Radio Transmitters

Radio Transmitters

As the Earth rotates, our more powerful radio transmitters slowly sweep the sky. A radio astronomer on a planet of another star would be able to calculate the length of the day on Earth from the times of appearance and disappearance of our signals. Some of our most powerful sources are radar transmitters; a few are used for radar astronomy, to probe with radio fingers the surfaces of the nearby planets.

The size of the radar beam projected against the sky is much larger than the size of the planets, and much of the signal wafts on, out of the solar system into the depths of interstellar space to any sensitive receivers that may be listening.

Most radar transmissions are for military purposes; they scan the skies in constant fear of a massive launch of missiles with nuclear warheads, and augury fifteen minutes early of the end of human civilization. The information content of these pulses is negligible: a succession of simple numerical patterns coded into beeps.

The mindless contents of commercial television and the integuments of international crisis and internecine warfare within the human family are the principal messages about life on Earth that we choose to broadcast to the Cosmos. What must they think of us? Image: Elena

Overall, the most pervasive and noticeable source of radio transmissions from the Earth is our television programming. Because the Earth is turning, some television stations will appear at on horizon of the Earth while others disappear over the other. There will be a confused jumble of programs. Even these might be sorted out and pieced together by an advanced civilization on a planet of a nearby star. The most frequently repeated messages will be station call signals and appeals to purchase detergents, deodorants, headache tables, petroleum and automobile products. The most noticeable messages will be those broadcast simultaneously by many transmitters in many time zones – for example, speeches in times of international crisis by the President of the United States or the Premier of the Soviet Union in the 1960’s.

There is no calling those television programs back. There is no way of sending a faster message to overtake them and revise the previous transmission. Nothing can travel faster than light. Large-scale television transmission on the planet Earth began only in the late 1940’s.

Thus, there is a spherical wave front centered on the Earth expanding at the speed of light and containing Howdy Doody, the Checkers speech of the Vice-President Richard M. Nixon and the televised inquisitions by Senator Joseph McCarthy. Because these transmissions were broadcast a few decades ago, they are only a few tens of light-years away from Earth. If the nearest civilization if farther away than that, then we can continue to breathe easy for a while. In any case, we can hope that they will find these programs incomprehensible.