Plausible observable products of reasonable technologies are examined. It is obvious that many possibilities, such as particle beams, are very unpromising candidates for detection. Electromagnetic radiation is most promising, but as is well known, the shortest and longest wavelengths are unpromising because of physical and galactic barriers to their detection. The optical radiation of the contemporary human civilization is shown to be very difficult to detect. As has been concluded for some 35 years, microwave transmissions are by far the most promising signatures to search for. Their striking power, compared to, say human light emissions or even stellar radio emission, makes them promising. Quantitative comparisons are given (see the next page), showing very large advantages in signal power for the microwaves. Furthermore, the intelligent, therefore biological, origin of microwave signals is readily and definitively established. This is manifest in the sharply defined bandwidth in which signals will likely occur, the fact that they will very likely be highly polarized, and the fact that they will likely exhibit a well-defined and "unnatural" modulation.

It would appear that the search for intelligent radio signals is an extremely promising, relatively inexpensive, means to search for biology in space. However, political opposition will probably cause such searches to continue to be supported entirely by private contributions.

Comparison of Power Levels for an "Earth-Sun-like" Pair

Total starlight reflected from the planet		1016 watts
Reflected power per unit bandwidth:	£	10 watt/Hz
Total stellar radiated power per Hz at 10-cm	Å	106 watt/Hz

Human-produced nighttime light emission:	«	1012 watts
	«	10-3 watts/Hz
	«	10-4 reflected starlight
Human radio emission: Arecibo:	EIRP= 2.7 (1013) watts
	£	2.7 (1013) watt/Hz
		3 (1012) reflected starlight
		3 (1016)human light emission
	Å	3 (107) stellar flux density
Typical TV Transmitter:	EIRP= 106 watts/Hz (carrier)
		105 reflected starlight
	»	109 human light emission
	Å	stellar flux density