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Past - Present - Future
by Dr. Alexander Zaitsev, IRE (alzaitsev @

There is only a moment between past and future,
Just it is called the Life.

Leonid Derbenev, Russian poet.
SETI, the quest for reasonable signals from Space, is aimed at the Past. We are really searching for signals that were sent to us (if any actually were) many, many years ago. Thus, we often search in the location where known exoplanets were at the time they transmitted toward us. In fact, the starry sky, as observed by us, is an image of the past, in the sense that we see celestial objects there where they were when they emitted the light now reaching the Earth. At present, each celestial body observed by us is actually in a slightly different place. This slight difference is related to PM, the proper motion of the celestial body, and is defined as the product of PM [in arc sec per year] times distance D [in Light Years] to the given body.

In contrast, any transmission of signals from Earth for reception by extraterrestrial civilizations (known as METI - Messaging to Extra-Terrestrial Intelligence) is directed toward the Future - our addressees will discover our messages (if they actually do discover them) only many, many years later, and in a location not where they are now, but where they will be at the moment our signals arrive. It is necessary to consider, when transmitting a directing a beam, that the target star will have moved slightly, between "now" and "then." This is similar to the proper motion considered for SETI, but with an opposite sign.

From here on Earth, we can say with good reason that conducting both searches (SETI) and transmission (METI) of reasonable signals, we appear poised just halfway between the Past and the Future: namely, in the Present! It is rather indicative of our place in time that in the Russian language, the concept "nastoiashchee" has two distinct meanings: it denotes "present," and "genuine."

An advanced civilization, terrestrial or extraterrestrial, may at some point attain such a high level of intellectual and technological development that it starts to feel the need to engage in both searches (SETI) and transmission (METI). The latter can be thought of as a purely messianic, unselfish activity, seeking to help our neighbors learn, that they are not alone in the boundless Universe. Such a socially mature civilization is worthy to be called "genuine." In implementing both SETI and METI, this civilization appears poised equidistant between the Past and Future, namely in the Present. Acting altruistically, unselfishly, not expecting any momentary benefits, caring only to enrich the existence of Others, such a civilization is engaged a worthy, genuine deed!

The figure below illustrates the concept of "Past - Present - Future."

illustration of proper motion

Consider OSETI, in which an optical system for transmitting and searching for reasonable signals in the Universe consists of lenses or reflectors for focusing either a beam of the powerful laser (during transmission), or radiation coming in from space (when searching). Accounting for the proper motion of a celestial body must be carried out at both ends! In the "Search" mode, we direct a telescope at the visible ("previous") position of the transmitting celestial body, and in the "Transmission" mode we enter a correction by pointing so as to hit where our target will be during the moment our signal arrives. It is interesting to note that our correction when transmitting is thus twice the product of PM and D.

In the case of transmission of radio signals, all of the above mentioned considerations will hold, except perhaps for the necessity to re-aim the antenna between the "Transmission" and "Search" modes. Consider that the beamwidth of even the largest radio antennas will be significantly larger than the angle of correction on antenna pointing. Therefore, the radio antenna can be directed continuously "on the center," that is, toward that point of space midway between "Past" and "Future," where there is actually a celestial body now ("at Present").

In the XVII century, French philosopher Blaise Pascal shared this emotional insight: "Eternal silence of these infinite spaces horrifies me!" The mature planetary consciousness, having felt as Pascal did, and having realized that this silence frightens not only us, but also all who inhabit the Universe, comes to an understanding that its mission is to contribute whatever it can to overcoming the silence of Space.

This editorial was submitted in response to Their Past, Our Future, by SETI League executive director emeritus H. Paul Shuch.

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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