Australian Sky & Telescope — November-December 2017

14 AUSTRALIAN SKY & TELESCOPE November | December 2017

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NRAO / AUI

MORETHAN100SEARCHESfor extraterrestrial intelligence

have been carried out since 1960, when Frank Drake first

pointedan26-metredishconnectedtoatabletopshortwave

radioreceiverattwonearbystars.ManyoftheseSETI

effortsviewedonlyahandfulofstarsforafewhours.Afew

ambitioussurveyscoveredmuchofthesky,buttheylistened

ateachspotforonlyafewminutes.

SomeofthemostfamousSETIexperimentshave

concentratedonarelativelyfewnearbystars—‘few’

meaninginthethousandsatmost—scrutinisingthem

intentlyonebyone.ThisapproachassumesthattheMilky

Wayisbuzzingwiththousandsormillionsofradio-signaling

civilisations,allofwhosetransmittersarerelativelyweak.

Butthere’severyreasontothinkthatanopposite

assumptionisthesmarterbet:thatthestrongestsignalsin

oursky,theoneswecanmosteasilydetect(assumingany

exist at all), come from rare civilisations extremely far away

who have harnessed energy on an extreme scale.

Withthisinmind,IusedtheVeryLargeArray(VLA)in

NewMexico,oneoftheworld’spremierradiotelescopes,to

examinethenearbygalaxiesM31inAndromedaandM33in

Triangulumforartificial,narrowbandsignals.Betweenthem,

theycontainroughlyonetrillionstars.

These stars probably host trillions of planets, including

billions of ‘habitable Earths’. Admittedly, they’re 2½ million

light-yearsdistant.Butifevenoneofthosestarshostsalong-

How I helped shrink the possibility that really

advanced aliens are broadcasting far and wide.

STHE LISTENERS The Karl G. Jansky Very Large Array in Socorro, New Mexico, comprises 27 radio dishes each 25 metres in diameter. Their data

are combined to create radio images with the sensitivity of a single 130-metre dish, but with much higher spatial resolution.

Searching a

Trillion Stars

SETI IN BULK by Robert H. Gray