Ask Dr. SETI ®

Tim, France

The Doctor Responds:
Your hypothetical alien radio astronomer would not be able to detect anything about our civilization from the Andromeda Galaxy, because we do not yet exist (in his/her/its timeframe). Remember that the Andromeda Galaxy is nearly 3 million light years distant. Thus, if the Andromedans pointed any kind of telescope (optical, radio, infrared, ultraviolet, x-ray, whatever) toward our planet, they would see our planet as it was just under three million years ago. That is not only long before humans started generating artificial electromagnetic emissions; it is long before our ancestors climbed down from the trees. All kinds of telescopes image things not as they are, but rather as they were.

Earth has been generating artificial signals in the FM and TV spectrum for perhaps sixty years. These signals would of course be detectable to sufficiently advanced civilizations within sixty light years of Earth. Our radio footprint is not now detectable at greater distances, simply because it has not yet traveled far enouth. These signals travel relatively unimpeded through the interstellar medium, at the fastest of all possible speeds (the speed of light). Thus, in another sixty years, they will be detectable out to 120 light years, and so on.

Transmission time notwithstanding, there are other artificial emissions from Earth that are even more powerful than our broadcast signals. During the Cold War, for example, our over-the-horizon search radars beamed massive signals into space, in search of incoming bombers and intercontinental ballistic missiles. Those signals (in the microwave spectrum, not all that far removed from the Hydrogen line and other popular SETI "magic frequencies") would be easily detectable over interstellar distances. If our cosmic companions have gone through a similar paranoid period, perhaps we could detect their radar signatures as well.