Howework 5: Explain in an essay of several pages how you might try to find and communicate with intelligent extraterrestrial life. Please list
the references that you use on a separate page.
There are billions of galaxies in the Universe. Andromeda (one of nearest galaxies)
lies at 1 million parsecs or 3 millions light-years! This shows that it would take 3 million years
for an electromagnetic signal to arrive to Andromeda, making very difficult
the communication with other civilizations in such galaxy. This would be even
worse for more distant galaxies. Let's focus then in our Galaxy (the Milky Way).
Our Galaxy encompasses enormous distances too. It would take 30,000 years
for a signal from us to get to the center of the Galaxy. Even the nearest
star to us (Proxima Centauri) is located 4 light-years from us. With the state-of-the-art
technology it would take us thousand of years to get there, so travelling
to other solar systems is not an efficient way to search for life.
This shows that distance is the major difficulty in searching and communicating with
other civilizations. The other problem is that our Galaxy has about 200 billion stars.
Where do we start our search?
We cannot rule out that life could take forms very different than the ones
we have on Earth. However, to make things easier let's focus on a search
of life like the one we know based on carbon, oxigen, and liquid water.
Based on these constraints we should probably restrict the sample of
our survey to those stars that are not too old (because they don't have
carbon, oxigen and water), stars that are not too hot (because they
evolve in a few million years and do give enough time for life to form),
stars that are not too cold (that do not have a habitable zone),
stars that do not belong to binary systems (since life needs a
planet with a stable orbit). A reasonable criterion is to select
stars like the Sun (of spectral type G) which have the right luminosity,
temperature, and age. The Sun is not a special type of star and many of the
200 billion stars of our Galaxy fulfill these criteria.
Another route to select our targets is to look at the stars that
are known to have planets. So far we know only 50 or so stars
that host planets but soon we will have a much larger sample.
Once we have selected the targets for our survey we have to decide
how to carry out the search. The logic here suggests us to use
detectors for electromagnetic signals. As we all know, electromagnetic
radiation is the fastest possible way to transmit information
and nothing can propagate faster than the speed of light.
The next thing to think is where in the electromagnetic spectrum
(Gamma rays, X rays, UV, optical, Infrared, Radio, Microwaves)
should we do our search? Since our galaxy is filled with
clouds of gas and dust that absorb and scatter radiation,
we must select frequencies that are not affected by the
interstellar medium. While dust clouds can easily stop
visible light, radio frequencies have the advantage
that they can easily penetrate these clouds.
If we look at background radio noise from Universe, noise level is lowest at Water Hole (radio frequency range from 1000 to
10,000 MHz that is close to the microwave emission lines of H and OH). This hole includes the 21-cm emission line (1,429 MHz)
from spin transition of hydrogen, which proves of obvious universal importance (H is the most abundant atomic element).
In 1960, radioastronomer Frank D. Drake, carried out the first attempt to detect interstellar radio
transmissions. His effort, Project Ozma, marked the birth of a new scientific field, the search for
SETI (Search for Extra-Terrestrial Intelligence)
is a research program consisting primarily in the search for narrow-bandwidth radio signals.
SETI has been scanning our Galaxy for signs of intelligent life, so far without success.
If we found a signature of intelligent life with our radio-telescopes, how could we communicate
with the ET's? The best way to transmit a message is, again, by using radio frequencies
(for the same reasons given above). What language should be use?
It has been suggested that a universal language would be that of mathematics.
A simple message with radio pulses containing prime numbers would be
a clear indication that we are an intelligent civilization with knowledge of math.
On November 16, 1974, a message was sent from the radio telescope in Arecibo, Puerto Rico.
The message contained 1,679 bits of information. This number is only divisible by two prime numbers (73 and 23),
suggesting that message should be laid out in those dimensions to reveal image.
This image was aimed at the M13 globular cluster because this cluster contains several hundred thousand stars
so that the probability that a civilization could intercept this message is higher.
However, it will reach its destination in about 25,000 years! and we will have to wait another
25,000 years to get a reply.
The Pioneer 10 and 11 are the first spacecrafts to leave the Solar System. Each Pioneer
carries a gold plaque that shows what humans look like, our size relative to Pioneer,
and Earth's position in the Solar System, with the hope that these probes could be
intercepted by some intelligent civilization. As mentioned above, it will
take so long before this message arrives to a nearby star that the chances
that somebody discovers this messsage are very, very small.