66 Science & technology The EconomistFebruary 29th 2020
2 versity of Maryland, worry that drone de-
liveries are open to a different sort of pira-
cy—hijacking. A drone in flight is easily
upset. A well-aimed stone, baseball or sim-
ilar missile is enough to bring it down, per-
mitting its payload to be purloined. Nor
need such stone-throwers have pecuniary
motives. Vandalism, or irritation with the
very presence of drones, might also pro-
voke pot shots. High-flying drones, like
those employed by the police for surveil-
lance, will normally be out of range of such
activity. But parcel drones will have to fly
low, at least for part of their journeys. To
counter this risk the pair therefore propose
to build a lightweight, low-power self-de-
fence mechanism which lets a drone sense
a missile fast enough to get out of its way.
That is nowhere near as easy as it might
sound. Drones are lean machines, provid-
ed with only enough battery strength, com-
puting power and payload-carrying capaci-
ty to do the job they are designed for.
Adding threat-detection measures, such as
heavy, power-hungry radar antennae, or
spinning lidar units, radar’s optical equiva-
lent, would either reduce range or prevent
lift-off in the first place. Dr Roy and Mr Garg
think, however, that they have hit on a low-
power, lightweight self-defence system
suitable for small drones. Instead of em-
ploying light or radio waves to detect in-
coming threats, it harnesses sound waves
and the Doppler effect.
The Doppler effect is the frequency shift
heard as a source of sound approaches or
recedes. It is, for example, the reason the
pitch of a police siren changes as a patrol
car passes in the street. To take advantage
of it Dr Roy and Mr Garg plan to fit drones
with diminutive loudspeakers, like those
found in smartphones. These would
broadcast an ultrasonic tone outward from
the drone. Similarly tiny microphones
would then listen for reflections from in-
coming objects. The Doppler shift of these
reflections, run through a bit of on-board
processing (but far less than that needed
for radar or lidar) would give the bearing of
the threat, and thus permit the drone to
take evasive action.
To test the principle of what they call
their DopplerDodge drone defence system,
Dr Roy and Mr Garg have constructed a stat-
ic version in their laboratory, and have
been throwing objects of various sizes and
shapes at it, as if it were a hovering drone.
At the moment, it can detect these objects
from distances of up to four metres away.
That is pretty close, but would still give a
drone a tenth of a second’s notice of an in-
coming missile. This would be sufficient
for it to move itself out of the way. Tests on
actual drones will take place shortly, and
then, if all goes as planned, the two re-
searchers will attempt to extend the sys-
tem’s range to 30 metres—a reasonable ap-
proximation of a stone’s throw. 7O
ral mythologyhastremendous
staying power. The Klamath, a group
of Native Americans who live in Oregon,
tell tales of an underworld god called
Llao who fell in love with a mortal wom-
an and grew furious when she refused
his advances. He emerged from a moun-
tain to cascade fire down onto her village,
but was then attacked by Skell, a sky god
who wished to protect human beings.
Skell forced Llao back into the earth, and
the mountain he had emerged from
collapsed on top of him during his re-
treat. Terrible rains followed, and the
hole left behind became a great lake.
An everyday story of deities the world
over, then. Except that the mountain in
question is Mount Mazama, a dormant
volcano, and the body of water is now
known as Crater Lake. Geological evi-
dence shows that the eruption which
created the lake happened 7,700 years
ago. The story of Skell, Llao and the earth-
ly maiden thus seems to be an interpreta-
tion of real events that has been passed
on intact for almost eight millennia.
Now, however, that record looks set to
be broken. For, in the Australian state of
Victoria, another group of indigenous
people, the Gunditjmara, also tell tales
about a local lake-filled volcano. And
evidence just published in Geologyby
Erin Matchan, a geochronologist at
Melbourne University, suggests theirtalesarefarolder.
The volcano in question is Budj Bim,
which is also the name of a central char-
acter in these myths. The tales speak of
the land and trees dancing as ancestral
beings came to life from deep within the
ground. One of these beings, Budj Bim,
spat liquid fire from between his teeth
when he revealed himself.
As is the case with Mount Mazama,
Budj Bim the volcano has erupted in a
past that is geologically recent but an-
cient in terms of human history. Exactly
how ancient has not, however, been
known. Dr Matchan therefore set out to
date this event precisely.
To do so she applied a method called
argon dating to samples of Budj Bim’s
rocks. Argon dating relies on the gradual
decay of a radioactive isotope of potassi-
um into non-radioactive argon. Because
argon is a gas, which escapes easily from
molten rock, the argon “clock” is reset
whenever molten rock solidifies. It is
therefore a reliable indicator of when
lava was erupted.
Dr Matchan’s calculations showed
that Budj Bim’s last eruption—which
presumably marks the origin of legends
about the eponymous being—was 37,000
years ago. That makes Budj Bim the being
almost five times older than Skell and
Llao, and thus the oldest known protago-
nist in human story telling.Fossilised folk tales
GeomythologyAn Australian legend may be 37 millennia oldThe once-fire-spitting mouth of Budj Bim