Where are all the aliens? | Stephen Webb


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. And if we learn to appreciate
how special our planet is, how important it is to look after our home and to find others, how incredibly fortunate we all are
simply to be aware of the universe, humanity might survive for a while. And all those amazing things we dreamed aliens
might have done in the past, that could be our future. Thank you very much. (Applause)

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