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How far do Earth radio/TV transmissions reach?
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puh32 Send message Joined: 8 Oct 99 Posts: 30 Credit: 4,746,631 RAC: 9 |
I’m having a hard time reconciling these three sentences: 1. “The signals radio/TV that are emitted from earth have a range of about 100 light years before the dissipate into the noise background.†[from https://setiathome.berkeley.edu/forum_thread.php?id=80585&postid=1833000#1833000] 2. "there isn't an instrument on Earth that can detect an Earth-like planet with Earth-like leakage of electromagnetic radiation." [from https://www.engadget.com/2017/05/25/listening-to-starlight-our-ongoing-search-for-alien-intelligenc/] 3. “It should be noted however that the most sensitive radio telescopes currently available on Earth would not be able to detect non-directional radio signals even at a fraction of a light-year,†[from https://en.wikipedia.org/wiki/Fermi_paradox] Am I to understand that whereas Earth leaks radio/TV transmissions as far out as 100 light years, the process of detecting such (weak? scattered?) signals is so difficult that we couldn’t do it ourselves right now (that is, we have no instrument that could pick up/identify radio/TV leakage from 100 light years away. In fact not even 1 light year away.) ? Or, in other words, right now we are only able to detect signals that were sent more or less directly to us (in our direction)? Sorry if I misunderstood something. It has happened before. |
Bob DeWoody Send message Joined: 9 May 10 Posts: 3387 Credit: 4,182,900 RAC: 10 |
I have detected contradictions too, both here in posts and in science programming on TV. Bob DeWoody My motto: Never do today what you can put off until tomorrow as it may not be required. This no longer applies in light of current events. |
puh32 Send message Joined: 8 Oct 99 Posts: 30 Credit: 4,746,631 RAC: 9 |
>I have detected contradictions too, both here in posts and in science programming on TV. I hear you here. Even so, this strikes me as a field where somebody would actually know the correct answers to these questions. Specifically, the question “are only able to detect signals that were sent more or less directly to us (in our direction)?†strikes me as being at the very heart of the SETI@home effort. Surely, someone will know? (Again, I’m very open to the possibility that I’m missing something obvious here.) |
Ghan-buri-Ghan Mike Send message Joined: 27 Dec 15 Posts: 123 Credit: 92,602,985 RAC: 172 |
It can be confusing. My understanding is that the Earth is "brighter" than the sun in the FM bands (lots of stations pushing signals) , but that is different than capturing a single discrete transmission. Aliens are not going to be watching a single episode of "I Love Lucy", no matter what the popular press has written. Analog TV was designed to push an earth-surface hugging signal to ranges of ~ 160 km. These were typically low powered transmissions for practical commercial reasons: (1) power costs money and (2) commercial spheres of influence. In the US it was set up so stations in Boston wouldn't step on the NY stations, etc. Nearby cities had staggered channels to limit interference. In my youth NY had VHF channels 2,4,5,7,9,11 and 13; while Philadelphia and 3,6,8,10 and 12. It was a frequency management issue. The typical single VHF TV broadcast signal will peter out quickly in space (brightness follows the inverse-square distance law). The figure I've seen is about 0.6 LY. UHF TV typically broadcast even less power. But TV isn't the only signal source. Cold war military radars were sky oriented and some pushed considerable power. The sensitivity of the receivers has grown by leaps and bounds, as has computing power. We have a chance of success. We are also constrained by the earth's atmospheric window, which blocks large portion of the RF spectrum. And no exoplanet is likely to have precisely the identical atmospheric window on their end. They could be broadcasting and we might never know it. Even if BTL comes up empty this time around, we have by no means exhausted the search possibilities Remember, we cannot see a single red dwarf star optically here from earth. Proxima Centauri is a little over 4 light years away, it is a massive fusion power source whose output dwarfs (no pun intended) any transmission man has made, and we can't see it without the aid of technology. |
Gary Charpentier Send message Joined: 25 Dec 00 Posts: 30639 Credit: 53,134,872 RAC: 32 |
One has to look at the assumptions behind the statements. An analog TV transmission isn't going to be detectable at a large distance because it is a very wide band transmission. But the audio portion even though lower power will be detectable at a farther distance because the signal has a much narrower bandwidth. Also while these are not omnidirectional signals, they are not beam signals. A military radar signal, essentially a short high power carrier wave pulse into a very directional antenna can be detected several orders of magnitude farther. The planetary radar at Arecibo operates by sending a radar pulse to another solar system body and listening to the reflection. Those reflections are extremely weak so we have considerable confidence we would pick up a similar radar out to a large volume of the galaxy. So ET won't be watching I Love Lucy, but they could be listening to our radar and even our Morse code transmissions. One other thing they may be able to detect is our power grid. Not directly, but all the transmitters on earth do vary slightly with the power grid so the sum total of our transmissions also does. Looking for ET is science and you should look to science papers, not general news for a answer to your question. |
Gordon Lowe Send message Joined: 5 Nov 00 Posts: 12094 Credit: 6,317,865 RAC: 0 |
One other thing they may be able to detect is our power grid. I agree; an Earthly power signature as a whole makes a lot more sense to me as something detectable from afar than random isolated transmissions. The mind is a weird and mysterious place |
Gordon Lowe Send message Joined: 5 Nov 00 Posts: 12094 Credit: 6,317,865 RAC: 0 |
Would anyone like me to ask the UK National Grid to boost their transmission power from from 33,000V transmission line to 35,00V just in case any Alphas are listening?? Yes! ;~) The mind is a weird and mysterious place |
Gary Charpentier Send message Joined: 25 Dec 00 Posts: 30639 Credit: 53,134,872 RAC: 32 |
Not the voltage, it is AC. Everything goes up and down, on your side of the pond 50 Hz, North America 60 Hz. That produces a rather obvious modulation. |
janneseti Send message Joined: 14 Oct 09 Posts: 14106 Credit: 655,366 RAC: 0 |
Not the voltage, it is AC. Everything goes up and down, on your side of the pond 50 Hz, North America 60 Hz. That produces a rather obvious modulation. The power grid doesn't use modulation. No carrier wave needed. Radio waves does like AM and FM. |
Gary Charpentier Send message Joined: 25 Dec 00 Posts: 30639 Credit: 53,134,872 RAC: 32 |
Not the voltage, it is AC. Everything goes up and down, on your side of the pond 50 Hz, North America 60 Hz. That produces a rather obvious modulation. It, the power grid, modulates everything! |
janneseti Send message Joined: 14 Oct 09 Posts: 14106 Credit: 655,366 RAC: 0 |
Yes Chris. You are right. If you have to deliver power offshore HVDC is better than AC. Britain have some islands like Sweden have. I know that HVDC cables are connected to both Gotland and Poland. In many cases, offshore transmission is better suited to HVDC applications than traditional AC. You could also could connect those cables to Internet:) |
William Rothamel Send message Joined: 25 Oct 06 Posts: 3756 Credit: 1,999,735 RAC: 4 |
It all depends on the antenna gain, transmitter power, absorption , polarization, signal to noise ratio, the atmosphere and a few other things as to the distance. In any event: after 50 years of listening it appears that eavesdropping on this type of signal is not possible since no one is in range of our detectors. It also means that If a SETI effort depends on repeated signal locations they aren't going to find any. The exceptions to my hypothesis may just be that planet in Alpha Centauri that has recently turned on their first radar or tv station; or the planet a few thousand light years away that regularly beams a high power message our way. For this reason I have said that every strong signal must be recorded and examined for content --I think that promising candidates could be screened right at the antenna as they were scanned and then sent on for further study for message type if any. I am sure that there are SETI practitioners, signal processing gurus and communication experts that would know exactly how to do this far better than I. I sincerely hope that we are or will soon apply this type of thinking to SETI efforts |
puh32 Send message Joined: 8 Oct 99 Posts: 30 Credit: 4,746,631 RAC: 9 |
From "An Introduction to Astrobiology" (ISBN 978 1 107 60093 5), p 288: "The myriad of weak radio transmissions from inside Earth's atmosphere is, in principle, detectable from space ... but the range would typically be no more than a few tens of light-years." |
puh32 Send message Joined: 8 Oct 99 Posts: 30 Credit: 4,746,631 RAC: 9 |
However, "SETI estimates, for instance, that with a radio telescope as sensitive as the Arecibo Observatory, Earth's television and radio broadcasts would only be detectable at distances up to 0.3 light-years, less than 1/10 the distance to the nearest star." (https://en.wikipedia.org/wiki/Fermi_paradox) I don't know what to make of all of these seemingly conflicting statements. Anyone? |
Gordon Lowe Send message Joined: 5 Nov 00 Posts: 12094 Credit: 6,317,865 RAC: 0 |
From "An Introduction to Astrobiology" (ISBN 978 1 107 60093 5), p 288: However, The way I interpret it is what they're saying is Arecibo is small. A much larger antenna from much farther away, could in theory pick up Earth's broadcasts. The mind is a weird and mysterious place |
William Rothamel Send message Joined: 25 Oct 06 Posts: 3756 Credit: 1,999,735 RAC: 4 |
If true about arecibo then what are we doing here at SETI trying to eavesdrop. |
Bob DeWoody Send message Joined: 9 May 10 Posts: 3387 Credit: 4,182,900 RAC: 10 |
I pick " They don't want to be found" especially by us. Bob DeWoody My motto: Never do today what you can put off until tomorrow as it may not be required. This no longer applies in light of current events. |
puh32 Send message Joined: 8 Oct 99 Posts: 30 Credit: 4,746,631 RAC: 9 |
>The way I interpret it is what they're saying is Arecibo is small ... Oh, good point. >We do the best we can with what we have. >... it is socialising with a group of like minded people. I certainly agree. |
William Rothamel Send message Joined: 25 Oct 06 Posts: 3756 Credit: 1,999,735 RAC: 4 |
The "They"' , if they exist, don't know about "Us" since they are not in our close neighborhood; and we are not sending any type of repeating beacon to promising directions in the sky. |
William Rothamel Send message Joined: 25 Oct 06 Posts: 3756 Credit: 1,999,735 RAC: 4 |
At 1000 feet (335 meters) across it is hardly small. There is at least one other that is larger. |
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