I saw UFO once I was eight or nine
playing in the street with a friend
who's 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 run
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 I
smile I saw a flying saucer I was coming
up with the pub after a few drinks
I stopped in there so I can explain that
sighting 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 that 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 here this
non observation 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 civilisation 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
into the white thick 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
what an alien's engages some
recognizable activity put worldlets
around a star to capture free sunlight
collaborate on a wikipedia Galactica or
just chat down 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 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
light
Goldilocks zone where water flows is 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 the
breakthrough starshot project plans to
send approached there we'd become a star
ferrying species but not all worlds a
habitable some will be too close to a
star no fry some will be too far away
and they'll freeze abiogenesis the
creation of life from non-life that's
Mariya the basic building blocks of life
aren't unique to earth amino acids have
been found in comments 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 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 the 2010
study showed that an octopus in
captivity can recognize different humans
2017 study showed that Ravens can plan
for future events wonderful clever
creatures but they can't contemplate a
breakthrough starshot project and if we
vanished today they wouldn't go on to
implement breakthrough starshot why
should they evolution she doesn't have
space travel as an end goal they'll 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
engineering small distances rather than
large or maybe they just don't want to
risk an encounter with a potentially
more advanced and hostile neighbor
they'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 unidentified we can't simply infer
their spacecraft you can't still have
some fun playing with the idea aliens
are here
some say some a 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
program is just haven't really all the
aliens yet most of my colleagues though
argue that et 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
Wadden grandly said there might be more
let's look at one possible barrier it's
a recent suggest you that David Walton 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 4 billion years of
good weather obviously downs but more or
less climate 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 and fááááá 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 fellers
right but one of them showed that if the
moon were just a few miles bigger things
would be different
Earth's spin axis would now wander
chaotically they'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 the barrier perhaps
you can imagine more barriers for
instance simple cells came into being
millions 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 for the
people said blocks the path to
communicative civilization again purely
for the purposes of illustration
supposes 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 Mica's the
project 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
a complex grammar one planet in a
billion galaxies and they have to avoid
disaster
not just self-inflicted but from the
skies to that planet around Proxima
Centauri last year it got blasted by a
flare one planet in a trillion galaxies
just does in the visible universe I
think we're alone those colleagues of
mine who agree were 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 where the
creatures who got lucky or 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 are aren't
simply to be aware of the universe if
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
you