SETI: Search for ExtraTerrestrial Intelligence

The Interstellar Distance Problem

The nearest star system, Alpha Centauri, is 4.4 light years away. That means it takes light, or any other electromagnetic wave such as radio, 4.4 years to travel that distance. A radio conversation with someone there would have some long pauses but might still be worthwhile, particularly if they had a lot to say.

Sending probes to even the nearby stars is much more difficult. Current interplanetary space probes move at speeds up to about 50 miles per second, which is 0.02% of the speed of light. At that speed, travel to the nearest star would take 22,000 years.

Some ideas for getting to much higher speeds:

The Sun-diver would use current technology and take advantage of the fact that a rocket is much more efficient when it is already moving at high speed.

a gravitational sling-shot maneuver using one or more of the planets puts the probe on a path that brings it very close to the Sun. By firing its rockets just as it swings past the Sun, it can gain the maximum possible final speed.

It might also jettison its rockets and fuel tanks and deploy solar sails to use the pressure of sunlight as it speeds away from the Sun.

With this kind of scheme (and some wild optimism) one can imagine getting up to 500 miles per second or 0.2% of the speed of light. Now the trip time is down to only 2,200 years.

Fusion-powered Ion Rockets are far in the future, but one can work out their limitations and find that it will be very difficult to get them much above 10% of the speed of light. At that speed, the Alpha Centauri run will take 44 years each way.

Evidently we are not going to be able to get probes to even the closer star systems any time soon. That will make it very difficult to inspect the planets of those stars for primitive life-forms.

Help from the other end

Instead of looking for primitive life, just look for intelligent, technology-using life that is actually trying to be found. The obvious way to be found is to broadcast some sort of message.

    Now we have three problems:

  1. What kind of radiation do we use?

  2. What frequencies do we listen to?

  3. How likely is it that there really is anyone out there sending us a message?

The Water Hole

So far, it appears that the most energy-efficient way to send a message is to use radio waves, which answers question 1.

The frequencies are limited by noise sources and absorption in both interstellar space and in the Earth's atmosphere.

In the following diagram, the vertical axis plots the amount of background noise as an effective temperature. As you can see, there is a nice minimum from 1 Gigahertz to about 10 Gigahertz. These are microwave frequencies, so the technology to listen to them is conveniently available.

That narrows down the answer to question 2, but we need to do better.

Within the quiet part of the radio spectrum there are two very prominent signals, one from the precession of interstellar Hydrogen and one from interstellar Hydroxyl ions. The region between these two absorption lines, from 1400MHz to 1727MHZ is called the water-hole..

It has been argued that any water and carbon-based technological civilization would be expected to give this region of the spectrum special significance, so it is the frequency band that we should listen to for broadcast messages. That answers question 2.

The Drake Equation

The Drake Equation was originally conceived as a device for organizing discussions about the Search for Extraterrestrial Intelligence. It does not predict anything, but just puts everything that we know or suspect together.

Frank Drake has referred to it as a way of "organizing our ignorance."

N = R*×fp×ne×fl×fi×fc×L

N=Number of communicating civilizations in our galaxy
R*=Average rate of star formation in our galaxy
fp=fraction of stars that have planets
ne=average number of habitable planets in each planetary system
fl=fraction of habitable planets that actually develop life
fi=fraction of life-supporting planets that develop intelligent life
fc=fraction of intelligent species that develop technological civilizations
L=the average length of time that a technological civilization
broadcasts signals

The values that summarized the 1961 meeting at which the Drake equation was conceived are as follows:

R*=10 per year
L=10,000 years

With these values, the Drake Equation gives

N = 10

communicating civilizations in our galaxy.

That is a little depressing since our galaxy is 100,000 light years across and the chance of a close neighbor to communicate with is pretty small.

How have the 1961 values held up?

The star formation rate R* is based on direct observation and is pretty good.

The value of  fp=0.5, the fraction of stars with planets is still regarded as a good guess.

The value of  ne=2, the number of habitable planets in a system, now looks much too high. A value of ne=0.5 might be closer.

The speed with which life arose on Earth suggests that the value of  fl=1 is about right. Whether life arose by abiogenesis or by panspermia, it appears to take hold as soon as the conditions are right for it.

The values of  fi,  fc and L are straight guesses based on no information at all.

The most vulnerable guess is probably the expected lifetime L of a communication-capable technological civilization. The amount of energy required for interstellar broadcasting implies the capability for efficient self-destruction by any number of different means. Our own civilization has had that status for about 100 years so far and it has not been easy.

It is not difficult to make very plausible changes in the values in the Drake equation that would make the value of N less than one. In that case, there is nobody out there to talk or listen to.

The Fermi Paradox

Fermi's question is "Where is everybody?"

Our limited experience as a technological civilization suggests that such civilizations expand exponentially, doubling such parameters as population and energy use every 20 years and finding new resources to exploit whenever scarcity threatens.

If such a civilization really lasts for even 10,000 years as the assumptions we put into the Drake Equation suggest, its impact on its environment should be overwhelming and obvious. If they do last that long, there seems no reason they should not last for millions of years, which would make them even more obvious --- they should be visiting us in person by now.

An example of the sort of impact that we might expect to see is the Dyson Star. That is a star that has become so surrounded by a swarm of orbiting solar power plants that all we can see is the infrared radiation from their cooling fins. Such a thing would indeed be overwhelmingly obvious and unnatural.

Another example is related to our earlier suggestion that interstellar space is actually home to many non-stellar planetary mass objects that could easily become bases for settlement and exploration. Although interstellar travel between stars may always be forbiddingly difficult, travel between these waystation planets might be rather easy. In that case the conjectured 10,000 year old civilization would surely expand at perhaps 10% of the speed of light, occupying a sphere 1000 light years in diameter and swallowing all of the stars in that region into its power grid. There is no way that we could miss seeing such a thing.

Carrier Modulation: The Vanishing Earth Civilization

All of the various SETI attempts assume that aliens are sending a rather narrow band signal with information modulating a carrier wave. That particular technology is just now becoming outdated here on Earth as we move from analog TV broadcasts to digital broadcasts.

Analog signals are very easy to spot because most of their energy goes into an obviously artificial carrier wave. Digital signals are useful precisely because they put much more of the signal energy into information content. That same feature causes them to look more like random noise to a receiver that does not know the correct decoding scheme.

Analog Television signals have, for many years, been the strongest signal that we broadcast to the universe. As they are replaced by digital signals, the "Earth broadcast" is becoming less and less detectable by distant civilizations.

The lesson here is that an advanced civilization may make less and less impact on its environment as it becomes more efficient.

SETI Design

Since we do not know what frequency the aliens are using, we need to listen to as many as possible. That implies a multi-channel receiver that can listen on many frequencies at once.

The amount of information gathered by a multi-channel, multi-directional system is enormous, so a very high capacity computer is needed to process it all and pick out signals that might be of interest.

SETI Attempts

The OSU Big Ear Radio Telescope operated from 1963 until 1997. It consisted of a tiltable flat reflector sending radio waves to a fixed parabolic reflector that would focus them onto a microwave receiver.

The Earth's rotation was used to scan in the east-west direction and the flat reflector was tilted to scan in the north-south direction.

It was used for the longest running SETI project so far, from 1973 until 1997.

In 1977 it received a signal that bore all the earmarks of being extraterrestrial. It is usually called the "Wow! Signal" because of the notation that a staff scientist made on the computer printout.

The frequency and direction of the Wow! Signal have been under constant surveillance ever since then, but the signal has never repeated.

SERENDIP (Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations) was started at UC Berkeley in 1979 and a successor program, SERENDIP II was begun in 1986 and was followed by several more upgraded programs. Each of these was a multichannel radio spectrometer that was attached to existing radio telescopes.

The initial SERENDIP was a 100 channel analog radio spectrometer covering 100kHz of bandwidth. The most recent version, SERENDIP IV, consists of a 168 million channel spectrometer covering 100 MHz of bandwidth between 1.37GHz and 1.47GHz and has been installed and operating at the Arecibo radio telescope since 1999.

The SETI@home system distributes chunks of the data obtained by SERENDIP to 5.2 million participating home computers. The program searches for

  • spikes in the power spectrum corresponding to carrier waves.
  • Gaussian rises and falls in power, corresponding to the telescope passing over a radio source as it turns.
  • Triplets or three spikes in a row.
  • Pulsing signals that might represent digital transmission.

So far, just one candidate signal was found on March 2003. That was Radio Source SHGb02+14a. The source was picked up three times at 1420MHz from a direction in which there are no stars for 1000 light years. Each time, its frequency started to drift in a fashion that is difficult to explain. It is generally thought to be of no significance.