At the radio-telescope observatory of the Cambridge University in England, an antenna was specially assembled in the mid-1960s for the purpose of conducting an investigation of the rapid variation, or scintillation , of celestial radio-wave sources. The array was made up of more than 2,000 smaller antennas that covered several acres of ground. Scintillations were observed and analyzed by a graduate student, Jocelyn Bell, and her professor, Anthony Hewish. Special electronic circuits were designed to scrutinize the scintillations, making graphs of the radio-wave strength as a function of time. This led to an unexpected discovery.
Little Green Men
As Bell examined the graphs, she noticed strange, inexplicable, extremely regular sequences of pulses. A radio source, having a sharply defined location in the sky, was emitting the bursts of energy at precise intervals. Scintillations caused by interstellar gas-and-dust clouds or by other natural effects were never so well timed. Neither Bell nor Hewish seriously believed that the pulses came from intelligent beings, but when the news of their discovery leaked out, certain people in the media immediately began spreading stories to the effect that the astronomers had received signals from an extraterrestrial civilization. The phenomenon was even given a tongue-in-cheek code name: LGM, an abbreviation for “little green men.” The radio source itself was called a pulsar and was given the designation CP1919, short for “Cambridge pulsar at right ascension 19 hours, 19 minutes.”
Before the results of their discovery could be released, Bell and Hewish had to be sure that the signals were coming from space and were not some form of Earthly radio interference. The position of the pulsar remained at RA 19 h 19 min and was fixed with respect to the stars, moving across the sky with them as the Earth rotated on its axis. Other tests were conducted to rule out the possibility that some internal combustion engine or electronic device was causing the signals, but the conclusion was inescapable: They were coming from somewhere in deep space, beyond the Solar System.
At the same time, tests were conducted in an attempt to find out if the signals were coming from a planet orbiting a star. In such a case, a Doppler shift would occur as the planet first moved away from the Solar System and then toward it in its circumnavigation of its parent star (Fig. 14-4). This would show up as a regular variation in the pulse interval. No such shift was found. Only if the planet’s ecliptic was at a right angle to our view would we observe no Doppler shift. (This would be a coincidence, but it could happen.) If numerous other pulsars could be found and their signals analyzed for Doppler shift, the question could be answered.
More pulsars were found in 1968, and since then, many of the objects have been catalogued. The periods, or intervals between pulses, vary greatly from pulsar to pulsar, but they are all regular timekeepers. A few pulsars show Doppler shifts, but this can be explained by supposing that such an object is one member of a binary system including one ordinary star and one pulsar. If pulsar signals really come from intelligent beings inhabiting planets, then we should expect to see Doppler effects in almost all of them—and this is not the case.
Although the “ticks” from pulsars always take place at regular intervals, the waveforms of the pulses are irregular. This is another argument against the theory that they are signals from extraterrestrial beings. A signal generated by a radio transmitter would be expected to have a smooth pulse shape, something like that shown in Fig. 14-5 a . However, the pulsars have rough traces; a hypothetical example is shown in Fig. 14-5 b .
When the angular diameter of CP1919 was measured, it turned out to be small. If it were visible, it would look like a flickering point of light. This did not come as a surprise to astronomers. The brevity of the pulse period and the shortness of the pulses themselves suggested a generating source that could be no more than a few hundred kilometers across. What sort of star could be that small and yet emit such powerful electromagnetic (EM) bursts? Scientists began to develop theories to account for the observations, but the task proved difficult.
Calculation of the distance to CP1919 indicated that it is about 400 light-years away. This determination was made using an ingenious scheme. At progressively longer wavelengths, the “ticks” from a pulsar arrive on Earth at greater and greater intervals. That is, the period of the pulsar depends on the wavelength to which a radio-telescope receiver is tuned. This takes place because the speed of EM wave propagation depends on the wavelength. In a perfect vacuum, all EM waves, however long or short, propagate at the same speed, approximately 2.99792 × 10 8 m/s. In interstellar space, however, which is not a perfect vacuum, radio waves travel slower than infrared, which in turn travels slower than visible light. (The effect is not peculiar to pulsars; it occurs for ordinary stars and emission nebulae as well.) This difference in propagation speed is tiny. Sophisticated instruments are required to detect it. This phenomenon is called dispersion , and it is the same effect that causes a prism to split white light into the colors of the rainbow.
The greater the distance to a pulsar, the more dispersion takes place, and the greater is the variation in the period as a function of wavelength. Knowing the extent to which dispersion occurs as a function of the wavelength, astronomers can calculate the distance to almost any pulsar because the pulse intervals are as regular as the output of an atomic clock. It is as if these objects were made-to-order distance-measurement devices.
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