36 | New Scientist |8 June 2019
across the divide, increasing the gravitational
attraction between particles in our own
universe and taking it down a different
historical path. That temperature difference,
in turn, would have made it much easier
for particles to cross over into the mirror
universe, oscillating out of our own world
for good. The best-developed mirror models
suggest five mirror particles for every regular
particle: exactly the prescription given by
our cosmological measurements of the ratio
of dark to “normal” matter.
What’s more, since the particles left behind
went on to form stars, planets and, eventually,
people, it seems reasonable to expect that
there is also a mirror version of life – and
significantly more of it than we can see.
“In the mirror universe, it would happen
five times more frequently,” says Berezhiani.
Who knows, there might even be a race of
mirror humans trying to work out why their
dark matter is five times less abundant than
their normal matter.left. According to this method, the neutron
lives for an average of 14 minutes and
39 seconds.
The beam experiment, by contrast,
counts the number of protons that emerge
from a beam of neutrons channelled out of
a nuclear reactor. Each proton can only appear
as a result of a decaying neutron. Using
calculations based on the beam intensity,
this method sets the neutron lifetime at
14 minutes and 48 seconds. And there is the
problem. “These two measurements should
be the same,” says Berezhiani.
At first, physicists thought these extra
9 seconds could be put down to experimental
error. But as we have improved our technical
abilities and narrowed down the errors in
the measurements, our certainty about
both results has only grown. There are, it
seems, two different neutron lifetimes.
The mirror world could be the culprit, if
it exists. A key feature of these models, says
Berezhiani, is that neutrons oscillate back and
forth between the two worlds. “When passing
through a magnetic field, the oscillation
probability increases,” says Berezhiani. The
jaw-dropping suggestion is that neutrons
are only a part-time resident of our universe.
The rest of their time is spent in a parallel
plane of reality, where any protons they emit
would go undetected.
If one in 100 neutrons swapped into the
mirror world before emitting a proton, that
would explain the longer measured neutron
lifetime in the magnetic fields of beam
experiments. “It’s a very natural explanation,”
says Berezhiani.Black mirrors
And that isn’t all the mirror sector can
do. “Many other puzzles can be naturally
explained using the same model with the
same parameters,” says Wanpeng Tan at
the University of Notre Dame in Indiana
(see “They do it with mirrors”, left). The
alternative universe could even provide a
hiding place for dark matter and explain why
it is so difficult to find. “The mirror neutron
seems like a good dark matter candidate,”
says Rabindra Mohapatra, a theorist at the
University of Maryland. “It’s very compelling.”
It is even more compelling when you
learn about the amount of mirror matter
that should exist. In order to be consistent
with our models of early universe evolution,
the mirror sector must have been much
cooler than our own. Too much heat, and
some mirror matter would have leakedThey do it
with mirrors
Some of the most vexing problems
in physics could be solved by the
discovery of a mirror universe
(see main story).Why is there something rather
than nothing?
The universe should have birthed
equal amounts of matter and
antimatter, and they should have
annihilated each other out of
existence. Perhaps they didn’t
because of the oscillation of particles
called neutral kaons between
our sector and the mirror sector.
Wanpeng Tan at the University
of Notre Dame in Indiana thinks
that oscillations in the early universe
between normal and mirror kaons
changed the balance of neutral
kaons and neutral anti-kaons, which
contain the building blocks required
for making matter and antimatter.Why is there so little lithium-7?
Physicists have long noted that the
real-world abundance of this isotope
doesn’t match the amount that
should have been created in the
first few minutes of the universe’s
existence. According to Alain Coc of
the Centre for Nuclear Science and
the Science of Matter in France, mirror
neutrons coming into our sector can
destabilise beryllium-7, the isotope
whose decay produces lithium-7,
causing the abundance of the latter
to fall below what would normally
be expected.Where are the ultra-high-energy
cosmic rays coming from?
Our telescopes are detecting
particles that come from outside
our galaxy – but with energies that
should be impossible after such a long
journey. However, Zurab Berezhiani
at the University of L’Aquila in Italy
has pointed out that the lower
temperature of the mirror sector
means that the particles can travel
further without expending as much
energy as they would in our sector.
If they then oscillate back into
our sector, we will see them as
anomalously energetic.