The SETI League, Inc., a membership-supported, non-profit {501(c)(3)}, educational and scientific organization Searching for Extra-Terrestrial Intelligence

Ask Dr. SETI ®

Chapter 3: Philosophy

How Far Can We Hear?

Dear Dr. SETI:
Could you please give me an indication of how far away we could detect a civilization with Earth-like technology? That is, if there were a society with radio, television, radar, etc., transmission power at a level equal to our own, radiating from omnidirectional antennas on an Earth-like planet, what would be the maximum distance in lightyears that we could detect these signals with our current SETI receiver sensitivity?

I would appreciate any clarification you could provide.
J. McA., Richmond, VA

The Doctor Responds:
My calculations indicate that incidental radiation from Earth-like technology could be detected out to at least 1000 LY by our larger SETI telescopes (such as the ones used in the SETI Institute's Project Phoenix targeted search). Since our members' own privately owned and operated radio telescopes are much smaller and less sensitive, it is unlikely that we would be able to detect such incidental radiation from any but the very nearest stars. For example, a one Megawatt transmitter driving a 100 meter diameter antenna can be detected by the typical amateur SETI station at a distance of 1 parsec, which is slightly less than the distance to the nearest star. Our private search hopes to encounter either significanlty more powerful transmissions, or highly directional beacons. Thus the two approaches (sky survey and targeted search) are complementary; see this article for elaboration.

The actual range over which we can hear a signal depends upon the transmit power and antenna gain of the civilization we are trying to detect -- factors over which we have no control whatever. It is interesting, however, to speculate, and to test the results of our best guesses as to extra-terrestrial technology. The spreadsheet programs "linkanal.xls" and "ranganal.xls" allow you to evaluate your own what if scenarios. They may be downloaded from here. Using such mathematical tools as these, your guess will be as good as mine!

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