Australian Sky & Telescope — November-December 2017

http://www.skyandtelescope.com.au 15

ADAM EVANS / WIKIMEDIA COMMONS / CC-BY-SA-3.

lived civilisation that wishes to make itself known to the

cosmos, it could plausibly build a transmitter that’s bathing

our entire Milky Way with a signal strong enough for the

VLA to detect.

Choosing a SETI strategy

Even using today’s most sensitive radio telescopes, the only

type of alien signal we have much hope of hearing is a

deliberate, powerful ‘beacon signal’ — one that somebody or

something designs to be detected by faraway listeners like

us. Any plausible ‘leakage’ from a civilisation’s own internal

communications will be much too weak and messy for us to

hear across many light-years.

Another key point: Any beacon that we pick up is almost

certain to have been broadcasting for millions of years. That’s

because shorter-lived ones would have only a minuscule

chance of overlapping our own moment in cosmic time. So

any ETs currently announcing themselves to the cosmos have

almost certainly had all the time in the world (literally!) to

build up their hailing transmitter to tremendous power.

If this logic is correct, then for listeners like us, playing

the numbers game makes sense.

Following this logic, Frank Drake and Carl Sagan searched

four galaxies in the early 1970s using the 300-metre Arecibo

dish in Puerto Rico. They examined frequency channels

1,000 hertz (Hz) wide, narrow enough to screen out natural

radio emissions, though not as narrow as alien radio

engineers might use for the best stand-out visibility. Drake

and Sagan looked at 212 spots covering M33 for about 1

minute each. They could not see M31; Arecibo’s big antenna

couldn’t point far enough away from Puerto Rico’s zenith.

I could point toward both galaxies easily with the VLA’s

27 fully steerable dishes. On the visible-light image of M

above, the five circles show the array’s ½°-wide field of view

for the five sky areas I observed.

Another daunting issue facing SETI efforts is the vast

range of radio frequencies that hypothetical aliens could

choose for their beacon: any of about 10 billion channels,

if their beacon broadcast is a highly efficient signal 1 Hz

wide. Where on this enormous dial should we tune? I chose

a narrow range around the ubiquitous hydrogen emission at

21 cm wavelength (a frequency of 1.420 gigahertz), as many

SETI projects have done. The hope is that radio astronomers

anywhere will think that radio astronomers anywhere else

will pay close attention to it.

From dream to reality

I proposed this project to the VLA — it is open to anybody

from anywhere in the world to use — and somewhat to

my surprise, the Time Allocation Committee granted me

12 hours.

It’s complicated to tell the 27-dish array what you want

it to do, but the help staff enabled my observations to go

successfully. Everything in SETI is a tradeoff. I had the

N

SANYBODY HOME? Overlaid on M31, the Andromeda Galaxy, are the five radio fields of view that the author studied with the Very Large Array

in his search for artificial radio signals. Billions of habitable planets should be within the circles. If any of them had radio engineers continuously

directing at least a 10^17 -watt signal to blanket the Milky Way at just the right frequency, he would have found it.