EDITORIAL REVIEW: NEW SCIENTIST (27 July 2002), REVIEWED BY MARCUS CHOWN

Galaxies, stars and planets are all very well, but most of the matter
in the Universe is entirely invisible. There are dozens of candidates
for this dark matter, but one has recently elbowed its way
to the front. It's called ``mirror matter'', and its chief
proponent, Australian physicist Robert Foot, sets out the
case in highly entertaining style in Shadowlands.

Nature, for reasons nobody understands, has chosen fundamental laws
that exhibit the maximum possible symmetry. The laws of physics are
the same in New York as in London, and they respect a myriad of other, 
more abstract symmetries besides. But in one respect nature is
not symmetrical. The laws are not the same when reflected in a 
hypothetical mirror. ``Electrons and other elementary
particles are, in a sense, left-handed'', says Foot. ``Although
most scientists have come to accept that God is left-handed, somehow
it has always bothered me.''

Foot's unease has led him to embrace a radical idea:
nature's left-right symmetry isn't really flawed, it only
looks that way. For every known particle, there is a ``mirror
particle'' that interacts in a right-handed way. We haven't 
noticed these mirror particles because they don't interact with normal
particles, and vice versa. It's as if they're ignoring each other. If
Foot is right, there is an invisible mirror universe
occupying the same space as our Universe, complete with
mirror galaxies, perhaps even mirror life. Hey presto, an
explanation for dark matter just like that.

All this would be science fiction if it wasn't for baffling experiments
with orthopositronium at CERN, the European centre for particle
physics. This intriguingly named stuff consists
of an electron and a positron-the antimatter counterpart of an
electron-orbiting each other and each spinning in the same
direction. Puzzlingly, it decays 0.1 percent faster than
predicted. What's speeding up the loss? Foot claims that the
orthopositronium is changing, or ``oscillating'', into mirror
orthopositronium and then decaying in the mirror world before
it oscillates back.

Foot believes there is evidence of mirror stars, mirror planets,
mirror meteorites and mirror neutrinos. However, he
reins in his enthusiasm towards the end of this intriguing book,
admitting the possibility that mirror matter might
not exist at all. Quoting Bertrand Russell, he says:
``It's good to have an open mind, but not so open that you brains fall
out.''