6 | New Scientist | 21/28 December 2019MedicineChelsea Whyte Clare WilsonORIGAMI can be deadly when made
of DNA. Tiny devices created from
intricately folded DNA strands can
boost the potency of antibacterial
chemicals by increasing their
contact with microbes.
When tested on two common
kinds of bacteria, the folded DNA
slowed their growth rate. Ioanna
Mela at the University of Cambridge
says a similar approach could
be directed against any kind of
microbe. “This is proof of principle.”
DNA is best known for storing our
genetic information, but it can also
be folded into 3D structures called
DNA origami. Additionally, small
lengths of DNA can be designed
to have the exact shape needed to
bind to other biological molecules,
like a key fitting into a lock.
Mela’s team combined these
two functions to create a flat
platform of DNA with five wells,
each loaded with two molecules
of lysozyme, an antibacterial
compound found in body fluids such
as tears. Sticking out from the edges
of the platform are short lengths
of DNA designed to bind to E. coli
or Bacillus subtilis bacteria.
The idea is that the platforms
lock on to bacteria and hold them
close to the lysozyme, increasing
its potency. Sure enough, when
bacteria were exposed to the
platforms, they grew more slowly
than when exposed to the
antibacterial compound alone
(bioRxiv, doi.org/dgzk).
Using the approach for an
antibiotic drug would allow lower
doses to be given, reducing side
effects for individuals and slowing
the rise of antibiotic resistance
in general, says Mela.
One potential flaw of the method
is that, unless DNA structures are
modified to make them more stable,
enzymes in the blood tend to break
them down, says Philip Tinnefeld at
the Ludwig Maximilian University
of Munich in Germany. ❚Tiny folded DNA
machines may
boost antibacterialsWOULD-BE time travellers
have long wrestled with the
grandfather paradox: if you
change things in the past and
prevent yourself ever existing,
how did you time travel in the
first place? In other words, if
Alice goes back in time and kills
her grandfather Bob, she won’t
have been born and can’t carry
out her murderous plot.
One way to avoid this is the
idea of branching universes, in
which the universe splits into
two with each instance of time
travel, leading to an infinite
number. Another idea is infinite
copies of parallel universes that
evolve differently over time.
But physicists would prefer
to avoid the infinite if possible.
Now Barak Shoshany and
Jacob Hauser at the Perimeter
Institute in Canada have come
up with a model of time travel
that merely requires a very
large, rather than infinite,
number of parallel histories
in order to avoid paradoxes.
The pair suggest that to
travel between these timelines,
a person could step through a
wormhole – essentially, a hole in
space-time – in a way that they
say is mathematically possible.
One way to think of the
universe is as a set of points
in space and time, with a
timeline or history of events
a mathematical function that
orders these points, says
Shoshany. In the branching
universe and parallel universe
ideas, you expand the number of
points to create multiple sets and
thus mathematically distinct
universes. But Shoshany and
Hauser have modelled a third
idea called parallel histories
(arxiv.org/abs/1911.11590).
“There are different parallel
universes where things are
roughly the same, and eachone is mathematically on a
separate space-time manifold.
You can go between those
manifolds when you travel
back in time,” says Shoshany.
Held within one universe,
or just one set of points, are
multiple timelines represented
by mathematical functions
rearranging those points.
Traversing a wormhole alters
the function and so the timeline
has a different arrangement.
This means Alice 1 can travel
from timeline 1 to timeline 2,
where she attempts to kill Bob 2by releasing a crocodile. If the
crocodile doesn’t kill him, he
can go on to have grandchildren
and Alice 2 can try again. She
travels to timeline 3 and tries
to drop a piano on Bob 3.
The key is that the Alices
can change their actions and so
aren’t locked into Alice 1’s failing
plan. If she were to travel within
a single timeline, she would have
to repeat that act because that
is the only way to keep history
consistent and eliminatethe possibility of a paradox.
It may seem a bit of a cheat
to add multiple timelines,
but Shoshany and Hauser have
calculated that this paradox-free
time travel can be achieved with
a finite number of histories in
one universe. Alice is made of a
finite number of particles that
can only be in a finite – but
incredibly large – number of
arrangements, he says.
There is one big flaw in the
pair’s model as it stands: it only
works with one dimension of
space, rather than our usual
three, says Geraint Lewis at the
University of Sydney. Shoshany
says he has plans to translate the
model to three dimensions to
better represent the real world.
Still, it isn’t as satisfying as
Lewis would like. “What time
travel means here is stepping
between those histories – that’s
even freakier,” he says. “At some
level it doesn’t even feel like
time travel anymore, because
what’s the point of going back
and killing Hitler if the second
world war still plays out in the
universe you’re from?” ❚Theoretical physicsParadox-free time travel
There may be a way to alter the past without breaking the future
2019MARV
EL
STUDIOSNews
Doctor Strange
searches alternative
timelines“ Alice 1 can travel from
timeline 1 to timeline 2,
where she tries to kill
Bob 2 with a crocodile”