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:
1. 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).
2. 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)).
3. 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).
1. Muir, Hazel. "There Could be Whole Worlds of Invisible Matter Out There," New Scientist, 17 (13 February 1999).