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00:00seeing is not believing our senses can deceive us even the stars are not what they appear to be
00:11the cosmos as revealed by science is stranger than we ever could have imagined light and time
00:19space and gravity conspire to create realities which lie beyond human experience that's where
00:29we're headed come with me back in 1802 on a night like this the astronomer william herschel strolled
00:41the beach on the english coast with his son john herschel was the first person ever to see into the
00:48deeper waters of the cosmic ocean there he glimpsed the magic trick that light does with time
00:55father do you believe in ghosts why yes my son you you do i would not have thought so
01:06oh no not in the human kind of ghost no not at all but look up my boy and see a sky full of them
01:16the stars father i do not follow every star is a sun as big as bright as our own just imagine how far
01:28away from us you'd have to move the sun to make it appear as small and faint as a star the light
01:35from the stars travels very fast faster than anything but not infinitely fast it takes time
01:42for their light to reach us for the nearest ones it takes years for others centuries some stars are so far
01:53away it takes aeons for their lights to get to earth by the time the light from some stars get here
02:01they are already dead for those stars we see only their ghosts we see their light but their bodies perished long long ago
02:15john i have seen further back in time than any man before me millions of years into the past
02:26william herschel was the first person to understand that a telescope is a time machine
02:33we cannot look out into space without seeing back in time
02:39in one second light travels 300 000 kilometers or 186 000 miles that's nearly the distance from the earth
02:49to the moon so the moon is about one light second away the next time you look at the moon
02:55you will be seeing one second into the past
02:59the moon is about the moon so the moon is about the moon and the moon is about the moon and the moon is about the moon and the moon is about the moon and the moon is about the moon and the moon is about the moon and the moon is about the moon and the moon is about the moon and the moon is about the moon and the moon is about the moon and the moon is about the moon and the moon is about the moon and the moon is about the moon and the moon is about the moon and the moon is about the moon and the moon is about the moon and the moon is about the moon and the moon is about the moon and the moon is about the moon and the moon is about the moon and the moon is about the moon and the moon is about the moon and the moon is about the moon and the moon is about the moon and the moon is about the moon and the moon is about the moon and the moon is about the moon and the moon is about the moon and the moon's about the moon and the moon is about the moon and the moon is about
04:59Before the physical sun is actually there.
05:01That sun behind me is a mirage.
05:06No more real than a shimmering image that hovers in the distance over a desert road on a hot day.
05:12Sunlight takes about eight minutes to reach Earth.
05:15So the sun is eight light minutes away.
05:18From Earth, we can only ever see the sun as it was eight minutes ago.
05:23And another thing, the sun doesn't really rise at all.
05:29The Earth turns, and we turn with it.
05:33It may not look like it, but right at this moment, I'm moving faster than a jet plane.
05:38And so are you and everyone on Earth.
05:40And while I'm at it, that horizon, it's not really there at all.
05:45There's no edge.
05:47The horizon is just another illusion.
05:49The distance between Earth and the outermost planet, Neptune, varies as the planets orbit the sun.
06:12On average, the light makes that trip in four hours.
06:15So for us on Earth, the Neptune we see is always four hours in the past, four light hours away.
06:22But the distances to the planets, even the farthest one, are mere baby steps on a much grander scale of the stars and galaxies.
06:36As soon as we leave the sun's immediate neighborhood, we need to change the unit of distance from light hours to light years.
06:44A light year is the yardstick of the cosmos.
06:46A single one is nearly 10 trillion kilometers, or about 6 trillion miles.
06:52It's a unit of distance just like a meter or a mile.
06:56It's the distance light travels in a year.
06:58The nearest star to the sun, Proxima Centauri, is a little more than four light years away from Earth.
07:05How far away is four light years?
07:08NASA's Voyager spacecraft moves at more than 56,000 kilometers an hour.
07:14Even at that astonishing speed, it would take Voyager more than 80,000 years to reach the nearest star.
07:22And the stars of the Pleiades Cluster?
07:27Four hundred light years away.
07:30The ship of the imagination is equipped with a highly unusual capability.
07:34One of a kind, actually.
07:36It makes it possible for us to see what was happening when the light from a distant star or galaxy first set out on its long journey to Earth.
07:45When that light left the Pleiades about 400 years ago, Galileo was taking his first look through a telescope.
07:57A few years later, he tried to measure the speed of light, but he couldn't do it.
08:01He had a very clever plan, but the technology of that era just wasn't good enough to measure the motion of anything that moves as fast as light.
08:09When we look at the Crab Nebula from Earth, we're seeing much farther back in time.
08:17The Crab Nebula was once a giant star ten times the mass of the sun, until it exploded in a supernova.
08:25At its heart is a pulsar, a collapsed star the size of a city, spinning 30 times a second.
08:31This pulsar's whirling magnetic field whips nearby electrons into a frenzy, accelerating them to almost the speed of light.
08:48They shine with a blue glow that lights up the tendrils of gas still unraveling from the supernova.
08:55The Crab Nebula is about 6,500 light years from Earth.
08:59According to some beliefs, that's the age of the whole universe.
09:06But if the universe were only 6,500 years old, how could we see the light from anything more distant than the Crab Nebula?
09:14We couldn't.
09:16There wouldn't have been enough time for the light to get to Earth from anywhere farther away than 6,500 light years in any direction.
09:23That's just enough time for light to travel through a tiny portion of our Milky Way galaxy.
09:31To believe in a universe as young as 6,000 or 7,000 years old is to extinguish the light from most of the galaxy.
09:38Not to mention the light from all the hundred billion other galaxies in the observable universe.
09:44The Crab Nebula
10:14The center of our own galaxy is about 30,000 light years from Earth.
10:20The light we see today, coming from the core of the Milky Way, left there when our ancestors were perfecting a way to vanquish death.
10:28By making art with the power to inspire those who would come long after they were gone.
10:39The light we see coming from the Sombrero galaxy is 30,000,000 years old.
10:53Our ancestors were living in trees when that light started out.
10:57They weighed about 5 kilos and had long tails.
11:00But even 30,000,000 light years away is still in our own cosmic backyard.
11:07That galaxy is part of the coma cluster, 320,000,000 light years away.
11:18What was going on back home when the light you were seeing began its trip to Earth?
11:22No familiar continents, oceans, or rivers.
11:28Our distant ancestors were just leaving the water for the land.
11:32It's pretty old light, but not nearly the oldest light we can see.
11:36The oldest light is very faint.
11:42A pale ghost in the night.
11:45See that red blob inside the circle?
11:47That's one of the oldest galaxies we've ever seen.
11:51You're looking at 13.4 billion year old starlight as captured by the Hubble Space Telescope.
11:58It's coming from the very first generation of stars.
12:11What was happening on Earth back then?
12:13Absolutely nothing.
12:15There was no Earth, no sun, no Milky Way.
12:19They would not come to be for billions of years.
12:22When we try to look even farther into the universe, we come to what appears to be the end of space.
12:31But actually, it's the beginning of time.
12:43Earth pulls on us.
12:45Our lives are a relentless struggle with gravity.
12:52That little girl is trying her best to climb out of our gravitational well.
13:07From our first efforts to stand to our final surrender,
13:11we are struggling to overcome the Earth's pull.
13:15We are born, live, and die in a force field.
13:19One that is almost as old as the universe itself.
13:25And how old is that?
13:27To visualize the 13.8 billion year age of the universe,
13:30we've compressed all of cosmic time into a single year-at-a-glance calendar.
13:35Midnight on December 31st is this very moment, right now.
13:40And January 1st is the beginning of time.
13:43See that glowing fog out there?
13:46It's radiation left over from the Big Bang.
13:48The explosion that made the universe 13.8 billion years ago.
13:55Right now, we're at the very edge of known space and time.
14:02So what happened before the Big Bang?
14:04Nobody knows.
14:06No evidence survives from before that moment.
14:09We've got some pretty crazy ideas about where the universe came from,
14:12which we'll get to in time.
14:14Where are we in the universe?
14:19At the very center.
14:21In the observed universe, everyone gets to feel special.
14:25No matter which galaxy you happen to live in,
14:28when you look out to the universe,
14:30you'll find yourself at the center of the cosmic horizon.
14:33But this is just an illusion.
14:36In reality, there is no center.
14:38And the cosmic horizon is no more real than the horizon at sea.
14:42It's what you get when you have a finite speed of light in a universe that had a beginning in time.
14:50A few hundred million years after the Big Bang,
14:57vast clouds of hydrogen and helium condensed into the first stars and galaxies.
15:03With these new sources of light, the long dark ages of the universe ended.
15:07As space continued to expand, cosmic evolution unfolded on grander scales.
15:14As the first generation of stars died, they seeded space with heavier elements,
15:20making possible the formation of planets and ultimately life.
15:30Matter and energy were formed in the Big Bang.
15:33But that's not all.
15:35Space and time were created, too.
15:37And all the forces that bind matter together, including gravity.
15:41Isaac Newton discovered a mathematical law that describes how gravity works.
15:46With that law, he could explain the motions of the planets.
15:49More than a hundred years later,
15:51William Herschel realized gravity could do much more.
16:03John, can you keep a secret?
16:05Yes, Father.
16:06I've made a discovery and have yet to tell another soul.
16:12The gravity that holds us to the Earth,
16:16the same gravity that Newton showed keeps the planets in their orbits,
16:20I've discovered that it also rules the distant stars.
16:25Father, but how can you know this?
16:29Can you find the constellation of the Lion?
16:31There.
16:32Well done.
16:38Can you now find the star that joins the lion's head to his body?
16:43That one.
16:45That star is really two stars,
16:48so close together that they appear to be one.
16:52I've been watching them through my telescope since long before you were born.
16:57They dance around each other, very slowly.
17:02More slowly than any planet moves around the sun.
17:08Many of the stars we see tonight,
17:11perhaps most of them,
17:13dance with invisible partners.
17:17Gravity's empire governs all the heavens.
17:20A century earlier,
17:32Isaac Newton had been haunted by the same absence of a mechanism for gravity.
17:37How could distant bodies affect each other across empty space without actually touching?
17:42This action at a distance, as he called it, baffled him.
17:48In the 19th century, Michael Faraday discovered that we were surrounded by invisible fields of force
17:55that explained how gravity works.
17:57He imagined those lines of gravitational force radiating out into space from every massive body.
18:10The earth, the moon, the sun, everything.
18:14Here was the answer to that question that had stumped Newton.
18:20In 1865, James Clark Maxwell translated Faraday's idea about fields of electricity and magnetism
18:28into mathematical laws.
18:30He discovered that these fields move through space in waves.
18:35When he calculated how fast they move,
18:37it turned out to be the speed of light.
18:41We were beginning to discover the threads of the cosmic tapestry.
18:44But we were not yet able to discern the rich pattern that time, light, space, and gravity weave.
18:51As Albert Einstein worked in Berlin on his theory of gravity,
18:55he kept the portraits of these three men before him.
18:59He knew he was standing on their shoulders.
19:02Years before, as a teenager,
19:05he had an insight that was as earth-shaking as any idea of theirs.
19:09And it happened one summer,
19:11while he was daydreaming in Italy.
19:14In the summer of 1895,
19:21Einstein's father's business in Germany had failed,
19:23and the family had moved here to northern Italy.
19:27Young Einstein loved wandering these roads
19:29and giving his mind free reign to explore.
19:32There's something timeless about this place.
19:34Nothing here has really changed since the time of Einstein's early daydromes.
19:41One day, he began to think about light and how fast it travels.
19:52In everyday life, we've always measured the speed of a moving object with respect to something else.
19:57Something that's presumably not moving.
20:00Something in the cosmos that's not in motion.
20:03For example, I'm moving about 10 kilometers per hour relative to the ground.
20:08But as I mentioned earlier, the ground is moving.
20:13Earth is turning at more than 1,600 kilometers per hour.
20:17While it orbits the sun at more than 100,000 kilometers per hour.
20:21And the sun is moving through the galaxy at a half a million miles per hour.
20:27And the Milky Way is moving through the universe at nearly one and a half million miles an hour.
20:32There is no fixed place in the cosmos.
20:36All of nature is in motion.
20:37It was hard even for the young Einstein to imagine some absolute standard to measure all those relative motions against.
21:01This is the very book that inspired Einstein as a young boy.
21:04You give a kid a book, and you change the world.
21:10In a way, even the universe.
21:13Look at this.
21:14The very first page.
21:16It describes the astonishing speed of electricity through wires and light through space.
21:23Einstein remembered what he'd learned as a child from this book.
21:26And perhaps for the first time, right here, wondered what the world would look like if you could travel at the speed of light.
21:34The more Einstein thought about it, the more troubled he became.
21:42If you imagine traveling at the speed of light, paradoxes seem to pop up everywhere.
21:49Einstein was shocked to realize that so much of what had been uncritically accepted as truth by even the greatest authorities on the subject was just plain wrong.
21:58When traveling at high speeds, there are certain rules which must be obeyed.
22:05Einstein called these rules the principles of relativity.
22:09Imagine that young woman who just blew past us on the motorbike.
22:12Imagine she was riding her bike through the cosmos.
22:15Light from a moving object travels at the same speed, no matter whether the object is at rest or in motion.
22:24Her speed is not added to the speed of light.
22:28The light from her motorbike still travels at the speed of light.
22:31Nature commands, thou shalt not add my speed to the speed of light.
22:39Also, no material object can travel at or faster than the speed of light.
22:44There's nothing in physics that prevents you from traveling as close to the speed of light as you like.
22:4999.9% of the speed of light is just fine.
22:52But no matter how hard you try, you never gain that last decimal point.
22:58For reality to be logically consistent, there must be a cosmic speed limit.
23:10The crack of that whip is due to its tip moving faster than the speed of sound.
23:15It makes a shockwave, a mini sonic boom in the Italian countryside.
23:18A thunderclap works the same way.
23:24So does the sound of a passing supersonic jet.
23:28So why is the speed of light any more a barrier than the speed of sound?
23:32The answer is not just that light travels about a million times faster than sound.
23:37And it's not merely an engineering problem like building a first supersonic jet.
23:41Instead, the light barrier is a fundamental law of nature.
23:45It's basic, it's gravity.
23:46Einstein found his absolute framework for the world.
23:50This sturdy pillar among all the relative motions within the motions of the cosmos.
23:55Light travels just as fast, no matter how fast or slow its source is moving.
24:01The speed of light is constant relative to everything else.
24:05Nothing can ever catch up with it.
24:07The thing about the laws of nature is that they're unbreakable.
24:14The job of physicists is to discover these commandments.
24:16The ones that do not vary culture to culture, time to time.
24:21And hold true throughout the cosmos.
24:22That's why, as Einstein showed, funny things happen close to the speed of light.
24:29Traveling close to the speed of light is kind of an elixir of life.
24:39Because your biological clock slows down relative to those you leave behind.
24:43This phenomenon may provide us humans who only live for a century or so a practical means to travel to the stars.
24:51Where the magic show of space-time really gets crazy.
25:04The 19th century astronomer William Herschel loved to share the wonders of the universe with his son John.
25:11I once had a friend, a very clever fellow, an astronomer and a parson at Leeds by the name of John Mitchell.
25:31Poor man died when you were a babe, God rest his soul.
25:35He held that some stars are invisible.
25:39They really exist, but we shall never see them.
25:43Dark stars, Mitchell called them.
25:48With all due respect, Father, surely your friend was mistaken.
25:52If no one can see them, then how can we possibly know they exist?
25:58Do you see the man who left those footprints, John?
26:03I know, Father. I did not.
26:06But do you know that he exists?
26:09John Mitchell is one of the greatest scientists you've probably never heard of.
26:30He lived and worked in England in the 18th century.
26:33If he ever sat for a portrait, it no longer exists.
26:37He was once described by an acquaintance as a short little man of black complexion and fat.
26:45Mitchell imagined a star so big, so massive, that nothing, not even light, could escape its gravitational grip.
26:52Can you find the dark star?
26:55Can't see with your eyes, not directly, but it may leave a kind of footprint on the cosmic shore.
27:03Mitchell realized that we might be able to detect some of these dark stars because of their extreme gravity.
27:08If one happened to be near a smaller luminous companion star, that star would appear to travel in a tight orbit around nothing.
27:16Even though we can't see it, we know something with a lot of mass has to be right there, a dark star, or what today we call a black hole.
27:27What does a black hole look like?
27:31And what would it be like inside?
27:34We'll get there.
27:35But first, let's make a pit stop in my hometown.
27:40New York City.
27:41Where it's always seemed to me that everything is in constant motion.
27:45I've lived here most of my life.
27:48There's always something new to see.
27:51One thing never changes.
27:53Gravity.
27:53Gravity on Earth has been the same for the past four and a half billion years.
27:58What if today we could alter it?
28:01Gravity is a distortion in the shape of space-time, as Einstein has showed.
28:07Space can expand and contract and warp without limit.
28:15If the Earth's size or density were even a little different, its gravity would be too.
28:22There's an infinite range of possibilities.
28:24New Yorkers feel right at home in the gravitational pull of the Earth, called 1G.
28:33Suppose we turn off the gravity on one of its streets.
28:37People and objects that were already in motion are launched into flight.
28:57Now what if I turn the gravity up to, say, eight or nine Gs?
29:02Out of compassion, let's evacuate the area.
29:05This is about the same G-force that a fighter pilot in a high-speed turn would feel.
29:12A few minutes of this wouldn't hurt you, but it wouldn't be comfortable.
29:17At 100,000 Gs, even fire hydrants become crushed by their own enormous weight.
29:24But at millions of Gs, even light bows to gravity.
29:29The light still moves at its constant speed, but it cannot escape.
29:33Mitchell's dark star, our black hole.
29:38And the nearest one may be closer than you think.
29:41Not every star can become a black hole.
29:52Only about one in a thousand is massive enough.
29:55The nearest one could be within a hundred light years of Earth.
29:59Black holes aren't the mythic cosmic vacuum cleaners of science fiction.
30:03They don't go around gobbling up unsuspecting worlds.
30:06You've got to come to them.
30:08But if you do, it might be the last thing you'll ever see.
30:12That was us, resisting a few million Gs of gravity.
30:19Don't forget, that thing swallows light.
30:23We'll keep our distance.
30:24When giant stars exhaust their nuclear fuel, they can no longer stay hot enough to fend off the inward pull of their own gravity.
30:35The most massive stars collapse into darkness, leaving only their gravity behind.
30:41This black hole enshrouds the shrunken corpse of a supergiant star.
30:45The star itself has shriveled into something even smaller than this darkness, only 64 kilometers wide.
30:55This is the first black hole ever discovered.
30:59Cygnus X-1.
31:01How did we on Earth ever find something so small and dark and far away?
31:07We looked at it in another kind of light.
31:10X-rays.
31:10In X-ray light, we lost sight of the blue star because its surface is a tepid 30,000 degrees.
31:18But the disk of gas around the black hole glowed brilliantly in X-rays at 100 million degrees.
31:26As William Herschel discovered, many stars have close companions forming a binary star system.
31:32But if one member of such a pair is enormous and the other is compact, the smaller star can drain and consume the atmosphere of its larger sibling.
31:42This neurotic relationship can last for millions of years.
31:46The atmosphere of the larger star was being siphoned onto a glowing hot accretion disk that revolves around and spirals into a black hole.
31:56The overwhelming gravity was accelerating the blue star's gas into a death spiral, crossing the space-time boundary never to be seen again.
32:05The fateful boundary that separates a black hole from the rest of the universe is called an event horizon.
32:13From our point of view, the substance in the disk slows down as it approaches the event horizon, never quite reaching it.
32:20But if you were riding on that spiraling gas, and I don't advise it, you would sail past the event horizon in a matter of seconds into the undiscovered country from which no traveler returns.
32:32We have searched the hearts of dozens of galaxies, and in every case, we have found a supermassive black hole.
32:50Our own galaxy is no exception.
32:55The stars nearest the center of our galaxy whip around at more than 40 million kilometers an hour.
33:02What can make them move so fast?
33:05The only logical explanation is that something with the mass of 4 million suns lies at the center.
33:14So where are the blazing light of 4 million suns?
33:18Since we can't see it, it must be imprisoned inside a black hole.
33:22Earth is far enough away to be perfectly safe.
33:32Other worlds might not be so lucky.
33:37If you somehow survived a perilous journey across the event horizons,
33:42you'd be able to look back out and see the entire future history of the universe unfold before your eyes.
33:48How?
33:53Because when space-time is warped by the extreme gravity of a black hole,
33:57time is stretched to the horizon.
33:59But what would be in front of you?
34:04Before we go there, I should warn you that we're entering uncharted scientific territory.
34:12For all we know, there may be undiscovered laws of physics that govern events at the center of the black hole.
34:17But until the next Einstein comes along, let's perform a thought experiment.
34:27That's how John Mitchell first imagined dark stars in the 18th century.
34:32And how Einstein can see the mysterious reality.
34:35Hey!
34:56Hey!
34:56I don't know.
35:26Father, do you believe in ghosts?
35:31Oh, no. Not in the human kind of ghosts. No, not at all.
35:36But look up, my boy, and see a sky full of them.
35:44If you could survive the trip into a black hole, you might emerge in another place and time in our own universe, circumventing the first commandment of relativity.
35:54Thou shalt not travel faster than light.
35:59Nothing can move through space faster than light.
36:02But space is not mere emptiness.
36:05It is properties.
36:06It can stretch and shrink. It can be deformed.
36:10And when that happens, time is deformed, too.
36:12Einstein discovered that space and time are just two aspects of the same thing.
36:23Space-time.
36:24Space-time itself can deform enough to carry you anywhere at any speed.
36:30Black holes may very well be tunnels through the universe.
36:33On this intergalactic subway system, you could travel to the farthest reaches of space-time.
36:54Or you might arrive in some place even more amazing.
37:00We might find ourselves in an altogether different universe.
37:04But how can a whole universe fit inside of a black hole?
37:07Which is only a small part of our universe.
37:12It's another magic trick of space-time.
37:15The phenomenal gravity of a black hole can warp the space of an entire universe inside it.
37:21Our local gravity may be a drag to us, but it's really feeble compared with what goes on inside a collapsed star.
37:37As far as we know, when a giant star collapses to make a black hole,
37:42the extreme density and pressure at the center mimic the Big Bang,
37:46which gave rise to our universe.
37:48And a universe inside a black hole might give rise to its own black holes.
37:53And those could lead to other universes.
38:00Maybe that's how our cosmos came to be.
38:02For all we know, if you want to see what it's like inside a black hole,
38:21just look around you.
38:24William Herschel went on to discover that the sun and its planets are moving through the Milky Way.
38:34And whatever became of his son John, he grew up to become a great scientist.
38:40His deep space observations built on those of his father
38:43to become the basis for the standard catalog of galaxies we use today.
38:48When William was in failing health,
38:51John stayed with him through the long nights at his telescope
38:53to help him sweep the stars.
38:56And when his father died,
38:58John wrote his epitaph.
39:01He broke through the walls of heaven.
39:04John often reminisced about those summer nights with his father.
39:20Maybe that's why he sought a way to preserve the past.
39:25John Herschel was one of the founders of a new form of time travel,
39:29a means to capture lightened memories.
39:31He actually coined a word for it.
39:36Photography.
39:42When you think about it,
39:44photography is a form of time travel.
39:47This man is staring at us from across the centuries,
39:51a ghost preserved by light.
39:53It's not hard to imagine that in the near future,
39:56we'll be able to capture the past in all three dimensions.
39:59We'll be able to step inside a memory.
40:08It may not be possible to travel backward in time,
40:11but perhaps one day we can bring the past to us.
40:14Here's a moment from my past.
40:19Like John Herschel,
40:21I'm remembering a younger version of myself.
40:24December 20th, 1975.
40:27A snowy day in Ithaca, New York.
40:30A branch point on the road
40:31that brought me to this moment with you.
40:34It was the day I met Carl Sagan.
40:39Reminds me of those ghost stars in the sky.
40:45You know,
40:46the ones that still shine their light upon us
40:49long after they're gone.
40:51We're going to see you next time.
41:16You know,
41:16we'll be right back.