What if SETI@home Gets Lucky?by Brian McConnell, author of Beyond Contact
From March 18 to March 20, SETI@home researchers will be conducting follow-up observations of the 200 most promising signals detected by the SETI@home project (out of more than 5 billion radio sources identified). Since its launch in 1999, the SETI@home system has enlisted millions of computers worldwide to analyze data collected by the Arecibo telescope in the search for transmissions from extraterrestrial civilizations.
University of California at Berkeley scientists Dan Werthimer, David Anderson, and Eric Korpela led the effort to narrow more than 5 billion hits to a list of the 200 most promising candidate signals.
Phase II of the SETI@home project will focus on those candidates deemed most likely to be bona fide extraterrestrial transmissions. With billions of candidates to choose from, the team narrowed its search to focus on signals that met the following criteria:
- Signals observed in the same location in the sky on two or more telescope passes
- Signals observed on similar frequencies during different passes (some frequency drift is allowed to compensate for Doppler shift)
- Strong signals that generated a power-versus-time curve closely matching that expected for the Arecibo telescope (this is known as a Gaussian curve)
- Signals that were not associated with known sources of terrestrial interference or satellite transmissions
- Sources adjacent to a known star or galaxy (sources in empty space were discarded)
- Sources adjacent to a main sequence star similar to our sun (other types of stars are thought to be unlikely to support habitable planets)
- Sources closer to our solar system
The Planetary Society has published a more detailed description of the process used to identify the most promising candidates.
With three days of telescope time, the SETI@home team hopes to re-observe at least 100-150 of the best candidates in detail. The team leaders estimate that the odds of confirming a signal are low, perhaps 1 in 10,000 according to project leader Dan Werthimer; or less than 1 percent according to SETI@home developer David Anderson. The team expects the vast majority of the candidate signals to be the result of man-made interference.
A long shot, to be sure, but this is the first time SETI researchers have conducted a targeted search to evaluate known sources with a real chance of success. Phase I of the search was not targeted. The SETI@home project simply observed whatever the Arecibo telescope was looking at as the Earth rotated. This allowed the project to observe a large percentage of the northern sky several times over four years. Interesting signals were scored and cataloged for future analysis and re-observation.
Phase II is the first targeted SETI search following up on known signals that met the criteria above. Even if this search fails, it marks an important step forward in the search for evidence of extraterrestrial civilizations. SETI researchers are no longer blindly looking at randomly chosen targets in the hope of picking up a signal, but are now methodically working through many candidate signals that may not pass the final tests required to confirm an ET signal. If the search does fail, it is not a definitive failure, as 25 million candidates were discarded for each one that will be re-observed in this short run. Only when facilities dedicated to SETI come online, such as the Allen Telescope Array, will researchers be able to examine a significant number of these candidate signals.
Confirming an ET Signal
What will happen if the SETI@home team discovers a promising candidate this week? The team will be recording data for detailed offline analysis in the weeks following the re-observations. SERENDIP IV, another Berkeley-based SETI program, will provide real-time results as the team conducts its observations. The SERENDIP system will guide the team in its observations, and will provide an early indication if the team is on to something interesting. A signal may not be visible to SERENDIP as the SETI@home system can conduct a more sensitive search for certain types of signals. If a signal is picked up by SERENDIP, the confirmation may come quickly. If not, we will have to wait several weeks for the offline analysis to be completed.
If they detect a signal that matches prior observations, the next step will be to train other SETI-capable telescopes on the target. This step is necessary to provide third-party confirmation of the Arecibo observations, though the SETI@home/SERENDIP re-observation will be strong evidence since the candidate signal would have been observed at several different times. Because the media is watching this experiment closely, it is unlikely this step would remain a secret for very long. If the signal passes this test, we will know fairly soon that a radio signal from another solar system has been detected.
Although unofficial word will spread quickly, official confirmation of a signal will take time. There will be strenuous debate about the veracity of the claim, and about other explanations for the signal's origin. It is possible that instead of an ET origin, they will have discovered a rare and previously unknown astronomical phenomenon. It is interesting to recall that when the first pulsar was detected, scientists initially speculated that it was an extraterrestrial beacon because of the precise timing of the signal pulses. They later determined that the source was a neutron star whose rotation was slowing ever so gradually (this deceleration was the giveaway that it was a natural radiation source instead of an extraterrestrial beacon).
How quickly this debate is settled will depend on the type of signal intercepted. If the signal is obviously artificial, we will know fairly quickly. Such a signal would do something that clearly indicates an intelligent origin (for example, by cycling through a series of special numbers such as primes, or numbers with integer square roots). If, on the other hand, the signal appears to contain no such information, we will know nothing about its purpose or origin, except that it appears to be generated by technology instead of a natural process. In this scenario, it will take longer to rule out explanations besides extraterrestrial communication.
What Happens If SETI Confirms an Alien Signal?
Merely confirming the existence of such a signal would have profound implications, some good, some quite ominous.
Most people assume that if SETI succeeds in detecting an ET signal that this will be a benign event. We'll finally know that we're not alone in the universe, science books will be rewritten, and so on. This view naively ignores human nature, specifically the ability of charlatans to con gullible people into doing stupid things. Rubes have never been in short supply and are currently more plentiful than ever. Successful detection could be a very good thing, if the contents of the signal are comprehensible. But what happens if we discover a signal, only to find that its contents are completely unrecognizable? All we would know is that there is an alien signal at 1.42 GHz, but what it says would be a mystery.
Fairly quickly an industry will form around the business of claiming to interpret the contents of this signal. Many people will see an alien civilization as a god-like entity, and will be easily conned by people who offer convincing explanations of what the signal says. Even today, in the absence of real evidence of alien contact, millions of people believe that aliens have contacted us, or are even living among us today.
What will happen if a real ET signal is added to this mix? Will the cults that form around it be benign groups of people with a shared interest in New Age mysticism, polygamy, and bad fashion? Or will some of them assume a more malevolent form? If history is any guide, it is a bad idea to bet on good intentions. (In my view, the worst-case scenario is that SETI succeeds in detecting a signal, and then fails in deciphering its contents.)
Detecting an ET signal will merely be the first step in an ongoing process. Once a signal is verified, the focus will shift to determining whether the signal is conveying data, and if so, what that data represents. The best way to insure that SETI's success is not hijacked by frauds is to determine the basic structure and contents of the signal as rapidly as possible.
This may not be possible depending on the type of transmission we intercept. Success will depend on whether the signal is an intentional attempt to communicate with other civilizations. If it is an intentional signal, it is possible that it will be coded so that it can be deciphered relatively easily (at least the basic parts of the message). If it is a randomly intercepted transmission, its purpose will probably remain a mystery. If "ET" were to intercept a digital cellular phone call, with no knowledge about our cellular networks or what a human voice sounds like, would they have any idea what they were looking at? Probably not.
If SETI succeeds, we should hope that they find an intentional transmission that is designed to be decoded relatively easily. Most people assume that meaningful communication between civilizations would be impossible, and that, even if it were, the long delays imposed by the speed of light would make interstellar communication impractical and pointless. This is not true. In principle, it is possible to construct messages based upon universal physical and mathematical concepts, and that can interact with their recipients, without requiring two-way communication between sites.
Between Worlds, an upcoming book from the SETI Institute and MIT Press, discusses the challenge of interstellar message composition at length. While this book focuses on how we might compose a message to send to other civilizations, it is also informative about what we should look for in any message we receive.
Demodulating an ET Signal
The first step following detection will be to examine the signal closely to determine if it is modulated (changes over time in a structured manner). For example, if the signal hops between two nearby frequencies, this would enable it to transmit a sequence of binary numbers (frequency A = 0, frequency B = 1). There are many ways to modulate a radio carrier to transmit analog and digital information, each with relative merits and disadvantages.
One way to embed data within a signal is to modify its strength over time (high power = 1, low power = 0). This is known as amplitude modulation (AM). It can be used to transmit both digital and analog information. AM is simple to read, but is not ideal for interstellar communication, where the carrier may be weak and vary in strength due to atmospheric effects and interference. An AM-encoded carrier will appear to vary in strength or cycle on and off, probably at regularly timed intervals.
Frequency modulation (FM) works by modifying a carrier's frequency while maintaining constant power output. FM can also be used to transmit both digital and analog information. When used to transmit digital information, this is known as frequency shift keying. In principle, FM is a simple system, but it too poses problems for interstellar communication because shifting frequencies makes the carrier itself harder to detect.
To compensate for weak signals and background noise, SETI searches filter radio signals into very narrow channels, usually 1 Hz or less per channel. A precisely tuned carrier will appear to be much stronger than background noise in such a narrow channel, even if the signal itself is weak. However, if the carrier jumps across many frequencies, its energy is no longer so concentrated by frequency, and it is more likely to be confused with background noise.
Phase modulation works by modifying the carrier's phase. A precisely tuned carrier is described mathematically by a sine wave. In phase modulation (or phase shift keying), the carrier jumps between a discrete number of states (in phase, or 180 degrees out of phase). The frequency and power output remain constant, only the phase of the sine wave varies over time.
This method is thought to be one of the more promising ways to embed information in an interstellar signal because it does not make the carrier itself harder to detect, even if information is transmitted at a rapid pace.
Polarity modulation is another method that may be used to transmit information. Radio transmissions, although they are invisible, are light-wave signals comprised of photons. Photons vibrate in a specific plane as they travel. A polarized receiver or filter will allow photons that vibrate in one plane to pass through while photons that vibrate in a perpendicular plane are blocked.
To see this for yourself, look at a flat-panel LCD monitor with polarized glasses. As you tilt your head, you'll notice that the light from the display is almost completely blocked at some angles. The polarity of a signal can also be used to represent a state or digit, and thus to convey information.
How much information could an ET signal transmit? A phase or polarity modulated carrier could transit information at a high rate without making the carrier itself much harder to detect in the first place. However, because of the techniques used to reject background noise in the detection process, this information will probably be invisible during the initial analysis. Information transmitted very slowly, at a few bits per second or less, may be visible. Information transmitted at a faster pace will be blurred out by the algorithms used to reject background noise.
Hopefully the sender of such a signal will structure it so that some information is transmitted at a very slow pace using an obvious modulation scheme, and thus is visible immediately. This low-speed channel would serve primarily to notify any recipient that the signal contains additional information. It is important to note that all of this assumes the signal is designed to be easy to decode. If it is not, we will probably not detect it in the first place, or if we do, we would have no idea how to extract data from it.
This work will be conducted by astronomers in collaboration with telecommunications experts, and will be done in several phases. In the first phase, scientists will use existing equipment to look for obvious signs of low bit-rate AM and FM modulation. This work would not take much time to complete and may lead to early results. In the second phase, the detection equipment would be modified to look for other modulation schemes (phase modulation, polarity modulation, etc.). In a third, longer-term phase, new telescopes may be constructed to provide enough gain to amplify the signal to detect high-speed modulation or lower power side channels that may be used to transmit larger amounts of data.
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