TED科学:Stephen Webb: Where are all the aliens?

I saw a UFO once.I was eight or nine,playing in the street with a friendwho was a couple of years older,and we saw a featureless silver dischovering over the houses.We watched it for a few seconds,and then it shot away incredibly quickly.Even as a kid,I got angry it was ignoringthe laws of physics.We ran inside to tell the grown-ups,and they were skeptical —you'd be skeptical too, right?I got my own back a few years later:one of those grown-ups told me,"Last night I saw a flying saucer.I was coming out of the pubafter a few drinks."I stopped him there.I said, "I can explain that sighting."


Psychologists have shownwe can't trust our brainsto tell the truth.It's easy to fool ourselves.I saw something,but what's more likely —that I saw an alien spacecraft,or that my brain misinterpretedthe data my eyes were giving it?Ever since though I've wondered:Why don't we seeflying saucers flitting around?At the very least,why don't we see lifeout there in the cosmos?It's a puzzle,and I've discussed itwith dozens of expertsfrom different disciplinesover the past three decades.And there's no consensus.Frank Drake began searchingfor alien signals back in 1960 —so far, nothing.And with each passing year,this nonobservation,this lack of evidencefor any alien activity gets more puzzlingbecause we should see them, shouldn't we?

The universe is 13.8 billion years old,give or take.If we represent the ageof the universe by one year,then our species came into beingabout 12 minutes before midnight,31st December.Western civilizationhas existed for a few seconds.Extraterrestrial civilizationscould have started in the summer months.Imagine a summer civilizationdeveloping a level of technologymore advanced than ours,but tech based on accepted physics though,I'm not talking wormholesor warp drives — whatever —just an extrapolationof the sort of tech that TED celebrates.That civilization could programself-replicating probesto visit every planetarysystem in the galaxy.If they launched the first probesjust after midnight one August day,then before breakfast same day,they could have colonized the galaxy.Intergalactic colonizationisn't much more difficult,it just takes longer.A civilization from any oneof millions of galaxiescould have colonized our galaxy.

Seems far-fetched?Maybe it is,but wouldn't aliens engagein some recognizable activity —put worldlets around a starto capture free sunlight,collaborate on a Wikipedia Galactica,or just shout outto the universe, "We're here"?

So where is everybody?It's a puzzle because we do expectthese civilizations to exist, don't we?After all, there could bea trillion planets in the galaxy —maybe more.

You don't need any special knowledgeto consider this question,and I've explored itwith lots of people over the years.And I've found they oftenframe their thinkingin terms of the barriersthat would need to be clearedif a planet is to hosta communicative civilization.And they usually identifyfour key barriers.

Habitability —that's the first barrier.We need a terrestrial planetin that just right "Goldilocks zone,"where water flows as a liquid.They're out there.In 2016, astronomers confirmedthere's a planet in the habitable zoneof the closest star,Proxima Centauri —so close that Breakthrough Starshotproject plans to send probes there.We'd become a starfaring species.But not all worlds are habitable.Some will be too close to a starand they'll fry,some will be too far awayand they'll freeze.

Abiogenesis —the creation of life from nonlife —that's the second barrier.The basic building blocks of lifearen't unique to Earth:amino acids have been found in comets,complex organic moleculesin interstellar dust clouds,water in exoplanetary systems.The ingredients are there,we just don't knowhow they combine to create life,and presumably there will be worldson which life doesn't start.

The development of technologicalcivilization is a third barrier.Some say we already share our planetwith alien intelligences.A 2011 study showed that elephantscan cooperate to solve problems.A 2010 study showedthat an octopus in captivitycan recognize different humans.2017 studies show that ravenscan plan for future events —wonderful, clever creatures —but they can't contemplatethe Breakthrough Starshot project,and if we vanished today,they wouldn't go onto implement Breakthrough Starshot —why should they?Evolution doesn't havespace travel as an end goal.There will be worlds where lifedoesn't give rise to advanced technology.

Communication across space —that's a fourth barrier.Maybe advanced civilizationschoose to explore inner spacerather than outer space,or engineer at small distancesrather than large.Or maybe they just don't wantto risk an encounterwith a potentially more advancedand hostile neighbor.There'll be worlds where,for whatever reason,civilizations either stay silentor don't spend long trying to communicate.

As for the height of the barriers,your guess is as good as anyone's.In my experience,when people sit down and do the math,they typically conclude there arethousands of civilizations in the galaxy.But then we're back to the puzzle:Where is everybody?By definition,UFOs — including the one I saw —are unidentified.We can't simply infer they're spacecraft.You can still have some funplaying with the idea aliens are here.Some say a summer civilizationdid colonize the galaxyand seeded Earth with life ...others, that we're livingin a cosmic wilderness preserve —a zoo.Yet others —that we're living in a simulation.Programmers just haven'trevealed the aliens yet.Most of my colleagues thoughargue that E.T. is out there,we just need to keep looking,and this makes sense.Space is vast.Identifying a signal is hard,and we haven't been looking that long.Without doubt, we shouldspend more on the search.It's about understandingour place in the universe.It's too important a question to ignore.

But there's an obvious answer:we're alone.It's just us.There could be a trillionplanets in the galaxy.Is it plausible we're the only creaturescapable of contemplating this question?Well, yes, because in this context,we don't know whethera trillion is a big number.In 2000, Peter Ward and Don Brownleeproposed the Rare Earth idea.Remember those four barriersthat people use to estimatethe number of civilizations?Ward and Brownlee saidthere might be more.

Let's look at one possible barrier.It's a recent suggestion by David Waltham,a geophysicist.This is my very simplified versionof Dave's much moresophisticated argument.We are able to be here nowbecause Earth's previousinhabitants enjoyedfour billion years of good weather —ups and downs but more or less clement.But long-term climatestability is strange,if only because astronomical influencescan push a planettowards freezing or frying.There's a hint our moon has helped,and that's interestingbecause the prevailing theory isthat the moon came into being when Theia,a body the size of Mars,crashed into a newly formed Earth.The outcome of that crash could have beena quite different Earth-Moon system.We ended up with a large moonand that permitted Earthto have both a stable axial tiltand a slow rotation rate.Both factors influence climateand the suggestion is that they've helpedmoderate climate change.Great for us, right?But Waltham showed that if the moonwere just a few miles bigger,things would be different.Earth's spin axiswould now wander chaotically.There'd be episodesof rapid climate change —not good for complex life.The moon is just the right size:big but not too big.A "Goldilocks" moon arounda "Goldilocks" planet —a barrier perhaps.

You can imagine more barriers.For instance,simple cells came into beingbillions of years ago ...but perhaps the developmentof complex lifeneeded a series of unlikely events.Once life on Earthhad access to multicellularityand sophisticated genetic structures,and sex,new opportunities opened up:animals became possible.But maybe it's the fate of many planetsfor life to settleat the level of simple cells.

Purely for the purposes of illustration,let me suggest four more barriersto add to the fourthat people said blocked the pathto communicative civilization.Again, purely for the purposesof illustration,suppose there's a one-in-a-thousand chanceof making it across each of the barriers.Of course there might bedifferent ways of navigating the barriers,and some chances will be betterthan one in a thousand.Equally, there might be more barriersand some chancesmight be one in a million.Let's just seewhat happens in this picture.

If the galaxy contains a trillion planets,how many will host a civilizationcapable of contemplating like usprojects such as Breakthrough Starshot?Habitability —right sort of planetaround the right sort of star —the trillion becomes a billion.Stability —a climate that stays benign for eons —the billion becomes a million.Life must start —the million becomes a thousand.Complex life forms must arise —the thousand becomes one.Sophisticated tool use must develop —that's one planet in a thousand galaxies.To understand the universe,they'll have to develop the techniquesof science and mathematics —that's one planet in a million galaxies.To reach the stars,they'll have to be social creatures,capable of discussingabstract concepts with each otherusing complex grammar —one planet in a billion galaxies.And they have to avoid disaster —not just self-inflictedbut from the skies, too.That planet around Proxima Centauri,last year it got blasted by a flare.One planet in a trillion galaxies,just as in the visible universe.

I think we're alone.Those colleagues of minewho agree we're aloneoften see a barrier ahead —bioterror,global warming, war.A universe that's silentbecause technology itselfforms the barrierto the developmentof a truly advanced civilization.Depressing, right?

I'm arguing the exact opposite.I grew up watching "Star Trek"and "Forbidden Planet,"and I saw a UFO once,so this idea of cosmic lonelinessI certainly find slightly wistful.But for me,the silence of the universe is shouting,"We're the creatures who got lucky."All barriers are behind us.We're the only speciesthat's cleared them —the only species capableof determining its own destiny.And if we learn to appreciatehow special our planet is,how important it is to look after our homeand to find others,how incredibly fortunate we all aresimply to be aware of the universe,humanity might survive for a while.And all those amazing thingswe dreamed aliensmight have done in the past,that could be our future.

Thank you very much.


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