How Far Away Could We See An Alien Civilization Transmitting a Powerful Radio Signal in Our Direction?

-- | May 10, 2016

Question: If the most powerful transmitter currently used on Earth was used by an alien civilization, what is the farthest it could be to still receive an intelligible signal by the most sensitive current receivers on Earth? Perhaps the signal could be a pulse of length 1 and a pulse of length 2*Pi. Maybe assume that the the transmitter broadcasts a spherical signal to assure coverage, or a shaped signal that takes advantage of Earth’s rotation. — Larry

Answer: The most sensitive radio telescopes for this purpose operate at low frequencies close to the same frequencies that we use for general radio communication. Assuming that the alien civilization transmits at these same radio frequencies, there are several calculations that we can make. The following is derived from a very nice article on just this subject by Abraham Loeb and Matias Zaldarriga. First, just looking at the “leakage”, or general “static” produced by Earth-like radio communications and powerful radar signals on Earth, one could see the radio communications signals produced here on Earth out to a distance of about 10 pc with current radio telescopes and out to about 500 pc using radio telescope facilities currently under development. Note that there are some reasonable assumptions regarding how much time is spent observing a given direction in the sky and what the spectral shape of the signal would be.

Jeff Mangum

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