Mirror matter is a hypothetical form of matter, not to be confused with antimatter,
that would balance out the fact that ordinary matter has a slight left-hand
bias in its interactions at the subatomic level; in other words, mirror
matter would restore parity to the Universe. The idea that every particle
in nature has an elusive, unseen mirror partner was first put forward in
the 1980s. Then, in 1999, came the suggestion1 that a small number
of MACHOs, which had been detected on the outskirts
of our Galaxy, might be stars composed of this exotic stuff.
Mirror matter would be subject to its own distinct set of physical laws.
Although it would feel gravity in the ordinary way and therefore be able
to condense into mirror stars and planets, its versions of the three other
basic forces – electromagnetism and the strong and weak forces –
would be different. One consequence is that mirror stars would be invisible
because they would not emit electromagnetic radiation.
The only way their presence could become known to us is through their gravitational microlensing effects and their subsequent
identification as MACHOs. Theoretical considerations suggest that the maximum
mass of a stable mirror star would be about 0.5 solar mass – just
right to explain the dozen or so MACHOs that have been singled out as candidates.
Some researchers, notably Robert Foot (see external site below) at the University
of Melbourne, Australia, have gone further and suggested that the presence
of mirror matter in the Solar System might explain the Tunguska
event, possible anomalous movements of the deep space probes Pioneer
10 and 11, and even some unusual material at the bottom of crater floors
on Eros. The possibility of mirror
stars and planets opens up the bizarre possibility of mirror organisms which
would be completely invisible to us (and we to them).
In response to my question (originally supplied by Dan Handlin) "Are mirror
matter particles in any way related to supersymmetry partner particles?" Robert Foot kindly answered as follows:
There's really no relation. Mirror symmetry is a different type of symmetry
to supersymmetry. The only similarity is that both ideas require a "doubling"
of the number of elementary particles. (in mirror symmetry, the mirror
particles form an almost decoupled sector - similar to ordinary particles
but where left and right are interchanged). Mirror symmetry is a discrete
symmetry (i.e. not a continuous symmetry), which allows this type of theory
to exhibit space-reflection as a symmetry, while supersymmetry is nothing
to do with space-reflection, but is a continuous symmetry relating particles
with different spin: each ordinary particle has a hypothetical superpartner.
However supersymmetry must be broken because if it was unbroken the SUSY
particles would have been discovered already). Nevertheless, supersymmetry
is very popular, but there really is no evidence for it (despite multi-billion
dollar searches for it!!). It survives only because it is popular. As
you know, mirror symmetry is not so popular but I like to think there
is a lot of evidence for it – certainly more than for supersymmetry.
If I can give you an example: both theories claim to provide an explanation
for dark matter, but I would argue that the mirror symmetry explanation
is the more natural. Why? Because it explains the basic properties of
dark matter. Mirror particles couple extremely weakly to photons, so mirror
matter is dark. mirror atoms are also stable for the same reason that
ordinary ones are. In other words, with the one hypothesis, mirror symmetry,
one predicts the existence of invisible stable matter in the Universe.
The abundance is not predicted but this depends on initial conditions
and early evolution of the universe. SUSY, on the other hand, requires
3 hypothesis to "explain" the dark matter:
- SUSY exists. (Actually it has to be broken symmetry because constraints
from experiments rule out SUSY particles if they have the same mass
as their partners).
- To make a SUSY particle stable it is hypothesized that "r-parity"
exists. This is an ad hoc discrete symmetry which is independent of
supersymmetry (i.e. hypothesis (1)).
- SUSY with r-parity, has the lightest SUSY particle being stable,
and thus may be the dark matter. But, there are many SUSY particles,
so one must hypothesise that the lightest one will be a neutral particle
(to explain the darkness of dark matter).
Clearly, the basic properties of dark matter (dark and stable) are not
explained by SUSY but motivate hypothesis (2) & (3).
- Muir, Hazel. "There Could be Whole Worlds of Invisible Matter Out
There," New Scientist, 17 (February 13, 1999).
Matter on the asteroid Eros?