Econ_Sep_22

74 Science and technology

The Economist

September 22nd 2018

SMISMATCHES go, it’s a big one. When

physicists bring the StandardModel of

particle physics and Einstein’s general the-

ory of relativity together they get a clear

prediction. In the very early universe,

equal amounts of matter and antimatter

should have come into being. Since the

one famously annihilates the other, the re-

sult should be a universe full of radiation,

but without the stars, planets and nebulae

that make up galaxies. Yet stars, planets

and nebulae do exist. The inference is that

matter and antimatter are not quite as

equal and opposite as themodels predict.

This problem has troubled physics for

the past half-century, but itmaynowbe ap-

proaching resolution. At

CERN

, a particle-

physics laboratory near Geneva, three

teams of researchers are applying different

methods to answer the same question:

does antimatter fall down, or up? Relativ-

ity predicts “down”, just like matter. If it

falls up, that could hint at a difference be-

tween the two that allowed a matter-

dominated universe to form.

Testing this idea is tricky. Antimatter

needed for experiments is usuallymade in

accelerators, in the form of particles travel-

lingat nearly the speedof light. Touse it ina

gravitational test, it must be slowed down,

isolated from all other forces and kept in-

tact for long enough to track its fall (or rise).

All without allowing any matter to touch

it, and thus annihilate it.

This is possible only at

CERN

, which

has dedicated one of its smaller accelera-

tors to creatinga streamof low-energyanti-

protons. It does this by smashinga high-en-

ergy beam of protons into a block of

iridium, and then feeding the resulting

antiprotons into a ring 180 metres in cir-

cumference. This ring acts as a particle ac-

celerator in reverse, slowing the antipro-

tons to amere tenth of the speed of light.

Since antiprotons are electrically

charged (theyare negative; normal protons

are positive), gravity’s effect on them is

swamped by any electromagnetic forces

around, which will be much larger. All

three experiments deal with this by com-

bining their antiprotons with positrons

(antielectrons) to create antihydrogen at-

oms. These are electrically neutral, and

thus immune to such disturbances.

One team,

AEGIS

(Antihydrogen Ex-

periment: Gravity, Interferometry, Spec-

troscopy), guides the resulting beam of an-

tihydrogen into something called a Moiré

deflectometer. This splits the beam up and

then recombines it to create an interference

pattern. Gravitational effects should show

up as small shifts in the pattern.

A second group,

GBAR

(Gravitational

Behaviour of Antihydrogen at Rest), runs

twomore stages of deceleration, then adds

two positrons to each antiproton, to create

a positively charged antihydrogen ion.

Such ions, precisely because they are

charged, can be slowed down even further

using an electric field. This brings them to

something approaching human walking

pace. The field then guides them towards a

target point where they are pinned in place

by a second field and the extra positron is

shot off by pulses of laser light. At this

point, the ions become electrically neutral,

so gravity is the only remaining force act-

ing on them. The direction of their fall can

thus be observed directly.

The third team,

ALPHA

, maybe the clos-

est to success. This group has spent the past

18 years measuring every possible proper-

ty of antihydrogen, and has become adept

at trapping it and chilling it towithin half a

degree of absolute zero. (A cold atom is a

slow-moving atom.)

ALPHA

’s best attempt

so far gathered antihydrogen atoms in a

trap and turned off the magnetic field to

watch which way they drifted. But even at

this temperature, there was still plenty of

noise in the system. The team could con-

clude only that antimattermight fall down-

wards anywhere up to110 times faster than

normal matter does, or fall upwards any-

where up to 65 times faster than normal

matter falls down.

Now,

ALPHA

’s researchers may be able

to do better. As a consequence of work

they reported inAugust on a phenomenon

called the Lyman-alpha transition, they

think they can bring a technique called la-

ser cooling to bear on antihydrogen. This

should bring the antiatoms towithin a few

hundredths of a degree of absolute zero.

With antihydrogen that cold, a similar drift

test would give a much more precise mea-

surement of gravitational behaviour.

All three teams are now in a race to per-

fect their apparatus in time to make obser-

vations before

CERN

’s accelerators come to

the end of their current run and take a two-

yearmaintenance break. That is scheduled

for November. There appears to be a good

chance of at least a “sign test” by then (ie,

one that provides an answer to the ques-

tion of whether antimatter falls up or

down), even if it does not show exactly

howfast the atoms are falling or rising.

Few really believe that antimatter falls

up, because so much existing theory pre-

dicts it will not. But in 1887 existing theory

predicted that the apparent speed of light

would vary with the speed at which the

observer was moving. When Albert Mi-

chelson and EdwardMorley showed that it

does not, they tore up the rule book of

physics. It could happen again.

Antimatter

I go up, or down...

Three experiments probewhymatter

and antimatter are notmirror images

YUSAKU MAEZAWA is a Japanese fashion billionaire. If he gets his way, in 2023 he will

become the first person to visit the Moon since Eugene Cernan, an American astronaut,

left in 1972. Unlike Mr Cernan, Mr Maezawa will not land. Instead, he will zoom round the

Moon aboard a spaceship built by SpaceX, an American rocketry firm. That, at least, was

the plan announced on September 17th by Elon Musk, SpaceX’s boss. Whether it will

happen is another question. SpaceX is already late with plans to fly astronauts to the

International Space Station, which is far easier. A previous lunar trip scheduled for this

year has been shelved. And the rocket that will do the job does not yet exist. Even Mr

Musk was careful to describe the timeline as aspirational.

Destination: Moon. Maybe