I do not search; I find.
Forty years of SETI projects have amassed a huge amount of data. Is it possible that somewhere in all that data there is a thumbprint of an ETC — a signal we have not yet recognized?
The SETI detectors can be fooled by a host of terrestrial signals: stray radiation from mobile phones, radar from military devices, and so on. The SETI astronomers are alert to these sources of interference and can usually identify them for what they are. But there remain a few tantalizing exceptions. For example, the META project logged several signals that were non-random and possibly intelligent transmissions.128 Zuckerman and Palmer examined 700 nearby stars and logged ten signals that could have been artificial.129 We have already discussed the famous "Wow!" signal.
The trouble is, whenever astronomers redirect their telescopes in the direction from whence the signal came, they find nothing. The signals never repeat. Perhaps these signals were indeed the intermittent broadcasts of ETCs, a lighthouse beam that swept across Earth before moving away. or perhaps they were simply a source of radio interference that has not yet been identified.
Another problem arises with the interpretation of the data from telescopes. We collect photons from gamma-ray bursters and explain their origin in terms of a cataclysmic fireball; we collect photons from stars with an infrared excess and deduce that the star is shrouded in dust; we find a thermal spectrum and infer that it comes from a black body. We could explain all these observations in terms of ETC activity. As we have seen, Ball suggested that ETCs might communicate by exchanging bursts of gamma-rays; one of the signatures of a Dyson sphere is an infrared excess; the most efficient mode of communication (which an ETC would presumably employ) is indistinguishable from black-body radiation to observers like us, who are not privy to the system used.
Ultimately, the difficulty is that we are stuck on a tiny rock, at the bottom of a thick atmosphere, trying to make sense of the Universe by interpreting the occasional photons our telescopes can catch. This is a challenge. Sometimes scientists may be wrong; but if we can explain observations in terms of natural phenomena, then we need not postulate the existence of ETCs. Occam, again. So when we observe, for example, that the spectra of almost all galaxies show a redshift, it is enough to explain it in terms of the expansion of the Universe — an explanation fantastic (and beautiful) enough in itself. We do not need to suppose, as did one SF story, that redshifts are the exhaust gases of alien craft fleeing from mankind.
We have to hope advanced ETCs will make their signals unambiguous and clearly distinguishable from noise. We have to hope their signals will be strong; if our present generation of detectors is insufficiently sensitive for the task, then 40 years of observation will have been wasted. And we have to hope they repeat their signals often. It would be a pity if we have already recorded a signal but cannot prove it is from an ETC.
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