16 AUSTRALIAN SKY & TELESCOPE November | December 2017
NLITTLE BROTHERM33, the Pinwheel Galaxy in Triangulum,
is smaller and much dimmer than M31, with only about 1% as
many stars. But it’s still the third largest galaxy in our Local
Group, after M31 and the Milky Way. Three VLA fields of view
were enough to cover it.ALEXANDER MELEG / WIKIMEDIA COMMONS / CC-BY-SA-3.◗For the author’s VLA project to have hit the jackpot, a
beacon transmitter in galaxies M31 or M33 would have
to be continuously emitting at least 10^17 watts of signal if
it were illuminating the entire Milky Way. Is such a thing
even conceivable?
Well, we could be building one ourselves in not too
many centuries.
If it were built in space, there’s really no limit to the
power a beacon transmitter could have. We could avoid
inconveniencing the Earth by, for instance, assembling a
grid of small modules in a close orbit around the Sun that
would collect solar energy on one side and emit radio
from the other, through small, phased-array antennas.
Such an assembly could grow indefinitely large, maybe
by robotic manufacturing of modules using asteroids as
raw material. If it grew to thousands of kilometres across
— and remember, a long-lived civilisation has all the time
in the world — our own radio equipment today could hear
it from as far away as other galaxies.To build a really loud hailer
BY ALAN MACROBERTarray dwell on each pointing direction for 5 to 20 minutes,
building up the signal-to-noise ratio like a long photographic
exposure. I searched two spectral windows near the hydrogen
frequency, blueshifted slightly to account for the motions of
M31 and M33 toward us. These windows spanned 1 MHz
and 125 kHz, admittedly very small slivers of the microwave
dial. The hardware divided these frequency windows into
8,192 channels 122 Hz and 15.3 Hz wide, respectively, much
narrower than Drake and Sagan’s 1,000 Hz but not as narrow
as the roughly 1 Hz channels often used in SETI.
To check that this experiment really could detect a weak
signal from far away, I pointed the VLA at the most distant
known transmitter — the Voyager 1 spacecraft. Launched
in 1977, it was 130 astronomical units away and its signal
power was down to about 10 watts, not much more than a
flashlight’s. But it came in extremely loud and clear from far
out past Pluto.
I spent the next year analysing the resulting several
hundred gigabytes of data using the software package AIPS.
Pat Palmer at the University of Chicago advised me on many
baffling problems, and Kunal Mooley, a recent Caltech
astrophysics PhD, joined the analysis as my co-author using
the software package CASA.
After analysing well over 100,000 radio images, one for
each of 8,192 channels per bandwidth per field, we found
nothing much stronger than noise.
Too bad — it seemed like such a good idea!The meaning of a negative result
The value of a scientific experiment can often be judged by
how meaningful a negative result will be. Mooley and I think
we’ve shown that there are no technological supercivilisations
broadcasting very powerfully toward the Milky Way
continuously in the 21-cm band from any of a trillion stars.
What’s the problem? Maybe there is nobody like that
in M31 or M33. Or maybe they’re not transmitting radio
beacons toward the Milky Way, or not in the narrow
frequency window we chose, or not strongly enough, or their
transmitter wasn’t turned on when we looked, or... the list
goes on.
We can, however, say one thing: There were no easy
pickings. Disappointing, but worth knowing.
A more thorough followup should be coming along
behind us. Outclassing all previous SETI efforts, if it goes as
planned, will be Yuri Milner’s $100 million Breakthrough
Listen project, now in its beginning stages (see https://is.gd/
leapseti). Breakthrough Listen will, among other things,
examine 100 galaxies (including M31 and M33) using many
millions of channels across much of the entire microwave
spectrum — a search vastly broader and deeper. So the
case is far from closed on the possibility that somebody or
something is signaling our galaxy from another.ROBERT GRAY is author of The Elusive Wow: Searching for
Extraterrestrial Intelligence.SETI IN BULK