Sky & Telescope - September 2021 - 12

COSMIC RELIEF by David Grinspoon
The Prime of Their Lives
The strange cycles of cicadas offer timely insight into the search for ET.
come from naturally occurring processes,
she says.
Which brings us back to the cicadas.
They favor prime numbers. Some
species come up every year, but Brood X
and the other periodical cicadas spend
long intervals buried underground as
nymphs sucking on tree roots. After 17
years for Brood X and several others,
and 13 years for some other broods,
they all suddenly crawl to the surface
for a few weeks of sex and song. Their
synchronous appearance is part of
a survival strategy - with so many
appearing all at once, enough will avoid
being eaten and mate successfully.
But why the primes? The most promBrood
X cicadas
can not only look
like aliens but can
inform our search
for their intelligent
constituents.
AS I WRITE THIS, hordes of bug-eyed
monsters are invading our nation's capital.
Vast numbers of Brood X cicadas are
swarming the trees and filling the air
with their pulsing songs. So naturally
I'm thinking about aliens.
As far as we can tell, life arose on
Earth due to conditions that ought to
exist on myriad planets, and it evolved
into complexity and cognition through
evolutionary processes that should also
be universal. Many of us feel we're not
likely to be alone in this universe.
Yet we would love to confirm this
hunch with solid evidence. How will we
recognize the hallmarks of intelligence
in signals or artifacts? It's not enough
to look for regular geometrical forms or
repeating patterns. We know nature can
make those in abundance - no minds
needed. What kind of pattern would be
an unmistakable sign of intelligence?
One popular idea is that an extraterrestrial
intelligence could well construct
12 SEPTEMBER 2021 * SKY & TELESCOPE
a message using prime numbers, which
are divisible only by themselves and by
1. There's no simple, formulaic way to
generate primes, and it's often said that
no natural process produces them. It
has become canonical in the SETI field
that investigators should seek prime
numbers when searching for artificial
signals. Astronomer Frank Drake constructed
the famous Arecibo message,
broadcast in 1974, in a sequence containing
1,679 bits, which any numerate
alien would see is the product of two
primes and thus begs to be displayed as
a 73-by-23-bit image.
In Carl Sagan's novel Contact,
astronomers deduce that a radio signal
coming from near the star Vega is a
message from an extraterrestrial intelligence
because it contains a repeating
sequence of primes. In the book,
astronomer Ellie Arroway explains that
" prime numbers are very specific, very
artificial. " Sequences of primes don't
ising hypothesis is that these intervals
help the cicadas evade predators whose
life cycles peak at smaller intervals.
Primes have no small number factors,
so no critters following two- or threeor
five-year patterns will consistently
overlap with these longer-term cicada
broods. By evolving prime number
cycles, these bugs avoid cultivating
predators that can consistently lie in
wait for them.
So much for the idea that primes
must always be the hallmarks of a
math-savvy civilization. They may be
unlikely to arise from physics or chemistry,
but they can evolve biologically.
My point isn't that if we think we
see a message with alien primes it
could just be a brood of ET insects that
have somehow commandeered a radio
telescope. It's simply that everything
we think we can surmise about technosignatures
is still a web of educated
guesses, and that " we'll know it when
we see it " - however vague and unsatisfying
- is still the best principle for
recognizing that we've got company.
¢ Contributing Editor DAVID GRINSPOON
is an astrobiologist at the Planetary
Science Institute (psi.edu).
CHIP SOMODEVILLA / GETTY IMAGES
http://www.psi.edu
Sky & Telescope - September 2021

Table of Contents for the Digital Edition of Sky & Telescope - September 2021
