THE WORLD on the other side of the mirror is a favourite of fantasy writers:
a place where everything is subtly twisted, where things impossible in our world
can happen. Scores of fictional adventurers, from Alice onwards, have stepped
through into that reflected realm. But it could be that the world on the other
side of the mirror is more than a fantasy.
Physicists suspect that we might be surrounded by a parallel universe of
mirror matter, where mirror particles assemble themselves into mirror galaxies,
mirror stars and mirror planets—even mirror life. And two scientists will
soon try to get a first glimpse into this mirror world.
Just like the mirror worlds of fantasy, it is a dominion subtly different
from our own. If you look at yourself in a mirror, the reflected you seems to
behave in just the same way as the real one, only back-to-front. But look more
closely and you’ll find that nature’s symmetry is flawed. When physicists
examine some known processes involving fundamental particles, they find that the
mirror-image process is not always possible. For instance, when an atom decays
to create a neutrino, the neutrino always spins in the same direction,
left-handedly. If it were coming towards you, you’d see it spinning
clockwise—in other words it twists like a left-handed corkscrew.
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But we never see right-handed neutrinos. Wouldn’t it be more pleasing if both
types of neutrino existed, and nature were mirror symmetric? In the 1950s, Tsung
Dao Lee and Chen Ning Yang, the Nobel prizewinning physicists who had first
pointed out that asymmetry, speculated that perfect left-right symmetry could be
restored to the Universe if a “mirror” or “shadow” world existed, complete with
mirror-reflected particles. Mirror particles would differ only in the way they
interacted with each other—every interaction would work as if reflected in
a mirror. So processes that produce right-handed particles in our world, for
example, would produce left-handed particles in the mirror world.
Mirror particles would have the same masses as their ordinary counterparts,
and in the mirror world, mirror quarks and other particles would interact via
mirror forces very like ours. But mirror particles would interact with ordinary
particles only through gravity. Otherwise they would be essentially invisible to
us—just as our Universe would be all but invisible to them.
A hazy glimpse of the mirror world is provided by neutrinos. Physicists see
these ghostly particles coming from nuclear reactions inside the Sun and from
the upper atmosphere of the Earth, where they are created by cosmic rays. But
there are fewer than expected. These shortfalls could be explained if the three
known types of neutrino—the muon, tau and electron neutrinos—are
spontaneously changing into each other. Unless, that is, you include the results
of experiments at Los Alamos National Laboratory in New Mexico. If those results
hold up, we might need a new form of neutrino, one so shy it makes the other
three particles seem positively sociable
(New Scientist, 13 March 1999, p 32).
Could the fourth neutrino be from the mirror universe? “If neutrinos can
oscillate into mirror neutrinos, this can nicely explain the solar and
atmospheric neutrino anomalies,” says Robert Foot, a physicist at the University
of Melbourne in Australia.
Neutrino research hasn’t proved the existence of mirror matter, but more
convincing evidence may come from an ephemeral substance called
orthopositronium. Positronium is a type of matter that looks a little like an
atom, but instead of electrons orbiting a nucleus made of protons and neutrons,
there are just an electron and a positron orbiting each other. If the spins of
the two particles point in the same direction, the system is called
orthopositronium.
In 1990, physicists at the University of Michigan at Ann Arbor made a batch
of this short-lived substance by firing a low-energy beam of positrons into
matter, so that some of them slowed down and captured electrons. They then
measured its lifetime. Theorists calculate that orthopositronium should decay
into three photons after, on average, 142 nanoseconds—but the Michigan
physicists discovered that its lifetime is actually 0.1 per cent shorter.
At first, nobody worried too much. It was assumed that when theorists made
more detailed calculations, the predicted lifetime would agree with the one
observed. But when those calculations were finally done in March, by a team led
by Gregory Adkins of Franklin and Marshall College in Lancaster, Pennsylvania,
the discrepancy remained—leaving physicists scratching their heads (
Physical Review Letters, vol 84, p 5086).
Enter Sergei Gninenko of CERN, the European laboratory for particle physics
near Geneva, Switzerland. In a paper published with Foot in the 11 May issue of
Physics Letters B(vol 480, p 171), he claims that the lifetime can be
explained by a mirror universe.
The idea that orthopositronium might be peculiarly sensitive to the mirror
world was pointed out in 1986 by the Nobel prizewinning physicist Sheldon
Glashow of Harvard University. Bob Holdom of the University of Toronto had
proposed that ordinary matter and mirror matter might interact in ways besides
gravity: there might be heavy particles that interact with both ordinary and
mirror matter, carrying a tiny force between them. According to Glashow, this
coupling between ordinary and mirror orthopositronium would mix the two states.
The effect would be that orthopositronium could oscillate between mirror and
ordinary orthopositronium—jumping back and forth through the mirror.
Gninenko and Foot say these oscillations could explain the short lifetime. If
some orthopositronium oscillates into the mirror world, and decays there, it can
never oscillate back. “Extra losses mean a faster decay,” says Gninenko. “We
find that if the oscillations take about 3000 nanoseconds, it is possible to
explain the observed reduction in decay lifetime.”
It’s a remarkable claim, but Gninenko and Foot believe it is testable. They
are preparing a proposal for a vacuum experiment to be carried out at CERN
(see Diagram).
The idea is to enclose orthopositronium inside the evacuated cavity of
a calorimeter, which measures total energy changes. “If oscillations are
occurring, then particles will disappear from our world,” says Gninenko. “The
unmistakable signature which we will look for is missing energy. We could check
for the influence of a possible mirror world within a few years.”
The vacuum is crucial because frequent collisions with matter can stop the
oscillations. But achieving a vacuum in such an experiment is difficult, because
to create positronium in the first place you need a source of electrons—in
other words, some matter. According to Gninenko, this was enough to spoil a
similar experiment at the University of Tokyo in 1995, where grains of silicon
dioxide supplied the necessary electrons but masked the effect. Gninenko will
try to create enough positronium from collisions with electrons in the wall of
his vacuum vessel.FIG-mg22434701.JPG
Gninenko’s experiment isn’t the only way to seek the mirror world. Another
sign would be the existence of particles with tiny electric charges. All the
particles we know of have electric charges that are multiples of the charge on
an electron, or thirds of this (in the case of quarks). But if normal electrons
interact with mirror particles via the heavy intermediaries that Holdom
conjectured, they effectively have a small mirror electric charge. Similarly,
mirror particles would have a small normal charge—perhaps only a
ten-thousandth or a hundred-thousandth of that carried by an electron. Physicists
have searched for such “milli-charged” particles, both in oil-drop experiments
and in bubble chambers attached to particle accelerators, but so far in
vain.
If Gninenko does find mirror matter, it will surely tell physicists something
fundamental about the Universe—perhaps even providing them with the
ultimate theory. “A mirror world is predicted by some versions of superstring
theory,” says John Cramer of the University of Washington in Seattle. “It is
therefore pleasing to see that experimental tests of the ideas are possible,
since the predictions of super-string theories are usually untestable.”
If it exists, mirror matter wouldn’t just turn up in esoteric particle
experiments. Large amounts of it could fill our Universe. After all, astronomers
are crying out for a new type of matter: at least 90 per cent of the Universe’s
mass appears to be mysterious “dark matter”, which reveals itself only by the
gravitational pull it exerts on stars and galaxies. “Mirror matter could
certainly be the dark matter,” says Foot.
Cramer disagrees: “The mirror matter would only boost the gravitational
matter density around galactic clusters by a factor of two—we need about a
factor of seven.” Foot counters that Cramer is making an unjustified assumption,
namely that there should be only as much mirror matter as ordinary matter. We
know little about the initial conditions of the Universe, he asserts. Maybe the
big bang created more mirror than ordinary matter?
If there is indeed a mirror universe sharing our space, it might hold mirror
galaxies, mirror stars and mirror planets. They could even host mirror
life—real Tweedledums and Tweedledees.
According to Foot, mirror stars might be observable, but only when they
explode as supernovae. These would look very different from ordinary supernovae.
Because of the very weak coupling of mirror matter to ordinary photons, mirror
supernovae would be difficult to spot by looking for ordinary light. But in a
mirror supernova, oscillations of mirror neutrinos into ordinary ones should
produce a huge burst of observable neutrinos. “These could reveal a mirror
supernova to underground experiments, just as a burst of neutrinos revealed an
ordinary supernova in 1987,” says Foot.
Other worlds
So much for stars. What about planets? Whole mirror solar systems would be
impossible to detect, because even their parent stars would be almost invisible.
But mirror planets might orbit ordinary stars.
For example, some of the 40 or so extra-solar planets discovered in the past
few years might be mirror planets, as hardly any have yet been observed
directly. With the next generation of telescopes it should be easy to tell,
because if the extrasolar planets are mirror worlds then they should, unlike
normal planets, reflect none of the ordinary light from their stars. “The idea
that the extrasolar planets are made of mirror matter can be tested in the near
future as searches for reflected light improve in sensitivity,” says Foot.
Foot thinks it unlikely that our own Solar System contains mirror planets,
unless they are either so small or so far from the Sun that their gravitational
influence is negligible. “However, they may orbit in a different plane to the
ordinary planets, making them harder to detect,” he says.
Such mirror rogues may have a sinister side. In 1984, David Raup and John
Sepkoski of the University of Chicago suggested that the Sun might have a
distant stellar companion, dubbed Nemesis, which periodically stirred up the
Oort Cloud of comets, sending icy bodies on a collision course with the Earth.
This would explain an apparently regular pattern of species extinctions in the
geological record. So far no one has spotted Nemesis, so Zurab Silagadze of the
Budker Institute of Nuclear Physics in Novosibirsk, Russia, has proposed that it
could be an invisible mirror star. “It’s very hard to prove,” he says, “but fun
to speculate.”
Whether there’s a mirror star out there or not, the rocks that rain down on
Earth might be mirror missiles. Foot speculates that the object that devastated
the Tunguska region of Siberia in 1908, flattening 2000 square kilometres of
forest but leaving no crater, could have been a mirror asteroid or comet. With a
big enough body, even the tiny forces between mirror and ordinary matter could
have released enough energy to flatten the trees. Once it ploughed into Earth,
it should have melted a huge volume of rock. “The idea could be tested by taking
samples under the Tunguska site to look for evidence of a large release of
energy underground,” says Foot. “Of course, I have a vivid imagination, and
Tunguska may have nothing to do with mirror matter.”
The mirror world provokes a lot of speculation. Cramer has even written a
novel about mirror matter. Published in 1986, Twistor speculates on the
existence of a mirror Earth occupying the same volume of space as the ordinary
Earth. Tidal forces lock the spins of the two Earths together, and their cores
are fairly light, but in combination they add up to the Earth’s observed mass.
The hero even devises a way to escape into the mirror world. Not surprisingly,
Foot finds this unlikely.
Unfortunately, unless there is a parallel Earth to support you, stepping
through the mirror might be perilous. “You would probably fall straight to the
centre of the Earth,” says Foot. At least it should be painless, and you might
see some interesting things on the way. Rather like falling down a rabbit hole . . .