The Tweet from Earth

Thirty-five years ago, scientists at the Arecibo Radio Telescope sent out the first message deliberately designed to be picked up by alien life.  The message was written and encoded by Dr. Frank Drake with some help from Carl Sagan.  The entire message was a 73 x 23 “picture” that showed the basic design of DNA, a graphic representation of a person and the solar system we inhabit.  The entire broadcast wasn’t much larger than a “tweet”; a short greeting to anyone listening in the general galactic population.

So, how easy would it be for aliens to tune into Earth?

The Search for Extra-Terrestrial Intelligence has historically focused its efforts in picking up radio signals from other solar systems.  People on Earth have been broadcasting since the beginning of the twentieth century.  Thus, it is believed signals from Earth would be visible to any star in a sphere some 200 light years around.  However, the first transmissions were short wave and medium wave bands (AM radio).  The good news is signals in these areas of the spectrum bounce off the ionosphere and allow listeners to tune in from around the world.  The bad news is the signals never make it through the atmosphere, so alien listeners would be out of luck.

FM radio broadcasting was invented in the late 1930’s.  These frequencies don’t bounce against the Heaviside layer and radiate into space.  Television transmissions do the same.  Perhaps aliens missed Jack Benny on radio in the 1940’s, but couldn’t they pick up his TV show from the 1950’s?  The largest radio telescope on Earth – Arecibo – fills an entire valley in Puerto Rico.  It is sensitive enough to be able to pull in a TV signal two trillion miles away.  That is impressive, but the nearest star is close to twenty-six trillion miles away.  And the signal sent from Arecibo 35 years ago was aimed at a group of stars some 5,000 times farther away than that.

While specific data might be lost in the vast distances between the stars, the sheer amount of radio energy coming from the Earth would alert our aliens that something was going on, even if they didn’t know what exactly.  UHF carrier waves are thought to be detectable by Earth type equipment sixty light years away.  However, as the switch to digital television transmission takes place around the world, the total amount of energy being blasted into space is decreasing.  On top of that, the digital data transmissions look an awful lot like random noise, unless the aliens have a converter box.

With an infinite amount of spectrum available, SETI researchers have picked certain key areas to search.  One considered most likely is the 21 cm band (1,420.4 on your FM dial, if it went that high).  This frequency contains the hiss of neutral hydrogen from around the galaxy.  Astronomers use this measurement to “see” in radio what they can’t in visible light.  Ironically, aliens listening in on this frequency and tuning into Earth would not pick up much of anything as this particular frequency is not in use.  And the one signal sent out deliberately from Earth was broadcast at 2380 MHz.

In 1977, a SETI team picked up a 72 second signal that seemed to match what an alien transmission might look like.  On the printed paper copy, the scientist in charge wrote “Wow!” next to the data.  Upon further review, no follow-up signal was ever detected.  One wonders if some alien somewhere was just testing its equipment long ago before setting back to listen once again.



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