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Communicating Across SETI Time
by Valdemar Phoenix, KG2PM (email vphoenix @

I have recently read several articles and books written on SETI. Some of them have speculated about how we'll communicate with extra-terrestrial intelligence (ETI) once its presence is discovered. While these discussions are interesting, my present musings are that "communication with ETI" may be misleading because of a concept I'll call SETI time. This concept takes into account the speed of light, which also equals the speed of communication. Our physics of today is limited to this speed, so there is no point as yet arguing about black hole or worm hole communication possibilities. SETI time also relates to the age and technological level of civilizations and this limits our possibilities of ever "communicating" with ETI, even if there are many ET civilizations.

In standard astronomical usage, a light year is primarily a measure of distance, not of time. But SETI concepts do not necessarily conform to standard astronomical usage, and therein lies the problem. In SETI, a light year is also a measure of time, the amount of time since the signal left its source. A signal which has traveled one light year is now one year old. A signal from 1000 light years distant is 1000 years old, etc. If we assume, simply as a baseline, that a received SETI signal originated from its source at a time when its technology was approximately at our technological level today, then we can use the concept of SETI time as a measure of how advanced such a civilization would be. An ET signal found to be emanating from a distance of 100 light years, with a corresponding SETI time of 100 years, could therefore be assumed to have come from a civilization now 100 years more advanced than we are today.

1000 or even a 1,000,000 light years is nothing in standard astronomy. But in SETI, its implications are enormous! Let's put some SETI times into anthropological perspective. If we hear a signal that is 1000 light years distant in time and space, then its senders will now be 1000 years more advanced than we are, at a minimum. Compared to them, we are still in the Dark Ages. If we detect a signal with a SETI time of 100,000, we are Neanderthals or Early Modern Humans at best, still making stone tools and wearing bear skins.

I have heard estimates of the size of the Milky Way Galaxy as 70,000 to 100,000 light years across. Any intelligences that we detect from that far away will dwarf our technology and probably our brains as well. A signal of SETI time 1,000,000 puts us back to Homo Erectus, just learning to use fire. A 10,000,000 light year ET signal means that we as a species don't even exist yet, by comparison, and our ancestors are still in the trees. It's unlikely that we'll ever be able to hear an ET signal from such a distance, but ETI may be out there nevertheless.

What are the implications of SETI time for our SETI searches? Surely one effect is that we'd better limit our searches to 100 light years or less if we have any hope of actually communicating with ETI, and even this is unlikely. Recognizing SETI time as a factor, we need to be cautious and precise when we talk of "communicating" with ETI. Borrowing from shortwave radio terminology, maybe "listening" is the best we'll ever do.

This doesn't mean we won't have some learning opportunities. SETI is likely to end up being a kind of paradoxical "archeology of advanced technological civilizations," and possibly of long dead civilizations. Archeologists and anthropologists dig up remains of ancient beings and civilizations that preceeded us and were primitive by comparison. SETI may uncover civilizations far in advance of our technological level. We may indeed find signals once our equipment is sensitive enough. But real-time communications? I think not.

Disclaimer: The opinions expressed in editorials are those of the individual authors, and do not necessarily reflect the position of The SETI League, Inc., its Trustees, officers, Advisory Board, members, donors, or commercial sponsors.

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