8 June 2019 | New Scientist | 37come back into our universe can be detected,”
says Broussard.
By varying the magnetic fields on both sides
of the wall, Broussard wants to see if she can
find a field strength and shape that increases
the number of neutrons passing through the
wall. “If my numbers are right, they should see
something,” says Berezhiani.Further reflections
The apparatus is built and ready to go.
Broussard is currently negotiating with the
neutron beam operators at Oak Ridge to find
a time when they can install the experiment
in the beam path and perform the tests.
Although excited, she isn’t expecting a
breakthrough on the first run – no one knows
what magnetic fields might sufficiently
enhance the likelihood of oscillations.
“I fully expect to measure zero,” she says.
Instead, for her, it is all about narrowing
down the possible range in size of the effects.
But if Kirch’s team sees a signal in its data
that could be consistent with the existence
of mirror neutrons, Broussard and her team
could search the corresponding magnetic field
with an independent approach. If the neutron
count changes with the presence or absence
of the magnetic field, that would suggest the
existence of a mirror universe.
Kamyshkov, who is collaborating with
Broussard, thinks we are reaching an
important milestone. “The probability of
finding anything is low, but it’s a simple and
inexpensive experiment,” he says. “When a
positive result would usher in a revolution
in physics, we have to try.”
Even if these experiments do find mirror
neutrons, Broussard says a lot of work is still
required to make them a fit for dark matter –
and to populate the rest of the mirror sector.
“I would say it’s a good first step, but I think
there are still challenges to work out,” she says.
And if we don’t find mirror neutrons? One
thing Broussard is sure of is that the mirror
universe won’t die. “Theorists are very good
at evading the traps that experimentalists
leave for them,” she says. “You’ll always find
someone who’s happy to keep the idea alive.”
But with the number of problems physicists
have failed to solve with their current theories,
you can excuse them looking in the mirror. ❚Great theories, but finding the clinching
proof is far from easy. A mirror sector
embedded in our own universe will have zero
interplay with three of the four fundamental
forces of nature: the electromagnetic, strong
and weak force. “It won’t interact with us
except by gravity, and gravity is too weak
to experiment with,” says Yuri Kamyshkov,
who researches mirror matter at the
University of Tennessee in Knoxville.
The answer might lie in better neutron
lifetime experiments. In 2012, Berezhiani
published a paper claiming that previous
experiments that held a bottle of neutrons
in a varying magnetic field had spotted a
signal consistent with mirror neutrons. His
suggestion is that a small amount of mirror
matter is dragged through our world by
the rotation of Earth. The motion of mirror
particles that carry charges – mirror electrons,
say – would create mirror magnetic fields,
and these could increase the chances of
neutrons oscillating out of our universe in
certain ordinary magnetic fields.
That idea intrigued Klaus Kirch and his
colleagues at the Paul Scherrer Institute in
Villigen, Switzerland. They used a more
sensitive apparatus with the potential to test
the possibility that mirror magnetic fields
affect the neutron lifetime in a bottle trap
as suggested by the claimed signal.
Kirch thought the claim far-fetched, but
interesting enough to investigate. “The
experimentalist’s view is, if it doesn’t look
completely crazy, can it be tested?” he says.
“I don’t really believe the signals are there,
and we have designed an experiment that can
disprove them, and we’ll see what comes out
of it.” The exercise involved applying magnetic
fields of varying strength to the apparatus
to see whether they affect the abundance of
neutrons in the trap. It is now complete, says
Kirch, but the team is still analysing the data.
Broussard is watching with interest. Along
with her colleagues at Oak Ridge, she is getting
ready to test Berezhiani’s predictions about
the magnetic fields that cause neutron
oscillations in a purpose-designed experiment
that should give more detail and control than
the apparatus in Switzerland.
The idea behind it is fairly simple: fire a
beam of neutrons at a thick wall that they can’t
penetrate. If a neutron detector behind the
wall detects any neutrons, it could be because
they have oscillated into mirror neutrons en
route, failed to see the wall because it exists
in a different sector of the universe, and then
oscillated back before hitting the detector.
“Only the ones that can oscillate and thenMichael Brooks is a consultant for
New Scientist. His latest book is The
Quantum Astrologer’s Handbook.
@DrMichaelBrooks“ It seems
reasonable
to expect that
there is also a
mirror version
of life”
ANDREWPERRIS