Radio or Light? How to Get in Touch

May 02, 2001

by Seth Shostak - Senior Astronomer

Whats the best way to hunt down the extraterrestrials? Traditionally, radio has been preferred. But is there some earthly bias in this assumption? Do researchers choose to scan the sky with large antennas only because, by historical accident, radio was developed decades earlier than the technology needed to communicate with light?

Actually, yes. To some degree, radio SETI has enjoyed the inside track thanks to our technological past. Your local electronics outlet is called Radio Shack, not Laser Shack.

But aside from history, theres another argument in favor of radio: its more efficient. It takes less energy per "bit" to send a message.

The reasoning behind this conventional wisdom is simple, at least if you made a passing grade in high school physics. The smallest unit of either radio or light, called a photon, has an energy that increases as the wavelength decreases. A single photon of green light at 5,000 angstroms wavelength packs 500,000 times as much energy as a 25 centimeter radio photon, simply because its wavelength is a half-million times shorter. That means that sending messages by radio ought to be a half-million times cheaper than using light, at least as far as your utility bill is concerned. For a conservation-minded Klingon, that could be important.

But in fact, the disparity isnt quite this imposing. Sending a single photon may not help the Klingons if it gets lost in background noise. At radio frequencies, theres a ubiquitous hiss of celestial static due to the leftover glow of the Big Bang. The once fearsome fury of the universes birth -- now muffled by 15 billion years of cosmic expansion -- is most bothersome at short radio wavelengths. A single radio photon, even at 25 centimeters, isnt adequate to stand out against this pervasive natural noise. A hundred or so radio photons, bunched together, are needed to convey a bit of information, although only one is required for visible light. So the energy-per-bit disparity drops to roughly a thousand, a little less daunting but still favorable to radio.

However, theres more to this story. If you have $50 million to spend on a telescope (or its Klingon currency equivalent), you can buy an optical mirror thats maybe 33 feet (10 meters) across. For the same cash, you can construct a radio reflector thats closer to 328 feet (100 meters) in size. The surface accuracy of the radio dish doesnt have to be nearly as good as the optical mirror, which accounts for the difference in cost. Therefore, for a given expenditure, you get 100 times the collecting area using radio telescopes. So chalk up this additional factor in favor yet again of radio.

Lamentably for you, dear reader, there is more. It sounds as if the arguments for using radio are as solid as a cement truck. However there is one more angle, and this time the light brigade really strikes back. Those aliens who choose to use optical transmitters can more accurately target the recipients of their messages. Consider the broadcasting end of any SETI experiment. The radio-inclined aliens might have a 100-meter antenna operating at 25-centimeter wavelength. That means their "beam" the patch of sky theyre illuminating with radio energy is about one-sixth of a degree across, or roughly 10 percent the area of the full Moon. (For those readers who have led sheltered lives and never seen the Moon, this beam is roughly the size of a pinhead held at arms length.)

But a 10-meter optical mirror, which the transmitting aliens could buy at the same cost, will focus green light into a beam thats a mere 0.000003 degrees across. Thats a pinhead at the height of a U-2 aircraft. Even if the aliens live under an atmosphere like Earths that tends to scramble light beams, its still a pinhead at the height of the Empire State Building. This enormous concentration of transmitted power -- the incredibly small beams achievable with light -- more than makes up for all the losses suffered by light versus radio in the battles described above.

Bottom line? The arguments for SETI experiments that look for light pulses, are as persuasive as those favoring a hunt for radio squeals. We should do both. So in the spirit of electromagnetic equality, well take this a bit further next time, and describe what methods SETI astronomers are using to search for laser-wielding aliens.