Have you ever found yourself looking at a starry night sky, and asking in your mind
“Is there any extra-terrestrial life out there?
And if so, how many other civilizations are there in our galaxy, and ultimately, in the
whole Universe?”
I bet you did!
Personally, I did it many times…
Ok, the question is very simple, but what about the answer?
Well, you may think that nobody knows it, and that is somehow true, but… you may be
surprised to hear that scientists actually tried to answer it by using… a mathematical
equation!
Yes, you’ve heard it right: there is a mathematical formula that attempts to estimate how many
other civilizations there are in the Universe!
Curious to hear more about it?
Stick with me and I’ll tell you more!
Since the very first moment scientists started to look at the sky, they also started to wonder
whether we are alone in the Universe or not.
Astronomers always had different (and sometimes, radically different) opinions on this topic.
There are several reasonable arguments used by both sides to support their respective
opinion.
Those who claim there are other civilizations out there, simply point out to the huge number
of stars and galaxies in the Universe, predicting that it’s nearly impossible that there is
no other planet with intelligent life except from us.
On the other hand, those who claim we are alone in the Universe, point out to the fact
that we have never received any message from other hypothetical civilizations, and this
seems very unlikely, because a very advanced civilization should have been able to “show”
its presence to us, somehow.
There are many other arguments in favour of one hypothesis or the other.
For instance, experts also claim that the average lifetime of an intelligent form of
life is very short: so, an hypothetical civilization could have already existed in the past, but
it’s now extinct, and this is why we don’t see them now.
All of this is to show you that in order to answer this big question, there are several
factors we have to take into account first.
But we are lucky: somebody already did it for us!
The guy who did the work for us is Frank Donald Drake, one of the most prominent American
astronomers, involved in several important projects for the search of extra-terrestrial
life: for instance, you may have heard about the SETI project.
He is also the co-creator (together with Carl Sagan, another famous American astronomer)
of the “Arecibo message”, an interstellar message broadcasted into space using the Arecibo
radio telescope, containing information about the Earth, the Solar System, humanity, and
much more, in an attempt to communicate with potential civilizations living far away from
us.
But let’s go back to our main question: how many other extra-terrestrial intelligent
civilizations are there in the Universe?
As we said previously, there are several factors to take into account when we try to estimate
the number of civilizations in the Universe.
In 1961, Drake tried to list all these factors, summarizing them into a formula, known as
“the Drake equation”.
The equation is very simple: the number of civilizations with which we may communicate
in our galaxy, indicated with N, is just the product of this bunch of variables:
Ok I know, that’s a lot of stuff.. but don’t freak out.. just stick with me and in a few
minutes everything will be crystal clear!
Let’s start from the left of the formula.
The first parameter, , is the average rate of star formation in our galaxy.
In simple words, it tells us how many stars form on average in our galaxy.
That makes sense: the number of possible civilizations, N, will depend on the number of stars; the
more frequently new stars form, the higher , and the higher the chances of other forms
of life to appear.
I see what you are about to object…
“Well, but not all stars have planets”…
Indeed!
In fact, this is exactly what the second parameter of the equation, , takes into account: is
the fraction of stars that have planets orbiting them.
This is very simple: it’s a number between 0 and 1.
When , no star has planets; if , all stars have planets.
Of course, the real value of this parameter will be something in the middle: for instance,
if , it means that only half of the stars in our galaxy have planets.
Easy, right?
And again…
I know what is your next objection!
“Ok, but not all planets are habitable”… that’s right: life could not exist on extremely
hot planets like Venus, where temperatures reach insane values (even 480 ), nor on very
cold planets, such as Uranus, where temperatures can be as low as (yes, you read it right).
This is because in order for life to exist, a planet must be in the so-called “habitable
zone”, a certain region around a star where temperatures are just right for water to exist
in the liquid form… not too hot, not too cold.
So the next question would be, “How many of those planets are actually in the habitable
zone?”
And that’s what the next parameter in the formula, , tries to address.
is the number of planets (on average) per star where the conditions are just right to
support life.
For instance, we know that in our Solar System, (because we have only one planet with life!).
Now you should start to become familiar with the process: every parameter in the formula
raises a question that leads to the following one!
So, what about the next one?
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So far, we explained the meaning of the first three parameters in Drake’s equation.
Can you spot what is the next one?
Think about it… if a planet it’s located in the habitable zone, it doesn’t necessarily
mean that there is actually life on that planet.
And that’s why we add the next parameter: . is a number between 0 and 1 telling what
is the fraction of planets in the habitable zone that could actually develop life at some
point.
If we feel very optimistic today, we can say , which means that 100% of the planets in
the habitable zone have developed life at some point.
If we say , it means that life appeared only in 50% of them, and so on.
It’s quite easy now to guess the meaning of the next parameter: in fact, with this
equation, Drake wanted to estimate the number of civilizations in our galaxy with which
we could communicate… this implies that we are only taking into account forms of life
that are intelligent!
Otherwise, you would have a hard time trying to communicate with a civilization consisting
of bacteria only…
And that’s why Drake added the next parameter: . tells in which fraction of those planets
with life an intelligent civilization actually appeared.
Speaking about numbers again: if , it means that all the forms of life that appear on
those planets are intelligent; if , none of them (sigh!).
Ok, hang on, we are almost there… next one is . What is it?
Well, in order for extra-terrestrial forms of life to communicate with us, it’s not
enough for them to just exist and being intelligent.
They should also be able to send us messages, and to do that, they should have developed
some advanced technologies.
That’s why we add this : it represents the fraction of those intelligent civilizations
that actually managed to develop a technology that allow them to send us messages.
Again, the meaning is the usual one: means none of them did it, while means all of them
are able to send us a message.
Hang on, we are almost over!
The fact that an intelligent civilization exists somewhere, and it has the technology
to send us messages, it’s not enough yet.
There is one more factor to take into consideration: time.
Yes: in fact, imagine that a hypothetical intelligent civilization existed but, let’s
say, it lived million years ago, and it is now extinct.
Would we be able to communicate with them?
Of course not!
We wouldn’t see any sign of them.
So, the lifetime also matters.
In fact, if such civilization exists but it only lives for a short time, there would be
no chance we could communicate with them.
So, we also have to take this into account: and this is done by adding the parameter in
the equation, which tells how long such civilizations exist for and are able to communicate with
us.
Wow, what an effort… we finally managed to see what is the meaning of each single
parameter in the Drake’s equation.
But now you may ask “Ok, this is all very interesting, but… how do we use that formula?”.
That’s a good question: indeed, our ultimate purpose was to find , the number of extra-terrestrial
civilizations we may able to communicate with in our galaxy.
You want to know the answer?
Well, the answer is… we don’t know.
Wait, what?
So we spent all this time for nothing?
Not really.
Drake’s purpose was not to give a precise estimate of the number of intelligent civilizations
other than us, rather to stimulate the scientific debate about the factors to consider when
searching for life outside the Earth.
In fact, the main problem with this formula is that in order for it to predict a precise
value of , we should be able to give a precise estimate for each of the factors in the equation,
and this is clearly not possible.
For instance: what is the value of , the average lifetime of an advanced civilization?
It can be anything between and years…
And of course by picking the two extreme values, the estimate of will change dramatically.
Or what about , the fraction of planets that develop intelligent life?
It can be as high as , or as low as , if this probability is small.
Of course these two values will lead to completely different estimates of .
Despite all these uncertainties, however, Drake and other scientists managed to give
a “reasonable estimate” about the values of all those parameters in the formula.
Without going into the details of the calculation, Drake and his colleagues obtained a range
between and for the value of . Yes!
This means that according to their estimate, there could be as low as civilizations with
which we may communicate in our galaxy, and this number can be as high as millions!
Quite fascinating, isn’t it?
Hold your enthusiasm, however…
As we said, these are only estimates, so they could be completely wrong.
Other scientists, for instance, estimated more pessimistic values for the parameters
in the equation, obtaining values of close to zero.
So in the end, this is a very difficult exercise, and nobody can really predict what the true
value of could be.
There is, however, one thing that we know for sure: cannot be exactly zero.
Why?
Because we know that there is at least one intelligent civilization that arose in the
Universe.
Guess who are we talking about?
Yes: humanity!
In fact, the very fact that we, humans, are here, implies that it must be possible for
other civilizations to be out there as well…
It’s true, we haven’t find them yet, and we don’t even know if they actually exist,
but it’s still very fascinating to think we are not alone in the Universe.
And who knows, maybe it’s only a matter of time, and one day we will finally receive
a message from a distant star.
That would probably be one of the most important days of our history, and it will be remembered
as the day when we finally answered one of the most mysterious question: are we alone
in the Universe?
"This video ends here!
Thanks for watching everyone!
Did you find Drake’s equation interesting?
Do you find Drake’s estimate is reasonable?
Do you think we are alone in the Universe, and if not, how many other civilizations do
you think are there?
Let me know in the comments below, be sure to subscribe, and I'll see you next time on
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