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SETI: Radio Searches and the Fermi Paradox

In the previous lecture, we looked at the Drake Equation, which can be used to estimate the number of communicating extraterrestrial civilizations present in the Galaxy at any one time. In this lecture, we will examine how we currently go about looking for the signals sent out by these civilizations. Throughout, we make the assumption that we are not alone in the Galaxy, so that it is worthwhile to search for signals.

  1. The Water Hole
  2. Historical Radio Searches
  3. SETI@home
  4. The Arecibo Message
  5. The Fermi Paradox

The Water Hole

When looking for electromagnetic signals from an extraterrestrial civilization, we must first decide what part of the electromagnetic spectrum to search in. Given present-day technology, it would be completely impossible to examine every single frequency in the spectrum; therefore, we must carefully choose carefully where we are going to look.

Radio frequencies are definitely the best choice, because radio waves are far less affected by attenuation than other types of electromagnetic radiation (e.g., visible light). However, the radio part of the electromagnetic spectrum itself has far too many frequencies to monitor simultaneously. Therefore, we must narrow our search further.

It turns out that a narrow range of radio frequencies, around 1,400 MHz, is usually chosen in searches for extraterrestrial signals. The reasons for this are twofold:

  • The amount of background noise in this frequency range is relatively low, meaning that we will be able to `hear' any signals reaching us. At lower frequencies, noise from the Galaxy would swamp out the signals; at higher frequencies, noise from our own atmosphere would also swamp out the signals.
  • This frequency range is already monitored by astronomers, since it includes radiation emitted by atomic hydrogen (at a frequency of 1,420 MHz) which is used to map our galaxy. Therefore, it is possible to piggyback SETI studies on other astronomical research programmes.

The range of frequencies around 1,400 MHz is usually known as the water hole, since as well as the 1,420 MHz frequency associated with hydrogen emission, it includes a frequency (1,640 MHz) associated with hydroxyl (OH) emission, and hydroxyl combined with hydrogen produces water (H2O).

The water hole

The water hole

Historical Radio Searches

Radio searches for extraterrestrial signals in the water hole began in 1960, when Frank Drake (of Drake Equation fame) was based at the Greenbank (West Virginia) site of the USA's [ National Radio Astronomy Observatory] (NRAO). Drake used the 85-foot Tatel radio telescope in his Project Ozma (named after the queen figure in the Wizard of Oz), which examined signals coming from two nearby stars, Tau Ceti and Epsilon Eridani. Looking at the hydrogen frequency (1,420 MHz), Drake searched for both repeated sequences of uniformly patterned pulses, and sequences of prime numbers; the detection of either of these would indicate the presence of an extraterrestrial civilization.

The Tatel radio telescope at Greenbank, WV

The Tatel radio telescope at Greenbank, WV

Project Ozma was unsuccessful, but paved the way for many subsequent radio searches. During the 1960s, the Soviet Union conducted many wide-field searches for anomalous radio power sources in the sky. At the beginning of the 1970s, a team of experts produced a report for NASA, known as Project Cyclops, which analyzed the science and technology of radio-based SETI, and provided the foundation on which much subsequent work was based.

Throughout the 1970s, interest in SETI grew, and feasibility studies were conducted at NASA's Ames Research Centre and Jet Propulsion Laboratory. By 1989, NASA was convinced of the usefulness of radio searches, and agreed to fund the High Resolution Microwave Survey (HRMS), a project designed to scan the sky at frequencies covering the water hole.

HRMS began operation in 1992, but was shut down one year later, when the US Congress ordered NASA to cease any radio searches for extraterrestrial signals. However, from the ashes of HRMS arose the appropriately-known Project Phoenix, a privately-funded initiative to conduct intensive radio investigations of 1,000 nearby Sun-like stars.

The demise of HRMS illustrates the fact that SETI programmes, being highly speculative, are difficult to secure funding for. However, in the mid-1970s, Stuart Bowyer (an astronomer at the University of California, Berkeley) came up with the idea of piggybacking, where a SETI-based detector can ride on a radio telescope which is being used for other astronomical purposes. By piggybacking, a detector can have nearly unlimited time on world-class telescopes, although there is no control over which parts of the sky are observed.

Perhaps the most famous piggyback programme is SERENDIP (Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations), which began in 1980. Since 1992, SERENDIP has been piggybacked on the 305-m Arecibo radio telescope in Puerto Rico, the largest radio telescope in the world. The SERENDIP detector is today in its fourth incarnation, and can look at 160 million separate frequencies around the 1,420 MHz hydrogen frequency every 1.7 seconds.

The Arecibo radio telescope

The Arecibo radio telescope

One of the biggest problems faced by SERENDIP is the analysis of the radio data which it takes. As well as looking for certain patterns associated with extraterrestrial civilizations, the analysis must filter out all of the human-made noise which dominates the data. This very computationally-intensive task led to the development of the largest computing project in mankind's history: SETI@home.


SETI@home is a distributed computing initiative, primarily sponsored by the Planetary Society, which is designed to analyze the data coming from the SERENDIP project, and look for signals produced by an extraterrestrial civilization. It takes the form of a screensaver, which can be installed on normal personal computer (PC). When the PC is inactive, the screensaver starts up, and downloads a chunk of data from the SETI@home servers. This data is scanned for any unusual artifacts, which cannot be attributed to natural or human-caused mechanisms. When the chunk has been completely analyzed, the results are sent back to the SETI@home server, and a new chunk is downloaded.

Screenshot of the SETI@home screensaver

Screenshot of the SETI@home screensaver

Over 4 million people around the world have installed the SETI@home screensaver on their computers, and nearly 1.4 million years of computing time have been consumed over the 4 'real' years of the project. No conclusive evidence for extraterrestrial civilizations has yet been found, but many 'interesting' candidate radio sources have been pinpointed. To investigate further the most promising of these sources, 24 hours of dedicated observing time on the Arecibo radio telescope were allocated to the SERENDIP project, split up into three 8-hour periods on the 18th, 19th and 20th of March, 2003.

The observations on 18th March discovered no unusual signals. Unfortunately, solar flares disrupted the observations on the 19th and 20th March, but 14 hours of observations have been scheduled for 24th March, to compensate for the lost time.

The Arecibo Message

As well as listening for radio signals from extraterrestrial civilizations, attempts have been made in the past directly to contact them, by sending coded messages into space. The most significant of these was the Arecibo broadcast, sent out in 1974 by the Arecibo radio telescope. The broadcast was the most powerful ever transmitted into space, and would be detectable by an Arecibo-sized equivalent from any point in the Galaxy. The message contained, amongst other things, pictures of the telescope, the Solar System, a human form, DNA, and some of the chemicals essential to terrestrial life. The message was broadcast toward the globular cluster M13, which is about 21,000 light years away, and contains a third of a million stars.

The message contained in the Arecibo broadcast

The message contained in the Arecibo broadcast

The Fermi Paradox

Coming down to lunch with other scientists one day in 1950, the famous physicist Enrico Fermi (discoverer of the neutrino) posed a question he had been thinking about: 'Where are they?'. This seemingly-simple question actually has profound implications for SETI. If we assume that advanced civilizations have developed elsewhere in the Galaxy, then given a short amount of time (relative to the age of the Galaxy), they should have spread and completely colonized the Galaxy. However, there is currently no scientific evidence whatsoever for either the signals or artifacts of such civilizations. Why is this?

A number of answers have been put forward to answer 'Fermi's paradox':

The filter answer
There comes a point at which any civilization destroys itself

through war, disease, pollution or some other mechanism. Therefore, no civilization exists for long enough to colonize the Galaxy.

The socioeconomic answer
Although civilizations may reach a level where they are

technologically capable of colonizing the Galaxy, they all choose not to for social or economic reasons.

The cosmic zoo answer
Civilizations have colonized the Galaxy, but they '''hide their

presence from us, perhaps observing the development of terrestrial life and mankind as part of some kind of zoological project'''.

The suburbia answer
Civilizations have colonized the Galaxy, but consider Earth to

be too boring to be worth visiting or communicating with.

The Hungarian answer
Civilizations have reached Earth and secretly integrated

themselves within society, disguised as Hungarians. This answer was put forward by the Hungarian physicist Leo Szilard (who discovered the nuclear chain reaction); Szilard claimed support for his hypothesis on the grounds that all Hungarians a) have wonderlust, b) speak a language completely unrelated to neighbouring languages, and c) are very intelligent.

The null answer
Mankind is the only advanced lifeform in the Galaxy, explaining

why there is no evidence for others.

Unfortunately for SETI projects, the null answer appears to be the most attractive solution to Fermi's paradox, indicating that we are truly alone in the Galaxy. However, it is important to remember that the absence of evidence (for extraterrestrial civilizations) is not the same as evidence for absence; therefore, SETI projects should still continue.

Updated 2009-10-14 04:12:55