STRANGE lumps of matter on the outskirts of our Galaxy may be stars from a
bizarre 鈥渕irror sector鈥� 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.
鈥淲e don鈥檛 know what these life forms would be like, but it鈥檚 a possibility,鈥�
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鈥檙e made of. With masses about half that of the Sun, they鈥檙e 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.
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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 鈥渕ix鈥� and flip from one
type to another. Some experiments hint that the three particles are mixing with
a fourth kind鈥攑erhaps from the mirror world
(快猫短视频, 26 April 1997, p 20).
Assuming that鈥檚 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鈥攋ust 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 鈥減hantom鈥�
explosion would be invisible.
鈥淭hat鈥檚 a possibility,鈥� says Mohapatra. However, he says there are hints that
nuclear reactions would burn stellar fuel faster in the mirror world. It鈥檚
possible that any mirror stars鈥攅ven small ones鈥攄ied young and
collapsed into black holes long ago.
鈥淭hey may be correct,鈥� 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. 鈥淚 don鈥檛 expect any of us,
in our lifetimes, to be able to confirm this.鈥�
