Box 2. The Kardashev classification

Kardashev²¹ classified possible ETCs according to the energy at their disposal (see table below). This scheme allows us to determine whether we are dealing with a civilization like our own (type I), a rather advanced civilization (type II) or a vastly more advanced civilization (type III). Humanity has sufficient resources at present to broadcast messages comparable to a type I civilization in a specific direction, although in practice the types of transmission are based on isotropic radiators. A Type II transmission might be transmitted by an ETC that had captured all of the power from its central star. These ETCs are referred to as Dyson civilizations²⁷. Type III civilizations have captured the power of an entire galaxy.

In a recent survey of SETI results, it is reported that searches rule out type II civilizations to a distance of 10 million light years and type I civilizations to 1,000 light years (ref. 22). However, this is based on two assumptions. First, that ETCs transmit at centimetre-scale radio wavelengths, and second, that the bands surveyed include the transmitter wavelengths of nearby galaxies. Conservatively, one can state that for a sizeable part of our galaxy we can probably rule out the presence of type II and III civilizations, if these ETCs are broadcasting messages in the centimetre wavelength range. However, there exist sources of radio emission that are about a million times as powerful as those at the disposal of hypothetical type I ETCs. These arise from regions smaller than 1,000 times the distance between the Earth and Sun, although a detailed analysis of the noise characteristics of these signals shows that they are natural sources of radiation³⁵.