Why I believe the search for extraterrestrial intelligence is best done optically

As there are 200 billion stars in our galaxy and more galaxies than stars, I cannot believe that we are the only intelligent life form in the universe. Whether we can make contact is another matter - I like to believe we will be able to do that, too, one day.

Our solar system is 4.5 billion years old, the universe about 15 billion years old, so there has been lots of time for advanced civilisations to come and go even within our galaxy.

If other civilisations are going to communicate with us, the chances are that they will be hundreds of thousands of millions of years ahead of us. They will have to come down to our level. Will they use a crude technology such as electromagnetic communication or something faster than the speed of light that we do not know about? If we assume that we are limited by electromagnetic technology and that is what they will use, would they use radio waves or laser technology? I think lasers are more likely.

It is objected that more energy is required to launch an optical photon than a microwave photon. That is true, but for an advanced civilisation, the economics could be different. What really matters is the signal-to-noise ratio at the receiving end. With lasers, the transmitted intensity is many orders of magnitude greater than with radio waves. If you want to send the Encyclopedia Galactica in bursts, you can support it on a laser beam but not on a microwave signal.

Also, with the optical Seti (search for extraterrestrial intelligence), you do not have to guess what is going to be the magic optical wavelength that these extraterrestrials would use for their lasers. You just need to know that it will be in the visible or the near-infrared part of the spectrum. And if there is a pulse there, you will receive it with excellent signal-to-noise ratio.

Nasa is developing free space laser communication for future deep-space probes - when men walk on the surface of Mars, the pictures of that one small step will come back on a laser beam, not on a radio beam. A laser transmitter is far more energy-efficient and compact than a microwave one. In fact, had Nasa had a bigger budget, it would have had deep-space probes with laser communication packages by now.

Stuart Kinglsey is director of the Columbus Optical Seti Observatory ( www.coseti.org ). He will chair the third conference on the Search for Extraterrestrial Intelligence in the Optical Spectrum in San Jose, California, in January.

* Interview by John Davies.