# How can we detect extra terrestrial radio signals? **URL:** \ **Category:** Factual Questions\ **Created:** [December 14, 2008, 3:22am UTC](https://boards.straightdope.com/t/how-can-we-detect-extra-terrestrial-radio-signals/477083 "2008-12-14T03:22:06Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![tzimmerman](https://avatars.discourse-cdn.com/v4/letter/t/278dde/32.png) [@tzimmerman](https://boards.straightdope.com/u/tzimmerman)\ **Post date:** [December 14, 2008, 3:22am UTC](https://boards.straightdope.com/t/how-can-we-detect-extra-terrestrial-radio-signals/477083/1 "2008-12-14T03:22:06Z")
As I understand it, the SETI program is constantly searching for artificial radio signals from intelligent life across the galaxy. I’ve also heard that alien life could know we are here because every radio and tv signal we’ve every produced is beaming through space. The question I have is: don’t radio signals weaken exponentially with distance? I mean if you drive more than 50 miles from a radio tower, that station’s signal quickly fades. How is it possible that any kind of discernable signal can be detected from light years away? ---
**Author:** ![anson2995](https://avatars.discourse-cdn.com/v4/letter/a/c77e96/32.png) [@anson2995](https://boards.straightdope.com/u/anson2995)\ **Post date:** [December 14, 2008, 4:22am UTC](https://boards.straightdope.com/t/how-can-we-detect-extra-terrestrial-radio-signals/477083/2 "2008-12-14T04:22:12Z")
> [@tzimmerman](#): > > The question I have is: don’t radio signals weaken exponentially with distance? I mean if you drive more than 50 miles from a radio tower, that station’s signal quickly fades. How is it possible that any kind of discernable signal can be detected from light years away? The reason you lose aradio or TV signal as you get further away isn’t because the signal degrades. It’s because the signal can’t bend around the curvature of the earth or things like mountains. When are signals go out into space, they go on forever. ---
**Author:** ![tzimmerman](https://avatars.discourse-cdn.com/v4/letter/t/278dde/32.png) [@tzimmerman](https://boards.straightdope.com/u/tzimmerman)\ **Post date:** [December 14, 2008, 4:31am UTC](https://boards.straightdope.com/t/how-can-we-detect-extra-terrestrial-radio-signals/477083/3 "2008-12-14T04:31:15Z")
> [@anson2995](#): > > The reason you lose aradio or TV signal as you get further away isn’t because the signal degrades. It’s because the signal can’t bend around the curvature of the earth or things like mountains. When are signals go out into space, they go on forever. What about the signal weakening exponetially? ---
**Author:** ![carnivorousplant](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnivorousplant/32/3563_2.png) [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)\ **Post date:** [December 14, 2008, 4:32am UTC](https://boards.straightdope.com/t/how-can-we-detect-extra-terrestrial-radio-signals/477083/4 "2008-12-14T04:32:18Z")
FM is propagated by a ground wave and is blocked by geographic features, but AM is propagated by a sky wave, bounces iff the D layer, I believe and is not blocked by geography. Radio waves do weaken by the square of the distance, as I recall. ---
**Author:** ![tzimmerman](https://avatars.discourse-cdn.com/v4/letter/t/278dde/32.png) [@tzimmerman](https://boards.straightdope.com/u/tzimmerman)\ **Post date:** [December 14, 2008, 5:42am UTC](https://boards.straightdope.com/t/how-can-we-detect-extra-terrestrial-radio-signals/477083/5 "2008-12-14T05:42:33Z")
> [@carnivorousplant](#): > > FM is propagated by a ground wave and is blocked by geographic features, but AM is propagated by a sky wave, bounces iff the D layer, I believe and is not blocked by geography. > Radio waves do weaken by the square of the distance, as I recall. I remember hearing something similar, which is why you can pick up am from a much greater distance. ---
**Author:** ![Wendell\_Wagner](https://avatars.discourse-cdn.com/v4/letter/w/8491ac/32.png) [@Wendell\_Wagner](https://boards.straightdope.com/u/Wendell_Wagner)\ **Post date:** [December 14, 2008, 8:01am UTC](https://boards.straightdope.com/t/how-can-we-detect-extra-terrestrial-radio-signals/477083/6 "2008-12-14T08:01:57Z")
I’m told that a radiotelescope as sensitive as the Arecibo array could detect TV signals from at most two light years away (as indicated in the section here called "Artificial Radio Spectrum): [http://www.rfreitas.com/Astro/ObservableETC1985.htm](http://www.rfreitas.com/Astro/ObservableETC1985.htm) Apparently beyond that distance the signal is lost in the noise. ---
**Author:** ![SenorBeef](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senorbeef/32/2220_2.png) [@SenorBeef](https://boards.straightdope.com/u/SenorBeef)\ **Post date:** [December 14, 2008, 9:38am UTC](https://boards.straightdope.com/t/how-can-we-detect-extra-terrestrial-radio-signals/477083/7 "2008-12-14T09:38:18Z")
Radio waves do get weaker quite fast. It’s not likely anyone would be able to detect our signals beyond a few light years, and that would be with some huge antenna arrays. SETI hopes to find stronger signals, possibly directional signals rather than omnidirectional signals. ---
**Author:** ![guizot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/guizot/32/3636_2.png) [@guizot](https://boards.straightdope.com/u/guizot)\ **Post date:** [December 14, 2008, 11:23am UTC](https://boards.straightdope.com/t/how-can-we-detect-extra-terrestrial-radio-signals/477083/8 "2008-12-14T11:23:30Z")
> [@Wendell\_Wagner](#): > > I’m told that a radiotelescope as sensitive as the Arecibo array could detect TV signals from at most two light years away. So all we’d pick up would be old radio shows from other planets. That hardly seems useful to me. Imagine how stale the jokes would be from the Jack Benny equivalent of Theta Centari-C. ---
**Author:** ![Baldwin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/baldwin/32/7264_2.png) [@Baldwin](https://boards.straightdope.com/u/Baldwin)\ **Post date:** [December 14, 2008, 12:48pm UTC](https://boards.straightdope.com/t/how-can-we-detect-extra-terrestrial-radio-signals/477083/9 "2008-12-14T12:48:10Z")
However, if another civilization was using an absurdly powerful broadcast, or for some reasons shooting off directional signals that happened to intersect us, maybe we’d get something. (Actually, that’s the premise of one of my screenplay outlines, in which ordinary television from an alien civilization becomes hugely popular on Earth.) We’ve occasionally sent out deliberate messages such as the 1974 Arecibo message. It was sent toward the M13 cluster, about 25,000 light years away, but I don’t know at what power, or whether it could realistically be detected at that distance. (Using our technology, that is. A more advanced civilization might have a means of creating a virtual radio telescope billions of miles across.) ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [December 14, 2008, 2:34pm UTC](https://boards.straightdope.com/t/how-can-we-detect-extra-terrestrial-radio-signals/477083/10 "2008-12-14T14:34:49Z")
Radio waves do **not** fade exponentially. They fade _quadratically_. (Which is in the category of _polynomial_ change.) An example of fading exponentially: if the strength halved after travelling distance x, then they would fade half again after travelling another distance x, etc. After 8x units, it would fade to 1/256th of it’s starting strength. After 9x units- 1/512th strength. With quadratic decrease, each doubling results in a reduction by 1/4th. So after 8x units it would be at 1/64th strength. Going to 9x units would be a small decrease to 1/89th. The difference is more dramatic the higher one goes. ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [December 14, 2008, 2:41pm UTC](https://boards.straightdope.com/t/how-can-we-detect-extra-terrestrial-radio-signals/477083/11 "2008-12-14T14:41:57Z")
Ditto ftg. If we radiate a signal with equal strength in all directions, any sphere you imagine centered on us will intercept exactly 100% of the signal on its inner surface. Since the area of the sphere is proportional to the square of its diameter or radius, the signal weakens according to distance squared. It isn’t hard to detect signals with strengths like our own from stellar distances, and as SETI progresses it gets practical to detect signals like our own from the distances of a greater and greater number of stars. While it is hard to say with a great deal of confidence, many people in and around astronomy think that it is probably only years or decades to the first discovery of extraterrestrial communication signals. Actually, it may be pretty correct to say that the number of stars close enough for us to detect humanlike emissions from their planets IS increasing exponentially with time. ---
**Author:** ![Wendell\_Wagner](https://avatars.discourse-cdn.com/v4/letter/w/8491ac/32.png) [@Wendell\_Wagner](https://boards.straightdope.com/u/Wendell_Wagner)\ **Post date:** [December 14, 2008, 3:29pm UTC](https://boards.straightdope.com/t/how-can-we-detect-extra-terrestrial-radio-signals/477083/12 "2008-12-14T15:29:30Z")
guizot writes: \> So all we’d pick up would be old radio shows from other planets. That hardly \> seems useful to me. Imagine how stale the jokes would be from the Jack Benny \> equivalent of Theta Centari-C. Did you read my post or did you just pick a random post to quote when you replied? The point of my post is that any star is too far away for even a huge radiotelescope to be able to pick out our television and radio signals from the noise. After two years no receiving apparatus can pick out a TV or radio signal. ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [December 14, 2008, 5:39pm UTC](https://boards.straightdope.com/t/how-can-we-detect-extra-terrestrial-radio-signals/477083/13 "2008-12-14T17:39:41Z")
> [@ftg](#): > > Going to 9x units would be a small decrease to 1/89th. Umm, that’s a typo there. 1/81. Sorry. ---
**Author:** ![diggleblop](https://avatars.discourse-cdn.com/v4/letter/d/ac91a4/32.png) [@diggleblop](https://boards.straightdope.com/u/diggleblop)\ **Post date:** [December 14, 2008, 6:55pm UTC](https://boards.straightdope.com/t/how-can-we-detect-extra-terrestrial-radio-signals/477083/14 "2008-12-14T18:55:32Z")
I don’t think aliens use radio waves personally. I think SETI using radio signals to contact aliens is comparable to me using smoke signals to contact the space station. ---
**Author:** ![Erdosain](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/erdosain/32/7284_2.png) [@Erdosain](https://boards.straightdope.com/u/Erdosain)\ **Post date:** [December 15, 2008, 12:16am UTC](https://boards.straightdope.com/t/how-can-we-detect-extra-terrestrial-radio-signals/477083/15 "2008-12-15T00:16:47Z")
> [@Napier](#): > > It isn’t hard to detect signals with strengths like our own from stellar distances, and as SETI progresses it gets practical to detect signals like our own from the distances of a greater and greater number of stars. While it is hard to say with a great deal of confidence, many people in and around astronomy think that it is probably only years or decades to the first discovery of extraterrestrial communication signals. [QUOTE=Wendell Wagner] The point of my post is that any star is too far away for even a huge radiotelescope to be able to pick out our television and radio signals from the noise. After two years no receiving apparatus can pick out a TV or radio signal [/QUOTE] Wait, aren’t you two saying exact opposite things? **Napier** says that we can detect signals from stellar distances and **Wendell Wagner** says we can’t detect anything beyond two light-years without a ridiculously large array. (Per Wikipedia, Proxima Centauri is 4.2 light-years from the sun.) So who is correct? This is fascinating to me. [QUOTE=Wendell Wagner] Did you read my post or did you just pick a random post to quote when you replied? [/QUOTE] Way to kill a joke, man! ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [December 15, 2008, 12:32am UTC](https://boards.straightdope.com/t/how-can-we-detect-extra-terrestrial-radio-signals/477083/16 "2008-12-15T00:32:39Z")
> [@Erdosain](#): > > Wait, aren’t you two saying exact opposite things? **Napier** says that we can detect signals from stellar distances and **Wendell Wagner** says we can’t detect anything beyond two light-years without a ridiculously large array. (Per Wikipedia, Proxima Centauri is 4.2 light-years from the sun.) So who is correct? **Wendell** is. To detect incidental RF activity (that is to say, signals like our commercial TV and radio broadcasts which aren’t intentionally beamed towards us) would require an antenna (or an equivalent array of antennae) spanning roughly the diameter of the solar system to pick it out of the background noise floor. ---
**Author:** ![Tim\_T-Bonham.net](https://avatars.discourse-cdn.com/v4/letter/t/46a35a/32.png) [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)\ **Post date:** [December 15, 2008, 12:36am UTC](https://boards.straightdope.com/t/how-can-we-detect-extra-terrestrial-radio-signals/477083/17 "2008-12-15T00:36:12Z")
> [@SenorBeef](#): > > Radio waves do get weaker quite fast. It’s not likely anyone would be able to detect our signals beyond a few light years, That’s still a pretty fair distance – after all, the furtherest any human object has traveled (_Voyager_, after decades of travel) is only about one and a half **light-days** away. ---
**Author:** ![SenorBeef](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senorbeef/32/2220_2.png) [@SenorBeef](https://boards.straightdope.com/u/SenorBeef)\ **Post date:** [December 15, 2008, 12:39am UTC](https://boards.straightdope.com/t/how-can-we-detect-extra-terrestrial-radio-signals/477083/18 "2008-12-15T00:39:16Z")
Well, yes, but there aren’t many stars that close, and I doubt there are random radio arrays in the middle of interstellar space nearby. ---
**Author:** ![Erdosain](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/erdosain/32/7284_2.png) [@Erdosain](https://boards.straightdope.com/u/Erdosain)\ **Post date:** [December 15, 2008, 1:25am UTC](https://boards.straightdope.com/t/how-can-we-detect-extra-terrestrial-radio-signals/477083/19 "2008-12-15T01:25:17Z")
> [@Q.E.D](#): > > **Wendell** is. To detect incidental RF activity (that is to say, signals like our commercial TV and radio broadcasts which aren’t intentionally beamed towards us) would require an antenna (or an equivalent array of antennae) spanning roughly the diameter of the solar system to pick it out of the background noise floor. I am disappointed. I was looking forward to an alien version of Lost, possibly titled Found. I guess I’ll just have to make do with **Baldwin’s** screenplay. ---
**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\ **Post date:** [December 15, 2008, 1:51am UTC](https://boards.straightdope.com/t/how-can-we-detect-extra-terrestrial-radio-signals/477083/20 "2008-12-15T01:51:43Z")
> [@Q.E.D](#): > > To detect incidental RF activity (that is to say, signals like our commercial TV and radio broadcasts which aren’t intentionally beamed towards us) would require an antenna (or an equivalent array of antennae) spanning roughly the diameter of the solar system to pick it out of the background noise floor. Then it would seem useless of SETI to even try. Yet there they are, hopelessly searching for one signal beamed directly to us. [Next page](https://boards.straightdope.com/t/how-can-we-detect-extra-terrestrial-radio-signals/477083.md?page=2)