Can alien beings listen in on TV and radio broadcasts from earth?

[alpe]

For the most part, this SETI supporter agrees with you, BUT.... I have to niggle about your line in Can alien beings listen in on TV and radio broadcasts from earth? that "The star closest to us, Proxima Centauri, is more than four light years away."

You got the rough distance to Alpha/Proxima Centauri right, but it isn't the closest star! That one is an average of just 93,000,000 miles and we call it SOL.
BTW, loved CONTACT - the movie and the book (except for the glaring error on the last page.) BILLIONS and BILLIONS of thoughts go to Sagan.

[jackdavinci]

Pedantic nitpicking is pedantic.

[Treesong]

"The aliens would be able to hear us at those enormous distances only if they already knew where we were and could point their telescope at us. If all they had was a hunch that we were out here somewhere, the likelihood they’d find us seems almost nil."

Actually, I'd put it almost 100% if they're in hearing distance at all. Wikipedia has a list of the nearest stars and there are about 50 within 16 light years. Assuming that's typical in our vicinity, and they could detect us at up to 500 light years, they would have about 1.4 million stars to check. If they threw out stars too young to have planets with evolved life, those with too small a zone around them where Xrpftmlike life could exist, etc., that could be trimmed down considerably. But even assuming that they do the whole lot, they could point their space-based antenna arrays to every star in turn and scan it for ten minutes (possibly not consecutive) and finish the search in 30 years. I don't know if ten minutes is a reasonable time, but if they're only scanning for anomalies in the microwave spectrum, so that they can follow up good possibilities, I'd think they would only need seconds. Times however many times they want to check, to detect intermittent signals.

And of course that's assuming only one technological species with one antenna array. Perhaps an alien species that detects us will try to open contact by beaming a signal back at us, increasing our chance of success.

"The age of pumping high-power terrestrial noise into the ether is likely to be a mere blip lasting less than a century."

Now, that's discouraging.

"[R]adar is a directed beam — alien observers might pick it up if it’s pointed their way and they know where to look for it, but realistically, how likely is that?"

I dunno; what is the spread of radar beams? Nowhere near as narrow as lasers, certainly.

And hey, alpe, if you're going to nitpick about the sun being the nearest star, why not go after 'into the ether'? You're not telling anyone something they don't know, you're just saying that the astronomical definition is the only right one and nobody should use the universally understood meaning of 'star' as 'a single twinkling nonplanetary light in the sky'. The two can coexist.

[whitetho]

I'd like to point out that, although the image of radio signals fleeing the surly bonds of Earth into the cosmos included in Contact makes for a great Hollywood visual, dating this expanding wavefront all the way back to the introduction of radio communication is scientifically inaccurate. In fact, until the introduction of VHF (Very High Frequency) transmissions, radio signals were pretty much trapped on Earth by the ionosphere.

Over the years the general pattern of radio development has been to use higher frequencies as more bandwidth becomes needed. The original longwave and mediumwave signals, used for Morse code communication, and, beginning in the 1920s, for broadcasting, did not escape Earth, but instead were either absorbed by the ionosphere (originally known as the "Heaviside-Kennelly layer") during the day, or reflected (okay, "refracted" to be technically correct) back to Earth at night.

Shortwave transmissions, developed in the early 1920s, also are predominantly reflected back to Earth. Thus, through the 1920s, there was speculation that the ionosphere had cut the Earth off from ever communicating via radio signals with the rest of the universe. Fortunately, it turned out that VHF and higher frequencies signals, used by FM and TV stations beginning in the late 1930s, can generally pass the ionosphere unimpeded.

Also, in order to reduce costs and increase efficiency, most broadcast FM and TV signals are to a degree focused. FM and TV antennas use stacked elements known as "bays", and the more bays, the more concentrated the signal into a narrow horizontal plane, aimed at the horizon. This is from memory of the 1980s, but if I recall correctly, a 16-bay antenna gave about a 10-times improvement -- i.e. a 500 kilowatt "Transmitter Power Output" (TPO), focused by a 16-bay antenna, resulted in a 5 megawatt "Effective Radiated Power" -- (ERP) i.e. the resulting effective signal strength for a receiver located in the plane of the transmitted signal. Thus, as the Earth rotates, for an individual station any extraterrestrial observer would see two pronounced signal peaks, as the turning Earth aimed first the front, and then the back, sections of the transmission plane. And of course there likely would be a jumble of signals, since multiple FM and TV stations share the same operating channels.

The higher the frequency, the more the signal wants to travel line-of-sight rather than follow the curve of the Earth, so, the U.S. Federal Communications Commission lets TV stations operating on higher frequencies use higher power in order to compensate. Thus, during the analog TV days, for low-VHF channels (2-6), the power limit was 100 kilowatts ERP, 316 kilowatts for high-VHF channels (7-13), and 5000 kilowatts for UHF (Ultra High Frequency) stations (originally channels 14-81). (The power limits were higher than needed for the UHF band to have coverage comparable to that of the VHF stations, as an incentive for people to build UHF stations). As Cecil noted, with the transition to Digital TV, most TV stations now operate on the UHF band (now contracted to channels 14-51), although with more stations crowded on fewer channels, their transmission powers have been reduced, but UHF signals are still generally stronger than VHF.

I bring this up because I believe the statement "The most powerful passive leakers are VHF television stations and military radar..." should actually read "UHF".

[whitetho]

To clarify something which may concern only me, I meant to say that I believe "VHF televison stations and military radar" should actually read "UHF televison stations and military radar".

[C K Dexter Haven]

Originally Posted by alpe

You got the rough distance to Alpha/Proxima Centauri right, but it isn't the closest star! That one is an average of just 93,000,000 miles and we call it SOL.

On t'other hand, if we got news that there had been a reply to our radio/TV signals from the third planet of Sol... well, the concept that there is some sort of life form on that planet wouldn't be, um, earth-shaking news. I suppose if it came from France, we could count it as alien life...

[Una Persson]

Originally Posted by whitetho

I'd like to point out that, although the image of radio signals fleeing the surly bonds of Earth into the cosmos included in Contact makes for a great Hollywood visual, dating this expanding wavefront all the way back to the introduction of radio communication is scientifically inaccurate. In fact, until the introduction of VHF (Very High Frequency) transmissions, radio signals were pretty much trapped on Earth by the ionosphere.

Cecil also recognizes that in the intro to Contact, they have 20 year old music playing by the time you get to Mars, which is insanely inaccurate (or, it was modern times tuned to an oldies station...right). I presume that showing nearby (relatively) empty interstellar space while timing the music would have been boring to the average viewer.

[anguilla]

I've never heard anyone address the notion that lots of TV stations use the same frequency to broadcast on. That's possible because TV signals travel in a straight line and the curvature of the Earth isolates one station's signals from the others. This means that even if an alien civilization was able to find Earth and chose a particular frequency to try to decode, the signal would be a blend of all the shows on all the stations on that frequency at that moment (for the stations on the side of Earth facing the alien receiver). That would made decoding pretty tough, if not impossible.

[user_hostile]

One aspect Cecil didn't talk about is the increasing number of planets that we find orbiting other systems. Many of those planets are uninhabitable; but as the technology improves, we will be able to discern planets that are in the sweet zone for life. That, in itself, would helps narrow the search for potential alien life. Conversely, presuming that aliens are of the same technology level or more advanced (what if they're 5000 years ahead of us?), it stands to reason that they've also searched for life outside of their neighborhood. And presuming they're within range, would tag our little speck as a possibility of intelligent life (insert oxymoron joke here) .

I'm not disagreeing with Cecil's assessment nor am I holding my breath we'll find other civilizations. But in the next 30 years, the possibility of finding a planet that indicates signs of life isn't outside the realm of possibility. The problem, of course, is whether that life evolved in to sentient beings who can master technology and figure out how to look for their own aliens.

[Chronos]

Quoth whitetho:

Thus, as the Earth rotates, for an individual station any extraterrestrial observer would see two pronounced signal peaks, as the turning Earth aimed first the front, and then the back, sections of the transmission plane.

Only one peak for most stations, since most of them are directional: The typical situation is a station with a bank of antennas on the outskirts of a populated area, with the array set up to transmit towards the populated area. Transmission in the other direction would be wasted power.

[johnpost]

Originally Posted by Chronos

Only one peak for most stations, since most of them are directional: The typical situation is a station with a bank of antennas on the outskirts of a populated area, with the array set up to transmit towards the populated area. Transmission in the other direction would be wasted power.

that all depends on geography and power. antenna farms are located based on height and the obstruction they cause. lower powered stations may be midcity. higher powered stations might be midcity or 20 miles out though intend on covering a large circular area.

[Musicat]

Originally Posted by whitetho

Shortwave transmissions, developed in the early 1920s, also are predominantly reflected back to Earth.

(Ex-Ham here, and I suspect you are an OM, too.) I think the percentage of reflection/transmission is important, but I don't know what that is. I was thinking this could be used in a Drake-like equation to calculate the signal strength on a distant planet.

[clairobscur]

Originally Posted by C K Dexter Haven

On t'other hand, if we got news that there had been a reply to our radio/TV signals from the third planet of Sol... well, the concept that there is some sort of life form on that planet wouldn't be, um, earth-shaking news. I suppose if it came from France, we could count it as alien life...

bip bipbip bipbipbip bipbipbipbipbip bipbipbipbipbipbipbip

[Gyrate]

Originally Posted by clairobscur

bip bipbip bipbipbip bipbipbipbipbip bipbipbipbipbipbipbip

bip?

Definitely alien life from France.

[Bill Dietrich]

http://en.wikipedia.org/wiki/Astronomical_radio_source says there are lots of radio-freq sources, including Sol. Anything accidentally emitted from Earth would be overwhelmed. Not much chance we'd be heard. http://www.billdietrich.me/Reason/Re...ceProgram.html

[gladeye]

Originally Posted by jackdavinci

Pedantic nitpicking is pedantic.

That reminds me, I need to get my toenails done.

[Chronos]

http://en.wikipedia.org/wiki/Astronomical_radio_source says there are lots of radio-freq sources, including Sol. Anything accidentally emitted from Earth would be overwhelmed.

"Radio" refers to an awfully wide band. Over the entire span of "wavelengths between a few centimeters and infinity", the Sun (and a number of other astronomical objects) are far louder than the Earth is. If you restrict it a bit more than that, though, it's not hard to find bands where the Earth is considerably louder than most natural objects.

[gnoitall]

Originally Posted by Chronos

"Radio" refers to an awfully wide band. Over the entire span of "wavelengths between a few centimeters and infinity", the Sun (and a number of other astronomical objects) are far louder than the Earth is. If you restrict it a bit more than that, though, it's not hard to find bands where the Earth is considerably louder than most natural objects.

Well, there's "loud", and there's "unnaturally modulated". It's conceivable that some alien radio astronomer would detect our emissions specifically because their spectral or modulation characteristics are completely unlike natural emitters.

Kind of like Terrestrial radio astronomers getting all excited about radio pulses from the stars with clock-like regularity... which turned out to be natural-source.

[Bozuit]

Originally Posted by Treesong

Assuming that's typical in our vicinity, and they could detect us at up to 500 light years, they would have about 1.4 million stars to check. If they threw out stars too young to have planets with evolved life, those with too small a zone around them where Xrpftmlike life could exist, etc., that could be trimmed down considerably. But even assuming that they do the whole lot, they could point their space-based antenna arrays to every star in turn and scan it for ten minutes (possibly not consecutive) and finish the search in 30 years.

If we're 500 light years from them they'll have to wait 400 years or so before they'll hear anything from us, though.

[Francis Vaughan]

Well, there's "loud", and there's "unnaturally modulated".

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 the touchstone for best possible information transfer in a given bandwidth is that it should be externally instiquishable from noise. 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. We are on the way there already. 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.)

[JKellyMap]

Originally Posted by 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?

[JKellyMap]

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?

[John W. Kennedy]

Originally Posted by 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.

[Francis Vaughan]

Originally Posted by 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.

[Powers]

Originally Posted by 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^]

[John W. Kennedy]

Originally Posted by 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.

[Francis Vaughan]

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.

[chorpler]

Originally Posted by 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.

[chorpler]

Originally Posted by 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, or sometimes METI, "Messaging to Extra-Terrestrial Intelligence."

One example was the 1974 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...

[John W. Kennedy]

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.

[Francis Vaughan]

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.

[John DiFool]

Yeah, I definitely subscribe to the "How not to be seen" side of that coin.

[JKellyMap]

Originally Posted by chorpler

This is what they call Active SETI, or sometimes METI, "Messaging to Extra-Terrestrial Intelligence."

One example was the 1974 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. "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.

[Irishman]

I can find a description but not a transcript of the TAM.