03:27it just crushes it on itself.
03:31Gravity pulls every speck of matter inwards, crushing lumps of solid iron the size of Mount
03:36Everest down to the size of grains of sand.
03:43You think about solid matter as being something that's so solid, so strong that it can't compress
03:48anymore.
03:49Think of iron.
03:51But really, iron is mostly space.
03:53The space between the protons and neutrons at the core of that iron atom is huge.
03:59So at some point, for some stars, the compression is so large that you can end up smashing these
04:04core neutrons and protons together.
04:08What happens next is not well understood.
04:11Gravity squeezes these subatomic particles together in ways that science has yet to explain.
04:18The inside of a collapsing star is unimaginable.
04:21There are temperatures and pressures that we don't really even have very good numbers
04:25for.
04:28We know for a fact that we will need new physics, things that we have not yet discovered, to
04:33explain what goes on inside a black hole.
04:38And we do know that the giant star is crushed down to a fraction of its former size.
04:46Although the amount of stuff inside it, its mass, remains the same.
04:52Such a massive object squeezed into such a small space creates a gravitational field
04:57so strong nothing can escape.
05:00The result is a black hole.
05:04Stars are roughly 20 times more massive than the sun, but at the center of galaxies, black
05:09holes grow to become billions of times bigger.
05:12Their size and power is unimaginable, earning them their own special term.
05:18The coolest word in astronomy is supermassive black hole.
05:22As you can tell by the name, this supermassive black hole is consuming a supermassive amount
05:28of material, and we know that there's one in the center of our Milky Way galaxy.
05:34This supermassive black hole, Sag A star, is the final destination of our imaginary
05:40probe.
05:42Supermassive black holes are actually quite omnivorous.
05:44They like to eat gas, but, you know, if a star comes within reach, they'll gobble that
05:50up too, or a planet, or a spaceship, whatever, they're not so fussy.
05:55They even eat each other.
05:58Over billions of years, many stars can die in one area, and the resulting black holes
06:03can move together and combine into bigger and bigger black holes.
06:09Sag A star is an omnivore and a cannibal.
06:15It has gorged on anything and everything within reach for at least 12 billion years.
06:24The reason that the black hole can fit within it all these enormous amounts of stuff it's
06:28eaten is because it's like the ultimate trash compressor.
06:31It just squeezes the matter down to ridiculously small scales.
06:37So the weight of a black hole is simply the sum of everything it's eaten since the beginning
06:42of our universe.
06:45After billions of years of eating, Sag A star has grown to 4.6 million times more massive
06:51than our sun, but it's only 20 times wider.
06:59Outside its black sphere spins a luminous veil of stars, dust, and gas.
07:09As the probe approaches, something strange starts to happen.
07:16The probe looks like it's veering off course, even though the probe's instruments maintain
07:21that it's traveling in a straight line.
07:24From the probe's perspective, nothing weird is really happening.
07:28But if you're on the outside looking at this probe, this straight path is not going to
07:32be a straight path.
07:33It's going to get pulled off the straight path by the actual space around the black
07:38hole.
07:40The probe's course hasn't changed, but the space around it has.
07:45And as space changes, so does time.
07:52Time flows differently when you get near a black hole.
07:55If the black hole is spinning, it drags space around with it.
07:59It is just bizarre.
08:01They take all of our concepts of common sense and intuition and everything and just chuck
08:05them right out the window.
08:08Sag A star distorts space and time.
08:12So as the probe draws closer, time begins to warp.
08:18At the black hole, the ultimate science fiction fantasy becomes reality.
08:23The probe really can travel back to the future.
08:37As the imaginary probe nears the supermassive black hole at our galaxy's heart, something
08:43strange starts to happen.
08:46The probe experiences one of the most extreme and fundamental forces in the universe.
08:51A force with the power to twist space and send the probe traveling through time.
08:58Gravity is something incredibly commonplace.
09:00We're always dropping things.
09:01We're not flying off the surface of the earth.
09:03It's a part of our lives from the moment that we're born.
09:06But it turns out it's a very mysterious and deep part of the universe.
09:09When something has mass, it has the ability to bend space.
09:18Before Einstein's theory of general relativity, scientists imagined space as a stage on which
09:24the action plays out.
09:29Einstein's genius was to realize that space itself was an active member of the cast.
09:37We tend to think of space as just the stuff around us, but it's actually kind of a thing.
09:42It has a physical component to it, like a fabric, like a sheet.
09:48The fabric of space is usually flat, so light travels through it in a straight line.
09:55But a massive object like the sun, with its strong gravitational pull, bends the fabric.
10:02So the light appears to bend as it passes the sun.
10:06The light bends not because the light bends, but the space in which the light is traveling
10:10curves itself.
10:12This is why, as the probe nears Sag A star, it looks like it's going off course.
10:18The immense gravity around the black hole is warping the fabric of space.
10:23So everything, even the light that travels through this warped region of space, must
10:28follow the same curved path.
10:31The amazing thing is that not only does gravity warp the light into the black hole, light
10:36coming from farther away gets bent around it.
10:39And so you could see this shimmering distortion in the stars in the background as a black
10:44hole moved in front.
10:45It would almost be like taking a piece of glass and moving it across the star field.
10:53The black hole isn't just bending space.
10:56It's also warping time.
11:02So if we send a probe going into a black hole, from our point of view, we see it falling
11:06in and we see it falling in more and more slowly because its clock is ticking slower
11:11and slower and slower.
11:14That's a bizarre concept and it's really hard to wrap our brains around, but that is what
11:19the math and physics are telling us.
11:25We experience time as hours, minutes, and seconds.
11:29It seems straightforward, but actually time is relative.
11:34It changes depending on where you are.
11:37It seems to the viewer that this shot has a picture of a moment in time and a moment
11:42in space, but it's actually not a moment in space and a moment in time.
11:45It's spread out in space because there's some depth of field behind me, but it's also
11:49spread out in time because the light from the whiteboard behind me left the whiteboard
11:54a little bit earlier than the light from my face.
11:57So when you see an image, you're really seeing something that's spread out in space and time.
12:05Space and time are inextricably linked.
12:09So when gravity bends space, it also bends time.
12:14And the stronger the gravity, the more time is distorted.
12:20Time does not flow at the same rate everywhere in a gravitational field.
12:24The farther down you are into the gravity, the slower time goes.
12:27Now we actually have clocks that are accurate enough today to tell the difference in time
12:32between being in the basement and being in the attic.
12:35That's really true.
12:37Time really goes faster in the attic than it does in the basement.
12:42A day spent at the top of the Empire State Building will make you a few billionths of
12:46a second older than someone who stayed on the ground floor.
12:53That's because the Earth's gravitational pull is slightly stronger on the ground floor.
12:59And the stronger the gravity, the more time is slowed.
13:04The existence of mass and energy in space curves space.
13:07So right now, I'm curving space-time around me.
13:11And as you get to more and more massive objects, that curvature becomes greater and greater.
13:15So the Earth is curving space-time, the sun is curving space-time, and when you get to
13:19black holes, the curvature is incredibly extreme.
13:23As the probe nears the supermassive black hole, it starts to travel through time.
13:28It's hard to wrap your mind around this concept of space-time and even of black holes.
13:33It's pretty insane.
13:36If I'm near a black hole videoconferencing my mom on a computer, she would actually see
13:42me go, hi, mom.
13:47But I would feel that my time is running normally, whereas she is acting weird, going, oh, my
13:52goodness, Max, I'm a little bit worried about you because you're speaking so funny.
13:56Close to a black hole, you really can go back to the future.
14:00And if you go back, people would say, oh, you look so youthful, this was a rejuvenating
14:05vacation.
14:06I would be like, yes, because my time slowed down.
14:10As the probe travels closer, gravity's grip increases, and time ticks progressively more
14:17slowly.
14:19But the increasing gravity has another effect.
14:24It accelerates the probe towards the black hole.
14:31But in its path is a cosmic kill zone, a ring of molten dust and gas tens of millions of
14:38kilometres wide, known as the accretion disk.
14:46It threatens to rip the probe to shreds.
14:51You've got this matter falling into a black hole.
14:53It forms a disk, and there's friction inside of this disk.
14:57Things are rubbing against each other.
14:59And the velocities are so huge that this disk becomes terribly hot, and it glows when it
15:04gets that hot.
15:05And so, ironically, even though black holes are dark, as matter falls in, it can light
15:10up and be the most luminous objects we see in the universe.
15:15The probe plunges headlong towards this seething disk of matter.
15:20Exploration seems inevitable.
15:22But then a remarkable thing happens.
15:26Without firing any thrusters or manoeuvring in any way, the probe swerves.
15:32Rather than colliding with the spinning disk, the probe joins it, its course altered by
15:38an unseen force.
15:40Almost everything in the universe spins in some way or another.
15:44That is due to angular momentum, this tendency that once you get something spinning, it's
15:48going to stay spinning.
15:56When you think of that classic example of an ice skater spinning on an ice rink, and
16:00as she draws her arms in, she begins to spin faster and faster.
16:05Think about the scale involved when you go from a giant star to a tiny, hyper-compressed
16:10black hole.
16:11That spin was sped up and sped up and sped up.
16:15As the matter falls towards the black hole, it whirls around faster and faster, reaching
16:21astronomically high speed.
16:26Travelling at close to light speed, the matter spins too fast to fall into the black hole.
16:34For example, there's a very famous ride in fairs where you can be in a room and it starts
16:38spinning and you're pushed against the wall and the floor recedes, but you stay up, you
16:41don't slide down.
16:43That same effect can stop you from falling into a black hole.
16:47Spin something fast enough and centrifugal force overpowers gravity.
16:52There's a sort of yin and yang tension between gravity trying to crush things and the centrifugal
16:59force of spin trying to blow things apart.
17:03But the black hole doesn't just spin matter around it, it spins the fabric of space around
17:10it too.
17:15Picture a dust devil, a kind of mini-tornado.
17:19The swirling dust reveals the movement of the unseen wind.
17:23In the same way, the spinning matter around the black hole reveals the unseen movement
17:28of space itself.
17:30The spinning black hole creates a kind of tornado in the fabric of space and time.
17:36Black holes are weird no matter how you look at them.
17:39Everything about them is just so strange.
17:41But I think the weirdest thing about them is the fact that they can drag space and time,
17:45that fabric around with them, wrapping themselves in a cloak of the material of the universe
17:51itself.
17:55General relativity tells us that any massive spinning object, even the Earth, should move
18:00the space around it in a process called frame-dragging.
18:02Now, when we go to the case of the extreme gravity of a black hole, when we talk about
18:11frame-dragging, we're talking about a region where the space is actually being dragged
18:17so strongly that it's impossible to remain at rest there.
18:25The probe is spinning through twisted space and time.
18:31Things will look very warped and distorted.
18:35And what you see in front of you might even, in fact, be in back of you.
18:39And much of our intuition for what's where would actually break down.
18:54Rotating wildly, twisting space and time, black holes are not simply monsters that feast
19:00on everything within reach.
19:04The thing about a black hole is that they eat like they're toddlers.
19:07They're messy.
19:08So as stuff is falling in, a lot of it doesn't ever actually make it inside the black hole.
19:15Physicists estimate that up to 40% of the dust and gas in the accretion disk never enters
19:20the black hole.
19:22It's only when you're inside the black hole that you're doomed.
19:25But just a little bit outside the black hole, if you're going around fast enough, you can
19:29exist that way for a long time.
19:36In fact, black holes don't just pull material in.
19:40They also blast it out.
19:45This accretion disk is whirling madly around the black hole.
19:48There's friction.
19:49There are magnetic fields.
19:50And it can twist these up and they can form sort of cone-shaped magnetic cannons that
19:54point out the top and the bottom of the black hole.
19:57And material can flow along that and be ejected outward at almost the speed of light.
20:04These galaxy-sized cannons are called quasars.
20:09At their heart, powering the blast, is a supermassive black hole.
20:16If there's a place in the universe you don't want to be, it's looking down the barrel of
20:20one of these jets coming from a black hole.
20:23We see images of galaxies with supermassive black holes with jets coming out.
20:28And they're hitting galaxies nearby.
20:31And the environment in that target galaxy is probably completely inhospitable for life.
20:38Quasars can rip vast cavities in galaxies, preventing stars from forming.
20:45But by blasting matter far across the universe, they may provide the raw materials for the
20:50formation of new stars.
20:53Black holes we think of as destroyers, certainly in movies, because everything falls into them.
20:58And literally, once you're inside, your future isn't very good.
21:02But we are beginning to think more and more that black holes play a central role in the
21:06evolution of everything we see.
21:09Without black holes, our solar system may never have formed.
21:15But if our solar system ever found itself in the line of fire of one of these jets,
21:20we would probably be annihilated.
21:22Luckily, Sag A star seems to be quiet.
21:27And our probe is unlikely to be blasted out into deep space on a giant magnetic jet.
21:32Still, progress towards the core is slow.
21:38Suspended between spin and gravity, the probe spends thousands of years staring into the
21:43black hole's blank face.
21:45However, gradually, gravity wins, dragging the probe ever closer to the edge of oblivion,
21:54to a place where time stands still, and a wall of fire threatens to obliterate everything.
22:12The probe is on the precipice, about to enter the gaping mouth of the black hole, the event
22:19horizon.
22:24Black holes have something called the event horizon, and it's the point at which there
22:28is no escape from the gravitational pull of the black hole.
22:33Once you cross the event horizon, it's similar to going over the edge of a waterfall.
22:37There's no return.
22:49If you look at this waterfall, you see it's sort of going flat, and then suddenly at the
22:53edge, water's falling steeply down.
22:56The edge of a black hole is sort of like that.
22:58At the event horizon, suddenly space begins to bend deeply in.
23:06Gravity pulls the water across the edge and down the waterfall in the same way that it
23:10pulls space and time over the event horizon and into the black hole.
23:17Just like a fish exists in water, everything exists in space-time, and that includes light.
23:23So a little light ray falling over the event horizon of a black hole is never going to
23:27be able to come back out.
23:31For a fish to travel back up the waterfall, or for light to escape from a black hole,
23:39it would have to travel fast enough to overcome the pull of gravity.
23:44And the stronger the gravity, the faster the object needs to travel.
23:56To escape the Earth's gravity, a rocket must travel at 11 kilometers per second.
24:01From the far more massive sun, the escape velocity soars to 618 kilometers per second.
24:10Black holes have gravitational fields that are so much stronger than that.
24:15In fact, rather than the speed of a rocket to escape, even if you go the speed of light,
24:20you'll never get off.
24:23In the few remaining seconds before the probe tumbles past the event horizon, the pull of
24:28gravity and its distortion of space-time increases dramatically.
24:35Suppose I have a clock, and the probe has a clock, and I can observe the probe's clock
24:39and my clock.
24:40As the probe gets nearer and nearer the event horizon of the black hole, I will observe
24:45the probe's clock actually stop.
24:50At the event horizon, time basically grinds to a complete halt.
24:54So if I were sitting on the event horizon of a black hole, or almost on it, and you
24:58looked at me, I would just seem frozen to you.
25:03You never actually observe an object falling into the event horizon from the outside, because
25:09it will take longer than the age of the universe for us to watch the object disappear.
25:16Near the black hole's event horizon, time seems to slow to a virtual stop, and objects
25:24appear to change colour.
25:26As you fell into a black hole, people would see you being redshifted.
25:30They would see light coming from you losing energy, because it has to climb out of this
25:34incredible well of gravity to get out of the black hole.
25:40The same phenomenon occurs with sound.
25:43When a fire engine speeds towards us, the sound waves compress.
25:48Racing away, they stretch out.
25:51Like sound, light travels in waves.
25:55But instead of changing pitch, light waves change colour.
26:01Climbing out of the black hole's gravity, light waves are stretched and turn red.
26:07That's why black holes in some sense sound like the stuff of science fiction.
26:10But the real universe is stranger than science fiction.
26:13That's what's so wonderful.
26:14And what's even more wonderful is we don't know exactly what happens at the event horizon
26:19of a black hole.
26:23Einstein's theory of general relativity predicts the probe will simply glide through the event
26:28horizon.
26:31So this probe, if it's going past the event horizon, there's not going to be a signpost
26:36there that says, welcome to the event horizon.
26:39It won't notice anything different.
26:41You just cross this boundary and then suddenly you're cut off from the universe, never to
26:46return again.
26:48That's pretty wild.
26:51But according to a radical new theory, the probe's descent into the abyss won't be a
26:56gentle free fall.
27:01It'll be a baptism by fire in the ultimate cosmic firewall.
27:08So a computer firewall is simply supposed to be an impenetrable barrier, whereas the
27:12black hole firewall is literally a wall of fire.
27:16And that means that it's not innocent near the event horizon.
27:18It's not just empty space with a few soft photons that you can barely notice.
27:23Rather, there's this hugely dense radiation bath of photons.
27:30This new theory turns the event horizon into the most dangerous place in the universe.
27:38An inferno of infinite energy, incinerating anything that touches it.
27:49There is no doubt that there is a place in space, the event horizon, the place from which
27:54nothing gets out.
27:55The million-dollar question is what actually happens there if you jump in.
28:00Some people think that nothing special happens there, and other people think that you actually
28:05get burned up by this firewall.
28:07And what's so embarrassing about this is if you actually want to find out and decide to
28:10jump into a black hole, you will learn, but you can't handle anybody else.
28:16The frustrating thing about black holes is because it has this event horizon and we know
28:21that light cannot be emitted past that threshold.
28:24Even with all of our advanced telescopes, it's very difficult to detect.
28:33To resolve what happens on Sag A Star's event horizon, we need to build a telescope as big
28:39as the Earth.
28:42It sounds impossible, but it's already taking shape at Hawaii's Mauna Kea Observatory.
28:51If you want to see the black hole at the center of the galaxy, you need a really, really
28:56big telescope.
28:57We can't build optical telescopes that big, and what we do is we use radio telescopes
29:02and we distribute those telescopes over thousands of miles apart from each other.
29:08So we're creating a telescope that's as big as the Earth itself.
29:18And over here is where it all comes down.
29:21And inside of this rack, all of our signals come in here at eight gigabits per second.
29:27It's the equivalent of four digital movies per second.
29:31If we sent this back by your home internet connection, it would take years to transmit
29:36all of this data.
29:37So what we have to do is specially package this up and ship it to our central lab at
29:43MIT, where we process this data.
29:48This is essentially the holiest of holies.
29:52This is ground zero.
29:53This is where all the data come together.
29:57By accumulating two more years' worth of data, scientists believe the Event Horizon could
30:02be brought into focus.
30:05So once the Event Horizon telescope gets its final data sets, we hope to image something
30:09like this.
30:11This is what we wind up seeing, though, because the intervening gas to the center of our galaxy
30:16blurs the image.
30:18But if we take out the scattering, we wind up being able to recover the shadow feature.
30:22This is what we're after.
30:23This is the holy grail of the Event Horizon telescope.
30:29The Event Horizon is already taking shape.
30:32A black sphere, 27 million kilometers in diameter.
30:39This is what the probe should see in the final moments before it enters the abyss.
30:46If there is a firewall lurking behind this black veil, the probe will be destroyed.
30:54If not, the probe will glide across the Event Horizon without incident, only to find itself
31:01in the most bizarre and most extreme place in the universe, where new worlds begin and
31:09the past becomes the present.
31:27The probe has passed the point of no return.
31:31It's now inside the supermassive black hole, plunging through a world of unimaginable extremes.
31:40Everything that falls into a black hole disappears, and if it never gets out, the question is,
31:50where does it go?
31:52Here gravity is so intense that the fabric of space itself is pulled inwards at speeds
31:58faster than the speed of light.
32:01The 13 million kilometer journey to the black hole's center will take less than 40 seconds,
32:08but only if the probe can survive the extreme gravitational forces.
32:14Now imagine that I've fallen into a black hole.
32:17The strength of gravity is going to be different if I'm going in feet first, because of the
32:21extreme curvature of space-time there.
32:23And because of that, my feet will have a stronger pull, and you'll get stretched into a form
32:29that's reminiscent of spaghetti.
32:32So we call it spaghettification.
32:41Spaghettification is real.
32:45It happened when Jupiter's gravity spaghettified comet Shoemaker-Levy 9 into a chain of fragments
32:50in 1994.
32:56In reality, the probe would be stretched into atom-sized filaments before it reached the
33:03black hole's center.
33:05I mean, of course I could flex and hold myself together anyway, but, you know, it's the laws
33:09of physics we're battling with here.
33:15But let's suspend these laws and equip the probe with a protective force field straight
33:20out of science fiction.
33:24It's now halfway from the event horizon to the black hole's center.
33:29From this vantage point, the probe sees something truly incredible.
33:36If you were talking about the exact mathematical horizon, you would see the entire paths of
33:41the universe go by.
33:43You would see back to the beginning of the formation of the black hole.
33:48According to Einstein's equations, the black hole is a colossal cosmic museum.
33:54All the light that's fallen in since the black hole's formation is still here, sped up and
34:01stacked in layers.
34:09As the probe plunges deeper, the battle between gravity and spin resumes and intensifies.
34:18The centrifugal force gets relatively stronger deeper inside the black hole, and that centrifugal
34:26force slows down the flow of space.
34:29And the place where it slows back down to the speed of light is called the inner horizon.
34:35And all kinds of crazy things happen at the inner horizon.
34:43In this chaotic cosmic blender, matter gets spun around and flung back out.
34:51If I actually jump into a big black hole, just like if I fell down a waterfall, you're
34:57going to be perfectly alive and well while you're on the way down.
35:02Problems only come when you reach what corresponds to the inner horizon, at the base, where other
35:07water and rocks and things come at you.
35:12Now the probe nears the inner horizon.
35:18The inner horizons of rotating black holes are the most violent places in our universe.
35:25Wildly energetic particles slam into the probe at near light speed.
35:33The black hole is an amazing kind of particle accelerator at the inner horizon.
35:43Particles spewing in all directions, being accelerated through each other, they collide
35:47together, they produce enormous concentrations of energy which vastly exceed any energy in
35:54any terrestrial particle accelerator.
35:58The collisions are comparable in strength to the Big Bang.
36:05If there is any place in our universe where our universe is making baby universes, where
36:11our universe is reproducing, I can guarantee to you that that place is the inner horizons
36:18of black holes, because that is the place where the energies that are needed to make
36:23a Big Bang, they are produced there.
36:35The idea that a universe can be born inside a black hole led scientists to question the
36:41origins of our own universe.
36:44It's actually completely possible that our Big Bang is the remnant of a black hole in
36:49some pre-existing universe crushing things together and then spitting out a baby universe.
36:57One of the reasons black holes are such fascinating objects for theoretical physicists is they
37:01may provide a guide to a new understanding of the universe.
37:04So if we understand black holes, we may understand the Big Bang.
37:08If we understand the Big Bang, we may understand our very origins.
37:16As the probe crosses the inner horizon, everything changes.
37:24The gravity pulling the probe inward is matched by the spin pushing it out.
37:30The two opposing forces are finally balanced.
37:36So what this creates in the center, in the very region of the inner event horizon of
37:39one of these black holes, is sort of like a safe haven, like the eye of a hurricane.
37:44And in there, things are a lot calmer and you can imagine actually navigating once you
37:48get through the crazy region on the outside.
37:53Inside the eye of the storm, the probe heads for the black hole's heart, the singularity.
38:02Here, space and time and our understanding of the universe ends.
38:15Inside the inner event horizon of a spinning black hole, you can look at the singularity.
38:19It's over there in a ring.
38:22The actual thickness of the ring is zero.
38:24It is a circle of points of infinite mathematical thickness and infinite curvature of space-time.
38:35This was once a massive star, crushed down to an infinitely small, infinitely dense point
38:41in space and time.
38:47Spinning at astronomically high speed, centrifugal force transforms this point into a ring, which
38:54warps the space and time around it.
38:58And it turns out that around the singularity, there's a region where we have what in the
39:04jargon is called a closed timelike curve, which is a fancy way of saying you can enter
39:09this region, you can zoom around, and then you can come out, and you come out before
39:14you went in, so you can basically visit the past version of yourself before you ever entered.
39:27General relativity predicts that the singularity will bend space and time to such an extent
39:32that if the probe took a trip around the ring, it could come back to a time before it set
39:37off.
39:41But this probe is heading straight through the ring.
39:47There's this idea that a black hole is punching a hole in the fabric of space and time, and
39:52that it might be punching a second hole someplace very far away.
39:56And that if you fall into the black hole, you will come out on the other side somewhere
40:00light years away, even though it didn't take you any time to traverse that distance.
40:06We call this a wormhole, because it's like a worm eating its way through an apple.
40:10But could the probe survive the journey?
40:14What goes into a singularity may come out somewhere else, but what comes out is not
40:17going to have much of a relationship to what went in.
40:20It will be not only spaghettified, but essentially destroyed.
40:27If you want to go somewhere else, take an airplane.
40:35There's another theory that a singularity isn't a wormhole, it's a white hole.
40:42A white hole is basically a black hole played backward in time.
40:46So a white hole is a singularity that sort of spits out into the universe and pushes
40:51things away from it rather than pulling them in.
40:54The singularity spits out a universe, a process bearing a striking similarity to the Big Bang.
41:01Our Big Bang emerged, at least classically, from a singularity.
41:05And it's not lost on people that the end point of a black hole is a singularity.
41:10Is there a connection between the two? Maybe.
41:16But in reality, scientists are not sure that this infinitely dense and infinitely small
41:21singularity at the center of a black hole exists at all.
41:26Infinity is popping up in physics, really just tell us that we have no clue what we're
41:30talking about. We've never seen anything actually infinite in physics.
41:35So my own personal guess is that there are no singularities.
41:39And that whenever we see an infinity pop out of our equations, it's just nature telling
41:44us that you were wrong.
41:49Einstein's general relativity commits suicide, if you will, at singularities.
41:56It can't continue spacetime beyond singularities.
41:59How that plays out?
42:02Don't know.
42:04We don't know what happens to it.
42:11This is the end of the probe's extraordinary journey.
42:15It has reached the end of space and time and the limit of human knowledge.
42:23Certainly, right now, black holes are the bad boys of physics.
42:30They break every rule we know.
42:32And that just shows us our own limitations.
42:35It's not the black hole's problem.
42:37We have not gotten far enough to explain them.
42:40And that will continuously challenge us and pull us in a real way into the heart of a
42:45black hole.
42:48On our own journey into the black hole's heart, we've discovered it's more than the voracious
42:53monster of science fiction.
42:57It's a time machine and a cosmic museum, a destroyer and a creator of worlds, and the
43:08universe's ultimate enigma.
43:13Explore black holes and we will truly understand how the universe works.
43:21The math is bizarre.
43:22The observations are difficult and the concepts are mind-shredding.
43:26Who wouldn't love that?
43:28I think a lot of people love black holes just because they're big and scary and fun.
43:32But we physicists love them also for a different reason, which is that it's the most extreme
43:37physical systems that usually give us the best clues about how nature works.
43:42Black holes have driven our imagination beyond where it would have gone before.
43:48Nature is much more imaginative than we are.
43:53And every time we explore nature, we're surprised.
43:56So truth is far stranger than fiction, and the real universe exceeds even the wildest
44:01imaginings of the wildest science fiction writer.