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 everyday.
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.
What's the 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 region
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.
Limited Radio-telescope 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 telescopes
Conduct SETI analyses of radio data acquired by other radio astronomers (piggyback or parasite searches)
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.
The 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.
The Allen 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 array)
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 the Links section at the end of the article.
The Nobel 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 percent)
ne - Average number of earth-type planets per planetary system (0 < ne<1, estimated at 0.5 or 50 percent)
fl - Fraction of those planets where life develops (0 < fl<1, estimated at 1 or 100 percent)
fi - Fraction of life that develops intelligence (0 < fi<1, estimated at 0.1 or 10 percent)
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 purposes)
Some forms of the Drake equation add an additional term after R* -- 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 equation becomes:
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 screensaver.
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 computer's microprocessor and amount of memory.
When finished, your PC uploads its results to the UC-Berkeley servers and flags any possible hits in the analysis.
After the upload, your PC requests another chunk of data from the server, and the process continues.
The 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 SETI@home
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 SETI@home screen
Graph window of SETI@home screen
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 day.
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 programs.
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 shown below:
Spectrum of light from a sun-like star, showing where visible and infrared laser beacons would shine above the background light.
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 out there?"
Dishes for the Sky
If ET is communicating by radio, how can we detect such signals? Radio signals are waves of light, 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 the radio that you have in your house or car. It has the following parts:
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.
The 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 weak signals.
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 frequencies simultaneously.
Amplifier - Electrical device that increases the strength of a weak electrical current caused by an incoming radio signal.
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 GPS 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 movements.
Mechanical systems - Gears and motors 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 dish.
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 interferometry).