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Skywatching

Searching for aliens

Our planet is surrounded by a sphere of radio waves. That sphere is expanding at the speed of light and now has a radius of about 100 light years.

A light year is the distance light travels in a year: just under 10 trillion kilometres.

In principle, if there is anybody on a planet orbiting a star closer to us than 100 light years, and has a sensitive radio receiver with a big antenna pointing in our direction, they would be receiving evidence of a technically-oriented species here on Earth.

In the 1960s, radio astronomer Frank Drake had the idea that if there are technically-oriented civilizations out there, at least some of them will be transmitting radio signals.

If so, a radio telescope here on Earth might be able to detect them. He was the one who formulated the famous Drake Equation, where one could put in estimates of numbers of stars with planets, and the proportion of those planets with life forms, and how many of them would be technically advanced, and how many would be using radio.

At that time most of those numbers were just blind guesswork, but the project seemed worth doing. So Drake decided to search for radio signals from alien civilizations.

We know there is life around at least one sun-like star, so he chose sun-like stars as being the best targets. Considering the huge cosmic distances, even the nearest star after the sun is 4.3 light years away, the signals are going to be very weak, so he chose pair of sun-like stars that are close to us: Tau Ceti (11.9 light years away) and Epsilon Eridani (10.5 light years).

The next big problem is what frequency to tune to. The radio spectrum is huge, with space for many billions of radio communication channels. For us here on Earth this is an advantage, because there is plenty of space for radio, TV, satellites, radar and all the other things we do with radio signals.

The problem is that not many of these will be easily detectable after travelling far out into space. These signals will be so weak that there is no question of putting on the headphones and tuning around until you hear something.

Each channel might have to be monitored for minutes or hours, just to collect enough signal. So all Drake could do is guess.

There is one frequency that most radio astronomers in the universe are likely to be monitoring. The cold hydrogen gas in the cosmic gas clouds radiates a radio signal with a frequency of 1420.405751 MHz — a wavelength of 21 centimetres.

If you wanted to send a signal to alien civilizations, you could probably count on someone observing at this frequency. So that is what Drake tried. He put a state-of-the-art receiver on a 26-metre dish and started to search. There were some false alarms, but nothing genuinely alien turned up.

Maybe that is not a good frequency to use. Here on Earth the hydrogen signal is so astronomically important nobody is allowed to transmit signals in the band. In addition, all that hydrogen emission in space could blanket weak alien signals. However, then what frequencies should we monitor? This leaves the choice more or less wide open.

Thanks to modern developments in digital signal processing, we can now monitor millions or even billions of radio frequencies at once. In addition, we have far more sensitive receivers. Of course, we still have to be pointing our antennas in the right direction at the right time.

However, we should be able to substantially increase the odds by using modern radio telescope designs that can monitor more or less the whole visible sky at once, covering millions of stars, compared with Drake’s “one star at a time.”

If those aliens are out there, working their radio transmitters, there is a pretty good chance that in the next decade, we will detect one of them.

Then, what?

  • Mars lies in the west after dark.
  • Jupiter, shining like a searchlight, rises around 2 a.m.
  • Saturn rises at 3 a.m.
  • The moon will be full on the 19th.


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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]



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The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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