4 April 2020 | New Scientist | 19Infectious diseases Chaos
Clare Wilson Leah Crane
THE hepatitis C virus was on a
global rampage a decade ago,
but is now being pushed back.
Egypt, once the country with the
highest prevalence of this virus, is
on course to slash infection rates
this year, eliminating hepatitis C
as a public health threat.
The hepatitis C virus, which
can cause liver failure and cancer, is
mainly passed on through sex or by
drug users sharing needles. In the
past, it was also widely spread by
healthcare staff reusing needles.
Practical curative treatments
arrived a few years ago, but the
drugs were initially costly. Cheap
generic versions now exist.
Egypt has led the way in their use.
Until recently, one in 10 adults in
the country had the virus, as a result
of needles being reused during past
mass-treatment campaigns against
parasitic worms. In 2018, the
country began offering all adults
free tests and treatment. By last
year, 80 per cent of the country had
taken part and more than 2 million
people had been treated.
If trends continue, the prevalence
is set to fall to below 0.5 per cent of
the population this year, say Imam
Waked at Menoufia University in
Shibin El Kom, Egypt, and his
colleagues (The New England
Journal of Medicine, doi.org/dqjt).
Some countries with low infection
rates are targeting high-risk groups,
such as people who are HIV-positive,
by offering frequent testing. In one
of the first studies to show the
effectiveness of this strategy,
new infections fell by 68 per cent
between 2015 and 2018 among
HIV-positive gay men attending five
clinics in the UK (Clinical Infectious
Diseases, doi.org/dqjv). ❚
Hepatitis C infection
rates fall thanks to
tests and treatment
FUNDAMENTAL limits on
the smallest possible lengths
of time and space mean that
some events obeying basic
laws of physics can never
be predicted, it now seems,
even with the most powerful
computer simulations.
The three-body problem,
which is the mathematical
question of how three objects
orbit one another according
to Newton’s laws of motion, is
notoriously different to solve
because of a property called
chaos. A chaotic system is one
in which even a tiny change
in the initial conditions of the
objects, like their positions or
speeds, has an enormous effect
on how they move over time.
This is often referred to as
the butterfly effect, and it makes
it very difficult to predict how
these systems will evolve, or to
mathematically rewind them
and find out where they began.
Much of that difficulty
comes from the fact that
our computers have limited
precision, so even tiny
uncertainties can ruin a
simulation of a chaotic system.But now Tjarda Boekholt
at the University of Coimbra
in Portugal and his colleagues
say that even the best possible
computer in the universe can’t
solve this problem.
His team used extraordinarily
precise simulations to probe
whether a lack of precision
is the only problem with
predicting chaotic systems.
They started a simulation
of three black holes orbiting
each other at a distance of one
parsec, or about 3 light years.
They let it run for a while, and
then tried to rewind it back to
its initial configuration. They
repeated this process 1212 times.
If it was impossible to rewind
back to the initial configuration
of the system, that would mean
the system was unpredictable.
The team used their results
to calculate just how precise
you would need to be in
order to return to the initial
configuration. They found that
for about 5 per cent of triplesystems, you would need to
measure that configuration to
a precision of less than a single
Planck length – the smallest
possible unit of measurement
for length, and about 10-51 times
the initial distance between
the black holes (arxiv.org/
abs/2002.04029).
That means those systems
are deeply unpredictable.
“Even if you have a Planck
length difference, which is
a ridiculously small amount,
some situations are still
irreversible,” says Boekholt.
“We can’t go more precise
than nature.”
In a practical sense, this
means that there is a limit
to our predictive power when
we try to examine the universe
precisely, because even the
most powerful computer that
could ever exist can’t simulate
below the Plank length.
“There really are systems
of three black holes and one
of the consequences is that
trying to follow the motion
of those systems in any detail
really is impossible,” says
Scott Tremaine at the Institute
for Advanced Study in New
Jersey. “You can’t predict
the motion just because of
fundamental quantum-size
uncertainties, even for
astronomical-sized bodies.”
This may also have larger
implications for how we think
about time itself. “If we could
have reversed 100 per cent
of cases, then for three bodies
it wouldn’t be possible
to determine the arrow of
time – you couldn’t distinguish
forward and backward,” says
Boekholt. “But for 5 per cent
of cases, you can go forward
but not backward, so there’s
this asymmetry which is closely
linked to the arrow of time.” ❚Some things are just
impossible to predict
MARK
GARL
ICK/SCIENCE
PHOT
O^ LIBRAR
YThe behaviour of three
black holes orbiting each
other is very hard to predict0.5%
The expected prevalence of
hepatitis C in Egypt later this year