by Chris Boyce (cb @ ndirect.co.uk)
Radio amateurs in the SETI League have recently been discussing Long Delayed Echoes (LDEs), radio signals detected by the sender seconds or minutes after being transmitted. This phenomenon has been observed for over 70 years, and is widely documented. One working hypothesis is that LDEs are evidence of interstellar probes trying to get our attention. Though there is little in the record to support this hypothesis, further study is warranted.
A natural explanation for LDEs was put forward by Anthony Lawton almost a quarter century ago ['Long Delayed Echoes: the Search for a Solution' by A.T Lawton and S.J. Newton, Spaceflight, vol. 6 no 5, pp 181-187, May 1974]. This was written in response to Duncan Lunan's hypothesis that the 1920s LDEs could be pattern interpreted and that they indicated a station-keeping probe from Epsilon Bootes lay in either the L4 or L5 Lagrange points in the Moon's orbit around the Earth (published in his book 'Man and the Stars', 1973 - 'Interstellar Contact' in the USA). Such a probe had been suggested by Bracewell who also suggested a message may lie in the echoes themselves.
Lawton's natural origins hypothesis was regarded as sufficiently impressive by the British Interplanetary Society for them to award the author Fellowship of the Society. Since that time it has been cited a number of times usually in the context of dismissing further investigation of the phenomenon.
That notwithstanding, the L4 and L5 points actually were subsequently investigated but with negative results ['A search for natural or artificial objects located at the Earth-Moon libration point', Icarus, vol. 42, pp 442-7, 1980].
Twenty years ago I wrote a book 'Extraterrestrial Encounter' (same title in USA) in which I presented a case for ET probe searching. This was based on the hypothesis that the replicator probe strategy outlined by Michael Arbib ['The Likelihood of the Evolution of Communicating Intelligences on Other Planets', in Interstellar Communication: Scientific Perspectives (C. Ponnamperuma and A. Cameron, Eds.), Boston: Houghton Mifflin Co., (1974) pp. 59-78] This appeared so logical a strategy that I assumed, then and now, that any alien society with sufficient technical proficiency would probably adopt it if they intended exploring the cosmos.
There is good reason to make further searches. The technological level of the probes need not be particularly advanced on our own whereas the average ET society will probably be hundreds of millions of years beyond ourselves with technologies possibly 'akin to magic', to paraphrase A.C. Clarke. Replicator probes need only be a few centuries beyond us in technological development even if they were released a billion years ago. They would also be autonomous, self-sufficient and self-aware systems closer to biological entities than any robot device we might imagine. They will also be highly intelligent.
And they will be local. They'll have been in the Solar System for tens of millions of years at least. So where they are concerned we are the new kids on the block!
There's much more material on my website on this theme in the article The Logical Contact.
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this page last updated 4 January 2003
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