The expanding sphere of radio signals traveling outward from the earth is often portrayed in TV shows and Hollywood movies to be like a time machine. The further you get from earth, the further back you go in the history of radio and TV broadcasts. Today we explore how far these signals have traveled, and more importantly, if radio signals actually work that way.

Earth’s Expanding Radio Bubble

As depicted in the beginning of the movie ‘Contact’, the earth has an expanding ‘bubble’ of man-made radio signals expanding outward at the speed of light. The first of these early radio transmissions were short range experiments that used simple clicks and interrupts to show transmission of information in the 1890s. In 1900, Reginald Fessenden made the first — though incredibly weak — voice transmission over the airwaves. The next year saw a step up in power as Guglielmo Marconi made the first ever transatlantic radio broadcast.

This means that at 110 light-years away from earth — the edge of a radio ‘sphere’ which contains many star systems — our very first radio broadcasts are beginning to arrive. At 74 light-years away, television signals are being introduced. Star systems at a distance of 50 light-years are now entering the ‘Twilight Zone’.

Will Any Extraterrestrial Life Within That Radio Sphere Detect Us?

While it’s interesting to imagine how far our radio signals have traveled into space, it’s extremely unlikely that an alien civilization will be able to catch the latest episode of ‘I Love Lucy’. This is thanks to the inverse square law. In Layman’s term, it’s a form of signal degradation.

As radio signals leave earth, they propagate out in a wave form. Just like dropping a stone in a lake, the waves diffuse or “spread out” over distance thanks to the exponentially larger area they must encompass. The area can be calculated by multiplying length times width which is why we measure it in square units – square centimeters, square miles, etc. This means that the further away from the source, the more square units of area a signal has to ‘illuminate’.

Another way to think of it, is that the strength of a radio signal will be only 1/4 as great once you are twice the distance from the source. At ten times the distance, the strength of the signal would only be one hundredth as great.

Because of this inverse square law, all of our terrestrial radio signals become indistinguishable from background noise at around a few light-years from earth. For a civilization only a couple hundred light-years away, trying to listen to our broadcasts would be like trying to detect the small ripple from a pebble dropped in the pacific ocean off the coast of California – from Japan.

So Why Does SETI Bother To Listen To Radio Signals In Space?

While no alien civilization is likely to pick up our television or radio broadcasts unless they’re within a few light-years, radio signals can be focused and amplified. Most of our broadcasts were not intended for detection in space. Radio signals can be aimed, focused and amplified to mitigate signal degradation for interstellar communication. These signals would also eventually degrade but are able to travel much, much further before degradation occurs. Hundreds of light-years or more depending on how much power is used.

It’s now becoming possible to detect the atmospheric composition of extrasolar planets. This breakthrough has allowed researchers to narrow down our hunt for earth-like worlds. It’s quite possible that an advanced alien culture can also do this, and detected an abundance of water in our atmosphere. If they have, they may have sent a focused radio message in our direction. If we’re not listening though, we may just miss it.

Citations:

Radiowave propagation, edited by M.Hall and L.Barclay, page 2, published by Peter Peregrinus Ltd., (1989)

Ismail Bullialdus, Astronomia Philolaica … (Paris, France: Piget, 1645), page 23

Dictionary of Electronics By Rudolf F. Graf (1974). Page 467.

Demetrius T Paris and F. Kenneth Hurd, Basic Electromagnetic Theory, McGraw Hill, New York

