Homing Signals: Synchronized SETI
Sending signals across the galaxy is always a question of where and when.
If it is desirable to be seen or more likely heard, the signal can be sent out as a beam or a sphere (omnidirectional). Timing such a signal similarly relies on either a burst (the 'beep') pattern or a continuous broadcast. Like any radio station, the final decision comes down largely to a question of available power and costs. A highly-advanced broadcaster might find it easy to do the expensive job of a spherical, continuous, high-power option.
If the transmitter wants to be heard anywhere, how such a civilization chooses their signature broadcast hinges on just such a tough balance between direction, power and timing.
Crafting a better radio conversation is part of the research recently accepted for publication in the Astrobiology journal from Dr. Robin Corbet of the Universities' Space Research Association.
Knowing When to Pick Up
Historically, the few deliberate transmissions from Earth have been beamed in only one direction at a time and sent as extremely short beeps. Indeed a message sent from the large Arecibo Radio Telescope (Puerto Rico) in 1974 in the direction of the globular cluster M13 lasted only 169 s.
But if powerful enough, the chances improve to stand out over the low-background of natural noises, pops and chirps that typically get picked up from large radio telescopes like Arecibo. [For illustration of power requirements, if the Arecibo 1 MW planetary radar is operated continuously then this corresponds to the same energy costs as accelerating 1 kg masses to 1% of the speed of light approximately every 50 days.]
So for exploration in the best case, both the transmitter and receiver are tuned and synchronized to send and receive together, once they allow for time delays and know where to point in the sky.
Along with Corbet's previous (1999) paper entitled "The Use of Gamma-ray Bursts as Direction and Time Markers in SETI Strategies", his forthcoming work has revisited the strategy proposed by a number of authors: How exactly to synchronize by using natural events as markers?
Duty Cycles and a Primer
Dr. Corbet notes: "Essentially all of the current SETI (Search for ExtraTerrestrial Intelligence) observing programs make only brief observations of any particular part of the sky. This means that transmitters with low duty cycles are very unlikely to be detected. "
In his paper "Synchronized SETI - The Case for Opposition", he writes: "In the simplest scheme omnidirectional signals would be transmitted at the occurrence of some particular event such as a nova outburst, maximum flux of a long period variable, specific binary phase or supernova occurrence. A signal would then be detected at the Earth delayed by a time corresponding to the difference between the event/Earth distance and the event/transmitter multiplied by the transmitter/Earth distances. " In other words, a smart civilization might triangulate with a big flash, in just such a way that we might know that it is an interesting time to tune in.
So knowing this inherent efficiency boost, one might increase success by listening in for just such 'low-duty cycle', transmitting civilizations. He noted: "Without synchronization it will be very unlikely that the potential recipient will detect the signal".
Conserving and Tapping Natural Powers
One assumption of the synchronization strategy is that a civilization will need to conserve its power. It thus would have already seen the wisdom in mainly sending beeps, or short beamed pulses. But the lack of a continuous broadcast also has the drawback that a primer signal or preface must be commonly 'readable' for the transmission to work at all. According to Corbet, "techniques that would both save energy costs and use less time with transmission facilities that could be used for other purposes would become more important."
|The changing intensity of a gamma-ray burst. On the left is an image of the gamma ray sky showing the burst becoming the brightest object. On the right is a plot of the changing brightness with time. The first gamma-ray burst was seen in the year 1967 (although it was not reported to the world until 1973) by satellite-borne detectors intended to look for violations of the Nuclear Test Ban Treaty. Credit: BATSE
In addition to big natural events like supernovae, a relatively recent astronomical candidate of much interest is gamma-ray bursts (GRBs). On average, about once per day, somewhere in the celestial sky, a bright flash of intense, high-energy gamma particles reaches Earth. The source then disappears except for a much fainter afterglow. While no one can yet predict where or when such a burst will occur, they are frequent and among the brightest sources in the sky in any wavelength region. Corbet proposed that gamma-ray bursts are "the best of the known potential synchronizers that are available primarily because of their large apparent luminosities and very brief durations."
Just in Time Delivery
Since the GRBs are short-term (ranging from about 3/100ths of a second to over 1000 s.), says Corbet, "The transmitter ideally needs to be able to respond on the time scale of the burst. I don't think that need be too difficult. However, this involves trying to guess what technology is available to the transmitter which seems impossible except that it's almost certainly much more advanced than ours! Being able to predict when a GRB was going to occur in advance might help but I don't think it's required at all."
While "the origin of GRBs is completely separate, and irrelevant to its use for synchronization", one striking feature that since their 1967 discovery, has surprised gamma-ray astronomers was that the signals don't seem to come from a known stage of a star's life cycle (unlike old stars that supernova). Instead, they come from all (random) directions, and not necessarily just from where the stars are (or civilizations might be) most concentrated.
Dr. John Horack, who led the assembly, testing and calibration program for scientific space flight hardware on NASA's Compton Gamma-Ray Observatory, noted that "you'd expect the 'distribution of civilizations in galactic coordinates' to have a very high quadrupole moment..." or to be spatially flattened "(like the disk of the galaxy, because that's where the stars are). So any observation 'in the plane' is much more likely to yield a civilization than anything out of the plane. There are simply more stars per steradian on the sky."
So Where are They?
"Searches for emission at around 1.4 GHz have covered essentially the entire sky and no persistent source has been found (see review by Tarter, 2001). While this may simply mean that extraterrestrial transmissions were too faint to be detected in these surveys, or that transmissions are not being made near this frequency, an alternative explanation may be that transmissions exist but only have low duty cycles," notes Corbet.
Keeping the Sun to Your Back
As early as 1924 an attempt was organized by David Todd, an associate of Percival Lowell, to listen for artificial radio signals from Mars during the time of opposition (when the Earth-Sun-Mars system is aligned and the receiver and transmitter are on the same side of the Sun). This, apart from the much smaller distance involved which makes signal travel time much less important, is very close to the proposal to look for transmissions from around other stars at opposition.
So one scheme discussed by Dr. Corbet's paper is to look for Gamma-Ray Bursts as the universal timing marker, taking account of transmission delays, and look for very smart civilizations that know enough about us to synchronize to the Earth's orbit around the Sun (at opposition).
As Horack notes: "Looking in the direction of the burst, you're going to have to pull their signal out of some subsequent radio emission which dominates after about two weeks, once the burst region gets optically thin to the radio. Looking in the opposite direction, you have no time problems, since you'll have several thousand years, minimum, to wait for the burst to pass the Earth, hit the other civilization, and for them to beam back in the direction of the burst."
But "with two or three per day, you'd have one heck of a job if you were the transmitting civilization: Let's say you want to broadcast for two years after a burst? -- try 1,000 pointable transmitters, all spitting signals out into space in isotropically distributed directions".
So while a synchronized SETI program makes the receiver's chances higher, the transmitter civilization carries the burden of making the right connection and any ensuing conversational success.
The remarkable progress made already in extrasolar planet discovery (100+ to date) helps refine the feasibility of a synchronized SETI plan. NASA missions propose to detect Earth-like planets in the next decade using the SIM (Space Interferometer Mission) and Terrestrial Planet Finder mission. The European Space Agency (ESA) similarly is planning two missions called GAIA and Darwin.
Concludes Corbet: "It is anticipated that within about the next 10 to 20 years it will be possible to directly detect nearby extra-solar planets of approximately terrestrial mass. As any extraterrestrial transmitters are expected to have significantly more advanced technology it is therefore not unreasonable to expect that these transmitters would be able to detect the presence of the Earth and measure its orbit at even greater distances."
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