In the breathtaking opening of the movie “Contact” – adaptation of the even more inspiring original Carl Sagan novel – we travel from Earth, faster than the speed of light and listening to the reverse history of our electromagnetic transmissions through the Universe.
The scene is literally poetic, namely it involves a big deal of poetic license, as we listen to transmissions from decades ago while still in the outer solar system – Pluto is not that far. It even has a small joke, as the Face on Mars make a very brief appearance.
But the cold (and perhaps surprising) hard fact is that the scene is impossible. No, not only because it’s impossible to travel faster than light, but because our radio transmissions have a very limited range, and our first radio transmissions are not in some place tens of light-years away. In fact the range of our transmissions is just… well, if you read the title you already know it.
Two light-years for almost all of our transmissions. After that, even ginormous antennas turned exactly to our planet and listening to the right frequencies applying the necessary corrections would not be able to capture an intelligible signal. Even a 3.000 km antenna wouldn’t be enough to capture our TV transmissions from a distance greater than only 0,01 light-years away!
Omicronians hooked on Ally McBeal are therefore only fiction, but keep this as a cherished secret. That our radio bubble is very small is a celestial gift, given all the trash we air and all the perils that may be out there. And it only reinforces the motivation for SETI.
Because if our unintentional transmissions vanish in a couple of light-years, our deliberate and specially powerful transmissions do reach out for hundreds of light-years. And even some of our powerful unintentional transmissions, like those from high-powered radars, also go very far.
Curiously, they would be detected by our neighbors somewhat like what we already detected on SETI, as anomalous, once in a lifetime signals.
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