Are we alone in the universe?
It's a question we've been asking ever
since we figured out that there may be
other worlds besides this one.
The famous Drake Equation tries to estimate
the number of technological civilizations currently
existing in our galaxy.
It gives that number as a combination
of astrophysical, biological, and sociological factors,
each of which narrows the range of stars in our galaxy
that may have produced a surviving civilization.
These factors include the rate of star formation in the Milky
Way, the number of planets per star
that could support life, the fraction of life bearing
planets that produce intelligent civilizations,
and of those, the fraction that develop technology
that we might detect.
And finally, you need to factor in the average lifetime
of the typical advanced civilization.
When Frank Drake formulated his equation in the early '60s,
most of the factors going into it
were hopelessly unconstrained.
Drake's original calculations gave a range
from 20 and 50 million current technological civilizations
in the Milky Way, although he felt there
were probably at least 1,000.
Still, that is a huge range.
At the low end, we're unlikely to ever see one,
assuming they don't come to us.
And at the high end, there should
be civilisations within 100 light years, which
may have detected our own radio transmissions by now.
In the last 50 years, we've learned a lot more
about some of Drake's factors.
There's some progress with the biological factors,
as we push the fossil record back closer and closer
to the formation of the Earth and we see just how quickly
life arose here.
We also have a slightly better understanding
of abiogenesis, the initial evolution of life
and perhaps even the subsequent evolution of intelligence,
although these are still a long way off giving us hard numbers.
The sociological factors, and in particular,
the average lifespan of a technological civilization,
are still the subject of wild guesswork.
The area where we've made the most progress
is in the astrophysical factors.
We now have a very good idea of how many planets
there are in the Milky Way that could potentially support life.
The Kepler mission allowed us to estimate that our galaxy boasts
something like 14 billion terrestrial planets
in the Goldilocks zone of their parent star.
So 40 billion rocky worlds capable of supporting
liquid water, which may be an essential ingredient for life.
We call these habitable planets, although we have
no idea how many are inhabited.
Around 11 billion of those are Earth-like planets
around Sun-like stars.
With such a tight constraint on the astrophysics,
but with the other factors being pretty hopeless,
some researchers have decided to reframe the Drake equation.
Astrophysicists Adam Frank and Woodruff Sullivan
asked a different question.
Instead of figuring out the number
of technological civilizations still existing in the Milky
Way, they asked the following-- In the deeply pessimistic
assumption that humanity is the only technological civilization
in the entire known universe, what would that
say about the biological and sociological factors
in the Drake equation?
In fact, they figured out for there
to be only a 1% chance for one advanced civilization
to have ever arisen in the history of the known universe,
the chance for each habitable planet
to produce such a civilization would need to be less than 2.5
by 10 to the power of minus 24.
So less than a 1 in 400 billion trillion chance.
That's so insanely small that we're
forced to conclude that there must have been civilizations
out there before.
They also ran the numbers for the Milky Way.
They got that in order for humans
to be the only advanced civilization
to have ever appeared in our galaxy-- ever-- then
there would need to be only a 1 in 60
billion chance for any suitable planet producing something
like us.
That's still a very small number.
If the true probability of producing civilizations
is higher, then there have been-- and perhaps still are--
advanced species in our galaxy.
OK.
So I have a challenge question for you.
The best chance we have of finding other civilizations
is if they're close.
I want you to follow Frank and Sullivan's method,
plus whatever other information you care
to include, to answer this.
If humanity is the only technological civilization
to have arisen on any habitable planet within 100 light years,
how low would that probability of technological emergence
need to be?
What does this tell you about how close our nearest
neighbors might be?
To answer this, you should take a look at Frank and Sullivan's
paper-- link below-- and check out
some of the other useful info I'll leave in the description.
Let's also do an extra credit question.
Answer the above but also the following--
consider Tabby's Star, an otherwise normal-looking
F-type star, 1,500 light years away,
which the Kepler mission revealed to be experiencing
some weird dimming.
Some people like to think this is
evidence of some sort of alien mega structure eclipsing
the star.
Of course, it's almost certainly not.
But just say it was.
If we now say that those guys are the one advanced Dyson
swarm building civilization in the entire Kepler sample,
how close to us is the nearest such civilization
not in that sample?
What does your answer say about the likelihood of Tabby's Star
hosting such a civilization?
Submit your answers with full work
to pbsspacetime@gmail.com within two weeks of release
of this video for a chance to win a Space Time t-shirt.
Make sure you use the subject line Galactic Civilization
Challenge to be in the running because we
filter by subject line.
See you next week for a new episode of Space Time.