by Paul Gilster | Oct 2, 2018 | Astrobiology and SETI |
A new paper from Jason Wright (Penn State) and colleagues Shubham Kanodia and Emily Lubar deals with SETI and the ‘parameter space’ within which we search, with interesting implications. For the researchers show that despite searching for decades through a variety of projects and surveys, SETI is in early days indeed. Those who would draw conclusions about its lack of success to this point fail to understand the true dimensions of the challenge.
But before getting into the meat of the paper, let’s talk about a few items in its introduction. For Wright and team contextualize SETI in relation to broader statements about our place in the cosmos. We can ask questions about what we see and what we don’t see, but we have to avoid being too facile in our interpretation of what some consider to be an ‘eerie silence’ (the reference is to a wonderful book by Paul Davies of the same name).
Image: Penn State’s Jason Wright. Credit: Jody Barshinger.
Back in the 1970s, Michael Hart argued that even with very slow interstellar travel, the Milky Way should have been well settled by now. If, that is, there were civilizations out there to settle it. Frank Tipler made the same point, deducing from the lack of evidence that SETI itself was pointless, because if other civilizations existed, they would have already shown up.
In their new paper, Wright and team take a different tack, looking at the same argument as applied to more terrestrial concerns. Travel widely (Google Earth will do) and you’ll notice that most of the places you select at random show no obvious signs of humans or, in a great many cases, our technology. Why is this? After all, it takes but a small amount of time to fly across the globe when compared to the age of the technology that makes this possible. Shouldn’t we, then, expect that by now, most parts of the Earth’s surface should bear signs of our presence?
It’s a canny argument in particular because we are the only example of a technological species we have, and the Hart-style argument fails for us. If we accept the fact that although there are huge swaths of Earth’s surface that show no evidence of us, the Earth is still home to a technological civilization, then perhaps the same can be said for the galaxy. Or, for that matter, the Solar System, so much of which we have yet to explore. Could there be, for example, a billion year old Bracewell probe awaiting activation among the Trans-Neptunian objects?
Maybe, then, there is no such thing as an ‘eerie silence,’ or at least not one whose existence has been shown to be plausible. The matter seems theoretical until you realize it impacts practical concerns like SETI funding. If we assume that extraterrestrial civilizations do not exist because they have not visited us, then SETI is a wasteful exercise, its money better spent elsewhere.
By the same token, some argue that because we have not yet had a SETI detection of an alien culture, we can rule out their existence, at least anywhere near us in the galaxy. What Wright wants to do is show that the conclusion is false, because given the size of the search space, SETI has barely begun. We need, then, to examine just how much of a search we have actually been able to mount. What interstellar beacons, for example, might we have missed because we lacked the resources to keep a constant eye on the same patch of sky?
The Wright paper is about the parameter space within which we hope to find so-called ‘technosignatures.’ Jill Tarter has described a ‘cosmic haystack’ existing in three spatial dimensions, one temporal dimension, two polarization dimensions, central frequency, sensitivity and modulation — a haystack, then, of nine dimensions. Wright’s team likes this approach:
This “needle in a haystack” metaphor is especially appropriate in a SETI context because it emphasizes the vastness of the space to be searched, and it nicely captures how we seek an obvious product of intelligence and technology amidst a much larger set of purely natural products. SETI optimists hope that there are many alien needles to be found, presumably reducing the time to find the first one. Note that in this metaphor the needles are the detectable signatures of alien technology, meaning that a single alien species might be represented by many needles.
Image: Coming to terms with the search space as SETI proceeds, in this case at Green Bank, WV. Credit: Walter Bibikow/JAI/Corbis /Green Bank Observatory.
The Wright paper shows how our search haystacks can be defined even as we calculate the fraction of them already examined for our hypothetical needles. A quantitative, eight-dimensional model is developed to make the calculation, with a number of differences between the model haystack and the one developed by Tarter, and factoring in recent SETI searches like Breakthrough Listen’s ongoing work. The assumption here, necessary for the calculation, is that SETI surveys have similar search strategies and sensitivities.
This assumption allows the calculation to proceed, and it is given support when we learn that its results align fairly well with the previous calculation Jill Tarter made in a 2010 paper. Thus Wright: “…our current search completeness is extremely low, akin to having searched something like a large hot tub or small swimming pool’s worth of water out of all of Earth’s oceans.”
And then Tarter, whose result for the size of our search is a bit smaller. Let me just quote her (from an NPR interview in 2012) on the point:
“We’ve hardly begun to search… The space that we’re looking through is nine-dimensional. If you build a mathematical model, the amount of searching that we’ve done in 50 years is equivalent to scooping one 8-ounce glass out of the Earth’s ocean, looking and seeing if you caught a fish. No, no fish in that glass? Well, I don’t think you’re going to conclude that there are no fish in the ocean. You just haven’t searched very well yet. That’s where we are.”
This being the case, the idea that a lack of success for SETI to date is a compelling reason to abandon the search is shown for what it is, a misreading of the enormity of the search space. SETI cannot be said to have failed. But this leads to a different challenge. Wright again:
We should be careful, however, not to let this result swing the pendulum of public perceptions of SETI too far the other way by suggesting that the SETI haystack is so large that we can never hope to find a needle. The whole haystack need only be searched if one needs to prove that there are zero needles—because technological life might spread through the Galaxy, or because technological species might arise independently in many places, we might expect there to be a great number of needles to be found.
The paper also points out that in its haystack model are included regions of interstellar space between stars for which there is no assumption of transmitters. Transmissions from nearby stars are but a subset of the haystack, and move up in the calculation of detection likelihood.
So we keep looking, wary of drawing conclusions too swiftly when we have searched such a small part of the available parameter space, and we look toward the kind of searches that can accelerate the process. These would include “…surveys with large bandwidth, wide fields of view, long exposures, repeat visits, and good sensitivity,” according to the paper. The ultimate survey? All sky, all the time, the kind of all-out stare that would flag repeating signals that today could only register as one-off phenomena, and who knows what other data of interest not just to SETI but to the entire community of deep-sky astronomers and astrophysicists.
The paper is Wright et al., “How Much SETI Has Been Done? Finding Needles in the n-Dimensional Cosmic Haystack,” accepted at The Astronomical Journal (preprint).
by Paul Gilster | Oct 1, 2018 | Astrobiology and SETI |
Can rapidly advancing laser technology and optics augment the way we do SETI? At the University of California, Santa Barbara, Phil Lubin believes they can, and he’s behind a project called the Trillion Planet Survey to put the idea into practice for the benefit of students. As an incentive for looking into a career in physics, an entire galaxy may be just the ticket.
For the target is the nearest galaxy to our own. The Trillion Planet Survey will use a suite of meter-class telescopes to search for continuous wave (CW) laser beacons from M31, the Andromeda galaxy. But TPS is more than a student exercise. The work builds on Lubin’s 2016 paper called “The Search for Directed Intelligence,” which makes the case that laser technology foreseen today could be seen across the universe. And that issue deserves further comment.
Centauri Dreams readers are familiar with Lubin’s work with DE-STAR, (Directed Energy Solar Targeting of Asteroids and exploRation), a scalable technology that involves phased arrays of lasers. DE-STAR installations could be used for purposes ranging from asteroid deflection (DE-STAR 2-3) to propelling an interstellar spacecraft to a substantial fraction of the speed of light (DE-STAR 3-4). The work led to NIAC funding (NASA Starlight) in 2015 examining beamed energy systems for propulsion in the context of miniature probes using wafer-scale photonics and is also the basis for Breakthough Starshot.
Image: UC-Santa Barbara physicist Philip Lubin. Credit: Paul Wellman/Santa Barbara Independent.
A bit more background here: Lubin’s Phase I study “A Roadmap to Interstellar Flight ” is available online. It was followed by Phase II work titled “Directed Energy Propulsion for Interstellar Exploration (DEEP-IN).” Lubin’s discussions with Pete Worden on these ideas led to talks with Yuri Milner in late 2015. The Breakthrough Starshot program draws on the DE-STAR work, particularly in its reliance on miniaturized payloads and, of course, a laser array for beamed propulsion, the latter an idea that had largely been associated with large sails rather than chip-sized payloads. Mason Peck and team’s work on ‘sprites’ is also a huge factor.
But let’s get back to the Trillion Planet Survey — if I start talking about the history of beamed propulsion concepts, I could spend days, and anyway, Jim Benford has already undertaken the task in these pages in his A Photon Beam Propulsion Timeline. What’s occupies us this morning is the range of ideas that play around the edges of beamed propulsion, one of them being the beam itself, and how it might be detected at substantial distances. Lubin’s DE-STAR 4, capable of hitting an asteroid with 1.4 megatons of energy per day, would stand out in many a sky.
In fact, according to Lubin’s calculations, such a system — if directed at another star — would be seen in systems as distant as 1000 light years as, briefly, the brightest star in the sky. Suddenly we’re talking SETI, because if we can build such systems in the foreseeable future, so can the kind of advanced civilizations we may one day discover among the stars. Indeed, directed energy systems might announce themselves with remarkable intensity.
Image: M31, the Andromeda Galaxy, the target of the largely student led Trillion Planet Survey. Credit & Copyright: Robert Gendler.
Lubin makes this point in his 2016 paper, in which he states “… even modest directed energy systems can be ‘seen’ as the brightest objects in the universe within a narrow laser linewidth.” Amplifying on this from the paper, he shows that stellar light in a narrow bandwidth would be very small in comparison to the beamed energy source:
In case 1) we treat the Sun as a prototype for a distant star, one that is unresolved in our telescope (due to seeing or diffraction limits) but one where the stellar light ends up in ~ one pixel of our detector. Clearly the laser is vastly brighter in this sense. Indeed for the narrower linewidth the laser is much brighter than an entire galaxy in this sense. For very narrow linewidth lasers (~ 1 Hz) the laser can be nearly as bright as the sum of all stars in the universe within the linewidth. Even modest directed energy systems can stand out as the brightest objects in the universe within the laser linewidth.
And again (and note here that the reference to ‘class 4’ is not to an extended Kardashev scale, but rather to a civilization transmitting at DE-STAR 4 levels, as defined in the paper):
As can be seen at the distance of the typical Kepler planets (~ 1 kly distant) a class 4 civilization… appears as the equivalent of a mag~0 star (ie the brightest star in the Earth’s nighttime sky), at 10 kly it would appear as about mag ~ 5, while the same civilization at the distance of the nearest large galaxy (Andromeda) would appear as the equivalent of a m~17 star. The former is easily seen with the naked eye (assuming the wavelength is in our detection band) while the latter is easily seen in a modest consumer level telescope.
Out of this emerges the idea that a powerful civilization could be detected with modest ground-based telescopes if it happened to be transmitting in our direction when we were observing. Hence the Trillion Planet Survey, which looks at using small telescopes such as those in the Las Cumbres Observatory’s robotic global network to make such a detection.
With M31 as the target, the students in the Trillion Planet Survey are conducting a survey of the galaxy as TPS gets its software pipeline into gear. Developed by Emory University student Andrew Stewart, the pipeline processes images under a set of assumptions. Says Stewart:
“First and foremost, we are assuming there is a civilization out there of similar or higher class than ours trying to broadcast their presence using an optical beam, perhaps of the ‘directed energy’ arrayed-type currently being developed here on Earth. Second, we assume the transmission wavelength of this beam to be one that we can detect. Lastly, we assume that this beacon has been left on long enough for the light to be detected by us. If these requirements are met and the extraterrestrial intelligence’s beam power and diameter are consistent with an Earth-type civilization class, our system will detect this signal.”
Screening transient signals from its M31 images, the team will then submit them to further processing in the software pipeline to eliminate false positives. The TPS website offers links to background information, including Lubin’s 2016 paper, but as of yet has little about the actual image processing, so I’ll simply quote from a UCSB news release on the matter:
“We’re in the process of surveying (Andromeda) right now and getting what’s called ‘the pipeline’ up and running,” said researcher Alex Polanski, a UC Santa Barbara undergraduate in Lubin’s group. A set of photos taken by the telescopes, each of which takes a 1/30th slice of Andromeda, will be knit together to create a single image, he explained. That one photograph will then be compared to a more pristine image in which there are no known transient signals — interfering signals from, say, satellites or spacecraft — in addition to the optical signals emanating from the stellar systems themselves. The survey photo would be expected to have the same signal values as the pristine “control” photo, leading to a difference of zero. But a difference greater than zero could indicate a transient signal source, Polanski explained. Those transient signals would then be further processed in the software pipeline developed by Stewart to kick out false positives. In the future the team plans to use simultaneous multiple color imaging to help remove false positives as well.
Why Andromeda? The Trillion Planet Survey website notes that the galaxy is home to at least one trillion stars, a stellar density higher than the Milky Way’s, and thus represents “…an unprecedented number of targets relative to other past SETI searches.” The project gets the students who largely run it into the SETI business, juggling the variables as we consider strategies for detecting other civilizations and upgrading existing search techniques, particularly as we take into account the progress of exponentially accelerating photonic technologies.
Projects like these can exert a powerful incentive for students anxious to make a career out of physics. Thus Caitlin Gainey, now a freshman in physics at UC Santa Barbara:
“In the Trillion Planet Survey especially, we experience something very inspiring: We have the opportunity to look out of our earthly bubble at entire galaxies, which could potentially have other beings looking right back at us. The mere possibility of extraterrestrial intelligence is something very new and incredibly intriguing, so I’m excited to really delve into the search this coming year.”
And considering that any signal arriving from M31 would have been enroute for well over 2 million years, the TPS also offers the chance to involve students in the concept of SETI as a form of archaeology. We could discover evidence of a civilization long dead through signals sent well before civilization arose on Earth. A ‘funeral beacon’ announcing the demise of a once-great civilization is a possibility. In terms of artifacts, the search for Dyson Spheres or other megastructures is another. The larger picture is that evidence of extraterrestrial intelligence can come in various forms, including optical or radio signals as well as artifacts detectable through astronomy. It’s a field we continue to examine here, because that search has just begun.
Phil Lubin’s 2016 paper is “The Search for Directed Intelligence,” REACH – Reviews in Human Space Exploration, Vol. 1 (March 2016), pp. 20-45. (Preprint / full text).
