But it occurred to me in 1960, I think, a year after Cocconi and Morrison, that there might be aliens who were not communicating, that they might not want to communicate or they might have other means of communicating, so what about... what about detecting them, how could we detect uncommunicative aliens? And there is another way, which I got interested in. At that time they had satellite pictures of the earth, I think, already - I'm not sure, no, that may have been later - but in any case, if you look at the earth in the infrared band at night, nothing that you see is natural. The whole earth is in fact emitting in the infrared band only through human activities. So if you look down from a couple of thousand miles up at night with infrared telescope, what you see are flares from oil fields primarily, that is, the brightest patches on the earth are in Kuwait; and then after that you see of course big cities where humans are congregated. Everywhere where there is large scale human activities you see infrared radiation, and that's just the result of the second law of thermodynamics, that wherever humans are doing big engineering projects they have to radiate away the waste heat, just because of the laws of thermodynamics. Waste heat... waste heat has to be got rid of and it gets radiated away in the infrared band which is the natural frequency band for activities at a temperature of a few hundred degrees. So why not apply that to the universe at large? If there are extraterrestrials who are expansionist or have not controlled their populations, who are doing enormously big engineering projects, they also will have to radiate away their waste heat, and so they should be detectable over huge distances. So I proposed looking for these artificial sources of infrared, and so I published a little one page letter in Science called Search for Artificial Infrared Sources, and that was fine, and in recent years there have been some activities in that direction. We haven't discovered anything artificial. We now have very good infrared telescopes and so large numbers of sources have been found - nothing that is suspected of being artificial, but it's another channel which should be exploited.