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A lot of people, including many scientists,
seem pretty confident that aliens have to exist somewhere
in the universe.
Is that just wishful thinking or can
you actually calculate whether intelligent aliens exist?
UFO sightings and Area 51 rumors aside,
we have never seen an alien, never heard an alien,
or had any kind of verifiable indication
that they're out there.
So how do you estimate how many intelligent aliens you should
see when you've never actually seen any?
With probability statistics, we just
need to get a handle on the factors
at play in the emergence of intelligent life as we know it.
So where do we start?
Well, the Milky Way galaxy has about 200 billion stars.
Based on recent analyses of exoplanet data,
we can now also estimate, without totally guessing,
how common planets are that might support life.
It looks like it's around one habitable planet per star,
on average.
So 200 billion stars, 200 billion habitable planets.
Now, for the unknown.
We need to know what percentage of the habitable planets
would actually sprout life and then
what percentage of those life-sprouting planets
would evolve intelligent species?
Multiply these two probabilities by the 200 billion planets
and, voila, you know how many Wookies, or Covenant,
or Vulcans you should expect.
What I've just described is similar in spirit
to what's called the Drake equation.
The details are different, since Drake, the founder of SETI,
was estimating the number of detectable alien radio signals.
But the overarching logic is the same.
Anyway, if we get the odds of intelligent life, we're golden.
But here's the problem.
Whenever anyone, even a scientist,
quotes a number for the "probability" of life arising,
it's pretty much a complete guess.
How come?
Because the only place we've ever
seen so much as an amoeba is Earth.
We have no clue how frequently life sprouts, much less
intelligent life, because we have a whopping sample
size of one.
So is this game over?
Is it impossible to take a rational statistical scientific
position on whether or not aliens exist?
Not exactly.
There are sensible arguments both for
and against the existence of aliens.
They rely on other numbers that we can calculate,
mixed in with a bit of logic.
None of those arguments is decisive.
So I'll present you with simplified versions
of some of the most famous ones and let you be the judge.
Let's start with a pro aliens argument,
that intelligent life should exist.
The argument says, look, 200 billion inhabitable planets,
that's a huge number.
So huge that it should compensate even
for crazy low odds of intelligent life
arising on any one of them.
The late, great Carl Sagan was a big proponent and popularizer
of this viewpoint.
Here's an analogy.
Suppose the odds of sprouting Daleks
was as low as the odds of winning the Powerball
jackpot with a single ticket.
That's about 1 in 175 million.
You would still expect over 1,000 alien civilizations
in our galaxy by now.
If the chances of intelligent life evolving
are even lower, well, 200 billion
is just the number of habitable planets in our galaxy.
If we sweep nearby galaxies into our planet account,
we can add trillions more planets to compensate.
And that is Sagan's point.
If the odds of intelligent life were so insanely low
that you couldn't compensate with billions or trillions
of habitable worlds, it would start
to look like a cosmic conspiracy,
like Earth and humanity are absurdly special.
And scientists hate special, any kind of special.
Our whole model of the universe, dating back to Copernicus,
is rooted in the democratic notion
that our planet, our Sun, our galaxy, none of them
are special.
The Sagan view then is not that alien life is guaranteed.
It's that the alternative is so contrary to a closely
held scientific principle, that it's
quite difficult to swallow.
Pretty persuasive?
Well, as it turns out, there's a way
to turn this argument entirely on its head.
One prominent no aliens argument begins by pointing out
that our galaxy is not just very big, but also very old, about
10 billion years old.
So say you grant Sagan logic.
Taken to its conclusion, there should
have been not only enough places for intelligent life
to arise, but also enough time for at some of that life
to spread around the galaxy.
That a species would spread out isn't some crazy assumption
on our part.
Remember, our species, which is only about 200,000 years old,
has already sent the Pioneer and Voyager probes out
of the solar system.
In fact, with current propulsion technologies
and no fancy sci-fi stuff, we could send probes or robots
across the whole galaxy.
Yeah, it would take us 20 million years or so.
But so what?
The galaxy is 10 billion years old.
That's the blink of an eye.
Our more advanced future selves might need even less time
to do this.
And we are just one species on one planet.
So given how old the galaxy is, shouldn't there
be evidence of at least one other such species that's
jetted around the cosmos, something like radio signals,
space station probes, a broken down Millenium Falcon,
I don't know, an alien "drink Pepsi" sign, something?
But we've seen squat.
We've see exactly zero evidence of current intelligent aliens
or residual evidence s extinct ones.
Now if our ability to send stuff into space isn't special,
and alien intelligence really is inevitable on the billions
and billions of habitable planets, then where the aliens?
Or least where's there's stuff?
This argument was put forth by the physicist Enrico Fermi,
among others.
And it now bears his name, the Fermi paradox.
The punchline is that precisely because we don't see aliens,
the odds of intelligent life must be, wait
for it, astronomically low.
Now you can, of course, explain the Fermi paradox away.
But it's never totally satisfying.
Maybe aliens hate exploring space, OK.
But all of them?
It only takes one, one Klingon Richard Branson
to hop around the galaxy and leave some kind of a trace.
Maybe we're not looking in the right places?
Sure.
But remember the numbers here.
The galaxy is 10 billion years old
and humanity, right now, would only need 20 million years
to spread all over.
And we're not special, right Saganites?
So someone should have left something, even
in our little corner of the galaxy, huh?
So it seems like choosing between the Sagan and Fermi
camps comes down to deciding which of two unlikely things
you think is less unlikely.
That intelligent alien life never
evolves on the billions of possible planets
or that intelligent aliens evolve,
none of whom ever spread out in any observable way
in the 10-billion-year history of the galaxy.
There is an interesting third option, a way for them
both to be right, without having to invoke weird coincidences.
But it's a little depressing.
It's an argument that also has many proponents.
But it's been articulated especially nicely
by Oxford University philosopher Nick Bostrom.
Here's the idea.
Maybe the odds of intelligent life arising are pretty decent.
But the odds of intelligent life going extinct
before it can spread into the galaxy are also high.
There very well might be an evolutionary "great filter"
that works against intelligent species.
It can be something natural, like a virus or Godzilla,
or species made, like a nuclear holocaust
or out-of-control nanobots.
But something that would tend to wipe any intelligent species
off the cosmic map, including, unfortunately, us.
But hey, extinction, expinction, right?
At least we can sleep better knowing
that it's possible to reconcile Sagan's billions and billions
argument with the fact that Chewbacca wasn't
at your office holiday party.
Aaaaaaarrrgh!
Anyway, now you know how legit scientists and scholars handle
the question of space aliens.
So what do you think?
Give us your two cents in the comments.
I'd like to hear which side of the fence you find yourself on.
Alien abduction story, also welcomed, with proof.
I'll report any interesting findings
on the next episode of "Space Time."
Last week we asked, what planet is Super Mario World?
Let's see what you guys had to say.
D. Moritz found that Sonica Hedgehog
lives on a planet with about 5.6 Earth gees, Closer to a planet.
Good work.
Gaetan commented, Science!
That's correct.
The Gentleman Physicist points out
that Super Mario World could be a platform accelerating
through space with rockets at 70 meters per second squared.
And it could be.
But think about how much fuel it would
take to keep that acceleration going.
Nicholas Feeley suggests that maybe Mario
is on a black dwarf.
Cool idea, but you're off by about 100,000 gees.
And James Morgan, you keep regulating.
Hey, thanks to everybody who watched and blogged
about the debut episode.
Remember, you can also follow us on Twitter and Facebook.
We'll see you guys next week.
And hey, subscribe already.
Don't make me send this guy after your cat.
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