BazBear
Possible Suspect
How on Earth was that not a valid question RecoveringYuppy? My goodness...
Earth-like exoplanet discovered around Proxima Centauri
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smartcooky
Penultimate Amazing
Not currently, but any receiving antenna can be used to transmit signals by modifying it (attaching a transmitter). This is precisely what was done at Arecibo in 1974.
https://en.wikipedia.org/wiki/Arecibo_message
Ok .. thanks. (I was just wondering whether or not they'd upgraded it).
As per post #118, its probably easier to talk about Arecibo's transmit capability for a baseline, (although I also wonder if the new big Chinese telescope will be able to transmit .. that'd be an interesting one!)
smartcooky
Penultimate Amazing
Digression but on topic..
A friend of mine, Bill, and I (we were both Ham Radio operators) did a little experiment some years ago. We rigged up a couple of old Sky TV satellite dishes (which are not designed for tranmission); one we connected to an HP 70340 Microwave Signal Generator set to 12 gHz (the transmitter) and the other to an HP8566 spectrum analyzer (the receiver). We tested them across the back garden on a very low power setting to make sure the system worked, and the receiver could receive the transmitter signal. Then Bill took the transmitter to the top of Takaka Hill (on the way to Golden Bay from Nelson) and I took the receiver to the top of Princes Drive on the Port Hills of Nelson. The two sites are about 25 miles apart and in direct line of sight. We used 2m Ham radios to communicate, and Bill set the signal genny to maximum power output (about +10 dbm or 10 milliwatts)
Our initial "Mk1 eyeball" alignment was quite good, and I was able to see a clear signal on the Spec Analyser. He keyed it on and off a few times to confirm that what I was seeing was him, Then we refined the dish pointing to maximise the signal, and after a bit of too and fro we were able to get the dishes pointing close to straight at each other. At this point, his signal was very, very strong, and I had to crank the sensitivity right down to get a nice (not clipped) peak on the screen at 12 gHz. The he started to drop the power output at about 30 second intervals, and when the signal got too low, I started to increase the sensitivity. I could still see the signal right down to -50dbm (about 100th of a microwatt!!!).
A friend of mine, Bill, and I (we were both Ham Radio operators) did a little experiment some years ago. We rigged up a couple of old Sky TV satellite dishes (which are not designed for tranmission); one we connected to an HP 70340 Microwave Signal Generator set to 12 gHz (the transmitter) and the other to an HP8566 spectrum analyzer (the receiver). We tested them across the back garden on a very low power setting to make sure the system worked, and the receiver could receive the transmitter signal. Then Bill took the transmitter to the top of Takaka Hill (on the way to Golden Bay from Nelson) and I took the receiver to the top of Princes Drive on the Port Hills of Nelson. The two sites are about 25 miles apart and in direct line of sight. We used 2m Ham radios to communicate, and Bill set the signal genny to maximum power output (about +10 dbm or 10 milliwatts)
Our initial "Mk1 eyeball" alignment was quite good, and I was able to see a clear signal on the Spec Analyser. He keyed it on and off a few times to confirm that what I was seeing was him, Then we refined the dish pointing to maximise the signal, and after a bit of too and fro we were able to get the dishes pointing close to straight at each other. At this point, his signal was very, very strong, and I had to crank the sensitivity right down to get a nice (not clipped) peak on the screen at 12 gHz. The he started to drop the power output at about 30 second intervals, and when the signal got too low, I started to increase the sensitivity. I could still see the signal right down to -50dbm (about 100th of a microwatt!!!).
Hmm ... 12Ghz is at the lower end of the Ku band. If it was about 60cm, the antenna gain would've been about 37dB?
(my bold)Bit late for that, isn't it?
If there were radio telescopes on Proxima b just like those here on Earth, when would unusual radio signals from our solar system have first been detected (assume the relevant radio telescopes were pointed in our direction at the time; also assume Proxima b has an ionosphere which blocks radio the same way ours does)?
My first guess would be that ~1970s radio astronomers would be able to detect ~1940s "Earth radio"; today's gear likely ~1900s, possibly earlier.
Making any sense of our radio transmissions ... that's a whole different kettle of wax.
My mistake.
As pointed out, before ~1940 there was very little in the way of artificial radio emissions that would have made it through the ionosphere.
Before writing my post, the one you're quoting, I vaguely remembered that "TV ~10^8 W".
I like BOTEs (Back Of The Envelope calculations), and prefer to work in OOM (orders of magnitude, ~1 dex).
So, I had the surface at Proxima b as ~5x10^33 m2 (that's ~0.5 dex different from your, more accurate, number).
So, using my (likely misremembered) TV number, that's ~2Jy if just 1 Hz. I didn't, at the time, recall what bandwidth "TV" has, so I took a punt that if it's anything less than ~1 MHz, we're talking at least ~microJy.
Googling found me "LIFE IN THE UNIVERSE", Proceedings of a conference held at National Ames Research Center Moffet Field, California June 19-20, 1979 (link). One presentation, "Eavesdropping Mode and Radio Leakage from Earth", by WOODRUFF T. SULLIVAN III (link), is particularly interesting. Figure 3, for example, gives an estimate of the "mean daily television transmitter power, W" ... looks like I may have been off by ~0.5-1 dex
By treating radio emission as not isotropic - i.e. like it really is - the author clearly shows that radio astronomy facilities on Earth at the time (~1980!), if on Proxima b (and with a bunch of reasonable-looking caveats), would have detected TV emissions. There were, at the time, far more powerful radio emissions ("Ballistic Missile Early Warning System (BMEWS) radars"), but detecting them would be "lucky".
Of course, since 1980, radio emission from cellphone towers has increased somewhat.
A rather nice part of this paper is that they actually tested some of their ideas ... using the Moon as a proxy.
In the paper I cited in my last post, there is a distinction made between what they call an "acquisition signal" and an "information signal".
I was referring more to the former than the latter.
By the ~1980s, radio astronomers had a fair idea of what sorts of beasties, out there in the universe, produce radio emissions; today a much better one (duh!).
"Radio stars" - stars which are sources of radio emission - exist, but are rare. This might seem odd - after all a strong solar flare can wreck havoc with radio reception here on Earth - but to a radio astronomer, at the scale of galaxies our Sun's radio emission is puny (at extragalactic distances, the Sun is utterly undetectable).
~1980s radio astronomers, on Proxima b, would have detected our Sun as a radio source; ~2010s radio astronomers would have noted it as a real anomaly, a boring G dwarf with very strange radio emission.
What's ironic is that, at the radio frequencies Earth is very loud, our cacophony of radio noise makes it very hard to detect an Earth only a few light-years away. Radio astronomers do very little observing at "TV frequencies"
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Belz...
Fiend God
This is the science section, not the philosophy section.
Not sure.
But Aricebo does (they do a lot of radar astronomy), as do the large dishes in NASA's Deep Space Network. Etc.
RecoveringYuppy
Penultimate Amazing
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Sorry, I meant to quote a different post. This one:
I was referring to the conversation you were having with JeanTate and then me. I don't understand what the "query" was in that conversation.
From her Table 1, "Detectability of Terrestrial Analog Signals by the SKA": the "1 MW" entry gives ~ Proxima Cen as "within range".
That's consistent - at an OOM level - with my BOTE (above).
ETA: re "barely detect a signal of flux density of 1x10-28 W/m2". What she actually wrote is "Table 1, assumes a detection threshold for narrowband signals of 1x 10-28 W/m2, two orders of magnitude improvement over current searches". TV signals are, of course, anything but narrowband. And radio astronomers spend a lot more time, using more sensitive equipment, on targets other than SETI ones.
Her paper is far more about an "information signal", per the Sullivan paper I cited earlier, than an "acquisition signal".
On cellphone towers, I learned that "In the United States, the FCC limits omnidirectional cell tower signals to 100 watts of power" (source). I wonder how many outdoor mobile phone transmission sites ("towers") there are, in the world?
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RecoveringYuppy
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All of Earth's TV stations? They are spread across something like 500 MHz.
Haven't read your link yet, but I recall that TV transmitter antenna focus to within "a few degrees" of the horizon. That sounds like we're still talking 1,000 square degrees of sky. That's only a 41 to 1 advantage over diffusing across the whole sky.
RecoveringYuppy
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I had assumed this too but your link says that most of the power of a TV signal is in a .1 HZ region. That would make a 7 orders of magnitude difference over what I calculated, but that still doesn't bring many stars in range. I'll read more of your link later, thanks.
So why are so many people using philosophical arguments to make their points?
Yet, (as you say, and I agree), this is a Science forum!
All I've done is respond to their posts using the chosen basis of argument ... And they don't like it for some strange reason .. Are you saying I should discontinue responding because they don't like it?
Anyway, what better place to discuss the Philosophy underpinning Science than in a Science forum? The application of philosophy is best experienced when viewed from the subject matter it attempts to grapple with, no?
smartcooky
Penultimate Amazing
Philosophy and Science are inextricably linked. The very basis of science was in the Philosophy of the ancients. After all, if you have a PhD in Physics, you might well be a scientist, but the "PhD" part stands for "Doctor of Philosophy"!!
Ok, thanks! .. (My question was worth it to understand where you were coming from).
I think Tarter also makes the distinction of nearby vs distant, more specifically:
Tarter said:
This combined with "acquisition" and "information" might work for communicating here, (although there is a strong sense of purpose (or application) implied in the latter terms, whereas Tarter's are more closely aligned with the observations, rather than the purpose for which those observations are intended to be used).
JeanTate said:
I'm not convinced yet, (about the underlined text above) ... Why should I be?
JeanTate said:What's ironic is that, at the radio frequencies Earth is very loud, our cacophony of radio noise makes it very hard to detect an Earth only a few light-years away. Radio astronomers do very little observing at "TV frequencies"
The issue for me, is not about what might generate the signals (ie: 'TV').
I see the issue being about trying to distinguish one from a background of noise.
I mean, who says some alien astronomer on Proxima b would have any conception of a 'TV signal' to recognise?
Well .. a difficult one for me to respond to, made so because others seem to think the Philosophy of Science should be excluded from science discussions.
I agree with it being included, (of course).
Nonetheless, one needs to continually distinguish which bits are based on philosophical viewpoints, and which bits are coming directly from objective testing. There's nothing wrong with having a belief about something .. but not seeing that it is a belief in the first place, is a real probelm in science discussions!
My observations so far from my experiences at this site, is that there appear to be zero attempts made at developing those distinguishing charateristics. In fact, those requesting 'exclusions' mentioned above, would seem to actually be deliberately inhibiting such development(?)
Sure .. just a different perspective, I think.
(Ie: the SETI perspective ..)
JeanTate said:
Re my underlines: Sure .. those magnificiently 'colourful' words were my touch!
On a more serious note though, once again, Table 1 is also denoted with: "Detectability of Terrestrial Analog Signals by the SKA" (My bold/underline). Ie: its the SETI ("information signal" viewpoint again).
Sure .. agreed.JeanTate said:
JeanTate said:
Lots ... but these cell sites employ many different transmission and encoding techniques to minimise interference and maximise signal reception. They use spread spectrum techniques for more efficient power distribution across the band, as well as other digital domain techniques. (Not to mention time division GSM and frequency division protocols). Its a completely different kettle of fish to look for these if you're an alien sniffing the spectrum on Proxima b. It would all look virtually indistinguishable from background noise, as would its power spectrum.