Box 1. Power requirements
From the following article:
The search for extraterrestrial intelligence
T. L. Wilson
Nature 409, 1110-1114(22 February 2001)
doi:10.1038/35059235
The measurement of interest for a transmitter–antenna combination is the effective radiated power (ERP). ERP is proportional to the product of the effective power fed to the antenna and the squared diameter of the antenna in wavelengths. This can amount to a very large amount of energy: for example, if a 1-MW transmitter operating at 10 cm is connected to a 100-m antenna, the peak ERP is equivalent to the power used for all technological activities on Earth around 1970. And if there were another 100-m radio telescope receiving this signal at a distance of 100 light years, detection would occur in less than 30 s.
Spreading the bandwidth makes detection more difficult, as the figure below shows. A signal transmitted over a 1-Hz band can be detected from much further away than a signal having the same ERP but transmitted in a bandwidth of 1 kHz. If an ETC is sending a constant signal, then we increase our sensitivity by summing the result in time. Such a process increases the distance at which we can detect a given signal with a given ERP. Summing the signal for 20,000 s instead of 1 s allows us to detect a transmitter 12 times as distant.
All the above assumes that signals from ETCs would be sent intentionally for the purposes of communication. Much more problematic is the detection of 'inadvertent' signals from an ETC, such as from television transmitters33, interstellar navigation beacons or the microwave emissions from orbiting satellites used for solar power production34.
Box 1 Figure Signal detection as a function of transmitter power, signal bandwidth and integration time. The diagonal lines represent the distance at which a given signal can be detected at five times the noise for a given bandwidth (in hertz) and a given integration time (in seconds). If, for example, a 200-MW transmitter operating at 3 cm is coupled to a 100-m radio telescope with an efficiency of 0.6, the ERP is 104 TW (1010 MW). The calculation is based on the assumption that the receiving antenna has a diameter of 100 m, an efficiency of 0.6, and is equipped with a 20-K noise temperature receiver. There are two sets of curves, for bandwidths of 1 Hz and 1 kHz. The dashed curves shown have shorter integration times than the corresponding solid curves18.
As an example, at a distance of just 10 light years, and using a 100-m telescope, we would need to sum a 6-MW signal for 10,000 years to detect it. We can shorten this time by increasing the size of the telescope. There was an ambitious proposal to build 'Project Cyclops', an array of up to 1,500 radio telescopes each of 100-m diameter17. With Cyclops, television broadcasts from ETCs could be detected by summing the signal for 4 h. But even in this optimistic case, detection of an inadvertent signal would not be easy.
