Over a 1950 summer lunch at the Los Alamos National Laboratory, the great physicist Enrico Fermi asked his colleagues an unexpected question – “Don’t you ever wonder where everybody is?” Laughter went around the table as everyone immediately knew that he was talking about extraterrestrial intelligence . If life arises fairly commonly, as Fermi believed, it follows that there should be advanced civilizations with the desire to visit and colonize Earth close enough to do so. However, there is no incontrovertible evidence of aliens on Earth, either now or in the past. This is called the Fermi Paradox. The lack of observational evidence for extraterrestrial intelligence is known as the ‘Great Silence.’
Giuseppe Cocconi and Philip Morrison published a paper in Nature in September 1959, in which they suggest a probable frequency at which alien civilizations would attempt to communicate: 1.420 GHz. This is the frequency of electromagnetic radiation emitted by neutral Hydrogen during a change of energy state. The frequency is an important physical and astronomical value, would almost certainly be known by any civilization capable of communication, and it requires only relatively simple technology to broadcast at this frequency. As Cocconi and Morrison put it, “It is reasonable to expect that sensitive receivers for this frequency would be made at an early stage of the development of radioastronomy. That would be the expectation of the operators of the assumed source, and the present state of terrestrial instruments indeed justifies the expectation.” In other words, it makes sense that aliens would come to the same conclusion about the 1.420 GHz frequency. If the aliens want to communicate with us, they should be broadcasting at this frequency. 
Extraterrestrial civilizations may have no wish to communicate with us. In the wild, creatures do not necessarily want to be found. Many in fact evolve mechanisms to hide. A noisy planet may put itself at risk of interstellar predators. Human beings certainly make no effort to mask our footprints; constant probes and signals emanate from planet Earth in hope of satisfying our insatiable curiosity and relieving our stellar loneliness, but there is no reason to believe that other civilizations necessarily have the same tendencies we do. Luckily, the laws of physics allow the detection of civilizations that may not wish to be found.
A sufficiently advanced alien civilization may construct a massive device around its star in order to capture most or all of its radiant energy. This is known as a ‘Dyson Sphere,’ suggested in 1960 by Freeman Dyson. Such devices would be employed by a Kardashev Type II or III civilization. Due to the inescapable laws of thermodynamics, the energy utilized by the civilization must eventually be radiated in the form of heat. Dyson’s 1960 paper proposed searching for advanced civilizations by examining signals in the infrared spectrum,...