Owing to radio's aptitude in transporting information, our planet is endlessly peppered by man-made low-frequency radiation. Phone conversations, computer data, text messages, radar echoes, sitcoms, and morning DJ chatter are all electromagnetically belched in every direction at the speed of light-- including straight up into outer space.
Purveyors of science fiction are fond of exploring the ramifications of this radio leakage, suggesting that someday an advanced alien race might materialize to befriend, enslave, or destroy humanity after a little electromagnetic eavesdropping from afar. Indeed, if there happen to be any radio-savvy civilizations within 114 light years of Earth-- an area which encompasses roughly fifteen thousand stars-- humanity's earliest meaningful transmissions will have already reached them.
Similar speculation appears in science non-fiction, such as the Search for Extra-Terrestrial Intelligence (SETI) project, which strains its giant radio ears for extraterrestrial signals. When consulting the wisdom of probability, one finds that the universe ought to be teeming with technology-toting aliens; but aside from a couple of interesting-but-inconclusive detections, no discernibly intelligent patterns have ever been observed by Earth's space-listening instruments. One might surmise that the conspicuous silence is "evidence of absence," but such a conclusion might be a bit premature under the circumstances.
Outer space, as it was aptly put by the late Douglas Adams, is vastly, hugely, mind-bogglingly big. Astronomy's most up-to-date observations and calculations number the stars in the visible universe at somewhere around seventy sextillion (7 x 1022), an incomprehensible value which is seldom welcome in polite company. This figure is so formidable that any attempt to scale it for human consumption results in such impotent analogies as "ten times as many stars as grains of sand on all the world's beaches and deserts;" or, "ten trillion stars for every man, woman, and child on Earth."
At least two hundred billion of these stars reside within our own 13-billion-year-old galaxy, along with millions or billions of planets and moons. Considering the abundance of potential habitats and the amount time our galaxy has been around, it seems inconceivable that our ordinary planet is the only one which has produced intelligent, signal-radiating life. Even if a solar system's odds of developing intelligent life is only one-in-a-billion, that means that the Milky Way should be home to two hundred or so past or present civilizations, in addition to some seventy billion amongst the other galaxies.
In 1950, famed physicist Enrico Fermi was one of the first to popularize the discrepancy between probable and observable life in the universe. While lunching with colleagues and discussing the notion of interstellar neighbors, Fermi summed up the question by wondering aloud, "Where is everybody?" Thereafter the inconsistency was known as the Fermi Paradox. The paradox is a product of science's mediocrity principle, the observation that the Earth seems to be an ordinary planet orbiting an ordinary star within an ordinary galaxy. It follows, therefore, that Earth-like planets are probably somewhat common.
In 1961, a collection of ten distinguished scientists and engineers known as The Order of the Dolphin set upon a quest to remedy this astronomical shortcoming in our knowledge. They pondered the possibility of employing massive radio telescopes to scan the sky for stray extraterrestrial signals, a concept which eventually evolved into SETI. During these early discussions, astronomer Dr. Frank Drake first described a formula which effectively estimates the number of technologically advanced civilizations within the galaxy at a given time. To this day the Drake Equation remains as a framework for extraterrestrial speculation. There is much fist-shaking and spittle-making debate regarding most correct inputs, but as we gradually increase our knowledge of the universe, our guesses for these values become increasingly educated.
Even when using somewhat conservative inputs, the Drake Equation illustrates that our own humble galaxy is probably home to at least one other advanced civilization at present, along with the lingering physical and electromagnetic remains of many others. Massive radio telescopes have scoured the sky for such alien signals, including efforts by the Big Ear Observatory in Ohio; the Very Large Array (VLA) in New Mexico; and the famous Arecibo Observatory in Puerto Rico, the largest single-aperture telescope ever constructed. In forty-seven years of signal-seeking, SETI twice detected signals of possibly intelligent origins-- The "Wow!" signal in 1977, and Radio Source SHGb02+14a in 2004. But both had plausible Earthly explanations, so science must assume for now that they were not of extraterrestrial origin. The failure to find any stray radio evidence is taken by some as an indication there may indeed be something special about our planet and its location in the cosmos.
The Rare Earth hypothesis is the antithesis to the mediocrity principle, suggesting that complex life requires an extremely uncommon combination of astrophysical and geological events and circumstances: a slightly tilted planet with just the right chemistry, a large moon, a suitably metallic sun, and an orbit at just the right distance.
The hypothesis also advances the notion that there is a narrow galactic habitable zone where radiation levels are survivable, rogue meteors are few, and gravitational perturbations from neighboring stars are negligible. If all life relies upon such factors, then Rare Earth resolves the Fermi Paradox. The hypothesis carries the faint odor of anthropomorphic bias, however, since it assumes that all complex life must be very much like humans.
All these factors aside, there is one additional daunting obstacle which complicates any effort to tune in to intergalactic radio. Even if the universe is thick with signal-slinging civilizations, including some old enough that their indiscriminate electromagnetism has had sufficient time to reach Earth, not even the most massive and sensitive equipment of science is currently capable of plucking the signal from the static. When any non-focused electromagnetic signal is generated-- such as a television broadcast or a cell phone conversation-- the energy propagates as a spherical wavefront at the speed of light. When a sphere is doubled in diameter, its surface area increases by a factor of four; but in a spherical wave the "surface area" is the energy itself. This means the signal's energy is spread over four times more area at twice the distance, resulting in a 75% loss in intensity. To put it another way, in order for a broadcasting tower to double its effective range for a given receiver, it must quadruple its transmitting power.
To demonstrate the degrading effect of distance on an everyday omnidirectional signal, one might imagine a spacecraft equipped with an Arecibo-style radio receiver directed towards the Earth. If this hypothetical spacecraft were to set out for the interstellar medium, its massive 305-meter wide dish would lose its tenuous grip on AM radio before reaching Mars. Somewhere en route to Jupiter, the UHF television receivers would spew nothing but static. Before passing Saturn, the last of the FM radio stations would fade away, leaving all of Earth's electromagnetic chatter behind well before leaving our own solar system. If a range-finding radar beam from Earth happened to intersect the ship's path, it would be observable from a much greater distance; though its short duration and smooth, Gaussian meaninglessness would make it an inconclusive detection-- much like the Wow! signal and Radio Source SHGb02+14a. A highly focused beam such as that used to communicate with space probes would also remain detectable for some distance beyond the edge of the solar system.
If, hypothetically, A) a race of extra-intelligent extraterrestrials happened to reside in the nearby Alpha Centauri star system, B) they happened to broadcast a 5 Megawatt UHF television signal, and C) we were fortunate enough to be pointing the mighty Arecibo telescope directly towards the source when it arrived four years later, we would still be unable to enjoy the zany capers of the Alpha Centauri equivalent of Mork & Mindy.
In order to detect such a signal from this relatively proximate star, a dish with a diameter of 33,000 kilometers would be required. Even using Very Long Baseline Interferometry to link two Arecibo-style radio telescopes on opposite sides of the planet-- thereby providing a virtual radio telescope the size of the entire Earth-- our antenna area would still be 20,244 kilometers too small.
By coupling the laws of probability with our best current observations, we can be reasonably confident that some fraction of the 70,000,000,000,000,000,000,000 star systems in the visible universe are home to radio-sending species. It may indeed be that our planet is subjected to an unending spray of alien TV and radio signals, though they'd be attenuated beyond our best hardware's receiving extremes. Unless we dramatically improve our interstellar listening skills, or some alien race makes a specific and vigorous attempt to send us a message, there is little chance that we Earthlings will be trading messages with our astronomical neighbors anytime soon.