How can we detect signs of extraterrestrial (ET) life? One
way is to basically eavesdrop on any radio communications
coming from beyond Earth. Radio is not
only a cheap way of communicating, but also a sign of a
technological civilization. Humanity has been unintentionally
announcing its presence since the 1930s by way of the radio
waves and television
broadcasts that travel from Earth into outer space
The Search for Extraterrestrial Intelligence (SETI)
is conducted by dedicated scientists everyday. In the movie "Contact,"
Jodie Foster's character, Ellie Arroway, searches the
heavens with several large radio telescopes. When she receives
a radio message from a distant star, there are profound
implications for humanity.
SETI is an extremely controversial scientific endeavor.
Some scientists believe that it is a complete waste of time
and money, while others believe that detection of a signal
from ET would forever change our view of the universe. In this
edition of HowStuffWorks,
we will examine the SETI program. We'll look at how radio
telescopes work and how they are used for SETI searches, what
the probabilities of detecting alien life are, what might
happen if or when such a signal is detected and how you can
participate in SETI yourself.
Search the Skies The universe is an awfully
big place. How can you best search the huge sky for a radio
signal from ET? There are three basic dilemmas:
How to search such a large area of sky
Where to look on the radio dial for ET
How to make the best use of the limited radio-telescope
resources available for SETI
Large vs. Small Areas of
Sky Because the sky is so big, there two basic
approaches to SETI searches:
Wide-field search - In this method, you survey
large chunks of the sky, one at a time, for signals. A
wide-field search allows the entire sky to be searched at a
low resolution in a short period of time. However, if a
signal is detected, it would be difficult to pinpoint the
exact source without a subsequent high-resolution search.
Targeted search - In this method, you make
intensive investigations of a limited number (1,000 to
2,000) of sun-like stars for
ET signals. The targeted-search allows for more detailed
investigations of small areas that we think might be
probable locations of ET, such as stars with planets and
conditions favorable for life as we know it. However, this
approach ignores large portions of the sky and might yield
nothing if the guesswork is wrong.
Frequency? When you're in an unfamiliar area and
want to find a station on your car radio, you have to turn the
dial until you pick something up, or press the "search" or
"scan" button if your radio has these features. Well, the
question is, where might ET broadcast? This is perhaps the
biggest challenge for SETI researchers because there are so
many frequencies -- "billions and billions," to quote Carl
Sagan. The universe is filled with radio noise from naturally
occurring phenomena, much like a summer night is filled with
the sounds of crickets and other insects. Fortunately, nature
does provide a "window" in the radio spectrum where the
background noise is low.
Radio spectrum, showing the window, or "water
hole," in the microwave
In the 1- to 10-gigahertz (GHz) range of frequencies, there
is a sharp drop in background noise. In this region, there are
two frequencies that are caused by excited atoms or molecules:
1.42 GHz, caused by hydrogen atoms, and 1.65 GHz, caused by
hydroxyl ions. Because hydrogen and hydroxyl ions are the
components of water, this area has been called the water
hole. Many SETI researchers reason that ET would know
about this region of frequencies and deliberately broadcast
there because of the low noise. So, most SETI search protocols
include this area of the spectrum. Although other "magical"
frequencies have been proposed, SETI researchers have not
reached a consensus on which of these frequencies to search.
Another approach does not limit the search to any one,
small range of frequencies, but instead builds large,
multichannel-bandwidth signal processors that can scan
millions or billions of frequencies simultaneously. Many SETI
projects use this approach.
Resources The number of radio telescopes in the
world is limited, and SETI researchers must compete with other
radio astronomers for time on these instruments. There are
three possible solutions to this problem:
Conduct limited observing runs on existing radio
Conduct SETI analyses of radio data acquired by other
radio astronomers (piggyback or parasite
Build new radio telescopes that are entirely dedicated
to SETI research
Much of SETI research has been done
by "renting" time on existing radio telescopes. This is the
way it was done in the movie "Contact." In the real world, Project
Phoenix (the only targeted SETI search) has rented time on
the Parkes radio telescope in Australia, the 140-meter
telescope in Green Bank, West Virginia and the Arecibo radio
telescope in Puerto Rico. Project Phoenix has a
tractor-trailer full of signal-analysis equipment that it
attaches to the telescope for the search.
SERENDIP Project piggybacks an extra receiver onto a radio
telescope (Arecibo) that is used by someone else. The SERENDIP
researchers then analyze the signals acquired from the target
of interest. Project SERENDIP takes advantage of large amounts
of telescope time, but its researchers do not have control
over which targets are studied and cannot conduct follow-up
studies to confirm a possible ET signal.
Telescope Array is a new radio telescope being built by
the SETI Institute. Located northeast of San Francisco, in the
"radio quiet area" of the University of California at
Berkeley's Hat Creek Observatory, the array will be dedicated
entirely to SETI, using hundreds or perhaps thousands of
backyard-type satellite dishes to collect radio signals by interferometry
(see the section Dishes for the
Sky for information on radio telescopes). The Allen
Telescope Array is projected to cost about $26-million.
Photo courtesy Seth Shostak/SETI
Institute The Allen Telescope
Array (top: prototype seven-dish array; bottom: artist
concept of completed
SETI Projects Several
SETI projects have been conducted since 1960. Some of the
major ones are:
Project Ozma - The first SETI search, conducted
by astronomer Frank Drake in 1960
Ohio State Big Ear SETI Project - Launched in
1973, detected a brief but unconfirmed signal called the
WOW! signal in 1977 and was shut down in 1997 to make way
for a golf course
Project SERENDIP - Launched by the University of
California at Berkeley in 1979
NASA HRMS (High-resolution Microwave Survey) -
Launched by NASA in 1982 and discontinued in 1993 when the
U.S. Congress cut its funding
Project META (Mega-channel Extraterrestrial
Assay) - Launched at Harvard University in 1985 to search
8.4-million 0.5-Hz channels
COSETI (Columbus Optical SETI) - Launched in 1990
as the first optical SETI search for laser signals from ET
Project BETA (Billion-channel Extraterrestrial
Assay) - Launched at Harvard University in 1995 to search
billions of channels
Project Phoenix - Launched in 1995, SETI
Institute's continuation of the NASA SETI effort
Southern SERENDIP - Launched in Australia in
1998, piggyback project to search the southern sky
SETI@home - Available as of 1999, screensaver
program for analyzing SETI data using home computers
For details on these and other SETI projects, see
section at the end of the article.
Prize-winning physicist Enrico Fermi reasoned that if it
takes life billions of years to develop intelligence and
signal or travel to the stars, and if there are billions
of worlds in the universe, and if the universe is over
13-billion years old, then why haven't we been visited
by ET, or why isn't the galaxy crawling with ETs? This
argument has been used to question the value of SETI,
and author David Brin has expanded upon it in an essay
called "The Great Silence" (see "Are
We Alone in the Cosmos?: The Search for Alien Contact in
the New Millennium").
If a signal is detected,
there are a series of steps that follow to confirm that the
signal is extraterrestrial:
The radio telescope is moved off the target (off-axis)
-- the signal should go away, and it should return when the
telescope is pointed back to the target. This confirms that
the signal is coming from the telescope's field of view.
Known Earth or near-Earth sources, such as satellites,
must be ruled out as originators of the signal.
Known natural extraterrestrial sources, such as pulsars
and quasars, must be ruled out.
The signal must be confirmed by another radio telescope,
preferably one on a different continent.
What are the possibilities that we will find ET signals? To
address this issue, astronomer Frank Drake introduced
an equation to calculate the number of ET civilizations in the
galaxy in 1961. The equation, now referred to as the Drake
Equation, considers astronomical, biological and
sociological factors in its estimates:
N = R * x f p x
n e x f l x f i x
f c x L
N - Number of communicative civilizations
R* - Average rate
of formation of stars over the lifetime of the galaxy (10 to
40 per year)
fp - Fraction of
those stars with planets (0 < fp<1, estimated at 0.5 or 50
ne - Average
number of earth-type planets per planetary system (0 <
at 0.5 or 50 percent)
fl - Fraction of
those planets where life develops (0 < fl<1, estimated at 1 or 100
fi - Fraction of
life that develops intelligence (0 < fi<1, estimated at 0.1 or 10
fc - Fraction of
planets where intelligent life develops technology such as
radio (0 < fc<1, estimated at 0.1or 10 percent)
L - Lifetime of the communicative civilization in
years (estimates are highly variable, from hundreds to
thousands of years, approximately 500 years for example
Some forms of
the Drake equation add an additional term after
fs, for the
fraction of stars formed that are sun-like stars.
Non-zero values of fs vary between zero and 1, but
are estimated at 0.1 or 10 percent.
The fractions in the
Drake equation have non-zero values between zero and 1. The
first three terms on the right side of the equation are the
astronomical terms. The next two are the biological terms. The
final two are the sociological terms.
The Drake equation has been a guideline in SETI research.
The value of N has been calculated to be anywhere from
thousands to billions of civilizations in the galaxy,
depending upon estimates for the other values.
If we use the estimates listed above, and decide
R* equals 40 , then the drake
N = (40 stars per year) x (0.5) x (0.5) x (1) x
(0.1) x (0.1) x (500 years) = 50 civilizations
As you can see, the results of the Drake equation are
highly dependent upon the values that you use, and values of N
have been calculated at anywhere from 1 to in the thousands.
Some aspects of SETI and general astronomical research have
been devoted to gathering data for reliable estimates of the
terms in the Drake equation, such as the number of extrasolar
planets. See the Links
section for more details on the Drake Equation.
SETI and You In 1999, University of
California at Berkeley scientists Dan Werthimer and David P.
Anderson worked on Project SERENDIP. They recognized that a
limiting factor in analyzing the data from the Arecibo dish
used by SERENDIP was the available computing power. Instead of
using one or more large supercomputers to analyze the data,
many smaller desktop PCs could be used to analyze small pieces
of data over the Internet. They devised a screensaver
program called SETI@home that could be downloaded from UC
Berkeley over the Internet and reside on a participant's home
computer. The program can work in residence or as a
Data are collected from the Arecibo dish in Puerto Rico,
where Project SERENDIP is presently conducted.
The data are stored on tape or disk along with notes
about the observations, such as date, time, sky coordinates
and notes about the receiving equipment.
The data are divided into small chunks (approximately
107-second blocks) that desktop PCs can utilize.
The SETI@home program on your PC downloads a chunk data
from the computer servers at UC-Berkeley.
Your PC analyzes the chunk of downloaded data according
to the algorithms in the SETI@home program. It takes about
10 to 20 hours to analyze the data, depending on the
and amount of memory.
When finished, your PC uploads its results to the
UC-Berkeley servers and flags any possible hits in the
After the upload, your PC requests another chunk of data
from the server, and the process continues.
screen saver is divided into three sections: the data-analysis
window (upper left), the data/user information (upper right)
and the frequency-power-time graph of the data as it is being
analyzed (bottom). The chunk of data is analyzed by spreading
the data out over many channels using a mathematical technique
called a Fast Fourier Transform (FFT). If the data are
random, then the signal in all of the channels will be equal.
If a signal (spike) is present, then one or more FFT
channels will stand out above the rest, above a certain
power-level threshold. Next, the program looks to see if the
frequency of any spike is shifted slightly to other
frequencies -- this shift would be caused by the Earth's
rotation, indicating that the spike is of extraterrestrial
origin. Finally, since the Arecibo dish is stationary -- does
not track objects with the Earth's rotation -- an ET signal
would drift over the dish's surface, from edge to center to
edge, and a plot of the spike over time would look like a
bell-shaped curve. The program tests to see if the spike fits
this curve. If these three criteria are met, the program flags
the information for later analysis by UC-Berkeley.
Data Analysis window of
The data/user information section of the screen contains
the notes on the observations that obtained the data chunk, as
well as notes on the user.
Data/user information portion of the
Graph window of SETI@home
The graph screen allows the user to see the progress of the
analysis at a single glance. The program notes all of the
observed spikes and relays this information back to UC
Berkeley for further analysis. Each data set is processed
independently by two users for corroboration. If a spike
passes the criteria for a possible signal, then other SETI
projects will examine the coordinates in greater detail to
confirm the finding.
With SETI@home, a computer and an Internet connection, you
can participate in SETI research. To date, the SETI@home Web
site receives one-million hits and 100,000 unique visitors per
The Future of SETI It appears that the
public is greatly interested in SETI research, if interest can
be gauged from the monetary support of private foundations
like the SETI Institute and the SETI League and participation
in SETI@home. The future of SETI looks bright, with
developments in the following areas:
New SETI programs will exploit other areas of the radio
spectrum, such as the microwave regions.
With the technological advancements in
personal-computing power and the Internet, there will
probably be more participation in SETI@home, as well as the
development of other distributing-power computing
New radio telescopes, like the Allen Telescope Array,
will be built for exclusive SETI research.
Using relatively inexpensive, off-the-shelf technologies
such as satellite dishes, computers and electronic
equipment, amateurs can implement their own SETI programs.
One such amateur program is Project
BAMBI (Bob and Mike's Big Investment).
Because ET might send light
signals as well as or instead of radio signals, more
optical SETI programs may spring up. To look for light
signals from ET around sun-like stars, it
may be best to look in the infrared portion of the spectrum,
where the star's background light may be less obtrusive, as
Spectrum of light from a sun-like star,
showing where visible and infrared laser beacons would
shine above the background
One such optical SETI program is called COSETI
(Columbus Optical SETI).
The possibility of
intelligent life existing elsewhere in the universe has
intrigued humanity for thousands of years. We are currently at
a time when our technology has advanced enough for us to
detect signals from ET and even broadcast our own signals to
the stars. With the advancements in technology and the
increasing interest in SETI, we may be close to finding the
answer to that age-old question, "Does intelligent life exist
Dishes for the Sky If ET is communicating by
radio, how can we detect such signals? Radio signals are waves
like visible light, infra-red light (heat) and X-rays. But
radio signals have longer wavelengths than these other forms
of light. To detect ET radio signals, you use a radio
telescope. A radio telescope is a radio receiver similar to
that you have in your house or car. It has the following
Diagram of the parts of a radio telescope
(Cassegrain design). Hover over the labels for a call-out
of each piece.
Antenna - Metal device (usually straight or
coiled wire) located at the focus of the radio telescope. It
converts the radio waves into an electric current when tuned
to the correct frequency
because the radio waves cause movements of electrons
in the antenna.
electronics in the radio telescope -- antenna, tuner,
amplifier -- are often cooled with liquid nitrogen or
liquid helium to reduce random electrical currents, or
noise. The lower the noise, the easier it is to detect
Tuner - Electrical device that separates a single
radio signal from the thousands that come into the antenna.
The tuner adjusts the frequency of the antenna to match a
specific frequency among the incoming radio waves. SETI uses
multichannel analyzers that allow them to tune multiple
Amplifier - Electrical device that increases the
strength of a weak electrical current caused by an incoming
Data recorders - Magnetic-tape or digital devices
that store the signals from the amplifiers.
Auxiliary data instruments - Additional devices
that encode information on the data tapes for
interferometry (see below). These instruments include
receivers that record the position of the radio
telescope and devices for precise time notations.
Computers - Computers are used to acquire and
analyze data, as well as to control the telescope's
Mechanical systems - Gears and
on the horizontal and vertical axes are used to point and
track the dish.
Interferometers combine images from several
radio telescopes to make one image that looks like it
was taken from one large
In general, large radio telescopes allow you to detect weak
signals and resolve them -- so, the larger the dish, the
greater the resolution of the signal. However, large dishes
are difficult and expensive to construct and maintain. To get
around this problem, radio astronomers use a technique called
interferometry. Interferometry combines the signals
from several small radio telescopes spread out over a large
area to achieve the equivalent of one large dish over the same
area (see the links on the next page for details on