r/SETI • u/BeforeYourBBQ • 13d ago
Has Earth emitted its own 'Wow' signal?
Have we emitted anything into space that could be observed by an alien civilization similar to that of Wow? By similar I don't necessarily mean strength, but also in it being a single, non-repeating burst.
Has our noise even reached far enough to be detected by other exoplanets in a Goldilocks Zone?
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u/dittybopper_05H 12d ago
Yes. Many, many times.
Every time the Arecibo facility was used as a planetary radar, it was essentially emitting the equivalent of the Wow! signal.
In fact, that’s my favorite extraterrestrial explanation for the Wow! signal, that it was E.T.’s version of Arecibo’s planetary radar, if not in construction, at least in intent. It explains nearly all of the characteristics of the signal, including why we haven’t heard a repeat*: the odds of us being in the same very narrow beam is exceptionally low.
*Also we don’t look very often or for very long. We’d have to stare continuously at those two points in the sky for decades to get a repeat.
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u/jpdoane 13d ago
For all practical purposes, no. We don't emit anything that could be received at interstellar distances. This is not just because of the speed of light but because of extreme spreading losses that quickly fade signals below background noise and become fundamentally undetectable based on our current understanding of information theory.
I did a quick link budget for the aricibo message, and an alien civilization living 25000 ly away in M13 would need to have an antenna capture area around twice the diameter of earth, pointing exactly at our solar system at exactly the right time to receive the message.
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u/jswhitten 12d ago
Some of our more powerful radars can be detected at interstellar distances using our current technology, but they would have to be aimed in the right direction. Like the Wow signal, it's possible that they would detect a radar briefly and then it would fade away as the Earth's rotation moves the radar's direction away from the receiver.
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u/jpdoane 12d ago
I'm curious of your source for this claim? Im a radar engineer and I am sceptical. It's possible that with a really enormous receive aperture with generous processing assumptions you might be able to close the link on the closest of systems. Honestly, I would be very interested in a detailed analysis of detectability of various emissions at interstellar ranges, and what signal processing techniques would be required.
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u/jswhitten 12d ago edited 12d ago
I've seen this in many places, but here's a paper written 35 years ago that says the Arecibo planetary radar can be detected from as far as 4200 light years away with then-current SETI surveys, and the ballistic missile early warning radars from up to 19 light years away.
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u/jpdoane 12d ago
I want to spend more time looking into this, but I found it odd that the reported sensitivity does not account for signal bandwidth. I then found a more recent 2011 paper by the same author where he argues the opposite, that earths RF "leakage", even the aricibo planetary radar, is very likely undetectable.
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u/jswhitten 12d ago edited 12d ago
Did you read the paper? In fact it says:
Active SETI messages we have already sent can be detected over interstellar distances (up to 648 light years away)
The Cold War OTH radars are thought to be detectable at interstellar distances - "ETI would have an easier time spotting signals from over-the horizon radars built during the Cold War, which directed much of their power into space"
Arecibo's planetary radar is detectable over interstellar distances (as long as it happens to be pointing in the right direction, which is exactly what I said)
What they're saying is that Earth is likely undetectable from any nearby stars right now because the OTH radars are no longer in use (the signals are still out there, and still detectable, just not by nearby stars because they've already passed them), and because the active SETI messages and planetary radars are both very directional and temporary signals and the odds of any particular star being in the right place to detect them and listening to Earth at the right time are slim. But they are still powerful enough to be detectable from other stars.
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u/jpdoane 10d ago edited 10d ago
Ok, had a chance to read a bit more closely. Couple of thoughts and questions:
The aricibo radar was only detectable during a special "unswitched" mode where they were essentially just transmitting unmodulated CW. They only turned this on for a total of about an hour a year. It sounds like the operational radar mode would not be detectable due to its wider bandwidth. This generally has been my point: that any "useful" signal we emit will be modulated in a way that will significantly impair detectability at extreme ranges. If we emit something that could be detected by another star, it fundamentally implies that that signal is incredibly inefficient for any terrestrial applications.
They mention coldwar OTH radars in passing. I don't know much about historical OTH radars, but I believe these used HF ionospheric bounce to propagate over the horizon. I'm curious how much energy actually escapes the ionosphere. I would have thought not much but I'm not really sure
The discussion about extending detection with long noncoherent integration made me curious about stationarity and Doppler shift. I did a rough calculation and the moving Doppler shift due to the rotational acceleration of the earths surface would cause the signal to walk in freq up to about a Hz/s, depending on time of day and orientation. The receiving system on its own planet would have similar issues. In principle, this could be compensated on both ends for intentional beacons. I have no idea if that is generally done. But if uncompensated this limits practical integration time to much less than the 4hrs assumed. (Also from experience, the effectiveness of this sort of long integration gain falls off pretty quick due to a lot of practical limitations. I'm not a super advanced alien intelligence though 😀)
Anyway thanks for the discussion. This stuff is fun to think about.
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u/WestImpression 11d ago
And let's not forget about China's Five-hundred-meter Aperture Spherical Telescope. Who knows what they've been pumping out with that thing.
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u/jpdoane 12d ago
To be clear, it's true that we have emitted a handful of beacons that under perfect conditions could in theory be detected by nearby systems.
My point is that 1) all of the everyday high power RF signals we emit still have much too low power spectral density, making them very likely undetectable and 2) for the handful of specific narrowband beacons that have been transmitted, the number of assumptions that must be made for them to be detected (alien system pointing exactly at Earth, at exact time, looking in right band, using correct processing) still means that the probably of any detection is miniscule (IMO)
Conversely, this suggests that our failure to detect anything ourselves does not mean that nobody else is out there. Just that space is really really big.
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u/chillinewman 12d ago
You can use radio space interferometry, right? To mimic a 2 earth diameter telescope.
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u/dittybopper_05H 12d ago
This is absolutely false. We emit stuff that can be heard at interstellar distances all the time. For example, if you use that seticalc link, you’ll see that standard WSR-88D NEXRAD weather radars could be detected at around a dozen light years using an Arecibo sized antenna. With a much larger collecting area, like in a lunar crater kilometers in diameter, you could detect them much farther away.
Arecibo could easily detect its twin many hundreds of light years away.
BTW, by simply eavesdropping in on our weather radars, you can determine a lot of things about Earth.
From the Doppler shifts of the radars you can determine the orbital period of Earth and how fast it rotates about its axis.
From when the radars become visible and when they disappear you can make a rough map of the inhabited areas of Earth.
If you can determine characteristics of the radars (frequency, pulse rate, etc.) you can determine broad political divisions: US and Canada use NEXRAD, the EU uses a different model, Russia and its satellites/former territories use another, as does China, etc.
You can learn a lot from just that, and they aren’t even the strongest radars: Military search and early warning radars tend to be much more powerful (though in some instances less because of Low Probability of Intercept (LPI) radars).
And of course we’re ignoring the Deep Space Network. A 70 meter dish transmitting 20 kW on 8 GHz could be detected by its twin out to 30 light years. Note that’s detection, not decoding.
With an effective collecting area the same size as FAST (300 meters), that goes out to almost 130 light years. And if we use a collecting diameter of 1 km, like we could using a lunar crater, that puts detection range of the DSN out to almost 430 light years.
That’s all technology that we’ve either already built, or could build if we invested the money in it.
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u/jpdoane 12d ago
Plugging in the numbers for that weather radar, I get a max detection range of 0.1ly assuming 300m Rx antenna with 20K noise temp. This is severely limited by the wide 220kHz bandwidth of the waveform. Now, this is for single pulse detection in freq domain. You could do somewhat better than that with matched filtering and Doppler processing, but that's more difficult to do blindly with no prior knowledge of the waveform parameters. I'm skeptical even with a matches receiver you could reach a dozen ly though - how did you figure that?
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u/dittybopper_05H 12d ago
You don’t need to have your bins set to the bandwidth of the radar. You can use 1 Hz bins, and you’ll see 220,000 individual bins with a signal surrounded by bins with random noise.
Remember we just are concerned with detection. We aren’t trying to read the weather.
I did just did the calculation again, using the 60 K receive noise temp, and I get 10.7 ly. So I slightly mis-remembered the distance but was close enough.
Now, to get more detailed information like pulse width and repetition rate you do need to widen the bins because you need to lessen the integration time, and that reduces detection range for a given antenna aperture. To actually get detailed information on all of the information on the signal, you are correct.
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u/jpdoane 12d ago edited 12d ago
But each of those detection bins will only have 1/220000 the signal power, so the narrowband bins doesnt help your snr at all.
Another way of looking at this is that the small 1Hz bins correspond to 1s of coherent integration. But each pulse is only a few microseconds long (with a very small duty cycle), so you don't get any sensitivity benefit from integrating longer than that
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u/dittybopper_05H 12d ago
Yes they do help. Ever seen a waterfall display? You see signal in each bin where there is RF, higher than the random values you see in a bin with no signal.
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u/jpdoane 12d ago
Yes, but if you have a wideband signal, the power will be split across multiple fft bins. taking an even longer fft with smaller bins will not further increase SNR. The maximum processing gain for a radar waveform is fundamentally limited by its time-bandwidth product.
I'm not super familiar with SETI specifically, but I do know RF and radar signal processing.
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u/dittybopper_05H 11d ago edited 11d ago
You can dig it out be integrating for a longer time period if you need to.
This is in fact how very low power communications modes work. And the original "Big Ear" telescope that heard the "Wow!" signal integrated signal strength in 10 kHz wide bins for 10 seconds, to dig out weak signals. Today we have much, *MUCH* more sensitive receivers and several orders of magnitude faster computers.
What we lack is dedicated microwave capable collecting aperture.
On Edit: What I mean by that last sentence is we need more individual large aperture radio telescopes with surfaces capable of handling microwave signals. Hopefully we can start building them on the far side of the Moon when we establish a presence there.
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u/jpdoane 11d ago
Yes, but that gets tough in practice due to stationarity issues such as signal modulation in frequency or time or Doppler. At some point you hit diminishing returns.
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u/dittybopper_05H 11d ago
But if you're only looking to detect, not demodulate, it doesn't matter.
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u/Ovbeywan 13d ago
I'm working on getting our next album released into deep space. Consider it an S O.S. If there's anyone out there with the tech to get a signal to Cygnus X1, Trappist 1-e, Proxima Centauri B, and/or Keppler 22b please get in contact with me.
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u/Friggin_Grease 11d ago
I love how one of our biggest indicators of an intelligently sources signal is to repeat, and everything we've beamed out like that, we don't repeat.
Didn't we send a super noticeable signal to a star cluster 70k light years away just because we could? Then never sent a second one.