- 25/05/2025
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00:00Hello, my name is Stephen Hawking, physicist, cosmologist, and something of a dreamer.
00:16Although I cannot move, and I have to speak through a computer, in my mind, I am free.
00:26Free to explore the universe and ask the big questions, such as, do aliens exist?
00:34If so, where could they be found?
00:40What do they look like?
00:44What are they made of?
00:47Are they intelligent?
00:51And if we met them, what would it mean for humankind?
01:14Wherever I go in the world, people ask me, do aliens exist?
01:19It's a good question, because it cuts to the heart of how we see our place in the universe.
01:28Are we alone on our small, round, blue ball?
01:38I think probably not, because of one fact.
01:43The universe is big, really big.
01:50Our planet is just one of eight in orbit around our sun.
01:59Which itself is hardly special, being one of about 200 billion stars in a vast spark.
02:10Our galaxy, the Milky Way, it's so big, some days I find it hard to comprehend.
02:23But even the Milky Way is just a tiny drop in the cosmic ocean.
02:36Just one of 100 billion galaxies, formed into an enormous web, stretching away in all directions.
02:49At this scale, each point of light is an entire galaxy, which not only puts our little world in perspective,
02:59but also makes it difficult to believe we really are alone.
03:09So to my mathematical brain, the numbers alone make thinking about aliens perfectly rational.
03:20The real challenge is to try and work out what aliens might actually be like, living on some far-off world.
03:32The possibilities are infinite, and infinitely intriguing.
03:50Alien life could range from simple green slime that doesn't do much but drip,
03:58to more advanced animals, something with a bit more bite.
04:08But of course, that's just the start of what could be out here.
04:14In such a massive universe, it's logical to wonder if there are intelligent beings.
04:21Perhaps even civilizations like those in science fiction TV shows and movies.
04:32Star Wars and Star Trek, two of my personal favorites, may be closer to reality than we think.
04:40Similar scenarios are at least conceivable.
04:48But think about it more, and even this is limiting the options.
04:54There 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.
05:20Or even huge communities of microorganisms that look like clouds of cosmic dust.
05:29Maybe 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, or for that matter, where to look for it?
05:58The answer is right back where we began.
06:13The information we need is here at home.
06:17For the simple reason that home harbors the only known examples of life.
06:30The laws of physics appear to be the same everywhere.
06:34So it follows that the laws of life should be universal too, even if the detail is different.
06:43We can use life on Earth as a kind of alien hunter's handbook, a field guide to what life actually is and how it works, no matter where it occurs.
06:57Chapter one, in our particular case, takes us back 4.5 billion years, to when the Earth was really quite young.
07:10Exactly what triggered life here is still a mystery, but there are several theories.
07:17The most common one is that life began purely by accident, in pools of primordial soup, full of chemicals called amino acids.
07:32These molecules would have collided at random for millions of years, until the perfect combination just happened.
07:43It was the ultimate lucky break that started the chain of life.
08:07It is extremely unlikely that life could spontaneously create itself.
08:13There is a problem with this theory.
08:15It's like winning a lottery, although the odds are astronomical.
08:21Most weeks, someone hits the jackpot.
08:25But there is another intriguing idea, called panspermia, which says that life could have originated somewhere else and have been spread from planet to planet, by asteroids.
08:43It seems possible that lumps of rock could carry frozen organisms inside them, organisms able to withstand extremes of temperature and the vacuum of space.
08:57If so, asteroids could even now be transporting life to other worlds.
09:13Transcribed by ESO, translated by —
09:43Survival.
09:49Survival links you, me, and ET, and it generates rules all of its own.
09:59Survival demands a source of energy, what we call food, or else it would grind to a halt.
10:07Once nourished, life can then copy itself to protect against the death of any one individual.
10:15Ultimately, that leads to evolution.
10:19Evolution that would happen even on alien worlds.
10:25Producing, in some instances, animals that I think we would recognize as being alive, even if they look a bit strange.
10:37Transcribed by ESO, translated by —
10:53So the next step on our alien hunt is to find a place, or places, where organisms might find food and replicate and evolve.
11:05And as far as we know, that requires one thing.
11:26Like most people, I find water both beautiful and fascinating.
11:32But it's also the key to all known forms of life, from bacteria to blue whales.
11:42Find water elsewhere, and aliens could exist nearby.
11:48The good news is that water is very common indeed.
11:55But in space, frozen water litters the universe.
12:01From tiny single crystals, to icy comets the size of mountains.
12:16But to find liquid water, we need somewhere at the right temperature.
12:22Every star is a region where it's not too hot or too cold, but just right.
12:28Like the porridge in the story of Goldilocks and the Three Bears.
12:34Around our sun, there are two planets that lie in this Goldilocks zone.
12:40The Earth, and Mars.
12:44Which is why one day I'm sure we'll pay it a visit.
12:52Robots have been exploring Mars since the 70s.
12:58But they have yet to find life.
13:02I don't think we should give up.
13:06Beneath the Martian surface, NASA's Spirit rover discovered these white salts,
13:12which are formed in contact with liquid water.
13:18These images reveal drainage patterns and erosion of the kinds caused by rivers and oceans.
13:26There may well still be moisture under Mars' surface.
13:30Moisture that perhaps could support life.
13:36I hope one day we will find the money to send men and women to Mars.
13:46It would capture the public's imagination just as the Apollo moon missions captured mine back in the 60s.
13:58If they found even a few Martian microbes, in my opinion,
14:03it would be one of the most exciting discoveries ever made.
14:13But even if Mars is barren, there are other places to look for liquid water.
14:25One of them lies a mere 30 million miles from Mars,
14:29on a small, mysterious moon that orbits the giant planet Jupiter.
14:45This is Europa.
14:49Europa is tiny, just under 2,000 miles in diameter,
14:55and it's very cold.
14:59Minus 260 degrees.
15:03The entire moon is covered in a layer of ice perhaps 15 miles thick.
15:09But Europa may have a hidden heat source beneath the surface.
15:19Europa orbits Jupiter once every 3.6 days in an egg-shaped path.
15:29The gravitational pull from Jupiter changes constantly,
15:33stretching, then compressing Europa.
15:41This process is like kneading a piece of clay to make it warm and soft.
15:49And the heat produced may be enough to melt the underside of the ice sheet,
15:55creating a hidden ocean of liquid water,
15:59protected from the vacuum of space by the solid ice above.
16:11If so, there could be aliens living here.
16:15Creatures that have evolved to exploit this dark and ancient water world.
16:21I think it's even reasonable to guess at some of their physical features.
16:45Aliens here would probably swim in a similar way to our own ocean life,
16:51since liquid water is the same stuff everywhere.
17:05They might use chemicals in their skin to generate their own life,
17:12as many deep sea creatures do back home.
17:30They might even swim in school-like colonies just as aquatic animals do on Earth.
17:36But even if advanced animals do live inside Europa,
17:40I think they're unlikely to be trying to make contact with us any time soon.
17:46They'd exist cocooned in an icy shell 15 miles thick,
17:50so they'd have to be very careful.
17:54To find them, we'd need to send a mission here,
17:58which would be even more risky and expensive than visiting Mars.
18:04I hope one day we will discover Europa's secrets,
18:08and find out what they are.
18:24But before then, it's worth continuing our journey
18:28to search for aliens with a wider outlook.
18:34I think we need to leave our solar system,
18:38and voyage into the vastness that lies beyond.
18:48Stars surround us in the universe,
18:52but until recently, no one knew how many had planets in orbit around them,
18:58let alone if any of those planets could support alien life.
19:04Finding out is tough, because stars are big and blindingly bright.
19:10Planets are tiny and dark.
19:14Spotting them requires technology on an enormous scale.
19:22The Binocular Keck Telescope in Hawaii,
19:26with its twin 30-foot mirrors,
19:28is one of the most powerful land-based telescopes ever built.
19:34But even this vast machine can't see distant planets.
19:40Instead, it looks for stars that wobble,
19:46the telltale sign of an unseen planet in orbit.
19:52Look.
19:56A hammer thrower demonstrates the principle.
20:05As he spins, the hammer pulls on his body,
20:09and he wobbles from side to side.
20:15The same thing happens as a planet swings round its star.
20:22Planets also reveal themselves if they pass between their star and us.
20:30The planet causes regular dimming,
20:34and from the timing we can even determine if it's inside the star's Goldilocks zone.
20:42The first distant planet was discovered in 1995.
20:48Since then, hundreds more have been found.
20:53This, I think, is a pretty exciting discovery.
20:57We could be on the verge of a major breakthrough.
21:02One that will both redefine our view of life in the universe,
21:06and be a real triumph for science itself.
21:11Somewhere out there, perhaps not so far away,
21:15is a rocky planet a bit like Earth.
21:20A planet with liquid water, where life has begun.
21:29Due to the power of evolution, aliens here might be surprisingly familiar,
21:34even if at first they seem anything but.
21:38Aliens that eat, for example, need an input orifice,
21:43or, as most people say, a mouth.
21:49Likewise, if they eat, they need a mouth,
21:53and if they eat, they need a mouth.
21:58And if they eat, they need a mouth,
22:02and if they eat, they need a mouth.
22:06Likewise, if they live on a solid surface, they'll probably have legs.
22:15The detail might be different, but legs are good things to have on land,
22:21especially if the animal is clinging to the side of a cliff.
22:36If the planet is well-lit, eyes are almost guaranteed.
22:41They let a creature accurately sense its environment.
22:46Even the position of the eyes will follow the same rules as on Earth.
22:51Prey animals tend to move in the opposite direction,
22:55and if they move in the opposite direction,
22:59their eyes will follow the same rules as on Earth.
23:04Prey animals tend to have eyes on either side of their head,
23:08allowing them to look out for predators.
23:19Predators, even alien ones, need forward-facing eyes
23:23to accurately judge distance, an essential skill when hunting.
23:34Predators, even alien ones, need forward-facing eyes
23:38to accurately judge distance, an essential skill when hunting.
23:43Predators, even alien ones, need forward-facing eyes
23:47to accurately judge distance, an essential skill when hunting.
23:52Predators, even alien ones, need forward-facing eyes
23:56to accurately judge distance, an essential skill when hunting.
24:00Predators, even alien ones, need forward-facing eyes
24:04to accurately judge distance, an essential skill when hunting.
24:18Alien struggles of life and death are probably happening right now
24:22thanks to the universal power of evolution.
24:26But in my opinion, evolution is so remarkable
24:29we can't really be sure of its ultimate limits.
24:37Life forms on Earth-like planets or in hidden oceans
24:41are not the only options.
24:50We can go even further into the depths of the universe
24:54in search of other kinds of aliens,
24:58extraterrestrials that are totally unlike us.
25:02Life, but not as we know it.
25:11I like to think of myself as an optimist.
25:15And so, in our vast, ancient universe,
25:19with its countless galaxies,
25:24everything possible is likely to exist somewhere.
25:32So there could be, perhaps should be, really bizarre aliens
25:36that have followed a different evolutionary path.
25:42Aliens that don't depend on water, but on other chemicals instead.
25:50Nitrogen is one possibility.
25:53It's like gas on Earth,
25:55but it can exist as a liquid when it's very, very cold.
25:59Minus 320 degrees Fahrenheit.
26:08So is there a world of nitrogen oceans,
26:12lapping frozen shores,
26:14where aliens have evolved in temperatures
26:16that would kill a human instantly?
26:23Life here would need chemistry very different than our own.
26:29A cold-weather remix of the ingredients that make us.
26:40Ingredient number one, of course, is water.
26:44The average male holds eight gallons.
26:48So let's swap water for liquid nitrogen.
26:53There's also about two pounds of phosphorus.
26:56Half a pound of salt.
26:58Enough iron to make a nail.
27:01Three pounds of lime.
27:03Fifteen trace elements,
27:05which might also work in alien biology.
27:08And then this.
27:12Carbon.
27:1445 pounds of it.
27:19But what if carbon was switched with something else?
27:22Silicon, perhaps.
27:24Silicon has slightly different properties,
27:27but it could do the same job.
27:37With the right ingredients,
27:39ultra-low-temperature life might be possible.
27:45If so, I think energy would be scarce.
27:48Things round here would move very slowly.
28:13Other possibilities are even stranger.
28:17The astronomers searching for far-off planets
28:20have discovered that many seem to be giant gas planets.
28:25Like our own Jupiter and Saturn.
28:33Perhaps there are aliens made of gas.
28:47Aliens living here would need to consume something.
28:57I imagine they could use the power of lightning storms
29:00that constantly rage on planets like these.
29:04If such extreme life-forms are possible,
29:07then life elsewhere in the universe could be very common indeed.
29:16But what if life was just a dream?
29:19What if life was just a dream?
29:22What if life was just a dream?
29:25What if life was just a dream?
29:28What if life was just a dream?
29:31There are certainly many more planets
29:34that fall outside the Goldilocks regions of stars
29:37than fall inside them.
29:41It suddenly seems like there could be life
29:44nearly everywhere you look.
29:49But ultimately, I think it doesn't really matter
29:52what aliens are made of.
29:55To me, it's what they can do that counts.
29:58Are they thinking about the cosmos too?
30:01Trying to unlock its secrets?
30:04Just as we are.
30:07In short, has alien life evolved as we have?
30:10In short, has alien life evolved as we have?
30:13Undeveloped intelligence.
30:28If the universe is full of intelligent, space-faring aliens,
30:31If the universe is full of intelligent, space-faring aliens,
30:34I think at least some of them might be interested in us.
30:39If only as a curiosity.
30:42Of course, many people believe they are already here.
30:49Tales of alien abduction have been common
30:52ever since I was an undergraduate in the 1950s.
30:55And I watched all those B-movies too.
31:00The story always goes the same.
31:03A lone individual on a quiet road at night
31:06happens to take an unscheduled detour
31:09and finds himself lost.
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32:12I'm always a bit suspicious when I hear these tales.
32:15Look at it from the alien's point of view.
32:18What's the point of crossing vast tracks of the universe
32:21in a high-tech ship
32:24just to abduct some lone earthling?
32:27In my opinion, if aliens are here,
32:30I suspect the newspapers would be full of the story.
32:34And if governments are involved in a cover-up,
32:37they're doing a much better job at it
32:40than they seem to do at anything else.
32:43So the lack of alien contact
32:46raises a serious scientific problem.
32:49Where is everybody?
32:52We've been listening to space for over 40 years.
32:59And in all that time, we've picked up nothing.
33:04Well, except for one mysterious occasion.
33:13On August 16th, 1977,
33:18a radio telescope in Ohio
33:21picked up a signal that became famous.
33:26The telescope listened to space
33:30by scanning the skies as the Earth rotated.
33:34And just once, it recorded a signal that got everyone excited.
33:39The Wow! signal, as it became known.
33:47The signal was a steady source of radio waves,
33:51just the kind an alien race might send,
33:54because it stands out from the radio static that fills the universe.
34:03A computer recorded the signal as six letters and numbers.
34:12Astronomer Jerry Ehrman saw the data
34:15and wrote one word in the margin.
34:25Ehrman and others subsequently searched the same patch of sky many times,
34:31but found nothing.
34:34The Wow! signal had vanished.
34:42The whole mysterious episode reveals
34:45that making contact with aliens via radio
34:48is always going to be difficult.
34:55In such a vast universe,
34:58messages take a long time to reach their destination.
35:03The Wow! signal appeared to come from a star system 200 light-years away,
35:09so it took at least 200 years to reach us.
35:14If we sent a reply, it would take another 200 years to reach them,
35:20by which time they might have forgotten they sent anything
35:24and stopped listening for a reply.
35:28Worse, they might well have destroyed themselves in the meantime.
35:33The human race very quickly discovered the power of the atom bomb.
35:40If the same holds for intelligent aliens,
35:44then they might not last long.
35:50Perhaps they all blow themselves up
35:54soon after they discover that E equals mc squared.
36:09If civilisations take billions of years to evolve,
36:13only to vanish virtually overnight,
36:16then sadly we've next to no chance of hearing from them.
36:21They are simply too far away in space and time to reach.
36:26But there is one last possibility,
36:30that aliens who have avoided destroying themselves
36:34are already colonising the universe.
36:47The human race has only two options
36:51when it comes to looking for advanced aliens.
36:55We can listen, or we can be more active
36:59and broadcast our willingness to talk.
37:06We'd have to think very carefully about what we might say.
37:12I think this might be just a little too risky.
37:23We only have to look at ourselves
37:26to see how intelligent life might develop into something more.
37:31We'd have to look at ourselves
37:35to see how intelligent life might develop into something more.
37:39We humans are already capable
37:42of manipulating the course of our own evolution.
37:46Exactly the same, presumably,
37:49would be true of advanced extraterrestrials.
37:53Ultimately, they could halt ageing
37:56and become virtually immortal.
38:04What's more,
38:06they might have reached that point millions of years ago.
38:16It might sound unlikely,
38:19but if you think about it logically,
38:22alien technology should be as extraordinary to us
38:26as a rocket ship to a caveman.
38:36I imagine they might exist in massive ships like these,
38:40having used up all the resources from the home planet below.
38:45Such advanced aliens would perhaps become nomads,
38:49looking to conquer and colonise whatever planets they can reach.
39:00If so, it makes sense for them to exploit each new planet for materials,
39:05to build more spaceships so they could move on.
39:12Who knows what the limits would be?
39:21Perhaps their capabilities would only be limited
39:25by how much power they could harness and control.
39:29And that could be far more than we might first imagine.
39:36For example,
39:38it might be possible to collect the energy from an entire star.
39:46To do that, they could deploy millions of mirrors in space,
39:51encircling the whole sun
39:53and feeding the power to one single collection point.
40:06MUSIC
40:18Such power might make it possible to warp the very fabric of space
40:24and create a portal called a wormhole.
40:29This portal would act like a shortcut,
40:32allowing them to travel huge distances in the blink of an eye.
40:37MUSIC
41:02Like us, they would probably have evolved from a species
41:06used to exploiting whatever it can.
41:24So if aliens ever visit us,
41:27I think the outcome would be much as
41:29when Christopher Columbus first landed in America.
41:37Which didn't turn out very well for the Native Americans.
41:44So the journey that started with the search for water
41:49has led us to far-off worlds, which I think could exist.
41:54MUSIC
41:57Worlds where the conditions and chemistry allow life to begin
42:04and then change into many different forms.
42:14Even so, little green men are probably pretty rare.
42:20But in such a massive place as the cosmos,
42:24we only have to look at ourselves
42:27for proof that extremely unlikely things can and do happen all the time.
42:33MUSIC
42:50Let's just hope that if aliens do find us,
42:55they'll come in peace.
42:58MUSIC
43:19MUSIC
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