Cheap Communication Schemes for ET

Apr. 01, 2004
by Seth Shostak - Senior Astronomer


When it comes to signaling across space, power is paramount.

Project Phoenix, which just wrapped up nine years of observations, was an acutely sensitive search for radio broadcasts. The experiment could have discerned an alien signal that was wafting a mere 0.00000000000000000001 watts onto the Arecibo telescopes Cyclopean, twenty-acre aluminum mirror.

Ill state the obvious: thats a small number. Indeed, if that incoming energy were collected for the length of time from the Big Bang until today (that is to say, for all time), the total would only be enough to blink a flashlight for a thousandth of a second.

Still, if you work out what sort of effort is required to produce that pipsqueak signal, the numbers get large. At 100 light-years distance, and assuming that the aliens are broadcasting in all directions equally, our cosmic buddies would need to pump their transmitter with 100 billion watts in order to drop that miniscule amount of power onto the Arecibo dish. Thats more than produced by all of Americas electric utility plants.

Of course, the extraterrestrials could be clever about their attempts to signal. For example, a rotating antenna might target only the flat part of the Milky Way Galaxy, sweeping it like a lighthouse beacon and doing so with considerably less juice than would be required by the all-sky, all-the-time approach. Another scheme would be for the aliens to use antenna arrays that sequentially "pinged" only interesting star systems. Again, this would save on the electric bills.

Still, theres no doubt that any of these transmitting schemes would be big-time undertakings, involving large structures, hefty electronics, and gobs of power.

But lets consider what would be required to get in touch using a laser, one that communicated with very short pulses of infrared light. Again, if the aliens are 100 light-years away, and are using a "bare bulb" approach that flashes light in all directions, we could discern these pulses if the transmitters could manage an average power of 100 billion billion watts, assuming that they are content to send only a modest 100 bits per second of information and that we are observing with a relatively small, college-grade telescope.

Thats such a daunting power drain that its safe to say even advanced societies for whom power bills are no object would eschew the cost, and use a mirror to focus their light sources in preferred directions. For instance, they might limit their broadcast beam to a circle thats the size of Jupiters orbit, on the reasonable assumption that planets able to produce intelligent beings would lie within that distance from a Sun-like star. The mirror required to do this (again, with infrared light) would be roughly 20 feet across, which is hardly a challenge. Using the mirror reduces the power requirement to approximately 10 million watts.

However, theres the bugaboo that we might not be looking when the aliens are flashing. SETI researchers have given thought to this "synchronicity" problem, and come up with ingenious (if uncertain) schemes that might ensure that we are receiving when the extraterrestrials are sending. One approach, variously proposed by Tim Castellano (NASA), Ray Villard (Space Telescope Science Institute), and yours truly, is to assume that some ambitious alien astronomers using the transit technique have detected Earth. In other words, they have found our world because it blocks a small amount of light as it passes in front of the Sun as seen by them. This once-a-year eclipse is an event that both we, and any extraterrestrials who have observed it, will know. They might choose to send their laser flashes in our direction during these transits, on the assumption that we were clever enough to figure out the synchronization scheme.

Of course, this requires that the aliens have precise knowledge of the distance to our solar system, and the Suns motion through space. For an advanced society, that might not be too much to ask. But the kicker is this: they will know that, during the transit, our world is somewhere in front of the Suns disk. So they could use a mirror array to focus their signal on that disk, thus reducing the power requirements for signaling to less than 10 watts, comparable to a bicycle headlamp! Yes, the mirror would now be a mile across, but it could be made up of a few small, cheap, and simple individual reflectors.

In other words, with a collection of mirrors, a small laser, and a computer to run it all, a knowledgeable and entrepreneurial extraterrestrial could produce detectable signals with only as much power as a handful of batteries could supply. No mammoth antennas, and no beefy transmitters are required. The broadcast could be an alien science fair experiment.

It's interesting to imagine that attempts by extraterrestrials to locate other intelligence in the Galaxy might be made not by officialdom in massive societal programs, but by the personal efforts of the young and the daring.