The Economist - USA (2020-11-07)

72 Science & technology The EconomistNovember 7th 2020

T

wo centuriesago Hans Christian Oer-

sted, a Danish physicist, demonstrated

that the motion of an electric charge pro-

duces magnetism. This was the first obser-

vation of a wide-ranging phenomenon.

The charged clouds of particles which float

through the cosmos generate vast inter-

stellar magnetic fields as they go. The

sloshing of molten metal in Earth’s core

produces the planet’s north and south

magnetic poles. Even the firing of nerve

cells in a human brain creates a minuscule

amount of magnetism.

The ubiquity of such electrically gener-

ated magnetic fields does, though, bring

problems ranging from the pragmatic to

the esoteric. Doctors looking at mriscans,

for example, have to compensate for back-

ground magnetism. Meanwhile, experi-

mentalists conducting precision tests may

have to build complex shields to obscure

the magnetic effect of something as simple

as an electric wire running through the

wall of their laboratory.

It would be useful, then, to be able to

control, limit or shape magnetic fields

from a distance. Useful, but apparently im-

possible. For, in 1842, Samuel Earnshaw, a

British physicist, demonstrated mathe-

matically that the maximum strength of a

magnetic field cannot lie outside its

source. Every such field must, in other

words, surround and radiate from the ob-

ject which generates it. And there matters

stood until Rosa Mach-Batlle of the Auton-

omous University of Barcelona spotted a

way around Earnshaw’s conclusions. She

has not actually proved him wrong. But she

has shown that multiple magnetic fields,

each obeying Earnshaw’s theorem individ-

ually, can collectively appear to bypass it.

As they describe in Physical Review Let-

ters, Dr Mach-Batlle and her colleagues

pulled off their trick in a surprisingly sim-

ple way, by arranging 20 straight wires next

to one another in the form of a cylinder

40cm tall and 8cm in diameter, with a 21st

running through the cylinder’s centre.

When they passed electric currents

through all 21 wires a complex pattern of

magnetic field lines blossomed in the sur-

rounding area, forming shapes which va-

ried with the strength and direction of the

individual currents.

By choosing the right combination of

currents the researchers found they were

able to create a field pattern which emanat-

ed from a virtual version of the 21st wire

that ran not through the middle of the cyl-

inder but, rather, 2cm outside it. In other

words, if the apparatus doing the generat-

ing were to be shielded from an observer,

Wizard of Oz-style, by a curtain, it would

look to that observer as if this field was ap-

pearing from nowhere.

Going from Dr Mach-Batlle’s demon-

stration to something which could be used

in practice to manipulate distant magnetic

fields will be a long journey. But if that jour-

ney can be made, potential applications go

way beyond cleaning up fuzzy mriscans.

Remotely cast fields of this sort might be

used to steer medical nanobots through

someone’s bloodstream to deliver drugs to

a particular tissue, or else to guide them to-

wards a malignant tumour and remotely

raise their temperature once they have ar-

rived, in order to cook it to death. There are

also likely to be applications in quantum

computing. Many designs for quantum

computers rely on trapping atoms at pre-

cise locations in space—a difficult feat

which this sleight of hand could simplify.

The trick still requires refining. To

achieve such desired applications the team

need to be able to sculpt intricate magnetic

fields in three dimensions. At present, lim-

ited as they are to emulating the field gen-

erated by a single electric wire, they cannot

do this. But it is worth remembering that

Oersted’s original experiment, from which

the whole of electrical engineering ulti-

mately descends, was even simpler. It in-

volved only a battery, a magnetic compass

and a single wire. Great oaks from little

acorns grow. 7

Earnshaw’s theorem says magnetism

cannot be created remotely. It can

Manipulating magnetism

Out of left field

This collection of stone tools from Peru upsets the idea that, in the ancient world of

hunter-gatherers, it was men who hunted and women who gathered. It comes from the

9,000-year-old grave of a woman in her late teens and includes seven spear tips, and a

knife and a set of scrapers for butchering large animals. In light of its discovery, reported

this week in Science Advances, Randall Haas of the University of California, Davis, and

his colleagues, who found it, searched the literature on early burials in North and South

America. There were 27 where the sex of the inhumed was known and hunting tools had

been discovered alongside. Of these 11 were of females. Two, admittedly, were of infant

girls. But the idea that only men did the hunting does now look, well, a bit old fashioned.

Think again