New Scientist - 29.02.2020

48 | New Scientist | 29 February 2020

Richard Webb is executive

editor of New Scientist

Antimatter exists in our

world – you just have to

be very alert to spot it.

Everyday antimatter

generally takes the form of

antimatter electrons, known

as positrons, produced in

radioactive beta decays. These

positrons lead transient lives

before annihilating with the

first electron they encounter,

producing energy in the form

of gamma rays.

Like the occasional

breaking of the most delicate

of winds, we all emit the

occasional positron, thanks

largely to traces of radioactive

potassium-40 in our tissues.

A medium-sized banana

produces one maybe every

75 minutes. A bag of Brazil

nuts, or a worktop or bedrock

made of granite, ups the ante

considerably.

None of this is a danger

to us. The energy released by

each annihilation amounts to

precisely two electron masses,

or 1.022 megaelectronvolts –

in the standard units of very

small energies, considerably

less than one millionth of the

energy of a flying mosquito.

It follows that technologies

to propel humanity further,

such as antimatter drives or

rockets, or blow us to kingdom

come, such as antimatter

bombs, aren’t exactly

immediate prospects.

The total energy of stable

antimatter contained so far

over decades of experiments

isn’t enough even to boil the

water for a cup of tea. Plus,

you would need a vessel a

couple of hundred metres

across to hold it in place.

So what is antimatter

good for? That is one of

the favourite questions of

Tara Shears at CERN’s LHCb

experiment. “Antimatter is

really esoteric, isn’t it?” she

says. “You probably don’t

expect antimatter to help

diagnose cancer or help with

heart problems, or have any

practical benefit at all – but

it does.” Positrons make up

the P in a PET scan, which

stands for positron emission

tomography. Here the

annihilation emissions of

positrons in the radioactive

tracer you swallow can

illuminate all sorts of potential

internal nasties. “That’s really

useful,” says Shears.

Everyday antimatter

how antimatter responds to gravity, we need

anti-atoms to be electrically neutral. As soon as

Hangst and his team made their antihydrogen-

trapping breakthrough in 2018, they activated

plans to essentially tip their horizontal

trapping apparatus at right angles to function

as a gravity detector, dubbed ALPHA-G. “I’ve

worked harder during that stretch in my life

than any other time,” says Hangst. “We started

in May and worked seven-day weeks until the

middle of November.” But at that point, with

just a couple of weeks more needed to make

their first measurements, the valve supplying

the antiprotons was turned off. “It was so

frustrating, you have no idea.”

Hangst isn’t the only one gagging for the

antiprotons to come back on stream, due for

early 2021. Another detector that Malbrunot

is working on, AEGIS, and a third experiment,

GBAR, are also gearing up to confirm whether

antimatter falls up or down, and also answer

the much more fiddly question of whether

matter and antimatter feel the same strength

of gravitational force.

Any deviation from expectations would

have huge repercussions. Hajduković has

shown how the existence of two opposing

gravitational charges would allow the

quantum vacuum itself to become a source of

attractive and repulsive gravitational effects.

That might account for both why there seems

to be a lot of gravitating stuff out there that we

can’t see – dark matter – and a mysterious force

that seems to be speeding up the expansion

of empty space, which we call dark energy.

Black holes sucking in matter could then

also be white holes spewing out antimatter.

Hajduković says the possible new source

of antimatter could explain strange

excesses of high-energy positrons and

antiprotons observed in cosmic rays, as well

as very-high-energy neutrinos seen coming

from our galaxy’s centre by the IceCube

detector in Antarctica back in 2014.

Existence of two opposing gravitational

poles might even lend weight to the idea

that the big bang wasn’t a beginning, but

the latest in a series of cycling matter and

antimatter universes. “Antimatter gravity

experiments might be much more than a

measurement of the gravitational acceleration

of antimatter,” says Hajduković. “They might

open a window towards a new physics and

a new model of the universe.”

Or perhaps not. When the upgraded LHC

and ELENA machines switch on next year,

their antimatter investigations could bring

a radical new understanding of how our

universe works, and how we come to be in it.

Or it could be, well, an anti-moment, one that

sends us back to the drawing board in our

quest to make sense of reality.

Hangst is philosophical. “In one direction,

you win the Nobel prize, in the other, people

say, ‘OK, well, we told you so’,” says Hangst.

“These are very challenging experiments,

and it’s rewarding to succeed no matter

what answer you get.” ❚

“ If it turns out that

antimatter falls

up, everything

we know about

the universe

would be to

play for”

ZOONAR GMBH/ALAMY