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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 Prepares to Revisit its Best Signals
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by Amir Alexander
A Special Day at Arecibo
Sometime early in 2003 the giant radio telescope at Arecibo will take a day off from its normal astronomical duties. For 24 hours it will devote all of its immense observational capacity to a single goal: searching for steady repeating signals from space, the telltale signs of intelligent transmissions.
Needless to say, SETI research at Arecibo is not limited to that one single day. The SETI@home receiver, mounted 500 feet above the enormous dish, scans the skies throughout the year in search of intelligent transmission. On most days, however, the receiver sits passively on its perch, and scans whichever part of the sky the telescope happens to point to. Over time, the receiver scans the entire celestial band visible from Arecibo. It can never, however, go back and listen attentively to a particular promising signal, to determine whether it might possible be an intelligent transmission.
This, however, is precisely what the radio telescope will do on that special day. Rather than searching the vastness of space at random, it will focus its attention on a list of 100 most promising locations, handed to the telescope operators by SETI@home scientists. All of these are locations in the sky where SETI@home had detected radio signals at least twice before.
Signal Types
Gaussians are the power curves produced when the Arecibo beam scans a steady celestial radio source. The signal is weak at first, strong when it is at the center of the beam, and then fades again. This produces a bell shaped power curve known as a gaussian.
A perfect gaussian
Spikes represent any celestial radio signal of a fixed frequency that is distinguishable above the background noise.
Triplets are a set of 3 equally spaced spikes. Whereas gaussians represent a constant signal from space, triplets may represent a series of pulses transmitted at fixed time intervals.
Scores, Stars, and Multiplets
In preparation for those very special 24 hours, SETI@home scientists have been making a concerted effort to compile the best list of signals. Since more than 5 billion(!) gaussians, spikes, and triplets have been detected so far by SETI@home users around the world, coming up with a list of the 100 most promising signals has been no easy task. First, the least reliable signals must be weeded out in a process called "data integrity check", and those that are most likely the result of detection or computer error are eliminated. Then all signals are compared to a database of known Radio Frequency Interference (RFI) sources. These are strong human-made radio transmissions generated by radars, satellites, and the like, which operate in the vicinity of Arecibo. If a SETI@home signal appears to match a known RFI source, then it too is removed from the list.
Once these obvious "false alarms" are eliminated, however, SETI@home scientists are still left with several billion signals. Each must therefore be assigned a score, representing the likelihood that it is, in fact, an alien transmission. The top 100 scorers will have their day at Arecibo, where they will earn a repeat visit to their location by the giant radio telescope.
To determine a signal's ranking in this multitude, the first step is to give it an individual or "detection" score. The factors that go into calculating this, naturally depend on the type of signal. Gaussians will be ranked according to how well their curve matches a perfect gaussian generated by the Arecibo dish, as well as by their strength. The stronger the signal, the more likely it is to be located once again. Triplets and spikes are not restricted to a particular shape, but their score also improves with their strength.
Another factor that contributes to a signal's score is its location in the sky. A signal that comes from the direction of a known star or galaxy will be given preference over one that appears to emerge from empty space. To check for this, the SETI@home crew relied on the Hipparcus catalogue - the most comprehensive list of celestial objects available. Hipparcus lists no less than 33,000 main sequence stars within Arecibo's observation band, and all of these are compared with each signal. To these are added the numerous distant galaxies that dot the skies at Arecibo's latitude, on the assumption that a signal might just possibly originate from one of them as well.
When it comes to scoring signals, however, not all stars are equal. This is because, according to SETI wisdom, some stars are more likely to host a communicating alien civilization than others. Thus, for example, only main-sequence stars are considered for signal-scoring purposes, excluding red giants and white dwarfs. Short-lived stars, whose lifespan is only a few million years, are also excluded from consideration, since complex life would not have had time to evolve in such an environment. Nearby stars, on the other hand, get "extra credit" in their scoring, since it would be comparatively easier to communicate with civilizations in our galactic neighborhood than with those in distant parts of our galaxy or beyond. Finally, the more similar a star is to our own Sun, the higher its score, since it would be more likely to host a civilization similar to ours.
The extrasolar planets discovered in recent years are also factored into the equation: a signal originating from the direction of a star with known planets will certainly receive special attention. The ideal signal, in other words, would originate from the direction of a nearby main-sequence Sun-like star with known planets.
So far we have only dealt with unique events - separate signals that have been detected by Arecibo at different times. But a signal that has been detected only once and never again is not a good candidate for an extraterrestrial communication. Consider the "Wow!" signal for example: detected in 1977, it was (and still is!) by far the strongest and clearest transmission ever detected by SETI. It was, however, never heard from again despite repeated efforts, and as a result we are still not sure what it truly was.
The Star Factor
The formula used to rank the different stars according to the likelihood that they would host a communicating civilization is:
score=
N*(bv-bv0)*exp(0.5*(bv-bv_sun)^2)/(par+
where
N is a normalizing factor, 1.65x10^7
bv is b-v color
bv0 is b-v color of the bluest star in the catalog (-0.41)
bv_sun is the b-v color of the sun (+0.65)
par is the parallax in milliarcseconds
The formula was developed by SETI@home scientist Eric Korpela.
Because of this experience, SETI@home scientists insist that signals must be persistent and reliable to be strong candidates for an extraterrestrial transmission. Only signals that have been detected more than once in the same location on separate SETI runs are to be considered. These signals, composed of two, and sometimes three separate observations, are referred to as "multiplets" by the SETI@home team.
But not only repeated signals of the same type are considered. In some cases a particular kind of signal, say a gaussian, was detected at a given location during one pass of the Arecibo dish, while a different kind of signal, say a triplet, was detected coming from the same direction during a later pass. SETI@home scientists combine the two (or more) signals into a single candidate, and refer to it as a "metacandidate."
Now that we have ranked all the different gaussians, spikes, triplet, multiplets, and metacandidates, each according to its own set of criteria, we are well on our way to selecting the "winning" signals that will be tested at Arecibo. A major problem nonetheless remains. A signal's "detection score" effectively compare gaussians to other gaussians and triplets to other triplets, and determines which ones are most likely to represent intelligent transmissions. But in order to come up with a list of the 100 best signals overall, it is also necessary to compare gaussians to triplets, and spikes to metacandidates, and decide which are the most promising. To resolve this, each signal is assigned not only a "detection score," which is specific to each type of signal, but also a "metascore," which can be compared with all the different types of signals. The 100 signals with the best metascores are the ones that Arecibo will aim for.
That Magical Frame of Reference
One characteristic of a radio signal will immediately make it stand out in the crowd, and send it to the top of the list of candidates for re-observation: if it remains at a fixed and steady frequency. Almost all celestial signals vary in frequency over time. That is because they originate on moving celestial bodies, whose velocity relative to the Earth changes constantly. This causes the signal's detection frequency on Earth to vary as well, in a phenomenon known as "Doppler drift."
In their analysis, SETI@home scientists compensate for the Doppler drift: they take into account the motions of the Earth around the Sun, including the effects of the Moon and the giant planets, arriving at a figure known as the "barycentric" frequency. That is the frequency at which the signal would be detected if the receiver was placed not on Earth, but at the center of gravity ("barycenter") of the Solar System.
Even so, because of the motion of the transmitting body, the barycentric frequency does drift and scientists cannot compensate for the motions of this unknown body. If, however, the barycentric frequency of a signal remains steady, this almost certainly means that it is designed to compensate for the movements of its own host planet. In other words, it would point to a deliberate intelligent design. This is a unique, and indeed "magical" state of affairs. SETI@home scientists like to refer to the frame of reference in which celestial signals are sent and received at a steady frequency as a "Magical Frame of Reference."
And so, the SETI@home crew, along with millions of users around the world, awaits the day at Arecibo when it will put its most promising results to the test. As is usual with SETI, the odds of finally detecting an alien transmission are long. Nevertheless, those 24 hours at Arecibo may represent one of the best opportunities yet to find that elusive signal.
Isn't it obvious?
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