Reminders:

EXTRATERRESTRIAL INTELLIGENCE
Read book chapters: 12

What are the odds of finding intelligent life in the Milky Way? The "solution" was formulated in the form of an equation by Frank Drake: The Drake "equation".

The Drake Equation is an equation developed by Astronomer Frank Drake. He worked at the National Radio Astronomy Observatory (NRAO) in Green Bank, West Virginia. He proposed a project to look for extraterrestrial intellegence using radio observations (SETI). He developed an equation to take into account the factors that would limit the number of intellegent civilizations in the Universe from which we could potentially get a radio signal. If we cannot recieve a signal from them, then we cannot detect that they are there.

Let's first do a more familiar problem: How many piano tuners are there in Chicago?

Assumptions:

- Every piano tuner can tune 200 pianos/yr

- A piano needs tuning every year.

- So we need one piano tuner for every 200 pianos that get tuned every year.

- Number of people in Chicago and its suburbs: 5,000,000

- Number of family homes: 30% of that or a fraction of 0.3.

-Most pianos are owned by families with school-age children: 50% of that, so fraction = 0.5.

-How many such families make enough money to own a piano? Let's say one in three, so 33% or fraction = 0.33.

-In how many such families do the children actually decide to play piano? Let's say one in ten, so 10% or fraction = 0.1.

-How many such families then decide to actually have the piano tuned every year? Let's say one in four, so 25% or fraction = 0.25.

So how many piano tuners do we need?
5,000,000 X 0.3 X 0.5 X 0.33 X 0.1 X 0.25 = 6188 pianos, so we need about 30 piano tuners.


Now, with that example we can try the same equation for the Milky Way to estimate the number of advanced civilizations that may be present:

Number of civilizations present today in the Milky Way Galaxy =
- (number of stars present today in our galaxy suitable to have planets in habitable zones for sufficiently long time),
times
- (fraction of those stars that actually have habitable planets),
times
- (fraction of habitable planets that have life),
times
- (fraction of habitable planets with life on which intelligent civilization capable of interstellar communication has developed at some point in the planet's history),
times
- (fraction of planets that have intelligent civilizations now)

Example.

- Suppose that every one in ten stars has a planet that is habitable,
- that one in 100 of those develops life on it,
- that one in 1000 of those has at some point developed intelligent life,
- and that one in 10000 of those has intelligent life form present now.

If our Galaxy has 100,000,000,000 stars with habitable zones, then:

10,000,000,000 would have habitable planets,

100,000,000 would have planet(s) with life on it,

100,000 would have at some point have had intelligent life forms on it,

and 10 would have a present day civilization capable of communicating with us.


The book formulates the Drake equation in a different way, but it is fundamentally the same concept.
Drake Equation

Obviously, it is all in the numbers! We don't know the odds of any of the terms in the equation yet, although we are making good progress with the first couple of items on the list.

The BIG questions:

- odds for finding planets with life elsewhere?
- odds for developing intelligent life?
- how long will an intelligent civilization last?

How common is intellegence?
Intellegence on Earth has taken a long time to develop, basic life forms existed long before human ancestors. And then we had to develop the technology to send and recieve signals that can be hear across the Universe.

Important aspect: idea of "convergent evolution". This is related to the question of what are the odds of developing advanced life if life is already present in more primitive forms. The idea of convergent evolution is that even if life evolves through different paths for different species, many share characteristics anyway, even if they followed different evolutionary (in a biological sense) paths. Under this scenario, the odds of developing advanced life forms may be large. See the graph in the book on brain mass versus body mass. Can you think of arguments in favor of this notion and against this notion?

Measuring Intellegence: encephalization quotient (EQ). This plots brain mass versus animal mass, and fits a line to the general trend. We measure intellegence by where on the plot an animal falls. This is a number that can be easily calculated, and can also be determined for extinct species. It does seem to be correlated with complex behavior. EQ has been found to increase over time on Earth. Intellegence and cleverness is also an evolutionary advantage.

Does intellegence always lead to technology? Depends on environment. Dolphins?

The reality of the Drake equation: with the example of only one planet with advanced life forms known, it cannot give us any reliable answer as to the number of civilizations that might be out there.


So, let us move on to a more active initiative:

How to find evidence of extra-terrestrial intelligence?

Past claims that we have been "visited" are highly spurious at best, and outright lies at worst. There is a lot of money to be made selling books about pseudo science, as any trip to a bookstore will tell you!