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Space Forum / SETI / November 2004Total radio noise sent out from Earth
I was hoping to get an idea of the *scale* of total radio noise that we send out into the cosmos through all our day to day activities like television and local radio broadcasts, mobile phones, satellite relays, etc. How much "noise" does the planet's total output amount to in 'Janskys' (if that is indeed the right measure?!) This might sound a bit of a dumb Q amongst all you experienced SETI people... but if an Earth-sized planet in the Alpha Centauri system - just a stone's throw across the other side of the cosmic "pond" - were to output the same level of radio noise as ourselves on a planet-wide scale, how easy would it be to pick up such signals from Earth? Last question: the radio transmitter onboard the Voyager spacecraft operates on just 23 watts of power. If (hypothetically speaking) Voyager was at Alpha Centauri and transmitting in our direction, would the Arecibo radio telescope have been able to pick up its weak signals at such a low output? Thanks Abdul Ahad > I was hoping to get an idea of the *scale* of total radio noise that > we send out into the cosmos through all our day to day activities like > television and local radio broadcasts, mobile phones, satellite I'd guess somewhere between 1GW and 50GW. > relays, etc. How much "noise" does the planet's total output amount to > in 'Janskys' (if that is indeed the right measure?!) Jansky's is power per unit area per Hz of the *received* signal. It is something like 0.000 000 000 000 000 000 000 000 01 W/Hz per square metre. Also the total noise doesn't really matter, as what is detectable is signals with high Jansky values, which means they must have a larger amount of power per Hz. > This might sound a bit of a dumb Q amongst all you experienced SETI > people... but if an Earth-sized planet in the Alpha Centauri system - > just a stone's throw across the other side of the cosmic "pond" - were Please see the FAQ. For SETI@Home to register it would need to have about 1GW of effective isotropic radiated power (EIRP), a bandwidth of less than 0.07Hz and be close to 1.42GHz. EIRP is the amount of power that an ideal (but theoretically impossible) transmitter that transmitted equally in all directions would have to have, in order to produce the same received signal. (Arecibo has a 1MW transmitter, but produces something like 22TW of EIRP, although not on 1.42GHz.) By averaging over a larger amount of time and tolerating a larger number of false positives, the SETI Institute say they could detect analogue TV carriers to beyond that distance. The SETI Institute could do both because they are controlling the telescope and can hold it on a target and immediately followup a possible detection. > to output the same level of radio noise as ourselves on a planet-wide > scale, how easy would it be to pick up such signals from Earth? Last It is odd signals that would be detectable not the total noise. The total noise would be difficult to distinguish from natural noise. > question: the radio transmitter onboard the Voyager spacecraft > operates on just 23 watts of power. If (hypothetically speaking) But with a reasonable amount of antenna gain. > Voyager was at Alpha Centauri and transmitting in our direction, would > the Arecibo radio telescope have been able to pick up its weak signals > at such a low output? Arecibo sized dishes are necessary to recover a usable signal at its current half a light day range. However, it is possible that there is a pilot carrier signal that might be detectable rather further than the range over which communication is possible. Also, you can use averaging to pull the signal out of the noise. The uplink to Voyager is more likely to be detectable at interstellar distances, as the uplink will have a higher EIRP because it can use larger dishes and larger amounts of power. Communication may not be possible as the signal won't be sent with that much excess over that needed to effectively communicate with the probe and range scales with dish diameter. (The receiver on Voyager is likely to have a conservative, and old design, so the ETI's might have lower noise receivers.) > > I was hoping to get an idea of the *scale* of total radio noise that > > we send out into the cosmos through all our day to day activities like > > television and local radio broadcasts, mobile phones, satellite > > I'd guess somewhere between 1GW and 50GW. That's a vast range! In its latest copy of the "Planetary Report", the Plantary Society mentions that this figure has been gradually coming down over the past couple of decades (humanity is effectively "quitening down") as many terrestrial broadcasting applications have moved to lower power, digital modes of transmission with advancing technology. So, if an ETI planet follows a similar pattern of "quitening down" shortly after its radio "boom" period like ours, then this may be one of the major factors in explaining the lack of intelligent signals to date from all across the sky. Since the SETI program is only listening for signals at *specific* wavelengths, if an Earth-sized planetary intelligence at Alpha Centauri was outputting the same amount of *overall* spurious radio noise as us, would it be fair to say that we cannot definitively claim that we'd listened to the end and can safely rule out the existence of any radio-broadcasting intelligence in the Alpha Centauri system? Put this another way: if we swapped places with Alpha Centauri and we were listenning for an intelligent signal from Earth (our planet) and exhausted the same 40 years of SETI research like we have, would we be able to conclude *for definite* that "there's no intelligent signals from Earth"? My own gut feel is that we can't, simply because we have not been fine-tuning our receivers across the entire band of radio frequencies, aimed constantly toward the habitable zones, at one or two planets, orbiting the two principal stars of Alpha Centauri, 24 hours per day, 365 days per year...over 40 years Do you have any thoughts on this? cheers Abdul Ahad > I was hoping to get an idea of the *scale* of total radio noise that > we send out into the cosmos through all our day to day activities like > television and local radio broadcasts, mobile phones, satellite > relays, etc. How much "noise" does the planet's total output amount to > in 'Janskys' (if that is indeed the right measure?!) But if you notice all of our progress in communication has resulted in reducing the "wasted" energy going into space. If we define progress as data transfer my local cabel delivers more channels than I pay for and I get well over two hundred a 5MHz each. And if I added the 15MHz hidef channels and all the pay channels which are there it would make the 15 broadcast channels in the area fall even deeper into the noise. And I get about 350 Mbits of internet on it. The higher the frequency the more we depend upon antenna gain rather than transmitter power. So actually the more advanced the technology the less wasted energy into space we would expect.  SignatureBeheadings work better than a pro-American TV network. Improvised munitions work better than smart bombs. And Americans think they can win Iraq. The Iron Webmaster, 3263 > > I was hoping to get an idea of the *scale* of total radio noise that > > we send out into the cosmos through all our day to day activities like [quoted text clipped - 13 lines] > than transmitter power. So actually the more advanced the technology > the less wasted energy into space we would expect. Great. I find it a struggle to understand precisely how the SETI 'listening' process works as I don't have a background in radio astronomy, but just picture this hypothetical scenario:- If you have an Arecibo style dish stationed on Pluto, the outermost planet in our solar system, and you pointed the antenna toward our inner solar system at each of three sources in turn: Mars, the Earth and then the Sun. At a given frequency in the radio waveband, Mars would be deadly silent. The Earth would give you a signal, due to intelligence on our planet. The Sun would give you a strong, "natural" noise signal. Would that sound about right? Now, if you repeat this listening process over and over with Mars, then the Earth, then the Sun again, each time tuning your receiver a different frequency. At a certain frequency of say 'X' GHz, the signal from Earth would outshine all three bodies in strength - including the Sun!? What is that "magic" frequency I wonder? If we knew what this 'X' GHz was, and we tuned into *that* particular frequency when listening for inteligent signals from "another Earth", then the results might *just* show something! Has this sort of thing already been tried I wonder...after 40 odd years of SETI I'm sure it has?! Abdul Ahad >Now, if you repeat this listening process over and over with Mars, >then the Earth, then the Sun again, each time tuning your receiver a [quoted text clipped - 7 lines] >already been tried I wonder...after 40 odd years of SETI I'm sure it >has?! My guess would be one of the big FM radio stations (except for those times when a narrow beam of something else happens to be pointing in just the right direction), so that's somewhere between 88 and 108 MHz. It's a bit tricky to look for SETI signals in the FM radio band because the receiver tends to get swamped with local FM radio signals that are many trillions of times stronger than any signal we might expect from ET. The same difficulty would arise for any frequency in which the Earth is bright. The obvious solution would be to build a big receiver on Pluto or on the far side of the Moon, but that's some way beyond current SETI budgets. SignatureMike Williams Gentleman of Leisure > >Now, if you repeat this listening process over and over with Mars, > >then the Earth, then the Sun again, each time tuning your receiver a [quoted text clipped - 11 lines] > times when a narrow beam of something else happens to be pointing in > just the right direction), so that's somewhere between 88 and 108 MHz. They, who live on the opposite shores of the interstellar ocean on a planet circling the G2V star Alpha Centauri 'A', could be broadcasting "Centaurian music" on their FM transmitters in full, digital 3D surround sound formats 24 hours a day... yet we'd have absolutely no hint of it! And that star system is virtually on our doorstep compared to all the other promising 'sun-like' stars tens of light years out... After 40+ years of SETI research, has anyone published a summary of some kind by candidate star - something like this, I wonder:- SETI PROGRAM RESEARCH SUMMARY 1960 - 2004 ================================================================================ Star: | Total No of Years | Peak Frequency: | Total System Output: | Active Listening: | | ================================================================================ Alpha Cen A 15.5* 4.2 GHz* 550 GWatt* Alpha Cen B ? ? ? 61 Cygni A ? ? ? 61 Cygni B ? ? ? Tau Ceti ? ? ? Epsilon Eridani ? ? ? Epsilon Indi ? ? ? Delta Pavonis ? ? ? Eta Cassiopeiae A ? ? ? ================================================================================ * These numbers are made up; not real. A simplistic summary like this would be really helpful to someone like me who lacks the technical know-how to interprete the heavy radio astronomy jargon. For example, if the total radio noise output (giga-watts) from a candidate star system is 10% above that expected from a star of that size and spectral class, may be that might indicate a "hot spot" of some kind. Also, if the frequency at which the total output from a star peaks is statistically significantly far removed from the "average" for a star of its kind, then maybe that would be another "hot spot", and so on. I am only interested in a *simplistic* overview summary laid out by *star*, and particularly keen on SETI results from sun-like stars within about 50 light years from Earth. I wonder if there's such a summary available? Another question I have is what about the radio noise received from a region *around* the candidate star? If the sky position of the star is denoted by 'S', is there any measure of radio noise received from say S + 1 arc-sec, S + 2 arc-sec, S + 3 arc-sec angular separations? This might flag up if ET has ventured outward from its parent star system, and is presently en-route on an interstellar voyage to a neighbouring system...?! Or is that too much in the realm of fantasy??? Abdul Ahad > After 40+ years of SETI research, has anyone published a summary of > some kind by candidate star - something like this, I wonder:- Only some SETI projects have candidate stars. The SETI Institute publishes their target list, although possibly not the details of the observation programme. > Alpha Cen A 15.5* 4.2 GHz* 550 Alpha Centauri isn't a serious candidate, because of being a double star. As far as I know, sun-like stars are not really naked eye stars. > * These numbers are made up; not real. > For example, if the total radio noise output (giga-watts) from a > candidate star system is 10% above that expected from a star of that As noted in my other reply, it will be very difficult to measure the output of a sun-like star - you are talking about a fraction of a percent of the total noise for the star alone, with lots of other stars, possibly radio stars, in the field of view. I doubt that it would be possible to predict the baseline noise level with any degree of accuracy. Also, that baseline can vary by two orders of magnitude over time. > denoted by 'S', is there any measure of radio noise received from say > S + 1 arc-sec, S + 2 arc-sec, S + 3 arc-sec angular separations? This Radio telescope resolutions are measured in arc minutes! david@djwhome.demon.co.uk (David Woolley) wrote in message: > Alpha Centauri isn't a serious candidate, because of being a double I wonder why Alpha Centauri isn't much of a candidate. It may be a binary which does complicate things from a radio resolution perspective I agree, but it sure as hell is the most interesting star in the whole night sky... if only because of its sun-like components and close proximity to us! Things that ought to matter *most* to us! > star. As far as I know, sun-like stars are not really naked eye > stars. Maybe the ones in the SETI target list aren't naked eye, but there is a huge list of sun-like naked eye stars that ought to be in such target lists. Check this:- http://www.starmapping.co.uk/sunlike.cfm > As noted in my other reply, it will be very difficult to > measure the output of a sun-like star - you are talking about > a fraction of a percent of the total noise for the star alone, > with lots of other stars, possibly radio stars, in the field > of view. So if there are two stars very closely spaced in the field of view, one at 20 light years and another one at 2,000 light years distance, the radio telescope has no way of distinguishing the signal from one star separate from the other? So how does the SETI program make its results credible?! Surely every antenna pointing is affected by this problem? I am beginning to get a vague (no doubt mistaken!) impression of the SETI program as just a random, all sky survey, hoping for a magic signal from someone... somewhere... at some frequency... some day! > > denoted by 'S', is there any measure of radio noise received from say > > S + 1 arc-sec, S + 2 arc-sec, S + 3 arc-sec angular separations? This > > Radio telescope resolutions are measured in arc minutes! Are there such things as 'optical' or 'infrared' SETI initiatives? Resolutions would be much finer at these parts of the electromagnetic spectrum, but you're going to say that would be even more like looking for a needle in a haystack... Abdul Ahad > I wonder why Alpha Centauri isn't much of a candidate. It may be a > binary which does complicate things from a radio resolution > perspective I agree, but it sure as hell is the most interesting star That's not the problem; the problem is that it would disrupt a planetary system. (There was, though, a novel with a SETI component in which there was a habitable planet round a gas giant.) As it happens, it is on the Project Phoenix target list because it is within 25 LY and they cover everything within that range. See <http://www.seti.org/site/pp.asp?c=ktJ2J9MMIsE&b=179075> (assuming that that URL doesn't contain short term session IDs - why can't people use straightforward URLs - they make the page cachable as well). I couldn't, quickly, find the actual target list, although I believe it is on the site somewhere. > Maybe the ones in the SETI target list aren't naked eye, but there is You can never say "the" about SETI because there is not one, but many, SETI programmes. > So if there are two stars very closely spaced in the field of view, > one at 20 light years and another one at 2,000 light years distance, > the radio telescope has no way of distinguishing the signal from one > star separate from the other? In terms of continuum noise, no. The beamwidth for SETI@Home is about a tenth of a degree and that is limited by the diameter of the dish. If a long term transmission were detected, interferometry could be used to refine the source direction, but that is expensive, and, depending on the resolution you need, will require a long observation. > So how does the SETI program make its results credible?! Surely every > antenna pointing is affected by this problem? All it needs to demonstrate is that the signal is intelligent and not of human origin. Initially, that does not require positive identification of the source. Generally SETI is extrememly cash limited and has to work with available technology and limited access to that technology. The technology is good enough to detect certain sorts of intelligent signal. (In fact, to the extent that the results are negative, there will never be a source to positively identify.) > I am beginning to get a vague (no doubt mistaken!) impression of the > SETI program as just a random, all sky survey, hoping for a magic > signal from someone... somewhere... at some frequency... some day! Which SETI programme? SERENDIP and therefore SETI@Home are exactly that; it gives them the advantage that they don't have to negotiate for limited telescope time so and observe much longer. Project Phoenix is targetted (see URL above), but only gets a few days a year. > Are there such things as 'optical' or 'infrared' SETI initiatives? Yes. Berkeley, Ohio and Harvard, amongst others, run them. > > I wonder why Alpha Centauri isn't much of a candidate. It may be a > > binary which does complicate things from a radio resolution [quoted text clipped - 3 lines] > a planetary system. (There was, though, a novel with a SETI component > in which there was a habitable planet round a gas giant.) There has been a lot of in-depth study carried out in recent years to show that the Alpha Centauri binary system is stable and non-disruptive to potential life bearing planets around the principal stars 'A' and 'B' of the system. I quote:- "In a binary system, a planet must not be located too far away from its "home" star or its orbit will be unstable. If that distance exceeds about one fifth of the closest approach of the other star, then the gravitational pull of that second star can disrupt the orbit of the planet. Recent numerical integrations, however, suggest that stable planetary orbits exist: within three AUs (four AUs for retrograde orbits) of either Alpha Centauri A or B in the plane of the binary's orbit; only as far as 0.23 AU for 90-degree inclined orbits; and beyond 70 AUs for planets circling both stars (Weigert and Holman, 1997). Hence, under optimal conditions, either Alpha Centauri A and B could hold four inner rocky planets like the Solar System: Mercury (0.4 AU), Venus (0.7 AU), Earth (1 AU) and Mars (1.5 AUs). Indeed, the AB system may be more than twice (1.3 to 2.3 times) as enriched in elements heavier than hydrogen ("high metallicity") than our own Solar System (Cayrel de Strobel et al, 1991, page 297; Furenlid and Meylan, 1984; and Flannery and Ayres, 1978). Hence, either stars A or B could have one or two "rocky" planets in orbital zones where liquid water is possible." More analysis here: http://www.solstation.com/stars/alp-cent3.htm Make no mistake... I am totally convinced that Alpha Centauri is *the* system that promises us a tiny ray of hope... where most other systems offer zilch. I have already charted my course there and I'm all set to go:- http://uk.geocities.com/aa_spaceagent/restricted/interstellar-propulsion.html And before I set off, I *know* there is a good chance of locating a planet or two in the habitable zones around one of these two stars:- http://uk.geocities.com/aa_spaceagent/restricted/extrasolar-planets.html It's all a question of time and hope... Abdul Ahad > david@djwhome.demon.co.uk (David Woolley) wrote in message: > [quoted text clipped - 14 lines] > > http://www.starmapping.co.uk/sunlike.cfm You resolved this nicely. I just want to add a pointer (and a note) here to the PlanetQuest website at http://planetquest1.jpl.nasa.gov/atlas/atlas_index.cfm They listed these naked eye stars which are also known to be orbited by extra-solar planets: 47 Ursae Majoris 51 Pegasi 55 Cancri 70 Virginis Epsilon Eridani gamma Cephei Gliese 777A HD 104985 HD 10647 HD 142 HD 147513 HD 160691 HD 160691 c HD 160691 d HD 169830 HD 19994 HD 219449 HD 27442 HD 3651 HD 38529 HD 39091 HD 47536 HD 59686 HD 89744 HR 810 Iota Draconis rho Coronae Borealis tau Bootis Upsilon Andromedae As you probably know, that last one, Upsilon Andromedae (along with others) is also an interesting system not only because it's known to have multiple planets, but because it's also binary star system! I discovered that fact independently (while doing a science fair project with my daughter) and posted it in a thread here in this NG awhile back. See the binary dwarf orbiting Upsilon Andromedae in real images at the following link: http://www.cfas.org/Library/upsilonandromedaedssdss2neat.gif Here's the science fair paper caption we wrote for that image sequence: "In the above three frames, notice the star moving against the background stars, this is a red dwarf star that orbits Upsilon Andromeda, the bright central star in these three images (this star is the first-ever known to have at least 3 giant planets orbiting it.) This is probably the first-ever visible light 3-frame sequence confirming that this system is also a binary star system, first discovered in Spring 2002 by professional astronomers using a near infra-red camera, here you are seeing it for the first time ever in a sequence of 3 visible light images! E-mails received from the initial discoverers acknowledged this find, which they had not done before in all visible light! (First image from Digital Sky Survey 1, second image from Digital Sky Survey 2 Red Filter, third image from Near Earth Asteroid Tracking project.)" If Alpha Centauri has planets, their radial velocity effects on the system will be detected by the time your spacecraft leaves the ground :^) Have fun! Perpetuating the unceasing interrogative, Jason H. >>>>> "AI" == AA Institute <abdul.ahad@ntlworld.com> writes: AI> After 40+ years of SETI research, has anyone published a summary AI> of some kind by candidate star [...] I think Jill Tarter has published a list of all known SETI programs as well as the best limits obtained thus far. However, you'll have to get ahold of her review article in last year's (?) Annual Review of Astronomy & Astrophysics. Off the top of my head, the current best limits on the received fluxes from ET emissions are around 1E-26 W/m^2. If you want to convert that to a *transmitted* power, you have to pick the distance to your favorite target (keeping in mind that the entire sky has not been surveyed to this level so that the result might be optimistic). Converting this to more reasonable units, the limit is 12 GW (D/10 pc)^2. That is, a transmitter at a distance of 10 pc would have to be broadcasting at least 12 GW to be detectable in the best case. SignatureLt. Lazio, HTML police | e-mail: jlazio@patriot.net No means no, stop rape. | http://patriot.net/%7Ejlazio/ sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html > >>>>> "AI" == AA Institute <abdul.ahad@ntlworld.com> writes: > [quoted text clipped - 5 lines] > get ahold of her review article in last year's (?) Annual Review of > Astronomy & Astrophysics. You mean this? http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.astro.39.1.511;jsess ionid=jNeVYQmKMZMd I think that's a *pay* per article service and I'm not even sure it will give me what I'm looking for. All I'm after is a total radio flux density number for each Sun-like star within 20 light years at a given frequency... a quantity referred to as 'S' and measured in Janskeys. Ideally, I'd like this radio flux (S) measured for each star at a microwave frequency at which the Earth would be expected to outshine our Sun when surveyed from deep space. I want to know, does Tau Ceti have a higher radio output than Epsilon Indi? Does Delta Pavonis have a higher radio output than the Sun (Sol)?, etc. so I can compare between them. I would have thought such elementary data would be available readily, but a google search does not throw up this kind of data. This is just topline info without even going to the depths of deciphering ET emissions. Abdul Ahad > Ideally, I'd like this radio flux (S) measured for each star at a > microwave frequency at which the Earth would be expected to outshine > our Sun when surveyed from deep space. I don't think you've been paying attention. The sun is an extremely weak radio source as far as insterstellar detections are concerned, and is extremely variable in output. Having done some of the calculations more accurately, you need about 280 kilo watts per Hz at its noisiest frequency (around 300MHz), at the times that it is noisiest, to outshine it. If you assume stable carriers and conservative SETI detection bandwidths, that is 28 kW EIRP. In the waterhole range that is dropping to around 2kW and under. At quiet times, at 21cm it is about 300 watts EIRP. On the 27MHz CB band, it is about 3 watts at quiet times (i.e. a full legal power CB transmitter will probably outshine the sun at astronomical distances, and a full legal power 23cm amateur transmitter into a simple dipole antenna will easily outshine it). If baseline star output were the issue, the sun would be outshone at spot frequencies throughout the whole of the microwave region. > I want to know, does Tau Ceti have a higher radio output than Epsilon > Indi? Does Delta Pavonis have a higher radio output than the Sun If these are sun-like, I'm not even sure if we'd be able to measure the output of the star within the measurement uncertainties. Tau Ceti seems to be 2.724 parsecs. That makes it about 5.618E5 AU. That puts the Sun's radio flux 3.16E11 times less than at the earth. At the earth, the maximum flux is about 1E8 Janskies, so you have about 3E-4 Janskies maximum (0.0003 Janskies). Minimum detectable signals for radio astronomy are of the order of 1 Jansky. Even allowing for an extremely long averaging time, I don't see how 3E-4 Jansky's is going to get above the measurement error. (Tau Ceti is actually tagged as sub-dwarf in my catalogue, so, presumably, isn't sun-like.) > This is just topline info without even going to the depths of > deciphering ET emissions. Nobody is trying to decipher the emissions. They are looking for characteristics of the signal that indicate artificiality and take them above the minimum detectable level. >>>>> "AI" == AA Institute <abdul.ahad@ntlworld.com> writes: AI> Joseph Lazio <jlazio@adams.patriot.net> wrote in message AI> news:<llhdo7w688.fsf@adams.patriot.net>... AI> After 40+ years of SETI research, has anyone published a summary AI> of some kind by candidate star [...] >> I think Jill Tarter has published a list of all known SETI >> programs as well as the best limits obtained thus far. However, >> you'll have to get ahold of her review article in last year's (?) >> Annual Review of Astronomy & Astrophysics. AI> You mean this? [...] AI> I think that's a *pay* per article service and I'm not even sure AI> it will give me what I'm looking for. All I'm after is a total AI> radio flux density number for each Sun-like star within 20 light AI> years at a given frequency... a quantity referred to as 'S' and AI> measured in Janskeys. Oh, you want the *natural* radio emission. I thought you were trying to find the best limits from SETI programs. AI> I want to know, does Tau Ceti have a higher radio output than AI> Epsilon Indi? Does Delta Pavonis have a higher radio output than AI> the Sun (Sol)?, etc. so I can compare between them. I would have AI> thought such elementary data would be available readily, [...]. These are not *elementary* data! This article summarizes stellar radio emission, <URL:http://www.ras.ucalgary.ca/SKA/science/node13.html>. A more recent version by Steven White is at <URL:http://www.aoc.nrao.edu/%7Eccarilli/CHAPS/stars.ps>. He estimates that a solar-type star at the distance of Alpha Centauri would have a flux density of no more than 0.4 mJy. Now a flux density of 0.4 mJy at frequencies around 1--10 GHz is reasonably easy to measure. With the VLA at 1.4 GHz, I've gotten to 0.1 mJy without a lot of effort in only 1 hr of observing time. *However*, that estimate is for a star at only the distance of Alpha Centauri. At 10 pc, White's estimate would be a flux density of 4 *microJanskeys*. That's out of reach with current telescopes. As White writes, "No main-sequence stars other than the Sun have been detected so far through their thermal atmospheres ...." The stars that have been detected in the radio are unusual ones, having some kind of enhanced magnetic activity. SignatureLt. Lazio, HTML police | e-mail: jlazio@patriot.net No means no, stop rape. | http://patriot.net/%7Ejlazio/ sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html Joseph Lazio <jlazio@adams.patriot.net> wrote in message > AI> I think that's a *pay* per article service and I'm not even sure > AI> it will give me what I'm looking for. All I'm after is a total [quoted text clipped - 4 lines] > Oh, you want the *natural* radio emission. I thought you were trying > to find the best limits from SETI programs. Yes, just the natural radio emissions. > He > estimates that a solar-type star at the distance of Alpha Centauri > would have a flux density of no more than 0.4 mJy. Now a flux density > of 0.4 mJy at frequencies around 1--10 GHz is reasonably easy to > measure. With the VLA at 1.4 GHz, I've gotten to 0.1 mJy without a > lot of effort in only 1 hr of observing time. *However*, that So based on these *projections* what are the *actual* measures of flux for each star within about 5 parsecs distance and is it possible to list those accurate to +/-0.05 mJy accuracy (may be on the extreme "edge" of capability)? And if the measurements are made across a range of frequencies (1 GHz, 3 GHz, 5 GHz,... etc) would the flux peak at the *same* frequency for each star? If 0.4 milli-Jansky is projected at the distance of Alpha Centauri (1.3 parsec) then this implies a flux of 0.1 mJy will be received from a Sun-like star out to 17 light years (5 parsecs). Within that distance range there are at least 5 Sun-like stars (obviously what you define as 'sun-like' is slightly arbitrary) but I would say: Alpha Cen A, B, Tau Ceti, Epsilon Indi, <<possibly>> Sigma Draconis - which is at 18 light years out. My bottom line question is: If the Earth had been circling around *one* of these stars and outputting the *same* level of radio noise that we do... would it *inflate* the natural radio flux coming from its parent star by a significant amount so as to make *that* star stick out like a "sore thumb" above all its peers!? And for the purposes of this question, you can assume that the Earth that we are listening *from* (here), is deadly "silent" in radio noise so as to facilitate such detection. I'd like a straight YES or a NO answer to this last question, please! Thanks Abdul Ahad >>>>> "AI" == AA Institute <abdul.ahad@ntlworld.com> writes: >> Oh, you want the *natural* radio emission. I thought you were >> trying to find the best limits from SETI programs. AI> Yes, just the natural radio emissions. >> He estimates that a solar-type star at the distance of Alpha >> Centauri would have a flux density of no more than 0.4 mJy. Now a >> flux density of 0.4 mJy at frequencies around 1--10 GHz is >> reasonably easy to measure. With the VLA at 1.4 GHz, I've gotten >> to 0.1 mJy without a lot of effort in only 1 hr of observing time. Thought as David and I have discussed, my measurements were with a large bandwidth (50 MHz). With a narrower bandwidth, this becomes increasingly difficult. AI> So based on these *projections* what are the *actual* measures of AI> flux for each star within about 5 parsecs distance and is it AI> possible to list those accurate to +/-0.05 mJy accuracy (...)? And AI> if the measurements are made across a range of frequencies (1 GHz, AI> 3 GHz, 5 GHz,... etc) would the flux peak at the *same* frequency AI> for each star? I thought I included that in my original post. I referenced an article by Steven White in which he commented that the thermal emissions from *no* main-sequence stars, other than the Sun, have been detected. Thus, we don't have measurements; the best we might have are upper limits. Also, in the radio part of the spectrum, we are measuring on the so-called Rayleigh-Jeans portion of the spectrum. The peak flux density of a star occurs around 500 THz. In the radio part of the spectrum, if we could measure the thermal emissions, we'd measure decreasing flux densities at lower frequencies. That is, the flux density at 1 GHz would be lower than that at 3 GHz which would be lower than that at 5 GHz (and it would be lower by the square of the ratio of the frequencies). Similarly, in comparing two stars, the hotter of the two stars would have a higher flux density at all frequencies. AI> If 0.4 milli-Jansky is projected at the distance of Alpha Centauri AI> (1.3 parsec) then this implies a flux of 0.1 mJy will be received AI> from a Sun-like star out to 17 light years (5 parsecs). Hmm, I get that an object with a flux density of 0.4 mJy at 1.3 pc will have a flux density of 0.02 mJy at 5 pc. AI> Within that distance range there are at least 5 Sun-like stars AI> (obviously what you define as 'sun-like' is slightly arbitrary) AI> but I would say: Alpha Cen A, B, Tau Ceti, Epsilon Indi, AI> <<possibly>> Sigma Draconis - which is at 18 light years out. AI> My bottom line question is: If the Earth had been circling around AI> *one* of these stars and outputting the *same* level of radio AI> noise that we do... would it *inflate* the natural radio flux AI> coming from its parent star by a significant amount so as to make AI> *that* star stick out like a "sore thumb" above all its peers!? Because our strongest transmissions are narrowband, if you had the right frequency at the right time, yes, the Sun would appear anomalous. SignatureLt. Lazio, HTML police | e-mail: jlazio@patriot.net No means no, stop rape. | http://patriot.net/%7Ejlazio/ sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html Joseph Lazio <jlazio@adams.patriot.net> wrote in message <snip> > AI> So based on these *projections* what are the *actual* measures of > AI> flux for each star within about 5 parsecs distance and is it [quoted text clipped - 7 lines] > emissions from *no* main-sequence stars, other than the Sun, have been > detected. Surely, if Alpha Centauri was in view (it probably doesn't rise above the horizon as seen from the plains of New Mexico where the VLA is based), with *two* sun-like stars at just 1.3 parsec away... it may have been possible to get a faint signal. The SKA looks promising, but that's also going to be a northern hemisphere facility. Is there a radio astronomy facility located in the southern hemisphere that approaches any where near the VLA in its size and capability? It's a shame that so many nearby, sun-like stars are only visible in the southern skies: Delta Pavonis, Alpha Centauri, Epsilon Indi,... > Also, in the radio part of the spectrum, we are measuring on the > so-called Rayleigh-Jeans portion of the spectrum. The peak flux [quoted text clipped - 13 lines] > Hmm, I get that an object with a flux density of 0.4 mJy at 1.3 pc > will have a flux density of 0.02 mJy at 5 pc. It's an inverse square-law relation, right? I just divided 0.4 mJy by (5/1.3) to get a straight line 0.1 mJy at 5 parsecs. > AI> Within that distance range there are at least 5 Sun-like stars > AI> (obviously what you define as 'sun-like' is slightly arbitrary) [quoted text clipped - 10 lines] > right frequency at the right time, yes, the Sun would appear > anomalous. "The right frequency at the right time", that's the operative bit I think. Since we haven't been monitoring each of the nearby sun-like stars *continuously*, we cannot entirely rule out the possibility that there *may be* radio broadcasting civilisations on a planet within 20 light years from us? I am beginning to appreciate the bigger picture... sorry if this seems a bit repetitive, I'm just learning. Thanks Abdul > It's an inverse square-law relation, right? I just divided 0.4 mJy by > (5/1.3) to get a straight line 0.1 mJy at 5 parsecs. The square in square-law means squared, i.e. the ratio is one over (5/1.3)*(5/1.3), giving 0.027. david@djwhome.demon.co.uk (David Woolley) wrote in message > > It's an inverse square-law relation, right? I just divided 0.4 mJy by > > (5/1.3) to get a straight line 0.1 mJy at 5 parsecs. > > The square in square-law means squared, i.e. the ratio is one over > (5/1.3)*(5/1.3), giving 0.027. Thanks, that figures with professor Lazio's calculation...only I get 1/[(5/1.3)*(5/1.3)] = 0.0676? So the strength of the transmitted radio flux, as with all electromagnetic emissions, is received diminished by a factor governed by the inverse square law, depending on the distance separating the transmitter and the receiver. An interstellar radio message was sent to Messier 13, a globular star cluster in the constellation of Hercules, on November 16, 1974, from the Arecibo Radio Observatory in Puerto Rico. As M13 is 24,000 light years (c. 7,500 parsecs) from Earth, I wonder if the signal would remain strong enough for the intended recepients (if any!) to be able to pick this up in 24,000 years from now... How much interstellar extinction are radio waves expected to suffer in general and I wonder if radio waves are more "extinctioned" than light waves? I bet this varies markedly with the frequency/wavelength of the transmitted signal? Abdul Ahad > Thanks, that figures with professor Lazio's calculation...only I get > 1/[(5/1.3)*(5/1.3)] = 0.0676? You haven't multiplied by 0.4. > An interstellar radio message was sent to Messier 13, a globular star > cluster in the constellation of Hercules, on November 16, 1974, from It was sent in that direction, but there was no real intention that it be received. It wasn't even compensated for proper motion. It was purely a PR exercise with the new radar transmitter; it was not intended as a serious SETI contact. david@djwhome.demon.co.uk (David Woolley) wrote in message > > An interstellar radio message was sent to Messier 13, a globular star > > cluster in the constellation of Hercules, on November 16, 1974, from [quoted text clipped - 3 lines] > It was purely a PR exercise with the new radar transmitter; it was > not intended as a serious SETI contact. To your knowledge, in the total history of SETI, has there ever been a serious attempt at "flashing them"!? i.e. deliberately sending out a targeted message to let them know we're here? > To your knowledge, in the total history of SETI, has there ever been a > serious attempt at "flashing them"!? i.e. deliberately sending out a > targeted message to let them know we're here? No. But there are serious political, and financial problems in doing that. The Encounter transmissions were properly targetted, but brief, and were basically done for the vanity message funding, with the science message only to give some semblance of legitimacy. The political problems are that some people believe that any detection of us will trigger a pre-emptive strike, and others are concerned that a biassed religious or political message would be sent. There are UN protocols on this, but I think Encounter ignored them. The financial problem is that high power transmissions burns up transmit tubes and electricity, and that radio telescopes are expensive resources have their use allocated to what academia considers the most productive research projects. Encounter bought time on a Ukrainian telescope and probably paid real money for it. Nobody is saying that SETI is easy. All they are saying is that current methods have the best chance of success given currently available technology and funding, and that success only requires engineering at our current level at the sending end. (Moreover, at the limits of our SETI capabilities, we could detect signals that we sent unintentionally.) "success" means finding an ETI, but SETI can also successfully produce upper bounds on what is out there without finding anything. SETI is very poorly funded and, essentially, is funded by private donations. > > To your knowledge, in the total history of SETI, has there ever been a > > serious attempt at "flashing them"!? i.e. deliberately sending out a [quoted text clipped - 4 lines] > and were basically done for the vanity message funding, with the science > message only to give some semblance of legitimacy. What are these "Encounter" transmissions? I was never aware of such a project. > The political problems are that some people believe that any detection > of us will trigger a pre-emptive strike, and others are concerned that A pre-emptive strike...what at faster-than--light speed??? Some people... > a biassed religious or political message would be sent. There are > UN protocols on this, but I think Encounter ignored them. Intelligent beings that develop capabilities to a level sufficient for interstellar travel will have only done so with deep thought, compassion and immense amounts of wisdom and understanding about themselves as well as others. You can't ignore the universe and become so evil as to seek out and destroy others. If you're like that, your own lot will finish you long before you have the chance to acquire interstellar spaceflight capabilities. The deeply mysterious miracles that govern the birth and death of "life" will make species so far apart from one another, that should they one day meet "the others" they could only greet one another with open arms! I've done my research to the *end* of knowledge on this THING we call *life*, and I can assure you that what governs the mysterious binding of non-living matter into living matter is indeed a "miracle":- http://tinyurl.com/5qhtp Therefore, I firmly believe "flashing them" with targetted radio signals is probably far better than to face extinction all alone... At least you can die *easy* knowing that you weren't alone! (And may be share a laugh or two with the aliens if they have a half decent sense of humour!) > The financial problem is that high power transmissions burns up > transmit tubes and electricity, and that radio telescopes are expensive [quoted text clipped - 12 lines] > > SETI is very poorly funded and, essentially, is funded by private donations. The quest to resolve one of the most *fundamental* mysteries of the cosmos is under-funded... surely not! There's plenty of money out there being poured into meaningless analysis of dust clouds and inter-galactic dark matter (what I like to call "intellectual masturbation"!). Yet looking for life takes a back stage. We've got the wrong people sitting in strategic positions, pushing the wrong buttons, in my view. Abdul Ahad > What are these "Encounter" transmissions? I was never aware of such a > project. See <http://antwrp.gsfc.nasa.gov/apod/ap010109.html>. This is by the science consultants to the project. The commercial side were rather bad about putting the science on their web site and probably have broken any links by now. Yvan Dutil has been known to post in this newsgroup. > A pre-emptive strike...what at faster-than--light speed??? Some > people... This is not my view, but it has been, frequently, expressed on this newsgroup. Planet killing is a lot easier than rapid interstellar transport, as you simply crash a relativistic ship into the target. See <http://www.setileague.org/general/reply.htm> for the proposed UN rules (not actually ratified, but probably because of not taking SETI seriously, rather than any objection). > The quest to resolve one of the most *fundamental* mysteries of the > cosmos is under-funded... surely not! There's plenty of money out It has a UFO taint and has the problem that there is no steady progress in knowledge, other than a reduction in the likelihood of detectable signals, as coverage and sensitivities increase. The US government funded it with a fairly small amount of money, for a couple of years, some years ago, then the US Congress voted the budget out. Currently, US SETI survives on private donations of the order of one or two million dollars per year. Even with NASA funding, it would only have been US$ 12 million a year. See, for example <http://www.setileague.org/editor/petition.htm>. Even the Berkeley people only work about a third of their time on SETI as it would be bad for their research careers, otherwise. To get a good idea of the politics of SETI funding, watch the first half of the film "Contact", or read the book. The second half gets a bit fantastical. The first half will have been spiced up for entertainment purposes, but I'm told there is a lot of truth in it. The Lovell telescope does a small amount of SETI, mainly followups for project Phoenix, but I doubt that the British government has a formal SETI budget. > there being poured into meaningless analysis of dust clouds and > inter-galactic dark matter (what I like to call "intellectual > masturbation"!). You don't have any sympathy from me if you reject this sort of basic research - much of which has a bearing on the likelihood of life, or on the nature of natural radio signals that could compromise a detection. Also, without it, there would not be the radio telescopes that could be used for SETI. > Yet looking for life takes a back stage. We've got the wrong people There is more to looking for life than SETI, and that does get funded, with respect to planetary exploration. David Woolley wrote:- > To get a good idea of the politics of SETI funding, watch the first > half of the film "Contact", or read the book. The second half gets > a bit fantastical. The first half will have been spiced up for > entertainment purposes, but I'm told there is a lot of truth in it. Contact is one of my all time favourite movies (always sits on top of my DVD collection!). I think the movie drops a strong hint of the kind of quiet struggles that Carl Sagan must have gone through in his life, as I know he was a huge proponent of SETI. Really love the scenes just before the ending, on that alien planet. The funny thing is, the movie actually "kind of" shows religion as a winning force in the end. Jodie Foster is reminded of Occam's Razor... a taste of her own medicine, as she was quoting this very confidently to that religious guy early on in the film. > The Lovell telescope does a small amount of SETI, mainly followups for > project Phoenix, but I doubt that the British government has a formal [quoted text clipped - 9 lines] > Also, without it, there would not be the radio telescopes that could be > used for SETI. Agreed. It's about prioritisation, that's all I was implying. "Theory of everything" and "String theory" and those kinds of things are just... intellectual! > > Yet looking for life takes a back stage. We've got the wrong people > > There is more to looking for life than SETI, and that does get funded, > with respect to planetary exploration. True, and I'm all for solar system exploration. But one just gets the impression that we aren't going to find anything too exciting *this side* of the boundless interstellar ocean. Hence, those invisible beams radioed down to us from heaven may just contain a whisper from some distant civilisation calling out into the darkness... Abdul Ahad [Regarding transmitting messages:] >>>>> "DW" == David Woolley <david@djwhome.demon.co.uk> writes: DW> The financial problem is that high power transmissions burns up DW> transmit tubes and electricity, and that radio telescopes are DW> expensive resources [...]. A problem that can be summarized succintly as "waveguides melt." :) J. Cordes (and maybe W. Sullivan) have speculated that one way to avoid this problem would be to transmit in such a way as to "pump" masers, then modulate the pump so that the masers varied in an "artificial" manner. Interstellar and circumstellar masers provide much more "gain" than could be achieved from planetary radio telescopes. Obvious maser lines to pump would be the OH lines around 1.7 GHz and the water lines around 22 GHz. Of course, this is well outside our current capabilities. SignatureLt. Lazio, HTML police | e-mail: jlazio@patriot.net No means no, stop rape. | http://patriot.net/%7Ejlazio/ sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html >>>>> "AI" == AA Institute <abdul.ahad@ntlworld.com> writes: AI> How much interstellar extinction are radio waves expected to AI> suffer in general and I wonder if radio waves are more AI> "extinctioned" than light waves? I bet this varies markedly with AI> the frequency/wavelength of the transmitted signal? Radio signals suffer very little extinction, that's one of their advantages. From about 0.3 GHz to well over 100 GHz, radio signals can travel across the Galaxy suffering essentially no extinction. As an example consider <URL:http://antwrp.gsfc.nasa.gov/apod/ap020803.html>, which is a radio image of the Galactic center. By contrast, one cannot make an image of the Galactic center at optical wavelengths because there is about 30 magnitudes of extinction at optical wavelengths. (30 magnitudes = factor of 1 trillion) SignatureLt. Lazio, HTML police | e-mail: jlazio@patriot.net No means no, stop rape. | http://patriot.net/%7Ejlazio/ sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html >>>>> "AI" == AA Institute <abdul.ahad@ntlworld.com> writes: AI> Joseph Lazio <jlazio@adams.patriot.net> wrote in message <snip> AI> Surely, if Alpha Centauri was in view (...), with *two* sun-like AI> stars at just 1.3 parsec away... it may have been possible to get AI> a faint signal. Yes, in a broad-band sense. AI> The SKA looks promising, but that's also going to be a northern AI> hemisphere facility. I wouldn't say that. The Australian and South African governments are competing strenuously to host the SKA, and, IMHO, they have a strong case. AI> Is there a radio astronomy facility located in the southern AI> hemisphere that approaches any where near the VLA in its size and AI> capability? It's a shame that so many nearby, sun-like stars are AI> only visible in the southern skies: Delta Pavonis, Alpha Centauri, AI> Epsilon Indi,... There's the Australia Telescope, which can approach the VLA's performance in some respects, but I don't think it could equal the VLA's sensitivity in this case. (However, I haven't run the numbers.) AI> My bottom line question is: If the Earth had been circling around AI> *one* of these stars and outputting the *same* level of radio AI> noise that we do... would it *inflate* the natural radio flux AI> coming from its parent star by a significant amount so as to make AI> *that* star stick out like a "sore thumb" above all its peers!? >> Because our strongest transmissions are narrowband, if you had the >> right frequency at the right time, yes, the Sun would appear >> anomalous. AI> "The right frequency at the right time", that's the operative bit AI> I think. Yes, I was quite careful to add that part. AI> Since we haven't been monitoring each of the nearby sun-like stars AI> *continuously*, we cannot entirely rule out the possibility that AI> there *may be* radio broadcasting civilisations on a planet within AI> 20 light years from us? Right, though we can use existing radio surveys and SETI work to place limits on how strong any transmissions from nearby civilizations might be. SignatureLt. Lazio, HTML police | e-mail: jlazio@patriot.net No means no, stop rape. | http://patriot.net/%7Ejlazio/ sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html > would have a flux density of no more than 0.4 mJy. Now a flux density > of 0.4 mJy at frequencies around 1--10 GHz is reasonably easy to Just to resolve any apparent conflicts with what I wrote, and please note that Dr Lazio is the professional here, I used the flux at the strongest frequency when the sun is most active to get a 0.4mJ figure for Tau Ceti. At that frequency, the galactic noise is significantly higher, so the minimum detectable signal will be higher. At 21cm, the signal would be under 0.2mJ for the disturbed sun and something like 3 micro Janskies, for the quiet sun. > measure. With the VLA at 1.4 GHz, I've gotten to 0.1 mJy without a > lot of effort in only 1 hr of observing time. *However*, that I stand corrected on what is achievable. I think I may have failed to account for this being a wide band measurement. However, whilst a TV transmitter might put out about 5MW/Hz when viewed in a 0.1Hz narrowband channel, which would outshine the sun, it might only put out about 200mW per Hz when treated as a wide band source, across its full bandwidth, which would be well below the solar background at TV frequencies. > estimate is for a star at only the distance of Alpha Centauri. At 10 > pc, White's estimate would be a flux density of 4 *microJanskeys*. Note that my 3 micro Janskies at 2.7 pc is based on a spot frequency at which the sun's signal is weaker than that averaged over the full microwave range. I've been using the 1982 edition of the Handbook of Space Astronomy and Astrophysics, by Martin V Zombeck, as my source, in particular, a diagram at the end of the radio astronomy chapter. A more recent version is, or was, available online. >>>>> "DW" == David Woolley <david@djwhome.demon.co.uk> writes: DW> In article <llwtwy1us1.fsf@adams.patriot.net>, DW> Joseph Lazio <jlazio@adams.patriot.net> wrote: >> would have a flux density of no more than 0.4 mJy. Now a flux >> density of 0.4 mJy at frequencies around 1--10 GHz is reasonably >> easy to DW> Just to resolve any apparent conflicts with what I wrote [...] I DW> used the flux at the strongest frequency when the sun is most DW> active to get a 0.4mJ figure for Tau Ceti. At that frequency, the DW> galactic noise is significantly higher, so the minimum detectable DW> signal will be higher. At 21cm, the signal would be under 0.2mJ DW> for the disturbed sun and something like 3 micro Janskies, for the DW> quiet sun. Yes, the numbers I have been quoting are for the thermal emission from the Sun. The Sun's nonthermal emission can be stronger, but, as you note, it is emitted at lower frequencies, at which the Galactic background is stronger, so sensitivity is lower. Thus, detecting a solar twin remains challenging across the radio spectrum. >> measure. With the VLA at 1.4 GHz, I've gotten to 0.1 mJy without a >> lot of effort in only 1 hr of observing time. *However*, that DW> I stand corrected on what is achievable. I think I may have DW> failed to account for this being a wide band measurement. Ah, yes, that's the source of our difference. My measurements were with a 50 MHz bandwidth. If one shrinks that to, say, a 50 Hz bandwidth (ignoring that the VLA can't produce such narrow bandwidths currently), then the noise level would rise to 0.1 Jy. SignatureLt. Lazio, HTML police | e-mail: jlazio@patriot.net No means no, stop rape. | http://patriot.net/%7Ejlazio/ sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html > and then the Sun. At a given frequency in the radio waveband, Mars > would be deadly silent. The Earth would give you a signal, due to Mars would emit thermal noise consistent with its surface temperature, but would occupy so little of the field of view that it would be difficult to detect. > intelligence on our planet. The Sun would give you a strong, "natural" > noise signal. Would that sound about right? The Sun is not a particularly strong radio source and is almost certainly irrelevant for an Arecibo sized receiver at interstellar distances. The active Sun illuminates the earth with around 1E7 and 1E8 Janskies. Typical SETI detection thresholds are around 1 Jansky (might be 10). The earth is 8 light minutes from the Sun. Power scales as the square of distance, so the Sun's signal is down to 1 Jansky at 80,000 light minutes, which is about 56 light days. The quiet Sun ranges between about 1E5 and 1E7 over the interesting range. A 1MW EIRP carrier will outshine the active Sun most of the time (might be 100kW). At 21cm, a 10kW(1kW) carrier would outshine it. That is well within amateur radio capabilities on the 23cm band. (At 10m - CB the quiet Sun is down to 1E3 Janskies, so could be outshone by a legal CB transmitter and certainly would be outshone by many illegal ones.) > different frequency. At a certain frequency of say 'X' GHz, the signal > from Earth would outshine all three bodies in strength - including the > Sun!? What is that "magic" frequency I wonder? There will be 100s if not 1000s of frequencies at which the earth outshines the Sun to any sized dish at interstellar distances (dish size doesn't matter until it becomes large enough to exclude the Sun from the beam). The problem is not the Sun but noise from the big bang (equivalent to thermal noise of about 3K) and from galactic cyclotron radiation (around 10K equivalent) to which you need to add noise picked up from the antenna and surrounding ground (about 10K) and receiver internal noise (not much less than another 10K). >>>I was hoping to get an idea of the *scale* of total radio noise that >>>we send out into the cosmos through all our day to day activities like >>>television and local radio broadcasts, mobile phones, satellite >>>relays, etc. How much "noise" does the planet's total output amount to >>>in 'Janskys' (if that is indeed the right measure?!) >> But if you notice all of our progress in communication has resulted >>in reducing the "wasted" energy going into space. If we define [quoted text clipped - 3 lines] >>would make the 15 broadcast channels in the area fall even deeper into >>the noise. And I get about 350 Mbits of internet on it. >> The higher the frequency the more we depend upon antenna gain rather >>than transmitter power. So actually the more advanced the technology >>the less wasted energy into space we would expect. > Great. I find it a struggle to understand precisely how the SETI > 'listening' process works as I don't have a background in radio > astronomy, but just picture this hypothetical scenario:- > If you have an Arecibo style dish stationed on Pluto, the outermost > planet in our solar system, and you pointed the antenna toward our [quoted text clipped - 3 lines] > intelligence on our planet. The Sun would give you a strong, "natural" > noise signal. Would that sound about right? > Now, if you repeat this listening process over and over with Mars, > then the Earth, then the Sun again, each time tuning your receiver a > different frequency. At a certain frequency of say 'X' GHz, the signal > from Earth would outshine all three bodies in strength - including the > Sun!? What is that "magic" frequency I wonder? > If we knew what this 'X' GHz was, and we tuned into *that* particular > frequency when listening for inteligent signals from "another Earth", > then the results might *just* show something! Has this sort of thing > already been tried I wonder...after 40 odd years of SETI I'm sure it > has?! I don't have the time tonight to think through the answer precisely so look for followups of people correcting me. The assumption is we are trying to detect a transmission made in hopes of it being detected. We are not talking about accidental signals. I believe the resolution to this question is looking for a very small frequency range and much narrower bands within the frequency range. Take for example all the radio noise being put out by the sun. Get an AM radio and tune between stations and listen to it. (Not all from the sun, there are a huge number of local noise sources including cars starting.) But in looking for such a narrow set of frequencies it will stand out against background noise. You can transmit (illegally) a 5 MHz TV signal at frequencies that will propagate around the world just like short wave radio. For amateur voice radio that can be done with less than 1 kW and patience. Sort of reliable shortwave voice broadcasts are in the 50 kW and up with directional antennas. Lets say 50 kW. And lets say 10 kHz wide voice band. At 50 kW per 10 kHz that is 500 times 50 kW. All of this is be have a usable signal above the background noise. It is that signal to noise ratio that matters. Add dozens of caveats to that but it is the general idea. So when looking for a narrow signal it has to be above the noise in that same frequency range. So one idea is to look for a quiet frequency range and assume anyone wishing to transmit would use it. And that is the relatively quiet "water hole" which is the range to use because a fixed power signal is in a low noise band so the signal to noise is higher by simply the choice of frequency. Note that assumption is someone it deliberately trying to transmit something to be detected. So in your example, the folks on Pluto would be looking for the inner planet trying to make contact. SignatureOsama bin Laden, he can run but he can't hide for three more years. -- The Iron Webmaster, 3255 |
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