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00:00Hello. My name is Stephen Hawking. Physicist, cosmologist, and something of a dreamer.
00:15Although I cannot move, and I have to speak through a computer, in my mind, I am free.
00:24Free to explore the universe and ask the big questions, such as, do aliens exist?
00:36If so, where could they be found?
00:40What do they look like?
00:43What are they made of?
00:46Are they intelligent?
00:50And if we met them, what would it mean for humankind?
01:00Check it out.
01:02Wherever I go in the world, people ask me, do aliens exist?
01:18It's a good question, because it cuts to the heart of how we see our place in the universe.
01:24Are we alone on our small, round, blue ball?
01:34I think probably not, because of one fact.
01:42The universe is big.
01:44Really big.
01:46Our planet is just one of eight in orbit around our sun.
01:58Which itself is hardly special, being one of about 200 billion stars in a vast spiral.
02:08Our galaxy, the Milky Way.
02:12It's so big, some days I find it hard to comprehend.
02:18But even the Milky Way is just a tiny drop in the cosmic ocean.
02:26Just one of 100 billion galaxies, formed into an enormous web, stretching away in all directions.
02:44At this scale, each point of light is an entire galaxy.
02:54Which not only puts our little world in perspective, but also makes it difficult to believe we really are alone.
03:03So to my mathematical brain, the numbers alone, make thinking about aliens perfectly rational.
03:19The real challenge is to try and work out what aliens might actually be like, living on some far off world.
03:31The possibilities are infinite.
03:35And infinitely intriguing.
03:37Alien life could range from a simple green slime, that doesn't do much but drip, to more advanced animals.
04:01Something with a bit more bite.
04:05But of course, that's just the start of what could be out here.
04:11In such a massive universe, it's logical to wonder if there are intelligent beings.
04:19Perhaps even civilizations like those in science fiction TV shows and movies.
04:29Star Wars and Star Trek, two of my personal favorites, may be closer to reality than we think.
04:37Similar scenarios are at least conceivable.
04:47But think about it more.
04:49And even this is limiting the options.
04:53There could be life forms so strange, we wouldn't even recognize them as life.
05:01Perhaps there are really exotic creatures that live at the center of stars.
05:07Or even huge communities of microorganisms that look like clouds of cosmic dust.
05:25Maybe aliens live and die so fast, that they come and go in the blink of an eye.
05:43So in such a vast universe, with so many possibilities, how do we know what to look for?
05:49Or for that matter, where to look for it?
05:54The answer is right back where we began.
06:14The information we need is here at home.
06:18For the simple reason that home harbors the only known examples of life.
06:23The laws of physics appear to be the same everywhere.
06:34So it follows that the laws of life should be universal too.
06:39Even if the detail is different.
06:42We can use life on Earth as a kind of alien hunter's handbook.
06:47A field guide to what life actually is and how it works, no matter where it occurs.
06:57Chapter 1, in our particular case, takes us back 4.5 billion years.
07:03To when the Earth was really quite young.
07:06Exactly what triggered life here is still a mystery.
07:14But there are several theories.
07:17The most common one is that life began purely by accident.
07:21In pools of primordial soup.
07:24Full of chemicals called amino acids.
07:32These molecules would have collided at random for millions of years.
07:36Until the perfect combination just happened.
07:40The ultimate lucky break that started the chain of life.
07:53It is extremely unlikely that life could spontaneously create itself.
08:11But I don't think that's a problem with this theory.
08:14It's like winning a lottery.
08:16Although the odds are astronomical.
08:19Most weeks, someone hits the jackpot.
08:24But there is another intriguing idea called panspermia.
08:29Which says that life could have originated somewhere else.
08:33And have been spread from planet to planet.
08:36By asteroids.
08:39It seems possible that lumps of rock could carry frozen organisms inside them.
08:51Organisms able to withstand extremes of temperature and the vacuum of space.
08:58If so, asteroids could even now be transporting life to other worlds.
09:09Regardless of which theory is right, once life begins,
09:38the next chapter starts.
09:40And that's all about survival.
09:50Survival links you, me and ET.
09:53And it generates rules all of its own.
10:00Survival demands a source of energy.
10:02What we call food.
10:04Or else it would grind to a hole.
10:06Once nourished, life can then copy itself to protect against the death of any one individual.
10:13Ultimately, that leads to evolution.
10:18Evolution that would happen even on alien worlds.
10:24Producing, in some instances, animals that I think we would recognize as being alive.
10:30Even if they look a bit strange.
10:32Even if they look a bit strange.
10:34So the next step on our alien hunt is to find a place, or places, where organisms might find food and replicate and evolve.
10:50And as far as we know, that requires one thing.
10:51One thing.
10:52One thing.
10:53One thing.
10:54One thing.
10:55One thing.
10:56One thing.
10:57One thing.
10:58One thing.
11:00One thing.
11:03One thing.
11:05And as far as we know, that requires one thing.
11:10Like most people, I find water both beautiful and fascinating.
11:38But it's also the key to all known forms of life, from bacteria to blue whales.
11:47Find water elsewhere, and aliens could exist nearby.
11:53The good news is that water is very common indeed.
11:59Out in space, frozen water litters the universe.
12:06From tiny, single crystals to icy comets the size of mountains.
12:12But to find liquid water, we need somewhere at the right temperature.
12:26Around every star is a region where it's not too hot or too cold, but just right.
12:33Like the porridge in the story of Goldilocks and the three bears.
12:39Around our sun, there are two planets that lie in this Goldilocks zone.
12:44The Earth and Mars.
12:49Which is why one day I'm sure we'll pay it a visit.
13:00Robots have been exploring Mars since the 70s, but they have yet to find life.
13:07I don't think we should give up.
13:09Beneath the Martian surface, NASA's Spirit Rover discovered these white salts,
13:16which are formed in contact with liquid water.
13:22Satellite images reveal drainage patterns and erosion of the kinds caused by rivers and oceans.
13:30There may well still be moisture under Mars' surface.
13:35Moisture that perhaps could support life.
13:39I hope one day we will find the money to send men and women to Mars.
13:54It would capture the public's imagination just as the Apollo moon missions captured mine back in the 60s.
14:05If they found even a few Martian microbes, in my opinion,
14:10it would be one of the most exciting discoveries ever made.
14:14But even if Mars is barren, there are other places to look for liquid water.
14:31One of them lies a mere 30 million miles from Mars,
14:35on a small, mysterious moon that orbits the giant planet Jupiter.
14:41One of them lies a few years ago.
14:51This is Europa.
14:58Europa is tiny, just under 2,000 miles in diameter,
15:03and it's very cold, minus 260 degrees.
15:09The entire moon is covered in a layer of ice perhaps 15 miles thick.
15:17But Europa may have a hidden heat source beneath the surface.
15:25Europa orbits Jupiter once every 3.6 days in an egg-shaped path.
15:31The gravitational pull from Jupiter changes constantly,
15:39stretching, then compressing Europa.
15:47This process is like kneading a piece of clay to make it warm and soft.
15:53And the heat produced maybe enough to melt the underside of the ice sheet.
16:00Creating a hidden ocean of liquid water, protected from the vacuum of space by the solid ice above.
16:07If so, there could be aliens living here.
16:20Creatures that have evolved to exploit this dark and ancient water world.
16:24I think it's even reasonable to guess at some of their physical features.
16:30I think it's even reasonable to guess at some of their physical features.
16:34Aliens here would probably swim in a similar way to our own ocean life.
16:37Aliens here would probably swim in a similar way to our own ocean life.
16:56Aliens here would probably swim in a similar way to our own ocean life, since liquid water
17:05is the same stuff everywhere.
17:16They might use chemicals in their skin to generate their own light, as many deep sea
17:22creatures do back home.
17:39They might even swim in school-like colonies just as aquatic animals do on Earth.
17:47But even if advanced animals do live inside Europa, I think they're unlikely to be trying
18:02to make contact with us any time soon.
18:06They'd exist cocooned in an icy shell fifteen miles thick, so they'd be blissfully unaware
18:12of the universe beyond.
18:14To find them, we'd need to send a mission here, which would be even more risky and expensive
18:22than visiting Mars.
18:23I hope one day we will discover Europa's secrets.
18:28But before then, it's worth continuing our journey to search for aliens with a wider outlook.
18:41I think we need to leave our solar system and voyage into the vastness that lies beyond.
18:53stars surround us in the universe, but until recently, no one knew how many had planets in orbit
19:09around them, let alone if any of those planets could support alien life.
19:17Finding out is tough because stars are big and blindingly bright, planets are tiny and dark.
19:26Spotting them requires technology on an enormous scale.
19:32The binocular Keck telescope in Hawaii, with its twin 30-foot mirrors, is one of the most powerful
19:41land-based telescopes ever built.
19:43But even this vast machine can't see distant planets.
19:51Instead, it looks for stars that wobble, the tell-tale sign of an unseen planet in orbit.
20:06A hammer-thrower demonstrates the principle.
20:17As he spins, the hammer pulls on his body, and he wobbles from side to side.
20:27The same thing happens as a planet swings round its star.
20:37Planets also reveal themselves if they pass between their star and us.
20:44The planet causes regular dimming, and from the timing, we can even determine if it's
20:50inside the star's Goldilocks zone.
20:56The last distant planet was discovered in 1995.
21:01Since then, hundreds more have been found.
21:06This, I think, is a pretty exciting discovery.
21:11We could be on the verge of a major breakthrough.
21:16One that will both redefine our view of life in the universe, and be a real triumph for science itself.
21:25Somewhere out there, perhaps not so far away, is a rocky planet, a bit like Earth.
21:32A planet with liquid water, where life has begun.
21:38Due to the power of evolution, aliens here might be surprisingly familiar, even if at first, they seem anything but.
21:51aliens that eat, for example, need an input orifice, or as most people say, a mouth.
22:04Likewise, if they live on a solid surface, they'll probably have legs.
22:19The detail might be different, but legs are good things to have on land, especially if the animal is clinging to the side of a cliff.
22:35If the planet is well lit, eyes are almost guaranteed.
22:54They let a creature accurately sense its environment.
23:09Even the position of the eyes will follow the same rules as on Earth.
23:13prey animals tend to have eyes on either side of their head, allowing them to look out for predators.
23:30Predators, even alien ones, need forward-facing eyes to accurately judge distance, an essential skill when hunting.
23:42对 trio
23:48与新游能
23:53共存在
23:57谁
24:01呃
24:03偉
24:09垃
24:10平
24:12Alien struggles of life and death are probably happening right now thanks to the universal
24:34power of evolution.
24:36But in my opinion, evolution is so remarkable we can't really be sure of its ultimate limits.
24:48Life forms on Earth-like planets or in hidden oceans are not the only options.
25:01We can go even further into the depths of the universe in search of other kinds of aliens,
25:08extraterrestrials that are totally unlike us.
25:13Life, but not as we know it.
25:27I like to think of myself as an optimist.
25:30And so, in our vast ancient universe, with its countless galaxies, almost any life form
25:38that is physically possible is likely to exist somewhere.
25:45So there could be, perhaps should be, really bizarre aliens that have followed a different
25:53evolutionary path.
25:58Aliens that don't depend on water, but on other chemicals instead.
26:06Nitrogen is one possibility.
26:08It's a gas on Earth, but it can exist as a liquid when it's very, very cold.
26:15Minus 320 degrees Fahrenheit.
26:24So is there a world of nitrogen oceans lapping frozen shores?
26:29Where aliens have evolved in temperatures that would kill a human instantly.
26:39Life here would need chemistry very different than our own.
26:46A cold weather remix of the ingredients that make us.
26:56Number one, of course, is water.
27:01The average male holds eight gallons.
27:05So let's swap water for liquid nitrogen.
27:09There's also about two pounds of phosphorus, half a pound of salt, enough iron to make a
27:16nail, three pounds of lime, 15 trace elements, which might also work in alien biology, and then
27:25this.
27:27Carbon.
27:2845 pounds of it.
27:32But what if carbon was switched with something else, silicon, perhaps?
27:41Silicon has slightly different properties, but it could do the same job.
27:48With the right ingredients, ultra-low temperature life might be possible.
27:59If so, I think energy would be scarce.
28:04So things around here would move very slowly.
28:07Other possibilities are even stranger.
28:30The astronomers searching for far-off planets have discovered that many seem to be giant gas
28:40planets, like our own Jupiter and Saturn.
28:45Earth.
28:46Perhaps there are aliens made of gas.
29:13The aliens living here would need to consume something.
29:20I imagine they could use the power of lightning storms that constantly rage on planets like
29:26these.
29:28Earth.
29:29If such extreme life forms are possible, then life elsewhere in the universe could be very
29:44common indeed.
29:47Earth.
29:48Earth.
29:49Earth.
29:50Earth.
29:51Earth.
29:52Earth.
29:53Earth.
29:54There are certainly many more planets that fall outside the Goldilocks regions of stars
29:57than fall inside them.
30:00Earth.
30:01Earth.
30:02Earth.
30:03Earth.
30:04Earth.
30:05Earth.
30:06Earth.
30:07Earth.
30:08Earth.
30:09Earth.
30:10Earth.
30:11Earth.
30:12Earth.
30:13Earth.
30:14Earth.
30:15Earth.
30:16Earth.
30:17Earth.
30:18Earth.
30:19Earth.
30:20Earth.
30:21to try to unlock its secrets just as we are in short has alien life evolved as
30:30we have undeveloped intelligence
30:51if the universe is full of intelligent space-faring aliens I think at least some
30:57of them might be interested in us if only as a curiosity of course many people believe they
31:06are already here tales of alien abduction have been common ever since I was an undergraduate
31:16in the 1950s and I watched all those B movies too the story always goes the same a lone individual
31:28on a quiet road at night happens to take an unscheduled detour finds himself lost
31:46so
31:55so
31:58Oh, my God.
32:28I'm always a bit suspicious when I hear these tales.
32:38Look at it from the aliens' point of view.
32:41What's the point of crossing vast tracks of the universe in a high-tech ship just to abduct some lone Earthling?
32:50In my opinion, if aliens are here, I suspect the newspapers would be full of the story.
32:56And if governments are involved in a cover-up, they're doing a much better job at it than they seem to do at anything else.
33:06So the lack of alien contact raises a serious scientific problem.
33:11Where is everybody?
33:16We've been listening to space for over 40 years.
33:20And in all that time, we've picked up nothing.
33:28Well, except for one mysterious occasion.
33:32On August 16, 1977, a radio telescope in Ohio picked up a signal that became famous.
33:47The telescope listened to space by scanning the skies as the Earth rotated.
33:58And just once, it recorded a signal that got everyone excited.
34:02The WOW signal, as it became known.
34:05The signal was a steady source of radio waves, just the kind an alien race might send,
34:17because it stands out from the radiostatic that fills the universe.
34:25A computer recorded the signal as six letters and numbers.
34:30Astronomer, Jerry Ehrman, saw the data and wrote one word in the margin.
34:48Ehrman and others subsequently searched the same patch of sky many times,
34:55but found nothing.
34:57The WOW signal had vanished.
35:00The whole mysterious episode reveals that making contact with aliens via radio
35:10is always going to be difficult.
35:17In such a vast universe,
35:20messages take a long time to reach their destination.
35:25The WOW signal appeared to come from a star system
35:29from two hundred light-years away.
35:30So it took at least two hundred years to reach us.
35:35If we sent a reply, it would take another two hundred years to reach them.
35:41By which time, they might have forgotten they sent anything and stopped listening for a reply.
35:47Worse, they might well have destroyed themselves in the meantime.
35:54The human race very quickly discovered the power of the atom bomb.
35:58If the same holds for intelligent aliens, then they might not last long.
36:04Perhaps they all blow themselves up soon after they discover that E equals MC squared.
36:21If civilizations take billions of years to evolve, only to vanish virtually overnight, then sadly, we've next to no chance of hearing from them.
36:38They are simply too far away in space and time to reach.
36:47But there is one last possibility.
36:50That aliens who have avoided destroying themselves are already colonizing the universe.
36:58The human race has only two options when it comes to looking for advanced aliens.
37:24We can listen, or we can be more active and broadcast our willingness to talk.
37:37We'd have to think very carefully about what we might say.
37:53I think this might be just a little too risky.
38:04We only have to look at ourselves to see how intelligent life might develop into something we wouldn't want to meet.
38:13We humans are already capable of manipulating the course of our own evolution.
38:19Exactly the same presumably would be true of advanced extraterrestrials.
38:24Ultimately, they could halt aging and become virtually immortal.
38:31What's more, they might have reached that point millions of years ago.
38:47It might sound unlikely, but if you think about it logically, alien technology should be as extraordinary to us as a rocket ship to a caveman.
39:04I imagine they might exist in massive ships like these, having used up all the resources from the home planet below.
39:12Such advanced aliens would perhaps become nomads, looking to conquer and colonize whatever planets they can reach.
39:23If so, it makes sense for them to exploit each new planet for materials to build more spaceships so they could move on.
39:39Who knows what the limits would be?
39:49Perhaps their capabilities would only be limited by how much power they could harness and control.
39:57And that could be far more than we might first imagine.
40:04For example, it might be possible to collect the energy from an entire star.
40:14To do that, they could deploy millions of mirrors in space, encircling the whole sun and feeding the power to one single collection point.
40:26Such power might make it possible to warp the very fabric of space and create a portal called a wormhole.
40:55This portal would act like a shortcut, allowing them to travel huge distances in the blink of an eye.
41:25Like us, they would probably have evolved from a species used to exploiting whatever it can.
41:32So if aliens ever visit us, I think the outcome would be much as when Christopher Columbus first landed in America.
41:39Which didn't turn out very well for the Native Americans.
41:46So the journey that started with the Native Americans.
41:47The journey that started with the Native Americans is not a different way.
41:53So if aliens ever visit us, I think the outcome would be much as when Christopher Columbus first landed in America.
42:01America, which didn't turn out very well for the Native Americans.
42:15So the journey that started with the search for water has led us to far off worlds that
42:20I think probably do exist.
42:31In such a massive place as the cosmos, we only have to look at ourselves for proof that extremely
42:42unlikely things can and do happen all the time.
42:52Let's just hope that if aliens do find us, they'll come in peace.
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