Discovery of the Pulsar

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The pulsar is a celestial object (thought to be a young, rapidly rotating neutron star) that emits radiation in the form of rapid pulses with a characteristic pulse period and duration. In August 1967, a then-graduate student Jocelyn Bell Burnell (1943- ) and her academic advisor, the British astronomer and Nobel laureate Antony Hewish (1924- ), detected the first pulsar. The unusual celestial object emitted radio waves in a pulsating rhythm. Because of the structure and repetition in the radio signal, their initial inclination was to consider the possibility that the repetitive radio signal was really from an intelligent extraterrestrial civilization. However, subsequent careful investigation and the discovery of another radio wave pulsar that December quickly dispelled the "little green man" signal hypothesis and showed the British scientists that the unusual signal was being emitted by a very interesting, newly discovered natural phenomenon—the pulsar.

Antony Hewish collaborated with Sir Martin Ryle (1918-84) in the development of the radio wave-based astrophysics. His efforts included the discovery of the pulsar—for which he shared the 1974 Nobel Prize in physics with Ryle. Although his graduate student Jocelyn Bell Burnell was actually the first person to notice the repetitive signals from this pulsar, the 1974 Nobel awards committee inexplicably overlooked her contributions. However, while giving his Nobel lecture, Hewish publicly acknowledged his student's observational contributions in the discovery of the first pulsar.

The detection of the first pulsar, a natural phenomenon of great interest in astronomy, was originally regarded (with a good degree of British humor) as a "little green man" signal. Some powerful radio telescopes, like the Arecibo Observatory in Puerto Rico (see chapter 9), are used in both capacities, namely in the role as a powerful radio transmitter and in the role as a supersensitive radio frequency receiver.

The social consequences of interstellar contact and the potential risks inherent in either SETI or CETI are discussed in chapter 11. This chapter concentrates on the technical and operational aspects of the more conservative "listen only" approach that is characteristic of modern SETI activities.

Humankind's search for intelligent life beyond Earth is an attempt to answer the important philosophical question: Is our species alone in the universe? The classic paper by Giuseppe Cocconi and Philip Morrison entitled "Searching for Interstellar Communications" (Nature, 1959) is often regarded as the start of modern SETI. With the arrival of the space age, the entire subject of extraterrestrial intelligence departed from the realm of science fiction and became treated—within many technical circles at least—as a scientifically respectable (although currently speculative) field of endeavor. Unfortunately, budget-axe-wielding politicians and visionless federal bureaucrats continue to find it fashionable and convenient to attack any attempt to commit even very modest quantities of federal research monies to the SETI effort. SETI scientists find it difficult to understand how research funding has been unquestionably approved by agencies of the federal government to investigate the mechanics of bovine flatulence but withheld from any serious attempt to pursue scientifically an answer to one of humankind's most enduring philosophical questions.

The current understanding of stellar formation leads scientists to postulate that planets are normal and frequent companions of most stars. As interstellar clouds of dust and gas condense to form stars, they appear to leave behind clumps of material that form into planets. Astronomers estimate that the Milky Way galaxy contains some 100 billion to 200

This artist's rendering depicts a young star encircled by full-sized planets and rings of dust beyond. These rings, or debris disks, arise when embryonic planets smash into each other. The sequence of events for one of these catastrophic planetary body collisions appears in the upper left inset. The illustration shows how planetary systems arise out of massive collisions between rocky bodies. NASA's Spitzer Space Telescope is collecting data, which indicates that these catastrophes continue to occur around stars when they are as old as 100 million years and even after the alien solar systems have developed full-sized planets. (NASA/JPL-Caltech)

This artist's rendering depicts a young star encircled by full-sized planets and rings of dust beyond. These rings, or debris disks, arise when embryonic planets smash into each other. The sequence of events for one of these catastrophic planetary body collisions appears in the upper left inset. The illustration shows how planetary systems arise out of massive collisions between rocky bodies. NASA's Spitzer Space Telescope is collecting data, which indicates that these catastrophes continue to occur around stars when they are as old as 100 million years and even after the alien solar systems have developed full-sized planets. (NASA/JPL-Caltech)

billion stars. Starting in the 1990s, the detection of extrasolar planets by astronomers has validated this important assumption that planets are a normal byproduct of stellar evolution from clouds of dust and gas. (See chapter 7.) Consequently, there may be billions upon billions of planets in the galaxy—some, as yet an undetermined percentage of which, could be suitable for life.

Given an anticipated abundance of suitable planets, current theories on the origin and chemical evolution of life indicate that life probably is not unique to Earth but may be common and widespread throughout the galaxy. Furthermore, some scientists suggest that, once started, life on alien worlds will strive to evolve—leading in creatures with intelligence, curiosity, and even the technology to build the devices needed to transmit and receive electromagnetic signals across the interstellar void. For example, many intelligent alien civilizations (should such exist) might, like humans on Earth, radiate electromagnetic energy into space. This can happen unintentionally, as a result of planetwide radio-frequency communications networks; or intentionally through the deliberate beaming of structured radio signals out into the galaxy in the hope some other intelligent species can intercept and interpret these signals against the natural electromagnetic radiation background of space.

SETI observations may be performed using radio (and other) telescopes on Earth, in space, or even (someday) on the far side of the Moon. Each location has distinct advantages and disadvantages. Until recently, only very narrow portions of the electromagnetic spectrum have been examined for "artifact signals" (that is, those generated by intelligent alien civilizations). Humanmade radio and television signals—the kind that radio astronomers reject as clutter and interference—actually are similar to some of the type signals for which SETI researchers are hunting.

The sky is full of radio waves. In addition to the electromagnetic signals that human beings generate as part of their information-dependent technical civilization (for example, radio, TV, and radar), the sky also contains natural radio wave emissions from such celestial objects as the Sun, the planet Jupiter, radio galaxies, pulsars, and quasars. Even interstellar space is characterized by a constant, detectable radio-noise spectrum.

And just what would a radio-frequency signal from an intelligent extraterrestrial civilization look like? The accompanying figure presents a spectrogram that shows a simulated "artifact signal" from outside the solar system. This particular signal was sent by NASA's Pioneer 10 spacecraft from beyond the orbit of Neptune and was received by a deep space network (DSN) radio telescope at Goldstone, California, using a 65,000-channel spectrum analyzer. The three signal components are quite visible above the always-present background radio noise. The center spike appear ing in the figure has a transmitted signal power of approximately one watt, about half the power of a miniature Christmas tree light. SETI scientists are looking for a radio frequency signal that might appear this clearly but are also preparing their search equipment for one that may actually be more difficult to distinguish from the background radio wave noise (static). To search through a myriad of radio-frequency signals, SETI scientists have developed state-of-the-art spectrum analyzers that can sample millions of frequency channels simultaneously and can identify automatically candidate "artifact signals" for further observation and analysis.

In October 1992, NASA started a planned decade-long SETI program called the High Resolution Microwave Survey (HRMS). The main objective of HRMS was to search other solar systems for microwave signals, using radio telescopes at the National Science Foundation's Arecibo Observatory in Puerto Rico, NASA's Goldstone Deep Space Communications Complex in California, and other locations. Coupled with these telescopes were HRMS's, dedicated high-speed digital data-processing systems that contained off-the-shelf hardware and specially designed software.

The search proceeded in two different modes: a targeted search and an all-sky survey. The targeted search focused on about 1,000 nearby stars that resembled the Sun. In a somewhat less-sensitive search mode, the all-sky survey was planned to search the entire celestial sphere for unusual radio signals. However, severe budget constraints and refocused national research objectives resulted in the premature termination of NASA's HRMS program in 1993—after just one year of observation. Since then, while NASA has remained deeply interested in searching for life within our solar system, SETI projects no longer receive government funding.

Today, privately funded organizations and foundations (such as the SETI Institute in Mountain View, California—a nonprofit corporation that focuses on research and educational projects relating to the search for extraterrestrial life)—are conducting surveys of the heavens, hunting for radio signals from intelligent alien civilizations.

If an alien signal is ever detected and decoded, then the people of Earth would face another challenging question: Do we respond? For the present time, SETI scientists are content to listen passively for artifact signals that might arrive across the interstellar void.

Frequency->-

A simulated signal from an extraterrestrial civilization (using the Pioneer 10 spacecraft to transmit an "artifact" signal from beyond the orbit of Neptune). (NASA)

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