Occasionally we do see evidence of simple geometrical shapes like those familiar from human artifacts—or visible on the Earth from space. But normally our explanations for these end up being short enough that they seem to leave no room for anything like intelligence. And when we see elaborate patterns, say in nebulas or galaxies, we assume that these can have no purpose—even though they may remind us to some extent of human art.

So if we do not recognize any objects that seem to be artifacts, what about signals that might correspond to messages?

If we looked at the Earth from far away the most obvious signs of human intelligence would probably be found in radio signals.

And in fact in the past it was often assumed that just to generate radio signals at all must require intelligence and technology. So when complex radio signals not of human origin were discovered in the early 1900s it was at first thought that they must be coming from extraterrestrial intelligence. But it was eventually realized that in fact the signals were just produced by effects in the Earth's magnetosphere.

And then again in the 1960s when the intense and highly regular signals of pulsars were discovered it was briefly thought that they too must come from extraterrestrial intelligence. But it was soon realized that these signals could actually be produced just by ordinary physical processes in the magnetospheres of rapidly rotating neutron stars.

So what might a real signal from extraterrestrial intelligence be like? Human radio signals currently tend to be characterized by the presence of sharply defined carrier frequencies, corresponding in effect to almost perfect small-scale repetition. But such regularity greatly reduces the rate at which information can be transmitted. And as technology advances less and less regularity needs to be present.

But in practice essentially all serious searches for extraterrestrial intelligence made so far have been based on using radio telescopes to look for signals with sharply defined frequencies. And indeed no such signals have been found. But as we saw in Chapter 10 even signals that are nested rather than purely repetitive cannot reliably be recognized just by looking for peaks in frequency spectra.