Cheap Communication Schemes for ET
By Seth Shostak
Senior Astronomer, Project Phoenix
posted: 07:00 am ET
01 April 2004
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 telescope's Cyclopean,
twenty-acre aluminum mirror.
I'll state the obvious: that's 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. That's more than produced by all of America's
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, there's no doubt that any of these transmitting schemes would
be big-time undertakings, involving large structures, hefty
electronics, and gobs of power.
But let's 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.
That's such a daunting power drain that it's 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 that's
the size of Jupiter's 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, there's 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 Sun's 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 Sun's 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.