E
VERY year, I attend the
Star Trek convention in Las
Vegas, and every year, I get
asked whether warp speed will
ever be possible. In the Star Trek
universe, humanoid species zoom
around the galaxy at speeds faster
than light, using warp engines
fuelled by antimatter. Travelling
faster than the speed of light is
unlikely, but antimatter is real.
Every particle has an antimatter
partner that we call an antiparticle.
So, as a particle physicist,
what I really want to be asked
about isn’t the likelihood of
travelling long distances quickly,
but instead about the particle
type that underlies this fictional
technology. Star Trek’s futuristic
antimatter engine touches on one
of the great unsolved mysteries
in particle physics: where is all
of the antimatter anyway?
The best known type of
antimatter is the positron, which
is the antielectron. The positron
has the same mass as an electron,
but the opposite electrical charge.
When matter collides with its
antimatter partner, they
annihilate each other. This isn’t
simply a matter of theory: we
have seen antimatter in the lab,
and not just with the electron
and its partner.
Positrons can be made
through radioactive decay.
They are also created in a pair
with electrons when extremely
energetic photons, better known
as gamma rays, interact with
atomic nuclei. Antiprotons
have also been produced, and,
in 1995, scientists were finally
able to directly combine
positrons and antiprotons
to create antihydrogen.
Although antimatter is real,
it is rather difficult to make in the
lab. Since matter and antimatter
annihilate one another on contact,
one has to wonder why we arehere at all. If they are each other’s
complete opposites, one might
expect the same amount of matter
and antimatter to have been
produced in the big bang, quickly
leading to annihilation and an
empty universe. Instead, we live
in a highly asymmetric version of
the universe, where the negatively
charged electron is a fundamental
particle that forms a core part
of all atoms, hovering in their
orbitals. Why did nature use
only half of the building blocks
available to it?
Efforts to make sense of this
asymmetry are under way in both
theoretical and experimentalphysics. Many theorists believe
that the lopsided bias towards
matter is connected to violations
of something called charge-parity
symmetry, more commonly
known among physicists as
CP symmetry. This is a property
that demands that all particles
are interchangeable with their
antiparticle when their spatial
coordinates are flipped, a kind of
mirror symmetry. Most observed
particles obey CP symmetry,
but it can be violated.
Though most famous for
being the facility where the
Higgs boson was first detected,
the Large Hadron Collider is also
home to experiments that areseeking to learn more about
CP symmetry breaking.
The Large Hadron Collider
beauty (LHCb) experiment, for
example, specifically focuses on
b-physics. B-physics refers not to
low-budget physics, something
that our governments surely
dream of, but instead to the
physics of beauty quarks
(sometimes referred to as
bottom quarks).
Beauty quarks are just one of
six flavours of subatomic quarks,
which are the constituents of
neutrons and protons. The
other five varieties have equally
delightful names: top, up,
down, strange and charm. The
fundamental “weak” nuclear
force can cause quarks to change
flavours, and it also causes the
quarks to break CP symmetry.
This gives us an important hint
that CP symmetry violations are
possible, leading theorists to
consider matter-antimatter
models that rely on it.
In addition to beauty quarks,
LHCb can also study the properties
of charm quarks. Excitingly, the
experiment recently found the
first evidence of CP violation
among them. In order to achieve
this result, LHCb looked at decays
of D0 mesons – short-lived
particles made of a charm
quark and an up antiquark.
This result is an exciting
affirmation of a phenomenon that
scientists had expected to find for
decades, but had yet to produce
in the lab. The discovery doesn’t
radically change our perspective
on physics yet because it matches
theoretical predictions – and it
certainly isn’t a warp engine. But
it suggests that, under the right
conditions, CP violation can occur.
Perhaps those conditions existed
during the big bang, producing
the nearly antimatterless universe
we see today. ❚24 | New Scientist | 15 June 2019
PA
RAMOUNT^ PICTURES/RGAThis column will appear
monthly. Up next week:
Graham Lawton“ Star Trek’s futuristic
antimatter engine
touches on one
of the greatest
unsolved mysteries
of particle physics”The charm of antimatter The warp engines of the Star Trek
universe may not be possible, but we are learning more about the
particles that fuel them, writes Chanda Prescod-WeinsteinField notes from space-time
What are you reading?
I was thrilled to pick up a
copy of The Confessions
of Frannie Langton by
Sara Collins, which is
a powerful mediation
on slavery, racism and
autonomy.What are you watching?
The new Ava DuVernay
miniseries about the
Central Park Five, When
They See Us, was really
great.What are you
working on?
I am trying to convince
people that we need more
X-ray space telescopes.Chanda’s week
Chanda Prescod-Weinstein
is an assistant professor of
physics and astronomy,
and a core faculty member
in women’s studies at the
University of New Hampshire.
Find her on Twitter
@IBJIYONGI and the web
at cprescodweinstein.comViews Columnist