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Consider a very different approach to finding aliens - the radio search for extraterrestrial intelligence. How is this different from fantasy and pseudoscience? In Moscow in the early 1960s, Soviet astronomers held a press conference in which they announced that the intense radio emission from a mysterious distant object called CTA-102 was varying regularly, like a sine wave, with a period of about 100 days. No periodic distant source had ever before been found. Why did they convene a press conference to announce so arcane a discovery? Because they thought they had detected an extraterrestrial civilization of immense powers. Surely, that’s worth calling a press conference for. The report was briefly a media sensation, and the rock group, The Byrds, even composed and recorded a song about it. (‘CTA-102, we’re over here receiving you./ Signals tell us that you’re there./ We can hear them loud and clear...’)

Radio emission from CTA-102? Certainly. But what is CTA-102? Today we know that CTA-102 is a distant quasar. At the time, the word ‘quasar’ had not even been coined. We still don’t know very well what quasars are; and there is more than one mutually exclusive explanation for them in scientific literature. Nevertheless, no astronomers today, including those involved in that Moscow press conference, seriously contend that a quasar like CTA-102 is some extraterrestrial civilization billions of light years away with access to immense power levels. Why not? Because we have alternative explanations of the properties of quasars that are consistent with known physical laws and that do not invoke alien life. Extraterrestrials represent a hypothesis of last resort. You reach for it only if everything else fails.

In 1967, British scientists found a much nearer intense radio source turning on and off with astonishing precision, its period constant to ten or more significant figures. What was it? Their first thought was that it was a message intended for us, or maybe an interstellar navigation and timing beacon for spacecraft that ply the space between the stars. They even gave it, among themselves at Cambridge University, the wry designation LGM-1 - LGM standing for Little Green Men.

However, they were wiser than their Soviet counterparts. They did not call a press conference. It soon became clear that what they were observing was what is now called a ‘pulsar’, the first pulsar to be discovered. So, what’s a pulsar? A pulsar is the end state of a massive star, a sun shrunk to the size of a city, held up as no other stars are, not by gas pressure, not by electron degeneracy, but by nuclear forces. It is in a certain sense an atomic nucleus a mile or so across. Now that, I maintain, is a notion at least as bizarre as an interstellar navigation beacon. The answer to what a pulsar is has to be something mighty strange. It isn’t an extraterrestrial civilization. It’s something else: but a something else that opens our eyes and our minds and indicates unguessed possibilities in Nature. Anthony Hewish won the Nobel Prize in physics for the discovery of pulsars.

The original Ozma experiment (the first intentional radio search for extraterrestrial intelligence), the Harvard University/ Planetary Society META (Megachannel Extraterrestrial Assay) programme, the Ohio State University search, the SERENDIP Project of the University of California, Berkeley, and many other groups have all detected anomalous signals from space that make the observer’s heart palpitate a little. We think for a moment that we’ve picked up a genuine signal of intelligent origin from far beyond our solar system. In reality, we have not the foggiest idea what it is, because the signal does not repeat. A few minutes later, or the next day, or years later you turn the same telescope to the same spot in the sky with the same frequency, bandpass, polarization, and everything else, and you don’t hear a thing. You don’t deduce, much less announce, aliens. It may have been