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00:00Oh
02:59Invent the universe.
03:02Thank you very much.
03:05Suppose I cut a piece out of this apple pie.
03:09Crumbly, but good.
03:25And now suppose we cut this piece in half, or more or less.
03:33And then cut this piece in half, and keep going.
03:40How many cuts before we get down to an individual atom?
03:45The answer is about 90 successive cuts.
03:51Of course, this knife isn't sharp enough, the pie is too crumbly, and an atom is too small to see in any case.
04:01But there is a way to do it, but there is a way to do it.
04:08It was here at Cambridge University in England that the nature of the atom was first understood, in part by shooting pieces of atoms at atoms and seeing how they bounce off.
04:23A typical atom is surrounded by a kind of cloud of electrons.
04:31The electrons are electrically charged, as the electrons are electrically charged, as the name suggests, and they determine the chemical properties of the atom.
04:38For example, the glitter of gold, or the transparency of the solid that's made from the atom of silicon and oxygen.
04:52But deep inside the atom, hidden far beneath the outer electron cloud, is the nucleus, composed chiefly of protons and neutrons.
05:04Atoms are very small.
05:05A hundred million of them, end to end, would be about so big.
05:11And the nucleus is a hundred thousand times smaller still.
05:16Nevertheless, most of the mass in an atom is in the nucleus.
05:20The electrons are, by comparison, just bits of moving fluff.
05:27Atoms are mainly empty space.
05:30Matter is composed chiefly of nothing.
05:35When we consider cutting this apple pie, but down beyond a single atom, we confront an infinity of the very small.
05:48And when we look up at the night sky, we confront an infinity of the very large.
05:56These infinities are among the most awesome of human ideas.
06:01They represent an unending regress, which goes on not just very far, but forever.
06:09Have you ever stood between two parallel mirrors in a barbershop, say, and seen a very large number of you?
06:19Or you could use two flat mirrors and a candle flame.
06:29You would see a large number of images, each the reflection of another image.
06:36You can't really see an infinity of images because the mirrors are not perfectly flat and aligned, and there's a candle or a candle flame, at least, in the way.
06:49And light doesn't travel infinitely fast.
06:52When we talk of real infinities, we're talking about a quantity larger than any number.
06:58No matter what number you have in mind, infinity is larger.
07:07There is a nice way to write large numbers.
07:11You can write the number 1,000 as 10 to the power 3, meaning a 1 followed by 3 zeros.
07:23Or a million is written as 10 to the power 6, meaning a 1 followed by 6 zeros.
07:34There's no largest number.
07:36If anybody gives you a candidate largest number, you can always add the number 1 to it.
07:41But there certainly are very big numbers.
07:43The American mathematician Edward Kasner once asked his young nephew to invent a name for an extremely large number, 10 to the power 100,
07:56which I can't write out all the zeros on this page for because there isn't room on the page.
08:02The boy called it a Google.
08:05If you think a Google is large, consider a Googleplex.
08:13It's 10 to the power of a Google.
08:16That is, a 1 followed not by 100 zeros, but by a Google zeros.
08:23Now, by comparison with these enormous numbers, the total number of atoms in that apple pie is only about 10 to the 26th,
08:35tiny compared to a Google and, of course, much, much less than a Googleplex.
08:41The total number of elementary particles, protons, neutrons, and electrons, in the accessible universe is of the order of 10 to the 80th,
08:51a 1 followed by 80 zeros, still much, much less than a Google and vastly less than a Googleplex.
08:59And yet, these numbers, the Google and the Googleplex, do not approach.
09:05They come nowhere near infinity.
09:08In fact, a Googleplex is precisely as far from infinity as is the number one.
09:17We started to write out a Googleplex, but it wasn't easy.
09:24It's a very big number.
09:38Writing out a Googleplex is a spectacularly futile exercise.
09:51A piece of paper large enough to contain all the zeros in a Googleplex couldn't be stuffed into the known universe.
09:59Fortunately, there's a much simpler and more concise way to write a Googleplex.
10:14Like this.
10:24And infinity
10:26can be represented like this.
10:32This is the Cavendish Laboratory at Cambridge University, where the constituents of the atom were first discovered.
10:41The realm of the very small.
10:44From the time of Democritus in the 5th century B.C., people have speculated about the existence of atoms.
10:54For the last few hundred years, there have been persuasive, but indirect, arguments that all matter is made of atoms.
11:01But only in our time have we actually been able to see them.
11:06Here, the red blobs are the random, throbbing motions of uranium atoms, magnified a hundred million times.
11:17How Democritus of Abdera would have enjoyed this movie.
11:28We pretty much take atoms for granted.
11:32And yet, there are so many different kinds.
11:36Lovely and useful at the same time.
11:40Look.
11:47There are some 92 chemically distinct kinds of atoms naturally found on Earth.
12:16They're called the chemical elements.
12:19Virtually everything we see and know, all the beauty of the natural world, is made of these few kinds of atoms, arranged in harmonious chemical patterns.
12:48Here, we've represented all 92 of them.
12:53At room temperature, many of them are solids.
12:55A few are gases.
12:56And two of them, bromine and mercury, are liquids.
13:02They're arranged in order of complexity.
13:03Hydrogen, the simplest element, is element number one.
13:07And uranium, the simplest element, is element number one.
13:08And uranium, the most complex, is element 92.
13:12Some elements are different.
13:13Some elements are different.
13:14Some elements are different.
13:15One of them are solids.
13:16A few are gases.
13:17A few are gases.
13:18And two of them, bromine and mercury, are liquids.
13:21They're arranged in order of complexity.
13:27Hydrogen, the simplest element, is element number one.
13:31And uranium, the most complex, is element 92.
13:38Some elements are very familiar.
13:43For example, silicon, oxygen, magnesium, aluminum, iron, the elements that make up the earth.
13:51Or hydrogen, carbon, nitrogen, oxygen, phosphorus, sulfur, the elements that are essential for life.
13:58Other elements are spectacularly unfamiliar.
14:02For example, hafnium, erbium, diprosium, praseodymium.
14:13Elements we're not in the habit of bumping into in everyday life.
14:16By and large, the more familiar an element is, the more abundant it is.
14:21There's a great deal of iron on the earth, not all that much yttrium.
14:27The fact that atoms are composed of only three kinds of elementary particles, protons, neutrons, and electrons, is a comparatively recent finding.
14:38The neutron was not discovered until 1932.
14:41And it, like the electron and the proton, were discovered here at Cambridge University.
14:48Modern physics and chemistry have reduced the complexity of the sensible world to an astonishing simplicity.
14:57Three units, put together in different patterns, make, essentially, everything.
15:11A neutron is electrically neutral, as its name suggests.
15:18A proton has a positive electrical charge, and an electron an equal negative electrical charge.
15:27Since every atom is electrically neutral, the number of protons in the nucleus must equal the number of electrons far away in the electronic cloud.
15:36The protons and neutrons together make up the nucleus of the atom.
15:41Now, the chemistry of an atom, the nature of a chemical element, depends only on the number of electrons, which equals the number of protons, which is called the atomic number.
15:52Chemistry is just numbers, an idea which would have appealed to Pythagoras.
15:58If you're an atom, and you have just one proton, you're hydrogen.
16:05Two protons, helium, three lithium, four beryllium, five protons, boron, six carbon, seven nitrogen, eight oxygen, and so on.
16:17All the way to 92 protons, in which case, your name is uranium.
16:24Protons have positive electrical charges, but like charges repel each other.
16:30So, why does the nucleus hold together?
16:33Why don't the electrical repulsion of the protons make the nucleus fly to pieces?
16:39Because there's another force in nature, not electricity, not gravity, the nuclear force.
16:46We can think of it as short-range hooks, which start working when protons or neutrons are brought very close together.
16:55The nuclear force can overcome the electrical repulsion of the protons.
17:01Since the neutrons exert nuclear forces, but not electrical forces, they are a kind of glue, which holds the atomic nucleus together.
17:13A lump of two protons and two neutrons is the nucleus of a helium atom, and is very stable.
17:22Three helium nuclei, stuck together by nuclear forces, makes carbon.
17:29Four helium nuclei, makes oxygen.
17:33There is no difference between four helium nuclei, stuck together by nuclear forces, and the oxygen nucleus.
17:39They're the same thing.
17:41Five helium nuclei makes neon, six makes magnesium, seven makes silicon, eight makes sulfur, and so on.
17:54Increasing the atomic numbers by two, and always making some familiar element.
17:59Every time we add or subtract one proton and enough neutrons to keep the nucleus together, we make a new chemical element.
18:09Consider mercury.
18:12If we subtract one proton from mercury and three neutrons, we convert it into gold, the dream of the ancient alchemists.
18:26Beyond element 92, beyond uranium, there are other elements.
18:32They don't occur naturally on Earth.
18:35They're synthesized by human beings and fall to pieces pretty rapidly.
18:40One of them, element 94, is called plutonium, and is one of the most toxic substances known.
18:47Where do the naturally occurring chemical elements come from?
18:52Perhaps a separate creation for each element?
18:57But all the elements are made of the same elementary particles.
19:00The universe, all of it, everywhere, is 99.9% hydrogen and helium, the two simplest elements.
19:08In fact, helium was detected on the sun before it was ever found on the Earth.
19:15Might the other chemical elements have somehow evolved from hydrogen and helium?
19:23To avoid the electrical repulsion, protons and neutrons must be brought very close together,
19:29so the hooks, which represent nuclear forces, are engaged.
19:34This can happen only at very high temperatures, where particles are moving so fast
19:38that there isn't time for electrical repulsion to act.
19:43Temperatures of tens of millions of degrees.
19:48Such high temperatures are common in nature.
19:51Where? In the insides of the stars.
19:59Atoms are made in the insides of stars.
20:15In most of the stars we see, hydrogen nuclei are being jammed together to form helium nuclei.
20:22Every time a nucleus of helium is made, a photon of light is generated.
20:26This is why the stars shine.
20:31Stars are born in great clouds of gas and dust, like the Orion Nebula, 1500 light years away.
20:46Parts of which are collapsing under gravity.
20:50Collisions among the atoms heat the cloud until, in its interior, hydrogen begins to fuse into helium,
21:06and the stars turn on.
21:09stars are born in batches.
21:16Later they wander out of the nursery to pursue their destiny in the Milky Way.
21:22Adolescent stars, like the Pleiades, are still surrounded by gas and dust.
21:28Eventually, they journey far from home.
21:32Somewhere, there are stars formed from the same cloud complex as the sun, five billion years ago.
21:44But we do not know which stars they are.
21:47The siblings of the sun may, for all we know, be on the other side of the galaxy.
21:53Perhaps they also warm nearby planets, as the sun does.
22:04Perhaps they too have presided over the evolution of life and intelligence.
22:10The sun is the nearest star, a glowing sphere of gas shining because of its heat.
22:39like a red-hot poker.
22:46The surface we see in ordinary visible light is at 6,000 degrees centigrade.
22:51But in its hidden interior, in the nuclear furnace where sunlight is ultimately generated,
22:57its temperature is 20 million degrees.
23:09In x-rays, we see a part of the sun that is ordinarily invisible.
23:14Its million degree halo of gas, the solar corona.
23:19In ordinary visible light, these cooler, darker regions are the sunspots.
23:28They are associated with great surges of flaming gas, tongues of fire,
23:34which would engulf the earth if it were this close.
23:37These prominences are guided into paths determined by the sun's magnetic field.
23:56The dark regions of the x-ray sun are holes in the solar corona,
24:01through which stream the protons and electrons of the solar wind
24:05on their way past the planets to interstellar space.
24:11All this churning power is driven by the sun's interior,
24:17which is converting 400 million tons of hydrogen into helium every second.
24:23The sun is a great fusion reactor into which a million earths would fit.
24:29Luckily for us, it's safely placed 150 million kilometers away.
24:53It is the destiny of stars to collapse.
24:58Of the thousands of stars you see when you look up at the night sky,
25:01every one of them is living in an interval between two collapses.
25:06An initial collapse of a dark interstellar gas cloud to form the star,
25:11and a final collapse of the luminous star on the way to its ultimate fate.
25:16Gravity makes stars contract unless some other force intervenes.
25:21The sun is an immense ball of radiating hydrogen.
25:25The hot gas in its interior tries to make the sun expand.
25:29The gravity tries to make the sun contract.
25:33And the present state of the sun is the balance of these two forces
25:37and equilibrium between gravity and nuclear fire.
25:41In this long middle age between collapses, the stars steadily shine.
25:47But when the nuclear fuel is exhausted, the interior cools,
25:52the pressure is no longer enough to support its outer layers,
25:55and the initial collapse resumes.
25:57There are three ways that stars die.
26:00Their fates are predestined.
26:02Everything depends on their initial mass.
26:05A typical star with a mass like the sun will one day continue its collapse
26:10until its density becomes very high.
26:14And then the contraction is stopped by the mutual repulsion
26:17of the overcrowded electrons in its interior.
26:20A collapsing star twice as massive as the sun isn't stopped by the electron pressure,
26:26and it goes on falling in on itself until nuclear forces come into play,
26:32and they hold up the weight of the star.
26:35But a collapsing star three times as massive as the sun isn't stopped even by nuclear forces.
26:41There's no force known that can withstand this enormous compression.
26:46And such a star has an astonishing destiny.
26:49It continues to collapse until it vanishes utterly.
26:54So every star is characterized by the force that holds it up against gravity.
27:00A star that's supported by the gas pressure is a normal run-of-the-mill star like the sun.
27:07A collapsed star that's held up by electron forces is called a white dwarf.
27:13It's a sun shrunk to the size of the earth.
27:17A collapsed star supported by nuclear forces is called a neutron star.
27:23It's a sun shrunk to the size of a city.
27:27And a star so massive that in its final collapse it disappears altogether is called a black hole.
27:34It's a sun with no size at all.
27:37But on their ways to their separate fates, all stars experience a premonition of death.
27:45Before the final gravitational collapse, the star shudders,
27:50briefly swells into some grotesque parody of itself.
27:56With its last gasp, it becomes a red giant.
28:04Some five billion years from now, there will be a last perfect day on earth.
28:11Then the sun will slowly change and the earth will die.
28:24There is only so much hydrogen fuel in the sun.
28:27When it's almost all converted to helium, the solar interior will continue its original collapse.
28:33The higher temperatures in its core will make the outside of the sun expand.
28:38And the earth will become slowly warmer.
28:42Eventually, life will be extinguished.
28:45The oceans will evaporate and boil.
28:48And our atmosphere will gush away to space.
28:52The sun will become a bloated red giant star, filling the sky, enveloping and devouring the planets Mercury and Venus.
29:05And probably the earth as well.
29:08The inner solar system will reside inside the sun.
29:13But perhaps by then, our descendants will have ventured somewhere else.
29:27In its final agonies, the sun will slowly pulsate.
29:31By then, its core will have become so hot that it temporarily converts helium into carbon.
29:38The ash from today's nuclear fusion will become the fuel to power the sun near the end of its life in its red giant stage.
29:49Then, the sun will lose great shells of its outer atmosphere to space, filling the solar system with eerily glowing gas, the ghost of a star outward bound.
30:02Perhaps half the mass of the sun will be lost in this way.
30:07Viewed from elsewhere, our system will then resemble the ring nebula in Lyra, the atmosphere of the sun expanding outward like a soap bubble.
30:18And at the very center will be a white dwarf, the hot exposed core of the sun, its nuclear fuel now exhausted, slowly cooling to become a cold, dead star.
30:32Such is the life of an ordinary star, born in a gas cloud, maturing as a yellow sun, decaying as a red giant, and dying as a white dwarf enveloped in its shroud of gas.
30:47Suppose, as we traveled through interstellar space in our ship of the imagination, we could sample the cold, thin gas between the stars.
31:06We would find a great preponderance of hydrogen, an element as old as the universe.
31:13We would find carbon, oxygen, silicon. The most abundant atoms in the cosmos, apart from hydrogen, are those most easily made in the stars.
31:22But we would also find a small proportion of rare elements, praseodymium, say, or gold.
31:29They are not made in red giants. Such elements are manufactured in one of the most dramatic gestures of which a star is capable.
31:39A star more than about one and a half times the mass of the sun cannot be a white dwarf.
31:46It may end its life by blowing itself up in a titanic stellar explosion called a supernova.
31:53There has been no supernova explosion in our province of the galaxy since the invention of the telescope,
32:01and our sun will not become a supernova.
32:04But in our imagination, we can fulfill the dream of many earthbound astronomers
32:10and safely witness, close up, a supernova explosion.
32:14Most of stellar evolution takes millions or billions of years.
32:23But the interior collapse that triggers a supernova explosion takes only seconds.
32:29Suddenly, the star becomes brighter than all the other stars in the galaxy put together.
32:44If the nearby star became a supernova,
33:12it would be calamity enough for the inhabitants of this alien system.
33:17But if their own sun went supernova, it would be an unprecedented catastrophe.
33:23Worlds would be charred and vaporized.
33:26Life, even on the outer planets, would be extinguished.
33:31In our ship of the imagination, we are now backing away from the star.
33:35But the explosion fragments, traveling almost at the speed of light, are overtaking us.
33:41Individual atomic nuclei, accelerated to high speeds in the explosion, become cosmic rays.
33:48This is another way that stars return the atoms they've synthesized back into space.
33:54The shockwave of expanding gases heats and compresses the interstellar gas,
34:00triggering a later generation of stars to form.
34:03In this sense also, stars are phoenixes, rising from their own ashes.
34:14The cosmos was originally all hydrogen and helium.
34:18Heavier elements were made in red giants and in supernovas,
34:21and then blown off to space, where they were available for subsequent generations of stars and planets.
34:28Our sun is probably a third generation star.
34:32Except for hydrogen and helium, every atom in the sun and the earth was synthesized in other stars.
34:39The silicon in the rocks, the oxygen in the air, the carbon in our DNA, the gold in our banks, the uranium in our arsenals,
34:48were all made thousands of light years away and billions of years ago.
34:54Our planet, our society, and we ourselves are built of star stuff.
35:07We're in a lava tube, a cave carved through the earth by a river of molten rock.
35:19To do a little experiment, we've brought a Geiger counter and a piece of uranium ore.
35:33Now, the Geiger counter is sensitive to high-energy charged particles, protons, helium nuclei, gamma rays.
35:42If we bring it close to the uranium ore, the count rate, the number of clicks, increases dramatically.
35:53We also have a lead canister here.
35:56And if I drop the uranium ore into the canister, which absorbs the radiation, and cover it up,
36:03I then find the count rate goes down substantially, but it doesn't go down to zero.
36:12What's the source of the remaining counts?
36:15Well, some of them come from radioactivity in the walls of the cave.
36:21But there's more to it than that.
36:23Some of the counts we're hearing right now are due to high-energy charged particles,
36:27which are penetrating the roof of the cave.
36:31We are listening to cosmic rays.
36:35Every second they are penetrating my body and yours.
36:42They don't do much damage.
36:43Cosmic rays have bombarded the earth for the entire history of life on our planet.
36:48But they do cause some mutations, and they do affect life on the earth.
36:53The cosmic rays, mainly protons, are penetrating through the meters of rock in the cave above me.
37:04To do this, they have to be very energetic, and in fact, they are traveling almost at the speed of light.
37:11Think of it.
37:13A star blows up thousands of light-years away in space,
37:18and produces cosmic rays, which spiral through the Milky Way galaxy for millions of years,
37:27until quite by accident some of them strike the earth, penetrate this cave, reach this Geiger counter, and us.
37:36The evolution of life on earth is driven in part, through mutations, by the depths of distant stars.
37:46We are, in a very deep sense, tied to the cosmos.
37:51Our ancestors knew this well.
37:57The movements of the sun, the moon, and the stars could be used by those skilled in such arts to foretell the seasons.
38:05So the ancient astronomers all over the world studied the night sky with care, memorizing and recording the position of every visible star.
38:13To them, the appearance of any new star would have been significant.
38:19What would they have made of the apparition of a supernova, brighter than every other star in the sky?
38:26On July 4th, in the year 1054, Chinese astronomers recorded what they called a guest star, in the constellation of Taurus the Bull.
38:46A star never before seen, burst into radiance, became almost as bright as the full moon.
38:52Halfway around the world, here in the American Southwest, there was then a high culture rich in astronomical tradition.
39:03They too must have seen this brilliant new star.
39:07From carbon-14 dating of the remains of a charcoal fire, we know that in this very spot, there were people living in the 11th century.
39:19The people were the Anasazi, the antecedents of the Hopi of today.
39:26And one of them seems to have drawn on this overhang, protected from the weather, a picture of the new star.
39:35Its position near the crescent moon would have been just what we see here.
39:38And the handprint is perhaps the artist's signature.
39:45This remarkable star is now called the Crab Supernova.
39:50Nova from the Latin word for new, and crab because that's what an astronomer centuries later was reminded of when looking at this explosion remnant through the telescope.
40:00The crab is a star that blew itself up. The explosion was seen for three months. It was easily visible in broad daylight, and you could read by it at night.
40:14Imagine the night when that colossal stellar explosion first burst forth.
40:24A thousand years ago, people gazed up in amazement at the brilliant new star, and wondered what it was.
40:41We are the first generation privileged to know the answer. Through the telescope, we have seen what lies today at the spot in the sky noted by the ancient astronomers.
40:57A great luminous cloud, the remains of a star violently unraveling itself back into interstellar space.
41:05Only the massive red giants become supernovas, but every supernova was once a red giant.
41:18In the history of the galaxy, hundreds of millions of red giants have become supernovas.
41:26The bit of the star that isn't blown away collapses under gravity, spinning ever faster like a pirouetting ice skater bringing in her arms.
41:35The star becomes a single massive atomic nucleus, a neutron star.
41:41The one in the Crab Nebula is spinning 30 times a second.
41:44It emits a beamed pattern of light, and seems to us to be blinking on and off with astonishing regularity.
41:51Such neutron stars are called pulsars.
41:53Neutron star matter weighs about a mountain per teaspoonful, so much that if I had a piece of it here and let it go, and I could hardly prevent it from falling, it would effortlessly pass to the earth like a knife through warm butter.
42:13It would carve a hole for itself completely through the earth, emerging out the other side, perhaps in China.
42:21The people there might be walking along minding their own business when a tiny lump of neutron star matter comes booming out of the ground and then falls back again.
42:31The incident might, uh, make an agreeable break in the routine of the day.
42:38The neutron star matter, pulled back by the earth's gravity, would plunge again through the rotating earth, eventually punching hundreds of thousands of holes before friction with the interior of our planet stopped the motion.
42:52By the time it's at rest at the center of the earth, the inside of our world would look a little bit like a Swiss cheese.
43:01There are places in the galaxy where a neutron star and a red giant are locked in a mutual gravitational embrace.
43:24Tendrils of red giant star stuff spiral into a disk of accreting matter centered on the hot neutron star.
43:33Every star exists in a state of tension between the force that holds it up and gravity, the force that would pull it down.
43:52If gravity were to prevail, a stellar madness would ensue more bizarre than anything in Wonderland.
44:02Alice and her colleagues feel, more or less, at home in the gravitational pull of the earth, called 1G, G for earth gravity.
44:12What would happen if we made the gravity less or more?
44:15At lower gravity, things get lighter.
44:17Near 0G, the slightest motion sends our friends floating and tumbling in the air.
44:22Little blobs of liquid tea are everywhere, curious.
44:28If we now return the gravity to 1G, it's raining tea and our friends fall back to earth.
44:35I've been to a couple of parties like that myself.
44:42At higher gravities, 2 or 3Gs say, things get really laid back.
44:48Everyone feels heavy and leaden.
44:51Except, by special dispensation, the Cheshire Cat.
45:02As a kindness, we remove them.
45:05At thousands of Gs, trees become squashed.
45:09At 100,000 Gs, rocks become crushed by their own weight.
45:12At all these gravities, a beam of light remains unaffected, continuing up in a straight line.
45:19But at billions of Gs, even a beam of light feels the gravity and begins to bend back on itself.
45:26Curiouser and curiouser.
45:30Such a place, where the gravity is so large that even light can't get out, is called a black hole.
45:37It's a star in which light itself is imprisoned.
45:39Black holes were theoretical constructs speculated about since 1783.
45:46But in our time, we've verified the invisible.
45:50This bright star has a massive unseen companion.
45:54Satellite observatories find the companion to be an intense X-ray source called Cygnus X-1.
46:01These X-rays are like the footprints of an invisible man walking in the snow.
46:09The X-rays are thought to be generated by friction in the accretion disk surrounding the black hole.
46:21The matter in the disk slowly disappears down the black hole.
46:25Massive black holes produced by the collapse of a billion suns may be sitting at the centers of other galaxies, curiously producing great jets of radiation pouring out into space.
46:41At high enough density, the star winks out and vanishes from our universe, leaving only its gravity behind.
46:53It slips through a self-generated crack in the space-time continuum.
46:57A black hole is a place where a star once was.
47:02Here we have a flat two-dimensional surface with grid lines on it, something like a piece of graph paper.
47:10Suppose we take a small mass, drop it on the surface, and watch how the surface distorts or puckers into the third physical dimension.
47:23Gravity can be understood as a curvature of space.
47:33If our moving ball approaches a stationary distortion, it rolls around it like a planet orbiting the sun.
47:39In this interpretation, due to Einstein, gravity is only a pucker in the fabric of space which moving objects encounter.
47:49Space is warped by mass into an additional physical dimension.
47:56The larger the local mass, the greater is the local gravity, and the more intense is the distortion or pucker or warp of space.
48:07So, by this analogy, a black hole is a kind of bottomless pit.
48:14What would happen if you fell in?
48:17Well, assuming you could survive the gravitational tides and the intense radiation flux,
48:23it is just barely possible that by plunging into a black hole,
48:29you might emerge in another part of space-time, somewhere else in space,
48:34somewhere else in time.
48:38In this view, space is filled with a network of wormholes, something like the wormholes in an apple,
48:46although by no means is this point demonstrated.
48:49It is merely an exciting suggestion.
48:51If it is true, then perhaps there exist gravity tunnels, a kind of interstellar or intergalactic subway,
49:02which would permit you to get from here to there in much less than the usual time,
49:07a kind of cosmic rapid transit system.
49:10We cannot generate black holes. Our technology is far too feeble to move such massive amounts of matter around.
49:21But perhaps someday, it will be possible to voyage hundreds or thousands of light years to a black hole like Cygnus X-1.
49:28We would plunge down to emerge in some unimaginably exotic time and place,
49:35our common sense notions of reality severely challenged.
49:41Perhaps the cosmos is infested with wormholes, every one of them a tunnel to somewhere.
49:47Perhaps other civilizations with vastly more advanced technologies are today riding the Gravity Express.
49:54It's even possible that a black hole is a gate to another and quite different universe.
50:24The lives and deaths of the stars seem impossibly remote from human experience,
50:36and yet we're related in the most intimate way to their life cycles.
50:40The very matter that makes us up was generated long ago and far away in red giant stars.
50:58A blade of grass, as Walt Whitman said, is the journey work of the stars.
51:08The formation of the solar system may have been triggered by a nearby supernova explosion.
51:13After the sun turned on, its ultraviolet light poured into our atmosphere,
51:18its warmth generated lightning, and these energy sources sparked the origin of life.
51:22Plants harvest sunlight, converting solar into chemical energy.
51:29We and the other animals are parasites on the plants,
51:33so we are, all of us, solar powered.
51:37The evolution of life is driven by mutations.
51:41They're caused partly by natural radioactivity and cosmic rays,
51:44but they are both generated in the spectacular deaths of massive stars thousands of light years distant.
51:53Think of the sun's heat on your upturned face on a cloudless summer's day.
51:59From 150 million kilometers away, we recognize its power.
52:04What would we feel on its seething, self-luminous surface, or immersed in its heart of nuclear fire?
52:13And yet, the sun is an ordinary, even a mediocre star.
52:18Our ancestors worshiped the sun, and they were far from foolish.
52:22It makes good sense to revere the sun and the stars.
52:24Because we are their children.
52:32We have witnessed the life cycles of the stars.
52:36They are born, they mature, and then they die.
52:41As time goes on, there are more white dwarfs, more neutron stars, more black holes.
52:46The remains of the stars accumulate as the eons pass.
52:50But interstellar space also becomes progressively enriched in heavy elements,
52:56out of which form new generations of stars and planets, life and intelligence.
53:02The events in one star can influence a world halfway across the galaxy,
53:07and a billion years in the future.
53:08The vast interstellar clouds of gas and dust are stellar nurseries.
53:22Here first begins the inexorable gravitational collapse,
53:26which dominates the lives of the stars.
53:29Massive suns may evolve through the red giant stage in only millions of years,
53:33dying young, never leaving the cloud in which they were born.
53:40Other suns, long-alived, wander out of the nursery.
53:45Our sun is such a star, as are most of the stars in the sky.
53:49Most stars are members of double or multiple star systems,
54:06and live to process their nuclear fuel over billions of years.
54:10The galaxy is 10 billion years old,
54:14old enough to have spawned only a few generations of ordinary stars.
54:24The objects we encounter in a voyage through the Milky Way
54:28are stages in the life cycle of the stars.
54:31Some are bright and new, and others are as ancient as the galaxy itself.
54:50Surrounding the Milky Way is a halo of matter,
54:53which includes the globular clusters,
54:58each containing up to a million elderly stars.
55:01At the centers of globular clusters,
55:04and at the core of the galaxy,
55:06there may be massive black holes ticking and purring,
55:10the subject of future exploration.
55:12We on Earth marvel, and rightly so,
55:34at the daily return of our single sun.
55:37But from a planet orbiting a star in a distant globular cluster,
55:40a still more glorious dawn awaits.
55:44Not a sunrise, but a galaxy rise.
55:48A morning filled with 400 billion suns,
55:52the rising of the Milky Way.
55:54An enormous spiral form,
55:57with collapsing gas clouds, condensing planetary systems,
56:01luminous supergiants, stable middle-aged stars,
56:05red giants, white dwarfs, planetary nebulas,
56:07supernovas, neutron stars, pulsars, black holes,
56:13and, there is every reason to think,
56:15other exotic objects that we have not yet discovered.
56:22From such a world, high above the disk of the Milky Way,
56:26it would be clear, as it is beginning to be clear on our world,
56:29that we are made by the atoms and the stars,
56:33that our matter and our form are determined by the cosmos of which we are a part.
56:40I only have a moment, but I wanted you to see a picture of Betelgeuse, that's what it's called,
56:55in the constellation Orion, the first image of the surface of another star.
56:59But the most exciting recent stellar discovery has been of a nearby supernova in a companion galaxy to the Milky Way.
57:07We are here witnessing chemical elements in the process of synthesis,
57:12and have had our first glimpse of the supernova through a brand new field, neutrino astronomy.
57:18And we are now seeing, around neighboring stars,
57:23disks of gas and dust, just like those needed to explain the origin of the planets in our solar system.
57:29Worlds may be forming here.
57:32It's like a snapshot of our solar system's past.
57:36And there are so many such disks being found these days,
57:40that planets may be very common among the stars of the Milky Way.
57:44I must see my bacteria for certain years in the Milky Way.
57:48Sol judicial settlement is the amount of씨 has aает of the daytime,
58:07that exists only for me.
58:08Even if you are seeing the mechanics of this tiger well and 별 developing the world,
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