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". To understand that, we need to talk a bit about probabilities and estimating quantities when we don't have knowledge of all points needed to answer a question.

Clicker question. What do you know about statistics and probabilities?

1. You roll a die. What is the chance of getting the number 3?

a. 1/2 b. 1/3 c. 1/6 d. 1/12

2. Now you role the die twice. What is the chance you the number 1 and then the number 4?

a. 1/6 b. 1/12 c. 1/18 d. 1/36

3. Now you role two dice at once. What are the odds you get a 1 and a 4?

a. Same as in question 2.

b. Smaller than in question 2.

c. Larger than in question 2.

Next, we solve a problem that you will see can be phrased in the same way we can discuss the prospects of finding intelligent life in our Galaxy. You can easily modify the example below to other situations, e.g. how many primary care physicians we would need in Las Cruces, or how many customers you might expect if you open a particular restaurant.

EXAMPLE: 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.

If any of these odds is very different what we assume here, the number of civilizations that could be present today would change drastically.

Obviously, it is all in the numbers! We don't know the odds of all of the terms in the equation, but we are making good progress with the first couple of items on the list. The goal is to increase our knowledge to the point where we can better estimate all the different probabilities that we need to know.

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

The BIG questions:

- What are the odds for planets to form life?
- What are the odds for life forms to evolve to intelligent life?
- Once it exists, how long can an intelligent civilization last?

Discussion topic: threats to human civilization. Which are the largest, what can we do about them?

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?

The reality of the Drake equation: with the example of only one planet with advanced life forms known, it cannot yet 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!

We are currently left with one option:

Search for signals from outer space that cannot be explained by natural phenomena (remember Ockham's razor!). This is done by project SETI, the Search for Extraterrestrial Intelligence.

Considerations:

We have already been broadcasting our existence to the outside world; they may be watching our soap operas as we speak. There are about 2500 stars within 50 light years from Earth. And we have been "leaking" t.v. and radio waves into space for longer than that...

Think about this: if aliens were listening for our radio signals, only those within about 100 light years would even know we exist. This region spans a tiny, tiny section of the entire MW, so most alien civilizations, if they exist, can not know about us from listening in....

Questions:

• should we be concerned about announcing our existence into space?

• what is the strongest signal we have been sending into space?                     

PROJECT SETI and here is how you can help: SETI at home

What to search for?

Radio waves makes most sense: lowest energy electromagnetic waves, cheap to produce, very good for communication since they are not blocked by atmosphere, interstellar dust, etc. Radio waves are part of the electromagnetic spectrum, just as visible light, and travel at the speed of light in vacuum.

Challenge: the radio spectrum is vast. What frequency ("color") of waves to search for, and what type of signal? The signal has to be artificial, e.g. restricted to very narrow band width (but not a known spectral line),  and likely periodic or distinctly repetitive. Maybe the signal would be close to a well-known frequency, such as that of the element hydrogen, which has a distinct spectral line at 21-cm wavelength in the radio spectrum. There is also some prominent OH lines lines around 18 cm, so some astronomers talk about looking in the "water hole" of the spectrum, between wavelengths of 18 and 21 cm.

If the signal is not sent on purpose, but e.g. just leakage of radio waves into space, like our radio, t.v., and radar signals, then we might be able to detect a periodic variation in the signal of such a planet due to its rotation and the location of its major cities/centers.

False alarm in the 1960's: pulsars.

Pulsars are fast-spinning neutron stars which emits beams of radiation, much like a lighthouse, that were at first mistaken for alien signals. E.g. the "Crab pulsars" spins 30 times per second and emits pulses of radation at that rate. It is the neutron star left over from the star that exploded in 1054 AD as a supernova (the explosition was recorded by the Chinese). The detection of signals from pulsars by then post-graduate student Jocelyn Bell is one of the great scientific discoveries of the 20th century.

Where to search?

Well, it makes sense to start with local stars, since the signals will quickly get fainter with distance (as the inverse of distance squared). Also, pick stars like Sun or somewhat later in spectral type. There are plenty to chose from. Recent results on extra-solar planet detections might help to focus the search on particularly interesting stars.

Challenge: there are many many stars to chose from, and it is quite possible the nearby ones won't have anything interesting: if they did, why aren't "they" here?

How close would they be? If there are 20,000 civilizations in the Milky Way, randomly distributed through stars in space, the closest one, on average, would be 1000 light years away. Signals get pretty faint from that far away!

How to search? See above section on radio waves.

Challenge: establishing contact is by no means trivial. Remember, we need to decode their signal, then send a response back. If they are 50 lightyears away from us, and it takes us 5 years to figure out the message and send a response, it will take another 50 years for our signal to get to them. The larger the distance to the star, the harder "communication" will be.

What might happen if it we were to establish contact....

How might signal be decoded? Hopefully they use "binary notation" like we do in all our computers and messages in the "digital age".

E.g. number 12,345 = 1x10⁴ + 2x10³ + 3x10² +
                                     4x10¹ + 5x10⁰

Likewise, we can write any number in binary notation, pick a simple one: 45 which equals 101101 in binary notation:

101101 = 1x2⁵ + 0x2⁴ + 1x2³ + 1x2² + 0x2¹ + 1x2⁰
            (= 32    + 0       + 8      + 4       + 0       + 1)

Clicker question: What does the binary number 1011 represent in decimal figures?

a. 8

b. 21

c. 15

d. 11

Every number can be written using the binary system, as can letters, even pictures (see book Figure 11.6; the squares should only be black or white!):

Arecibo message
And its explanation.