Would an alien radio pick up a cacophony or a damp fizzle?

February 23, 2012 5:09 AM

If an alien located on a planet 100 light years from here was to switch on a big, multi-frequency radio receiver, and record all the noises coming from outer space for the next hundred years, on all frequencies, how many soap operas, advertisements and new broadcasts would they pick up from Earth? Would a mass-market radio, similar to our Earthly equivalents, pick up anything? Over time, as the number of Earth transmissions increases exponentially, would the alien pick up a cacophony or a damp fizzle?

We've all heard the cliché that since the first radio broadcast, the Earth has been spewing all our bad soap operas, CB-radio call outs, airplane distress calls and re-runs of Boy Meets World into outer space. This front of radio waviness is now as many light-years from Earth, in all directions, as the number of years since it was first transmitted (or so the cliché goes).

Now, it's also a function of radio wave propogation, that the Earth's ionosphere is used to bounce some of those waves around the world. Thus people in Zimbabwe can pick up BBC World Service. So, presumably, not everything ever transmitted will have left this planet, bound for space?

So, my question is about the percentage of those waves actually are travelling out in space? As time goes on, would the increase in transmissions from Earth's past begin to overwhelm all alien radio equipment? In 100 light years of space, how much of the transmission would be dampened by gas, gravity, etc? As the 21st century portion of the wave arrived at the receiver, how long would it be before all the transmissions sounded like 0s and 1s (announcing Earth's digital era)? Would the alien need special equipment? Or would any old radio pick up something, whatever frequency it was tuned to?

If two planets coincidentally started broadcasting around the same time, would the alien pick up a mixture of the two planets' frequencies? Or would the waves somehow cancel each other out as they meet on their individual journeys through space-time?

posted by 0bvious to science & nature (10 answers total)

Doubling the distance from a transmitter means that the power density of the radiated wave at that new location is reduced to one-quarter of its previous value. Inverse-square law or somesuch. Read up on radio propogation here.

I think 100 light years away would get absolutely nothing from us. Unless we sent out a high powered, focused radio signal directly at them.
posted by Grither at 5:22 AM on February 23 [2 favorites]

This is a FAQ: http://www.faqs.org/faqs/astronomy/faq/part6/section-12.html

Short version: TV / radio broadcasts are impossible to detect from more than a few AU. Narrow band transmissions (Eg radar) could be detectable from 100s of light years.
posted by pharm at 5:26 AM on February 23 [4 favorites]

Aside from the inverse-square issue someone already pointed out, there's the other big issue:

Radio works here on the planet due to localization. Let's consider you're watching "TV Channel 3". You're getting signal from a tower within maybe 20-50 miles of your location. The guys two big cities over ALSO have a "TV Channel 3", but it's a completely different station. They see theirs and you see yours because of relative proximity and relative signal strength. If you're a hundred miles up, however, and able to receive both, you receive neither - at least not clearly. With a sufficiently directional antenna, you can single out one of them, but that's just a different variation of localization.

An alien located on a planet 100 light years from here won't be receiving any easily-deciphered soap operas, advertisements, or news broadcasts. It would require specialized gear, super-sensitive, to pick up anything at all, but even if you did, it'll be the radio equivalent of the background noise going on in the crowd at a stadium: a bunch of extremely difficult-to-distinguish stuff.
posted by jgreco at 6:05 AM on February 23 [1 favorite]

Previously.

Also, at these distance scales it doesn't really matter whether the beam is "focused." Any optical system is going to have some finite focal length, at which the beam diameter passes through a waist, and beyond which the beam diverges again. There's a relationship that I can't remember right now which relates light/radio wavelength, lens/mirror/dish size, and the maximum focal length you can have. While we can certainly aim a directed radio signal anywhere in the sky, I'm pretty sure we can't "focus" a radio signal outside the solar system using antennas smaller than Earth.
posted by fantabulous timewaster at 6:05 AM on February 23 [1 favorite]

What if the ETs don't use radio/TV? As I understand it, detecting longer-wave signals would require receivers lots bigger than anything we currently have. Like, something bigger than you could build on Earth. So, if the ETs out there are using long-wave communications (as opposed to the shorter wave radio transmissions we've been looking for with SETI) we won't be hearing them until we can build something that can pick up transmission a hell of a lot longer than a 21 cm wavelength, which is the section of the radio spectrum where we've mostly been looking. Who's to say that the lack of "visible" radio chatter in the galaxy is less a function of no one being there than of them using a common means of communication among themselves that we just haven't discovered because we are not advanced enough?
posted by Guy_Inamonkeysuit at 6:13 AM on February 23

You might find this post interesting.
posted by run"monty at 6:26 AM on February 23

Thank you fabulous timewaster thought I was losing my marbles
posted by From Bklyn at 6:42 AM on February 23 [1 favorite]

So effectively projects like SETI are hunting for focused transmissions originally intended (by ET) to be received?

Would the ambient radio noise increase towards a cacophony as the alien continued receiving the 20th century wave, or would it be so dispersed that it made no real impact on the ambient wavelengths already permeating space?
posted by 0bvious at 7:37 AM on February 23 [1 favorite]

You can do long wave signals with reasonably sized antenna. And by you, I mean a sixteen year old science fair winner.

SETI is looking for non-natural radio sources, not for ineligible signals, so there is a subtle difference between what you're asking and what they're up to.
posted by Kid Charlemagne at 8:05 AM on February 23

Also previously.
posted by Homeboy Trouble at 10:38 AM on February 23