# Can alien beings listen in on TV and radio broadcasts from earth? **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [June 23, 2012, 4:22am UTC](https://boards.straightdope.com/t/can-alien-beings-listen-in-on-tv-and-radio-broadcasts-from-earth/626027 "2012-06-23T04:22:49Z")\ **Posts on this page:** 14\ **Page:** 2
**Author:** ![JKellyMap](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jkellymap/32/15880_2.png) [@JKellyMap](https://boards.straightdope.com/u/JKellyMap)\ **Post date:** [June 26, 2012, 1:41pm UTC](https://boards.straightdope.com/t/can-alien-beings-listen-in-on-tv-and-radio-broadcasts-from-earth/626027/21 "2012-06-26T13:41:11Z")
> [@Francis\_Vaughan](#): > > This is one factor that is missing from the commentary about technological advancement of communications. Radio communications is quite quickly ceasing to use trivial modulation techniques, like AM, FM, PSK, CW, and moving towards techniques that are both more resistant to interference and use as much bandwidth as possible. This means such things as spread spectrum, and compression. As these become more and more advanced, they become more and more like noise…Indeed military radar systems are already there. In the near future, an alien civilisation may point an antenna at us, and see what appears exactly like thermal noise, and yet they could be seeing full strength radio transmissions, and be incapable of telling the difference. (One notes that a clue as to our presence might be a deliberate hole in our transmissions at some selected frequencies - for instance 1420MHz - to allow radio astronomers access to the Hydrogen emission line there without interference.) Fascinating! This works the other way, too, right? Most any alien civilization will only have a brief period of “noiseless” emissions, followed by millennia of “noisy” ones? Doesn’t this make the whole SETI effort a waste of time? ---
**Author:** ![JKellyMap](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jkellymap/32/15880_2.png) [@JKellyMap](https://boards.straightdope.com/u/JKellyMap)\ **Post date:** [June 26, 2012, 1:49pm UTC](https://boards.straightdope.com/t/can-alien-beings-listen-in-on-tv-and-radio-broadcasts-from-earth/626027/22 "2012-06-26T13:49:35Z")
ETA: I just realized that, while SETI may be useless for detecting unintended planetary transmissions (like our once less-noisy, now more-noisy entertainment and local communications emissions), it IS good for finding INTENTIONAL interstellar communications signals, which are sure to be relatively noise-free. Remind me – WE have been sending out intentionally noise-free signals lately as well, no? Perhaps as part of the same SETI program? ---
**Author:** ![John\_W.Kennedy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_w.kennedy/32/1031_2.png) [@John\_W.Kennedy](https://boards.straightdope.com/u/John_W.Kennedy)\ **Post date:** [June 26, 2012, 3:37pm UTC](https://boards.straightdope.com/t/can-alien-beings-listen-in-on-tv-and-radio-broadcasts-from-earth/626027/23 "2012-06-26T15:37:34Z")
> [@Francis\_Vaughan](#): > > Only if you know what the precise modulation/compression system is can you decode it, and when you do, there is no redundant information, everything is useful. That last part is self-evident piffle. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [June 26, 2012, 4:17pm UTC](https://boards.straightdope.com/t/can-alien-beings-listen-in-on-tv-and-radio-broadcasts-from-earth/626027/24 "2012-06-26T16:17:41Z")
> [@John\_W.Kennedy](#): > > That last part is self-evident piffle. Why? The idea is hardly new. If the entropy of the stream is maximal it is indistinguishable from noise, and yet has maximal information content. Redundancy in the information implies less than maximum entropy. Lack of redundancy implies that all information is thus useful, rather than some being redundant, and thus of no additional use beyond what has already been received. ---
**Author:** ![Powers](https://avatars.discourse-cdn.com/v4/letter/p/edb3f5/32.png) [@Powers](https://boards.straightdope.com/u/Powers)\ **Post date:** [June 27, 2012, 10:27pm UTC](https://boards.straightdope.com/t/can-alien-beings-listen-in-on-tv-and-radio-broadcasts-from-earth/626027/25 "2012-06-27T22:27:53Z")
> [@whitetho](#): > > originally known as the “Heaviside-Kennelly layer”) Wait, the Heaviside Layer is a real thing? And here I thought T.S. Eliot made that up. Powers &8^] ---
**Author:** ![John\_W.Kennedy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_w.kennedy/32/1031_2.png) [@John\_W.Kennedy](https://boards.straightdope.com/u/John_W.Kennedy)\ **Post date:** [June 27, 2012, 11:58pm UTC](https://boards.straightdope.com/t/can-alien-beings-listen-in-on-tv-and-radio-broadcasts-from-earth/626027/26 "2012-06-27T23:58:35Z")
> [@Francis\_Vaughan](#): > > Why? The idea is hardly new. If the entropy of the stream is maximal it is indistinguishable from noise, and yet has maximal information content. Redundancy in the information implies less than maximum entropy. Lack of redundancy implies that all information is thus useful, rather than some being redundant, and thus of no additional use beyond what has already been received. With the unfortunate side effect that a one-bit glitch destroys the entire signal. Information Theory is a two-edged sword. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [June 28, 2012, 1:11am UTC](https://boards.straightdope.com/t/can-alien-beings-listen-in-on-tv-and-radio-broadcasts-from-earth/626027/27 "2012-06-28T01:11:35Z")
Quite true. It is clearly a limiting case. However you don’t need to add much redundancy back to allow it recover. Further, the way in which that redundancy is added may not be obvious, and require understanding of the coding process to even detect. Clearly the absolute limit case would not be used - one will always have some amount of redundancy present. Whether there is enough for our alien listeners to realise that there is unnatural modulation in use or not is another matter. In the presence of lots of background noise we may find that it is simply never viable. ---
**Author:** ![chorpler](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chorpler/32/2888_2.png) [@chorpler](https://boards.straightdope.com/u/chorpler)\ **Post date:** [June 28, 2012, 2:18am UTC](https://boards.straightdope.com/t/can-alien-beings-listen-in-on-tv-and-radio-broadcasts-from-earth/626027/28 "2012-06-28T02:18:42Z")
> [@Powers](#): > > Wait, the Heaviside Layer is a real thing? And here I thought T.S. Eliot made that up. Oh, it’s real, all right. It’s up, up, up, past the Russell Hotel. ---
**Author:** ![chorpler](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chorpler/32/2888_2.png) [@chorpler](https://boards.straightdope.com/u/chorpler)\ **Post date:** [June 28, 2012, 2:25am UTC](https://boards.straightdope.com/t/can-alien-beings-listen-in-on-tv-and-radio-broadcasts-from-earth/626027/29 "2012-06-28T02:25:04Z")
> [@JKellyMap](#): > > Remind me – WE have been sending out intentionally noise-free signals lately as well, no? Perhaps as part of the same SETI program? This is what they call [Active SETI](http://en.wikipedia.org/wiki/Active_SETI), or sometimes METI, “Messaging to Extra-Terrestrial Intelligence.” One example was the 1974 [Arecibo message](http://en.wikipedia.org/wiki/Arecibo_message) that Frank Drake and some associates sent to celebrate the revamping of the Arecibo telescope. They sent it to the M13 globular cluster, so it’ll get there in only 25,000 years… ---
**Author:** ![John\_W.Kennedy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_w.kennedy/32/1031_2.png) [@John\_W.Kennedy](https://boards.straightdope.com/u/John_W.Kennedy)\ **Post date:** [June 28, 2012, 4:21pm UTC](https://boards.straightdope.com/t/can-alien-beings-listen-in-on-tv-and-radio-broadcasts-from-earth/626027/30 "2012-06-28T16:21:31Z")
A part of me suspects that ordinary alien signaling will be layered (like TCP/IP or SNA) and have something very similar to HDLC at the equivalent layer. Maybe we should be on the lookout for 0x7E bytes. 😉 Communications actually intended for interstellar distances, of course, require a full ECC level of redundancy. Can’t rely on sequencing or mere error detection when turnaround is measured in years. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [June 28, 2012, 10:32pm UTC](https://boards.straightdope.com/t/can-alien-beings-listen-in-on-tv-and-radio-broadcasts-from-earth/626027/31 "2012-06-28T22:32:51Z")
If you are designing an error correction code from scratch now one would turn to things like Reed Solomon codes, possibly with cross interleaving. The neat thing here is that you can tune the coding to be resilient to quite significant damage by spreading out the information across a wide distance in the stream, and adding enough redundancy, also spread widely, to be able to reconstruct the data in the face of long periods of interference or outage. Where long might be tens, hundreds, potentially even millions of consecutive bits lost in the base stream. You can tune it to be resilient to the expected distribution of interference or outages. The downside is that the parameters to the coding become much more difficult to deduce if you are presented with the raw stream. Although it contains redundant information actually detecting the redundancy is non-trivial. Active SETI on the other hand is as dumb as possible. Basically it says, “Hi there, we’re made of meat. Meat, you hear that! Meat! Here is the menu. Lovely meat!” and repeat, sometimes even with pictures on the menu. ---
**Author:** ![John\_DiFool](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_difool/32/19543_2.png) [@John\_DiFool](https://boards.straightdope.com/u/John_DiFool)\ **Post date:** [June 28, 2012, 10:48pm UTC](https://boards.straightdope.com/t/can-alien-beings-listen-in-on-tv-and-radio-broadcasts-from-earth/626027/32 "2012-06-28T22:48:58Z")
Yeah, I definitely subscribe to the “How not to be seen” side of that coin. ---
**Author:** ![JKellyMap](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jkellymap/32/15880_2.png) [@JKellyMap](https://boards.straightdope.com/u/JKellyMap)\ **Post date:** [June 29, 2012, 1:18am UTC](https://boards.straightdope.com/t/can-alien-beings-listen-in-on-tv-and-radio-broadcasts-from-earth/626027/33 "2012-06-29T01:18:26Z")
> [@chorpler](#): > > This is what they call [Active SETI](http://en.wikipedia.org/wiki/Active_SETI), or sometimes METI, “Messaging to Extra-Terrestrial Intelligence.” > > One example was the 1974 [Arecibo message](http://en.wikipedia.org/wiki/Arecibo_message) that Frank Drake and some associates sent to celebrate the revamping of the Arecibo telescope. They sent it to the M13 globular cluster, so it’ll get there in only 25,000 years… Fascinating, thanks! Gotta love the [Teen-Age Message](http://en.wikipedia.org/wiki/Teen_Age_Message). “Hey, little green dudes, you dudes are, like, totally awesome, like, dude, whoa!”. Interesting that the first of these intentional messages will reach a star in Libra in 2029. I realize they’re pretty amateur, in terms of the coding and physics parameters, but hey, it’s a start. ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [June 29, 2012, 8:05pm UTC](https://boards.straightdope.com/t/can-alien-beings-listen-in-on-tv-and-radio-broadcasts-from-earth/626027/34 "2012-06-29T20:05:34Z")
I can find a description but not a transcript of the TAM. [Previous page](https://boards.straightdope.com/t/can-alien-beings-listen-in-on-tv-and-radio-broadcasts-from-earth/626027.md?page=1)