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  • I saw a UFO once.

  • I was eight or nine,

  • playing in the street with a friend who was a couple of years older,

  • and we saw a featureless silver disc hovering 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 ignoring the 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 pub after a few drinks."

  • I stopped him there. I said, "I can explain that sighting."

  • (Laughter)

  • Psychologists have shown we can't trust our brains

  • to 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 misinterpreted the data my eyes were giving it?

  • Ever since though I've wondered:

  • Why don't we see flying saucers flitting around?

  • At the very least,

  • why don't we see life out there in the cosmos?

  • It's a puzzle,

  • and I've discussed it with dozens of experts

  • from different disciplines over the past three decades.

  • And there's no consensus.

  • Frank Drake began searching for alien signals back in 1960 --

  • so far, nothing.

  • And with each passing year,

  • this nonobservation,

  • this lack of evidence for any alien activity gets more puzzling

  • because we should see them, shouldn't we?

  • The universe is 13.8 billion years old,

  • give or take.

  • If we represent the age of the universe by one year,

  • then our species came into being about 12 minutes before midnight,

  • 31st December.

  • Western civilization has existed for a few seconds.

  • Extraterrestrial civilizations could have started in the summer months.

  • Imagine a summer civilization

  • developing a level of technology more advanced than ours,

  • but tech based on accepted physics though,

  • I'm not talking wormholes or warp drives -- whatever --

  • just an extrapolation of the sort of tech that TED celebrates.

  • That civilization could program self-replicating probes

  • to visit every planetary system in the galaxy.

  • If they launched the first probes just after midnight one August day,

  • then before breakfast same day,

  • they could have colonized the galaxy.

  • Intergalactic colonization isn't much more difficult,

  • it just takes longer.

  • A civilization from any one of millions of galaxies

  • could have colonized our galaxy.

  • Seems far-fetched?

  • Maybe it is,

  • but wouldn't aliens engage in some recognizable activity --

  • put worldlets around a star to capture free sunlight,

  • collaborate on a Wikipedia Galactica,

  • or just shout out to the universe, "We're here"?

  • So where is everybody?

  • It's a puzzle because we do expect these civilizations to exist, don't we?

  • After all, there could be a trillion planets in the galaxy --

  • maybe more.

  • You don't need any special knowledge to consider this question,

  • and I've explored it with lots of people over the years.

  • And I've found they often frame their thinking

  • in terms of the barriers that would need to be cleared

  • if a planet is to host a communicative civilization.

  • And they usually identify four key barriers.

  • Habitability --

  • that's the first barrier.

  • We need a terrestrial planet in that just right "Goldilocks zone,"

  • where water flows as a liquid.

  • They're out there.

  • In 2016, astronomers confirmed there's a planet in the habitable zone

  • of the closest star,

  • Proxima Centauri --

  • so close that Breakthrough Starshot project plans to send probes there.

  • We'd become a starfaring species.

  • But not all worlds are habitable.

  • Some will be too close to a star and they'll fry,

  • some will be too far away and they'll freeze.

  • Abiogenesis --

  • the creation of life from nonlife --

  • that's the second barrier.

  • The basic building blocks of life aren't unique to Earth:

  • amino acids have been found in comets,

  • complex organic molecules in interstellar dust clouds,

  • water in exoplanetary systems.

  • The ingredients are there,

  • we just don't know how they combine to create life,

  • and presumably there will be worlds on which life doesn't start.

  • The development of technological civilization is a third barrier.

  • Some say we already share our planet with alien intelligences.

  • A 2011 study showed that elephants can cooperate to solve problems.

  • A 2010 study showed

  • that an octopus in captivity can recognize different humans.

  • 2017 studies show that ravens can plan for future events --

  • wonderful, clever creatures --

  • but they can't contemplate the Breakthrough Starshot project,

  • and if we vanished today,

  • they wouldn't go on to implement Breakthrough Starshot --

  • why should they?

  • Evolution doesn't have space travel as an end goal.

  • There will be worlds where life doesn't give rise to advanced technology.

  • Communication across space -- that's a fourth barrier.

  • Maybe advanced civilizations choose to explore inner space

  • rather than outer space,

  • or engineer at small distances rather than large.

  • Or maybe they just don't want to risk an encounter

  • with a potentially more advanced and hostile neighbor.

  • There'll be worlds where, for whatever reason,

  • civilizations either stay silent or 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 are thousands 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 fun playing with the idea aliens are here.

  • Some say a summer civilization did colonize the galaxy

  • and seeded Earth with life ...

  • others, that we're living in a cosmic wilderness preserve --

  • a zoo.

  • Yet others --

  • that we're living in a simulation.

  • Programmers just haven't revealed the aliens yet.

  • Most of my colleagues though argue 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 should spend more on the search.

  • It's about understanding our 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 trillion planets in the galaxy.

  • Is it plausible we're the only creatures capable of contemplating this question?

  • Well, yes, because in this context,

  • we don't know whether a trillion is a big number.

  • In 2000, Peter Ward and Don Brownlee proposed the Rare Earth idea.

  • Remember those four barriers

  • that people use to estimate the number of civilizations?

  • Ward and Brownlee said there 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 version

  • of Dave's much more sophisticated argument.

  • We are able to be here now

  • because Earth's previous inhabitants enjoyed

  • four billion years of good weather --

  • ups and downs but more or less clement.

  • But long-term climate stability is strange,

  • if only because astronomical influences

  • can push a planet towards freezing or frying.

  • There's a hint our moon has helped,

  • and that's interesting

  • because the prevailing theory is

  • that 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 been a quite different Earth-Moon system.

  • We ended up with a large moon

  • and that permitted Earth to have both a stable axial tilt

  • and a slow rotation rate.

  • Both factors influence climate

  • and the suggestion is that they've helped moderate climate change.

  • Great for us, right?

  • But Waltham showed that if the moon were just a few miles bigger,

  • things would be different.

  • Earth's spin axis would now wander chaotically.

  • There'd be episodes of rapid climate change --

  • not good for complex life.

  • The moon is just the right size:

  • big but not too big.

  • A "Goldilocks" moon around a "Goldilocks" planet --

  • a barrier perhaps.

  • You can imagine more barriers.

  • For instance,

  • simple cells came into being billions of years ago ...

  • but perhaps the development of complex life

  • needed a series of unlikely events.

  • Once life on Earth had access to multicellularity

  • and sophisticated genetic structures,

  • and sex,

  • new opportunities opened up:

  • animals became possible.

  • But maybe it's the fate of many planets

  • for life to settle at the level of simple cells.

  • Purely for the purposes of illustration,

  • let me suggest four more barriers to add to the four

  • that people said blocked the path to communicative civilization.

  • Again, purely for the purposes of illustration,

  • suppose there's a one-in-a-thousand chance of making it across each of the barriers.

  • Of course there might be different ways of navigating the barriers,

  • and some chances will be better than one in a thousand.

  • Equally, there might be more barriers

  • and some chances might be one in a million.

  • Let's just see what happens in this picture.

  • If the galaxy contains a trillion planets,

  • how many will host a civilization capable of contemplating like us

  • projects such as Breakthrough Starshot?

  • Habitability --

  • right sort of planet around 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 techniques of science and mathematics --

  • that's one planet in a million galaxies.

  • To reach the stars, they'll have to be social creatures,

  • capable of discussing abstract concepts with each other

  • using complex grammar --

  • one planet in a billion galaxies.

  • And they have to avoid disaster --

  • not just self-inflicted but 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 mine who agree we're alone

  • often see a barrier ahead --

  • bioterror,

  • global warming, war.

  • A universe that's silent

  • because technology itself forms the barrier

  • to the development of 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 loneliness I 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 species that's cleared them --

  • the only species capable of determining its own destiny.