42 | New Scientist | 27 July 2019Honey, we
shrunk the
accelerator
Huge particle smashers have ruled the
roost for decades. Jon Cartwright meets
the physicists learning to think small
T
O GET to the very bottom of physics,
there has always been one rule: size
matters. The first particle smashers of
the early 1960s were little wider than a dining
room table. A decade later, the Tevatron, a
circular collider in the US, had a circumference
of 6 kilometres. Today’s largest machine, the
Large Hadron Collider (LHC), has one four
times as long. Now there are plans to build
colliders 100 kilometres in circumference:
about the size of New York City.
Physicists get a lot of flak for these
enormous – and enormously expensive –
aspirations. Nature is tenacious, however,
and wresting its most closely held subatomic
secrets from it has always meant accelerating
particles over longer and longer distances
before smashing them together. But a new
shortcut is emerging in a weird, cloud-like state
of matter known as a plasma. Inject particles
into this febrile stuff, and they can accelerate
a thousand times faster than before.
This is more than wishful thinking. Plasma
accelerators have been advancing steadily over
the past few decades, and while they have yet
to pose a serious threat to the dominance of
conventional facilities, that might be changing.
PASeveral recent developments suggest that
UL
BL
OW
Features
plasma accelerators could soon give big
beasts like the LHC a run for their money.
Ultimately, the hope is that these small
machines will let us tackle some of the
biggest questions in physics: why our universe
is filled with matter and not antimatter, for
instance, or what constitutes dark matter.
It seems the ironclad rule of particle physics
is about to be broken.
Certainly, technology everywhere else
is shrinking. But conventional particle
accelerators have always suffered from an
intrinsic unshrinkability. Beneath their hugely
complicated steering and control systems are
lots of little metal pipes. These individually
kick particles forward with short, strong
electric fields. The stronger the fields between
these pipes, the more powerful the kicks – but
only up to a point. Beyond a certain field
strength, the space between the pipes becomes
conducting, discharging the fields like a
lightning bolt and stopping the acceleration.
To make the overall accelerator more powerful,
therefore, you have to add not stronger kicks
but more of them – in other words, more pipes.
It was this limit that led to circular, instead
of linear, particle smashers: in a circle, particles
can keep going round until they reach the