Two new techniques for examining interesting SETI signals come into view this morning, one out of Breakthrough Listen work at UC-Berkeley, the other from researchers working with the Five-hundred-meter Aperture Spherical radio Telescope (FAST), the so-called ‘Heaven’s Eye’ instrument located in southwest China. In both cases, the focus is on ways to screen SETI observations from disruptive radio frequency interference (RFI), which can appear at first glance to flag a signal from another star.
The Chinese work relies upon FAST’s array of receiving instruments, each acting as a separate ‘beam’ to cover slightly different portions of the sky. FAST’s currently operational L-band receiver array consists of 19 beams, to which researchers led by Bo-lun Huang (Beijing Normal University) apply a technique called MultiBeam Point-source Scanning (MBPS). Here the instrument scans the target star sequentially with different beams of the instrument, setting up the possibility of cross-verification and allowing researchers to identify local interference quickly and accurately.
The paper on this work points to the SETI ON-OFF strategy as a more conventional way to analyze a target star. In this case, the star is observed for a short time, followed by a different target six or more beamwidths away from the primary. These become the ‘ON’ and ‘OFF’ of the method, the assumption being that an authentic signal from another civilization would appear only in the ON set of observations. MBPS, on the other hand, can be used by any radio telescope with a multibeam receiver and requires the telescope to slew during the observation periods to provide ongoing comparisons between each beam.
Let me quote the paper on this:
…we are effectively adding new parameters and the observation data can thus be interpreted from different perspectives. The additional parameters introduced by the MBPS strategy include the duration of signals in a single beam, intensity variation of signals, and the difference in central frequencies of different beams which are the results of the observation method of the MBPS. With the three newly introduced parameters, we are then able to put in the most rigorous restrictions on the RFI/ETI identifications by confining the characteristics of an ETI/RFI signal in a new multi-parameter space.
Having run a re-observation campaign on TRAPPIST-1 using this strategy (this followed a set of observations taken in 2021), the team was able to retrieve all 16,645 received signals (!) as RFI. The authors’ confidence level in the technique is high:
We speculate that it would be exceedingly rare for the MBPS strategy to return any suspicious signals, even over the course of several years, because the types of false positives found by other strategies are easily identifiable with the MBPS strategy. However, when a genuine narrowband ETI signal does arrive on Earth, the MBPS strategy is capable of identifying it even amidst a substantial influx of RFI.
Image: An illustration shows how FAST receives radio waves emitted by distant pulsars, the rapidly rotating cores of dead stars. At left, a photo shows the huge telescope in Guizhou province. Can the new methods in the Bo-lun Huang paper help us weed radio interference out of signals from another civilization? Credit: China Daily.
At UC-Berkeley, Bryan Brzycki and team have been analyzing interstellar ‘scintillation,’ the refraction or bending of electromagnetic waves that pass through cold plasma in interstellar space. Rising and falling in amplitude, the waves interfere when they reach Earth by different paths. The phenomenon has been well studied through analysis of pulsars and other distant radio sources, and an obvious analog occurs in the twinkling of starlight created by Earth’s atmosphere. In the case of interstellar scintillation, Brzycki has come up with algorithms that can analyze narrowband signals for this effect, quickly selecting for those that show the phenomenon and thus are not local.
On first glance, this appears extraordinarily useful, as co-author (and Brzycki thesis adviser) Imke de Pater (UC-Berkeley) points out:
“This implies that we could use a suitably tuned pipeline to unambiguously identify artificial emission from distant sources vis-a-vis terrestrial interference. Further, even if we didn’t use this technique to find a signal, this technique could, in certain cases, confirm a signal originating from a distant source, rather than locally. This work represents the first new method of signal confirmation beyond the spatial reobservation filter in the history of radio SETI.”
Image: The Green Bank Telescope, nestled in a radio-quiet valley in West Virginia, is a major listening post for Breakthrough Listen. Credit: Steve Croft, Breakthrough Listen.
A useful tool indeed, though bear in mind that it proves useful only for signals originating more than 10,000 light years from Earth, for to produce the needed scintillation, the signal must do a lot of traveling. If we do make a SETI detection with the aid of scintillation, in other words, it will not be of a civilization we’ll be likely to converse with (unless, of course, we find a way someday to actually visit it).
The Brzycki paper dovetails nicely with the FAST work, as witness its discussion of the ON-OFF strategy discussed above. The italics below are mine:
…RFI can also appear in only ON observations. For example, RFI signals could exhibit intensity modulations that follow the observational cadence of 5 minutes per pointing, a false positive that would pass the directional filter. While we observe false positives like this in practice, having directional requirements still serves as an interpretable basis for determining candidates, which would induce follow-up observations for potential re-detection.
This begs the question: can we differentiate narrowband signals as RFI based on morphology alone? Since ETI signals must travel to us through interstellar space, are there effects that would be observable and sufficiently unique compared to RFI modulations?
Thus the important result: The effect of scintillation does indeed provide a way to single out RFI simply because no local interference will produce the effect. Indeed, as the authors note, ETI might well consider the presence of scintillation in an artificial, narrowband signal as an announcement: ‘we are here.’ Where this work points is to further analysis of radio emission – the authors single out polarization, which they say is only beginning to be studied in the SETI context. Who can doubt their conclusion?
Whether it is because certain effects are stochastic or because human radio emission exploits every facet of radio light possible for communication, extracting non trivial information from a radio signal’s detailed morphology has been and will remain difficult. We may need to push the limits of detectability along hitherto unexplored axes to discover the first technosignature.
The paper from FAST is Bo-lun Huang et al., “A solution to persistent RFI in narrowband radio SETI: The MultiBeam Point-source Scanning strategy,” currently available as a preprint. The paper on scintillation is Brzycki et al., “On Detecting Interstellar Scintillation in Narrowband Radio SETI,” Astrophysical Journal 17 July 2023 (full text).
Any ETs would likely be using their own star as a solar focus.
I wonder if Earth passed through a focal line during the ” WOW” signal.
The message would get smeared throughout the Einstein ring, making it more difficult to detect and decode. I am not debunking the idea, an ET people could use this to select for receiving people above a threshold of technology and commitment. The notion that there is a most likely signaling strategy should be debunked though. The signaling strategy used will simply align with motivation and conversely, we could use the strategy to understand motivation. A loud, omni-directional beacon transmitting at the water line wants to reach everyone. A more difficult to find signal wants to be selective.
Imo, the solar focals are well suited to a communication network. They require less energy and have built in encryption that could be augmented.
While good for the instrumentation, this may be a problem for SETI funding. False positives are always good for keeping the public’s attention with a mystery that may or may not be solved. Think of the WOW! signal, or is ‘Oumuamua an alien spacecraft, or UFO sightings. (The credulous US Congress is “investigating” the claims of a government coverup.) If there are no true positives to be detected, perhaps for centuries or even millennia, if ever, this research will not generate any publicity as all the current mystery signals will be eliminated as RFI. The “eerie silence” will be maintained, interest will be lost, funding will dry up, and the search abandoned. Perhaps that is a good thing if SETI work was just piggybacked on other astronomical observations where RFI can be eliminated to remove this source of noise.
This style of cynicism contains a grain of truth, unfortunately. If you’ll permit me to be cynical in turn, I suggest that a paucity of candidate signals is not an impediment so long as we have Avi Loeb and others like him. Looking abroad, I see that there’s a suitably credulous public hearing currently taking place in the US.
It will be interesting if Loeb’s claim that they may have found a piece of the [“alien”] meteor pans out once it has been tested in the lab. Have We Found Fragments of a Meteor from Another Star?.
Loeb, of course, hypes this further: Loeb: Are fragments of interstellar meteor from alien gadget?
The UFO history might provide a different path for the non-reporting of SETI signals – a government coverup conspiracy. This has kept the publicity going despite the lack of hard evidence. Would it work for SETI if instruments excluded all false positives?
[Any cynicism is purely accidental. :-) ]
Oumuamua actually is from another star-system, at least.
Sending a craft to it is a de facto interstellar object mission.
As for Avi’s little grains of metal—the less said about that—the better.
I’ve only had time to read the FAST paper, and I had a bit of deja vu. The work they’ve done is important but perhaps the reason is not sufficiently highlighted. It’s an easy paper to read and short, and I think worthwhile doing so.
The point about deja vu is that the general technique of distinguishing signals from RFI using the multiple beams in large antennas is not new. Indeed, we discussed this many years ago right here on CD. I only recall it because I participated in the comments. If the subject is of interest, I suggest reading this article and especially the comments.
https://centauri-dreams.org/2015/04/06/puzzling-out-the-perytons/
(On a side note, the comments of these old articles are formatted oddly since the web site redesign.)
What has not been highlighted sufficiently in the FAST article is the enormity of data that these telescopes generate. As they state, there are terabytes of data in even brief observing periods. Identification of signals (and RFI) is beyond the scope of manual review. Like LHC, LIGO and other instruments, automation is mandatory.
Algorithms that can effectively do so with low rates of false negatives and positives is critically required. That’s the promise of this work out of FAST, and is hopefully also the case in the scintillation paper (which I intend to read later). As I believe most will know, the number of false positives is increasing due to LEO satellites, military satellites, terrestrial radio electronics and consumer electronics. Effective algorithms are needed since those extraneous signals will only increase in number. As will the quantity of data from these and future observatories.
I finally found time to read the much longer Brzycki paper. Very interesting work. Unfortunately I don’t see how it’ll be terribly useful distinguishing RFI and signals. This is no great insight on my part since the paper itself describes the many challenges. They’re very honest about it though perhaps leaning too much on hope.
Obviously, as Paul already stated, a radio signal from 10,000 ly is exceedingly unlikely. That’s a huge hurdle to jump! The technique itself is really only suitable for CW signals since any reasonable modulation would confound the detection: modulation and scintillation are easily confused.
You would need a favorable combination of modulation rate, detector integration time and signal processing parameters to adequately reduce the rate of false positives and negatives. In every case the raw data would require in-depth follow up analysis. With that necessity and the high detection error rate, the value of the detector is almost superfluous.
Hopefully this work leads to better detection algorithms. Scintillation is not likely to be a suitable attribute to work with.
I think we have to be realistic that we may never receive an authentic signal for hundreds of years or more. How would the Wow signal have been categorized had we had the technical ability back then? Another disturbing thought is that we may not have hundreds of years to look for ETI, or even a hundred years. The state of our recent climate and forecasts for the near future suggest we have only decades left before life is drastically affected on Earth by climate change. Alex mentioned it on an earlier post but the new findings about the deep ocean currents, in particular the Atlantic currents suggest that as they slow and possibly stop, the climate consequences will be catastrophic, especially in Europe as far as North Atlantic currents are concerned. Heat waves are everywhere now as are massive forest fires brought on by drought (take Canada as a very worrisome example). Many locations are receiving more rainfall in a 24 hour period than they previously received in a month or more, leading to massive flooding events. China is experiencing temperatures over 50 C. I could go on and on. Do we concern ourselves with this crisis here and everywhere else we can speak out and educate? I don’t know the answer to that. Paul and Alex will decide I guess but my view is don’t make assumptions about what the human race will achieve over the next century or more, in space or anywhere else.
In the news: this week Young-Wan Kwon’s group in Seoul released papers on Arxiv saying that they have developed a room temperature, ambient pressure superconductor. See links from https://phys.org/news/2023-07-korean-team-room-temperature-ambient-pressure-superconductor.html . I don’t know the field, but surely this advance (if true) should transform radio astronomy?
It could change everything. Excitement (Science) and skepticism (New Scientist). The response is similar to announcements of a possible alien signal from the stars. I remember the cold fusion debacle at the beginning of the 1990s that ultimately proved an error and letdown. The results need to be replicated by other groups before a world with high-temperature superconductors can be envisaged.
Hello,
In the field of radio frequencies, I recently read something that we don’t often think about, but which is worth thinking about: if we assume an expanding universe (the “big bang” theory), we can assume that a radio wave propagating in this medium will also “stretch”. In other words, there will be a frequency shift between what E.T. has sent us and what we could potentially pick up. So if we don’t have an element of correction – or if E.T hasn’t made this correction in space-time – we won’t be able to decode the message. It’s a bit like the idea of “red shift” applied to radio. I find the idea interesting…
Fred, from France
I think SETI should come up with a new strategy. Any civilization that would use a technology we currently have is not interesting enough to bother with yet so don’t waste resources looking for such signals. Any truly advanced civilization would have long figured out how to travel interstellar distances and have likely quietly been in our neighborhood for all of our history and in contact with some human elites being careful not to cause massive societal upheaval by large scale direct presence. Therefore, SETI should use a portion of their resources to study the UAP problem in a new light, no longer just trying to put it to rest because it interferes with their own paradigm but seriously taking a closer look.