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This story is a bit different, so bear with me. It starts with the Hello World Project that Bernd Hopfengärtner was working on this past summer. He spent a considerable amount of time cutting a semacode into a wheat field just outside of Ilmenau in Germany. (See wheat field semacode.)

A "semacode" is a special kind of a barcode, a two-dimensional barcode that is a computer-generated visual tag. It has been used for ubiquitous computing projects; semacode takes an URL (a web address) and converts it into a graphic that can be read with a suitably-equipped picture phone. You see the semacode pasted to a real world object, take a picture with your phone, the software decodes the semacode image and voila you go to the URL on your web-enabled phone. For example, you could put a semacode tag on a lamppost outside 1600 Pennsylvania Avenue that encoded "http://www.whitehouse.gov."

So now Bernd has tagged the Earth with a semacode that reads "Hello World." His semacode is 170 meters by 170 meters, and consists of an 18x18 grid of light and dark squares. People can use tools like Google Earth to read it.

Apparently, he doesn't realize that, since this object can be seen from space, it "tags" the Earth for visiting aliens in the same way that streetcorners or other objects can be tagged for people.

It just so happens that the explicit idea of somehow creating codes or symbols that are large enough to be read from space by aliens is an idea that is more than a century old. In his 1867 novel From the Earth to the Moon, nineteenth century science fiction writer Jules Verne wrote about a way to communicate with the inhabitants of the Moon: