STRANGE lumps of matter on the outskirts of our Galaxy may
be stars from a bizarre "mirror sector" of the Universe, say physicists
in the US and Australia. Although these stars could be burning fiercely in
the prime of their lives, the laws of physics that govern them would make
them invisible to human eyes.
Such stars could also be home to mirror planets and maybe
even mirror organisms that have adapted to see their mirror world, yet are
blind to ours. "We don't know what these life forms would be like, but
it's a possibility," says Rabindra Mohapatra of the University of Maryland
at College Park.
The suggestion concerns more than a dozen so-called MACHOs,
clumps of matter that seem to be on the fringes of the Milky Way. Although
invisible, their gravity bends light from background stars, betraying their
presence. But no one knows what they're made of. With masses about half that
of the Sun, they're too heavy to be failed stars known as brown dwarfs. Ageing
stars called white dwarfs are a possible candidate. But astronomers see no
sign of the heavy elements that white dwarfs would have shed into space in
earlier phases of their lives.
Now Mohapatra and his colleague Vigdor Teplitz have suggested
that MACHOs are made of a weird kind of mirror matter generated in the big
bang. The idea of mirror matter, first suggested in the 1980s, is that every
particle in nature has an elusive, unseen partner. Mirror particles would
bolster theories of how the four forces in nature acted as one in the early
Universe.
Mirror matter would come with its own unique set of laws.
It would feel the force of ordinary gravity, and could clump into mirror stars
and planets. But its versions of the strong, weak and electromagnetic forces
would be different from those we know. Although mirror stars would burn through
nuclear fusion just like normal stars, they would not emit photons, so they
would be invisible.
Mohapatra and Teplitz say the case for a mirror sector is
backed by data from experiments that suggest the three known neutrinos can
"mix" and flip from one type to another. Some experiments hint that
the three particles are mixing with a fourth kind-perhaps from the mirror
world (New Scientist, 26 April 1997, p 20). Assuming that's true, the researchers
have used neutrino data to calculate the strength of forces in the mirror
world. From this, they predict that the maximum mass of a stable mirror star
would be about half a solar mass-just right to explain the MACHOs.
The physicists, who have submitted their work to Physical
Review Letters, say further experiments with neutrinos may help confirm their
theory. Robert Foot of the University of Melbourne, who has independently
come to similar conclusions, suggests another test. If a mirror star exploded,
it would emit a burst of neutrinos. This could be detected, but the "phantom"
explosion would be invisible.
"That's a possibility," says Mohapatra. However,
he says there are hints that nuclear reactions would burn stellar fuel faster
in the mirror world. It's possible that any mirror stars — even small ones
— died young and collapsed into black holes long ago.
"They may be correct," says Charles Alcock of the
Lawrence Livermore National Laboratory in California, one of a team that has
discovered MACHOs. However, he thinks the idea is speculative and difficult
to test. "I don't expect any of us, in our lifetimes, to be able to confirm
this."
Author: Hazel Muir
New Scientist magazine, issue 13th Feb. 1999
