Slashdot Mirror
SETI@Home Revisits Its 100 Best Signals
cmbrothe writes "The Planetary Society is running an article about SETI@Home's plan to revisit its 100 most promising signal candidates. The article also outlines the criteria for selecting the candidates."
SETI@home Takes Stock of Its Most Promising Signals
by Amir Alexander
I. Millions and Millions
For almost three years now SETI@home users have been processing data on their home computers. Millions of Gaussians, spikes, triplets, and pulsed signals, have been detected by SETI@home's three and a half million users, and sent back to Berkeley for further analysis. It is time, the SETI@home crew in Berkeley decided, to take stock of what has been accomplished so far. Which of this plethora of signals is the most likely to be that epoch-making transmission we have been waiting for?
A perfect gaussian as it would appear on a SETI@home screen.
Sorting through this mass of data is no easy matter: with so many candidate signals, how does one decide which is the most promising? To simplify matters a bit, the SETI@home scientists decided to start by sorting through the gaussians, and leave the other signals for later. "Gaussians", it will be recalled, are the bell-shaped power curves that are typical of continuous signals coming from space. When the Arecibo radio telescope's detection beam scans the sky, any continuous transmission from space first appears as a faint signal on the margin of the beam, then grows in strength as it approaches the beam's center, and finally fades away as the beam moves on to other regions of the sky. This pattern produces the characteristic gaussian shape. Since most SETI scientists believe that an alien transmission would indeed announce itself in the form of a gaussian, it seemed natural to start with these. Even this choice, however, left the SETI@home team with no less than 20 million candidates to choose from.
II. Why Some Gaussians are More Equal than Others
As a first step in selecting the most promising signals, each gaussian was assigned a score, defined as its peak power divided by chi square. The first element simply indicates the maximum strength of the signal. Naturally, if a signal is strong, it is a better candidate for further analysis. The second element, chi square, is a measure of how closely the detected signal resembles a perfect gaussian. The smaller chi square is, the better the fit, and consequently, the higher the gaussian score.
But while processing the signals in this manner, the SETI@home scientists detected a disturbing pattern: the number of gaussians found, it seemed, was dependent on the speed at which Arecibo's beam was traveling through the sky at the time the signal was detected. This speed, known as the telescope's "slew rate," can vary significantly, depending on what the telescope is observing at the time. For the SETI@home sky survey it would have been best if the telescope had been pointed constantly straight up, and would traverse all points at a fixed speed. But since the big dish is always being used for various scientific observations and experiments, it is a fact of life for SETI that the slew rate varies significantly over time.
An analysis of all the gaussians clearly showed that the faster the telescope's slew rate, the more gaussians it detected. The reason for this was clear to SETI scientists early on. At faster rates, it takes the beam a shorter time to scan a point in the sky than at slower rates. When a received signal is then analyzed by the SETI@home program on a user's computer, the program breaks down the continuous signal into measured "points," each lasting a fixed number of seconds. These points are represented by the blocks on your screen. A short signal (fast slew rate) would have fewer points than a long signal (slow slew rate). When the program then proceeds to search for gaussians in the data, it looks for close fits between the measured points and a perfect guassian. Naturally, it is much easier to fit a small number of points onto a gaussian curve than it is to fit a large number of points onto the same curve. As a result, the SETI@home client was much more likely to detect gaussians in short signals (fast slew rates) than in long signals (slow slew rates). These fast signals, however, were in a sense "lower quality" gaussians, because they were based on a fit of fewer points. Not all gaussians, it seems, are truly equal...
This discovery posed a significant problem: if not all gaussians are truly of the same quality, how should SETI@home go about ranking them? How do we ensure that the best candidates to be a "real" signal do indeed get the highest scores? To address this, project scientist Eric Korpela devised a mathematical function that would compensate for the "slew rate" distortion. This "normalizing" function excluded all but the best gaussians at the fast slew rates, while preserving most of the gaussians from the slow slew rates. As a result, the same number of gaussians would now be included from any slew-rate, effectively eliminating it as a selection factor.
III. Multiplets and Frequencies
The slew rate correction left SETI@home scientists with "only" 1.25 million gaussians with a score of 1.0 or higher to process and rank. These were the strongest and best fitting gaussians of the lot. But if a signal, no matter how strong and clear, is detected in the skies only once, how can we ever hope to know what it is? One need only think of the famous "Wow!" signal to appreciate the problem: the most promising signal ever detected by SETI was heard just once and never again, and as a result remains an enigma to this day. If we are to believe that a signal is coming from an alien civilization, then we must be able to detect it repeatedly.
For this reason, the SETI@home team set a final and demanding test for the remaining gaussians. A signal must be detected at least twice on two separate passes for it to be considered a likely candidate for an alien transmission.
Although this standard sounds straightforward enough, applying it is far from simple. For how does one know whether a signal detected today is the "same" one detected on a previous pass months ago? After some deliberation, the SETI@home team decided on the following criteria:
First - the two signals must come from the same direction in the sky, to within 10 arc minutes. Location, of course, is the primary indication that the two signals are one and the same.
Second - the two signals must be detected at least 900 seconds apart. This is to ensure that the two are indeed separate detections, rather than a continuous one.
Third - the "barycentric" frequency of the two signals must be the same, within 125 Hertz.
This last criterion requires some explanation. A signal coming from space will most likely not be received at a steady frequency. Because both the Earth and the presumed alien planet will be in motion around their stars, they will also be in motion in relation to each other, and their relative speed will be changing constantly. As a result, the frequency of a transmission received on Earth will drift, either increasing or decreasing, depending on the relative motions of the two planets. The SETI@home program installed on users' computers takes this effect, known as "Doppler drift," into account, and searches each data set at different drift rates. When SETI Scientists want to determine the true frequency of a transmission they must first compensate for this drift rate. This corrected frequency is known as the "detection frequency."
Even this, however, is not enough. The reception frequency is also affected by the Earth's movement within the Solar System. In other words, the same signal might be detected at different frequencies depending on the position and direction of the Earth at the time of detection. To compensate for this SETI scientists must take into account not only the Earth's exact position and movement at the time of the detection, but also the position of the Moon, the gravitational effects of the giant outer planets, and the direction the telescope is pointing. When all these are accounted for, the resulting frequency is the one that would have been received at a fixed point at the center of gravity (or "barycenter") of our Solar System. That is the barycentric frequency, used to determine whether two signals detected at different times could, in fact, be one and the same.
When the gaussians detected by SETI@home are checked against these exacting standards, the vast majority fail the test. Of the 1.25 million only 1397 qualify as being likely cases of multiple detections. They are, appropriately, labeled "multiplets."
Having a "mere" 1397 candidate signals is a vast improvement over having one and a quarter million signals, not to mention the 20 million gaussians we started out with. But even so it is too many. The SETI@home team still needs to narrow the list further, to around 25 most promising signals. This list they can then present at Arecibo, and arrange for dedicated telescope time to go back and check each of the signals separately.
Finding the few best signals out of millions of possibilities is a tough task, and there is always the danger of missing that one true signal hiding among so many false ones. The people at SETI@home are working hard to make sure this doesn't happen. And if their choices prove correct, then there is always the chance that one of these anonymous gaussians will in fact bear a message from the stars.
Just for fun, I googled the 1977 "Wow" signal mentioned in the article and every so often in SETI news. Found this good BBC article on the subject.
This blatant karma whoring is brought to you by the letters "ET".
Never approach a vast undertaking with a half-vast plan.
score= N*(bv-bv0)*exp(0.5*(bv-bv_sun)^2)/(par+0.01)^3
How exactly do you test the validity of a formula like this?
That's easy -- it's clearly wrong. It's saying the Sun gets the lowest possible score according to the 3rd factor, when it should obviously get the highest score. (They left out a negative sign.)
Why do journalists put formulas online when they don't have a clue what they mean?
There's no proof there was life on mars. There's only a theory that mars could once have sustained life. That and a pile of rocks that looks like a smiley-face.
I don't need no instructions to know how to rock!!!!
Sure, the astrometry (positions) in Hipparcos are better than in Tycho 2, and Hipparcos contains more information about the stars than Tycho 2 (e.g. variability), but still. I would in fact think that Tycho 2 would be better for SETI than Hipparcos, but they may have their reasons.
Employee of Inrupt, Project Release Manager and Community Manager for Solid
I'm not talking about all the regular satellite communications. Are we intentionally broadcasting any messages for the universe at large?
Short answer is no - apart from at least one PR message sent out from Aricebo in the 70's IIRC.
And would regular satellite communications appear barycentric? It doesn't sound like it. So, if we're not broadcasting barycentric signals, why would we expect other lifeforms to broadcast them? Or are we braodcasting something barycentric?
The current SETI efforts assume that we will be receiving signals from a beacon aimed at least generally in our direction and which will be very high power. This is obviously a big assumption, but the problem is that we don't have the technology at the moment to detect "alien TV"-strength signals. Those signals would be utterly missed by the Aricebo effort, as they are too weak to resolve against the background noise. The Square Kilometer Array radio telescope might be able to pick up alien TV signals out to a dozen or so light years.
I tried to dig up the paper, but these guys are really publishing a lot of stuff. this may have something to do with it. The author's homepage is here, you can look through a list of some of his papers.
Employee of Inrupt, Project Release Manager and Community Manager for Solid
Nice scientific arguement. Read this page on NASAs site.
That should read:
nyy lbhe onfr ner orybat gb hf
All it takes is nukes and nerves.
The dish is fixed into the bowl-shaped depression,
but the detection apparatus in the focal plane (if
that's what they call it) can move around.
Regards, RGC.
I want to live as an honest man, to get all I deserve and give all I can, to love a young woman who I don't understand.