00:00:13
I saw a UFO once.
00:00:15
I was eight or nine,
00:00:17
playing in the street with a friend
who was a couple of years older,
00:00:20
and we saw a featureless silver disc
hovering over the houses.
00:00:25
We watched it for a few seconds,
00:00:27
and then it shot away incredibly quickly.
00:00:30
Even as a kid,
00:00:31
I got angry it was ignoring
the laws of physics.
00:00:35
We ran inside to tell the grown-ups,
00:00:37
and they were skeptical --
00:00:39
you'd be skeptical too, right?
00:00:42
I got my own back a few years later:
00:00:43
one of those grown-ups told me,
00:00:45
"Last night I saw a flying saucer.
00:00:47
I was coming out of the pub
after a few drinks."
00:00:49
I stopped him there.
I said, "I can explain that sighting."
00:00:52
(Laughter)
00:00:53
Psychologists have shown
we can't trust our brains
00:00:56
to tell the truth.
00:00:57
It's easy to fool ourselves.
00:00:59
I saw something,
00:01:01
but what's more likely --
00:01:02
that I saw an alien spacecraft,
00:01:04
or that my brain misinterpreted
the data my eyes were giving it?
00:01:10
Ever since though I've wondered:
00:01:11
Why don't we see
flying saucers flitting around?
00:01:14
At the very least,
00:01:16
why don't we see life
out there in the cosmos?
00:01:18
It's a puzzle,
00:01:20
and I've discussed it
with dozens of experts
00:01:22
from different disciplines
over the past three decades.
00:01:25
And there's no consensus.
00:01:27
Frank Drake began searching
for alien signals back in 1960 --
00:01:32
so far, nothing.
00:01:34
And with each passing year,
00:01:35
this nonobservation,
00:01:37
this lack of evidence
for any alien activity gets more puzzling
00:01:42
because we should see them, shouldn't we?
00:01:47
The universe is 13.8 billion years old,
00:01:50
give or take.
00:01:52
If we represent the age
of the universe by one year,
00:01:55
then our species came into being
about 12 minutes before midnight,
00:02:00
31st December.
00:02:02
Western civilization
has existed for a few seconds.
00:02:05
Extraterrestrial civilizations
could have started in the summer months.
00:02:10
Imagine a summer civilization
00:02:13
developing a level of technology
more advanced than ours,
00:02:17
but tech based on accepted physics though,
00:02:19
I'm not talking wormholes
or warp drives -- whatever --
00:02:23
just an extrapolation
of the sort of tech that TED celebrates.
00:02:28
That civilization could program
self-replicating probes
00:02:32
to visit every planetary
system in the galaxy.
00:02:35
If they launched the first probes
just after midnight one August day,
00:02:41
then before breakfast same day,
00:02:43
they could have colonized the galaxy.
00:02:46
Intergalactic colonization
isn't much more difficult,
00:02:49
it just takes longer.
00:02:51
A civilization from any one
of millions of galaxies
00:02:54
could have colonized our galaxy.
00:02:56
Seems far-fetched?
00:02:58
Maybe it is,
00:02:59
but wouldn't aliens engage
in some recognizable activity --
00:03:05
put worldlets around a star
to capture free sunlight,
00:03:09
collaborate on a Wikipedia Galactica,
00:03:12
or just shout out
to the universe, "We're here"?
00:03:16
So where is everybody?
00:03:18
It's a puzzle because we do expect
these civilizations to exist, don't we?
00:03:24
After all, there could be
a trillion planets in the galaxy --
00:03:27
maybe more.
00:03:29
You don't need any special knowledge
to consider this question,
00:03:34
and I've explored it
with lots of people over the years.
00:03:37
And I've found they often
frame their thinking
00:03:40
in terms of the barriers
that would need to be cleared
00:03:44
if a planet is to host
a communicative civilization.
00:03:49
And they usually identify
four key barriers.
00:03:52
Habitability --
00:03:54
that's the first barrier.
00:03:55
We need a terrestrial planet
in that just right "Goldilocks zone,"
00:04:00
where water flows as a liquid.
00:04:03
They're out there.
00:04:04
In 2016, astronomers confirmed
there's a planet in the habitable zone
00:04:09
of the closest star,
00:04:10
Proxima Centauri --
00:04:12
so close that Breakthrough Starshot
project plans to send probes there.
00:04:17
We'd become a starfaring species.
00:04:21
But not all worlds are habitable.
00:04:23
Some will be too close to a star
and they'll fry,
00:04:25
some will be too far away
and they'll freeze.
00:04:28
Abiogenesis --
00:04:30
the creation of life from nonlife --
00:04:31
that's the second barrier.
00:04:34
The basic building blocks of life
aren't unique to Earth:
00:04:37
amino acids have been found in comets,
00:04:40
complex organic molecules
in interstellar dust clouds,
00:04:43
water in exoplanetary systems.
00:04:47
The ingredients are there,
00:04:48
we just don't know
how they combine to create life,
00:04:51
and presumably there will be worlds
on which life doesn't start.
00:04:55
The development of technological
civilization is a third barrier.
00:05:00
Some say we already share our planet
with alien intelligences.
00:05:05
A 2011 study showed that elephants
can cooperate to solve problems.
00:05:10
A 2010 study showed
00:05:12
that an octopus in captivity
can recognize different humans.
00:05:17
2017 studies show that ravens
can plan for future events --
00:05:21
wonderful, clever creatures --
00:05:24
but they can't contemplate
the Breakthrough Starshot project,
00:05:27
and if we vanished today,
00:05:29
they wouldn't go on
to implement Breakthrough Starshot --
00:05:32
why should they?
00:05:33
Evolution doesn't have
space travel as an end goal.
00:05:37
There will be worlds where life
doesn't give rise to advanced technology.
00:05:43
Communication across space --
that's a fourth barrier.
00:05:45
Maybe advanced civilizations
choose to explore inner space
00:05:49
rather than outer space,
00:05:52
or engineer at small distances
rather than large.
00:05:56
Or maybe they just don't want
to risk an encounter
00:05:59
with a potentially more advanced
and hostile neighbor.
00:06:03
There'll be worlds where,
for whatever reason,
00:06:05
civilizations either stay silent
or don't spend long trying to communicate.
00:06:12
As for the height of the barriers,
00:06:14
your guess is as good as anyone's.
00:06:18
In my experience,
00:06:19
when people sit down and do the math,
00:06:21
they typically conclude there are
thousands of civilizations in the galaxy.
00:06:27
But then we're back to the puzzle:
Where is everybody?
00:06:32
By definition,
00:06:33
UFOs -- including the one I saw --
00:06:35
are unidentified.
00:06:37
We can't simply infer they're spacecraft.
00:06:40
You can still have some fun
playing with the idea aliens are here.
00:06:45
Some say a summer civilization
did colonize the galaxy
00:06:48
and seeded Earth with life ...
00:06:51
others, that we're living
in a cosmic wilderness preserve --
00:06:54
a zoo.
00:06:56
Yet others --
00:06:58
that we're living in a simulation.
00:07:00
Programmers just haven't
revealed the aliens yet.
00:07:03
Most of my colleagues though
argue that E.T. is out there,
00:07:08
we just need to keep looking,
00:07:09
and this makes sense.
00:07:11
Space is vast.
00:07:13
Identifying a signal is hard,
00:07:15
and we haven't been looking that long.
00:07:19
Without doubt, we should
spend more on the search.
00:07:22
It's about understanding
our place in the universe.
00:07:25
It's too important a question to ignore.
00:07:29
But there's an obvious answer:
00:07:31
we're alone.
00:07:33
It's just us.
00:07:34
There could be a trillion
planets in the galaxy.
00:07:38
Is it plausible we're the only creatures
capable of contemplating this question?
00:07:43
Well, yes, because in this context,
00:07:46
we don't know whether
a trillion is a big number.
00:07:50
In 2000, Peter Ward and Don Brownlee
proposed the Rare Earth idea.
00:07:55
Remember those four barriers
00:07:57
that people use to estimate
the number of civilizations?
00:08:01
Ward and Brownlee said
there might be more.
00:08:03
Let's look at one possible barrier.
00:08:05
It's a recent suggestion by David Waltham,
00:08:08
a geophysicist.
00:08:10
This is my very simplified version
00:08:12
of Dave's much more
sophisticated argument.
00:08:16
We are able to be here now
00:08:18
because Earth's previous
inhabitants enjoyed
00:08:20
four billion years of good weather --
00:08:23
ups and downs but more or less clement.
00:08:26
But long-term climate
stability is strange,
00:08:29
if only because astronomical influences
00:08:31
can push a planet
towards freezing or frying.
00:08:36
There's a hint our moon has helped,
00:08:38
and that's interesting
00:08:40
because the prevailing theory is
00:08:42
that the moon came into being when Theia,
00:08:44
a body the size of Mars,
00:08:46
crashed into a newly formed Earth.
00:08:49
The outcome of that crash could have been
a quite different Earth-Moon system.
00:08:54
We ended up with a large moon
00:08:56
and that permitted Earth
to have both a stable axial tilt
00:09:00
and a slow rotation rate.
00:09:03
Both factors influence climate
00:09:05
and the suggestion is that they've helped
moderate climate change.
00:09:09
Great for us, right?
00:09:11
But Waltham showed that if the moon
were just a few miles bigger,
00:09:15
things would be different.
00:09:16
Earth's spin axis
would now wander chaotically.
00:09:20
There'd be episodes
of rapid climate change --
00:09:23
not good for complex life.
00:09:25
The moon is just the right size:
00:09:27
big but not too big.
00:09:30
A "Goldilocks" moon around
a "Goldilocks" planet --
00:09:33
a barrier perhaps.
00:09:35
You can imagine more barriers.
00:09:37
For instance,
00:09:38
simple cells came into being
billions of years ago ...
00:09:42
but perhaps the development
of complex life
00:09:45
needed a series of unlikely events.
00:09:48
Once life on Earth
had access to multicellularity
00:09:51
and sophisticated genetic structures,
00:09:53
and sex,
00:09:54
new opportunities opened up:
00:09:56
animals became possible.
00:09:58
But maybe it's the fate of many planets
00:10:01
for life to settle
at the level of simple cells.
00:10:06
Purely for the purposes of illustration,
00:10:09
let me suggest four more barriers
to add to the four
00:10:12
that people said blocked the path
to communicative civilization.
00:10:17
Again, purely for the purposes
of illustration,
00:10:21
suppose there's a one-in-a-thousand chance
of making it across each of the barriers.
00:10:25
Of course there might be
different ways of navigating the barriers,
00:10:28
and some chances will be better
than one in a thousand.
00:10:31
Equally, there might be more barriers
00:10:33
and some chances
might be one in a million.
00:10:35
Let's just see
what happens in this picture.
00:10:39
If the galaxy contains a trillion planets,
00:10:41
how many will host a civilization
capable of contemplating like us
00:10:47
projects such as Breakthrough Starshot?
00:10:50
Habitability --
00:10:51
right sort of planet
around the right sort of star --
00:10:54
the trillion becomes a billion.
00:10:56
Stability --
00:10:58
a climate that stays benign for eons --
00:11:01
the billion becomes a million.
00:11:03
Life must start --
00:11:05
the million becomes a thousand.
00:11:07
Complex life forms must arise --
00:11:10
the thousand becomes one.
00:11:12
Sophisticated tool use must develop --
00:11:14
that's one planet in a thousand galaxies.
00:11:17
To understand the universe,
00:11:19
they'll have to develop the techniques
of science and mathematics --
00:11:22
that's one planet in a million galaxies.
00:11:24
To reach the stars,
they'll have to be social creatures,
00:11:27
capable of discussing
abstract concepts with each other
00:11:29
using complex grammar --
00:11:31
one planet in a billion galaxies.
00:11:35
And they have to avoid disaster --
00:11:36
not just self-inflicted
but from the skies, too.
00:11:40
That planet around Proxima Centauri,
00:11:43
last year it got blasted by a flare.
00:11:47
One planet in a trillion galaxies,
00:11:49
just as in the visible universe.
00:11:54
I think we're alone.
00:11:57
Those colleagues of mine
who agree we're alone
00:11:59
often see a barrier ahead --
00:12:02
bioterror,
00:12:04
global warming, war.
00:12:06
A universe that's silent
00:12:09
because technology itself
forms the barrier
00:12:12
to the development
of a truly advanced civilization.
00:12:16
Depressing, right?
00:12:19
I'm arguing the exact opposite.
00:12:22
I grew up watching "Star Trek"
and "Forbidden Planet,"
00:12:25
and I saw a UFO once,
00:12:26
so this idea of cosmic loneliness
I certainly find slightly wistful.
00:12:34
But for me,
00:12:36
the silence of the universe is shouting,
00:12:38
"We're the creatures who got lucky."
00:12:40
All barriers are behind us.
00:12:42
We're the only species
that's cleared them --
00:12:45
the only species capable
of determining its own destiny.
00:12:49
And if we learn to appreciate
how special our planet is,
00:12:53
how important it is to look after our home
00:12:56
and to find others,
00:12:57
how incredibly fortunate we all are
simply to be aware of the universe,
00:13:02
humanity might survive for a while.
00:13:05
And all those amazing things
00:13:06
we dreamed aliens
might have done in the past,
00:13:09
that could be our future.
00:13:11
Thank you very much.
00:13:12
(Applause)
