ext question is: what should we say?

Some early attempts to communicate with aliens - including the plaques attached to NASA's Pioneer 10 and 11 spacecraft, and the phonograph records on the Voyager probes - were really only symbolic efforts. More likely to be received one day is the powerful radio broadcast devised by SETI pioneer Frank Drake and sent from the Arecibo telescope in Puerto Rico in 1974. We have a long wait for contact if we rely on this, though: the message won't reach the distant star cluster it was aimed at, M13, for 25,000 years. It was also a very brief message, containing only 210 bytes of information.

"These are greeting cards," says Seth Shostak, who is a senior astronomer at the SETI Institute. "It is nice to get a greeting card, but hard to decipher if it's in a language you don't understand, because the amount of material is so limited."

Interstellar geek speak

Although the Arecibo message is cunningly constructed, it is difficult even for a human to understand. The signal contains a series of 1679 bits, a number chosen as it is equal to the product of two prime numbers - 23 and 73. The hope is this will prompt an acute alien recipient to arrange the 1s and 0s into a 23-by-73 rectangle. Doing so reveals a rather complicated picture, which is supposed to give some basic information about our chemical and biological make-up, our civilisation and the solar system (see illustration).

To me it looks a bit like a small person with a big head, four eyes and eight bushy eyebrows. So the only bit I've got right is the "person", and there I had a big advantage. "It is much worse if you don't have lot of context, if you don't even know about Homo sapiens," says Shostak.

So where might we find some common ground with ET? Perhaps in mathematics. This idea goes back about three centuries, when it was suggested that we could hail beings on the moon by cutting a diagram of Pythagoras's theorem into the forests of Siberia.

In 1960, Dutch mathematician Hans Freudenthal proposed an interspecies language called Lincos that starts with simple mathematical statements - a series of beeps to represent 1 + 1 = 2 for example. It then uses these to define logical relationships, eventually building up to more complex concepts of time and space.

More recently in the 1990s, mathematician Carl DeVito and linguist Robert Oehrle, both then at the University of Arizona in Tucson, made an attempt to speed up this process by assuming that respondents have some knowledge of physical concepts. In DeVito's scheme, once some mathematical and logical symbols have been defined, we would launch into a description of the periodic table, and then discuss energy in terms of specific heats of the elements and so on. They argue that any civilisation ca

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