It has been conventional wisdom that the fundamental laws of
physics are not invariant under parity. A small minority of
physicists, however, have taken a different view. They have
argued that a so-called mirror world could exist. Nature would
then be symmetric under parity. Their so-called exact parity
model predicts the existence of so-called ''mirror matter''. Each
particle is postulated to have a mirror partner with similar
properties (they behave exactly as the mirror image of their
partners). This is thus similar to anti-matter, the main
difference is that mirror particles and ordinary particles only
have very weak interactions, otherwise they would have been
detected already.
Mirror particles could thus act as dark matter. Because
mirror matter has similar properties as ordinary matter, you
could have mirror stars, galaxies, planets etc. Note that mirror
stars would be invisible because they would emit mirror photons,
which don't interact with ordinary electrons (to be precise there
could be a very weak interaction, see below).
Besides gravity, there are other ways that mirror matter could
interact with ordinary matter. E.g. a term like in the Lagrangian, where F (F') is the (mirror)
electromagnetic field tensor, gives every charged mirror particle
an effective ordinary charge that is epsilon times as small.
Epsilon would have to be smaller than about 10^(-6). This bound arises from experiments
measuring the lifetime of orthopositronium.
Positronium is a bound state consisting of an electron
and a positron. Positronium with total spin 1 is called
othopositronium. A nonzero value for epsilon would
cause the eigenstates of the Hamiltonian to be linear
combinations of orthopositronium and mirror orthopositronium. So,
if you start at t = 0 with orthopositronium, part of it will have
oscillated into mirror orthopositronium at a later time t > 0.
Once orthopositronium has oscillated into
mirror orthopositronium it has effectively disappeared, because it
will subsequently decay into three invisible mirror photons. Experiments measuring
the decay rate of orthopositronium should thus measure a faster decay rate than predicted.
However, it makes a difference if the experiment is performed in vacuum or
in some other kind of medium. In a medium containing, say, gas,
the frequent collisions between orthopositronium and the gas
molecules will inhibit the oscillation of orthopositronium into
mirror orthopositronium. This effect is known as the quantum Zeno
effect. A nonzero value for epsilon thus manifest itself in a shorter
lifetime of orthopositronium in vacuum, while leaving the lifetime in a medium unaffected.
If epsilon is larger than about 10^(-9), a small amount of mirror matter in our solar system would have observable consequences. Interestingly, there are a number of anomalies that could be caused by mirror matter. The anomalous slowing down of the pioneer spacecraft could be caused by a drag force exerted by mirror gas or mirror dust. Another anomaly that could be caused by mirror matter has been observed on the asteroid 433 Eros. It was observed that this asteroid has far fewer craters smaller than 70 meters than had been expected. This can be explained if there are a lot of small mirror matter spacebodies in our solar system. Unlike ordinary objects, a mirror matter object colliding with a body will only leave a crater provided it is large enough. This is because the mirror matter object will penetrate some distance into the body and thus its kinetic energy will be released more slowly than in the case of an ordinary object. If the mirror matter object is larger than this ''penetration depth'', a crater will be formed. The lack of craters smaller than 70 meters suggests that epsilon is between 10^(-7) and 10^(-9). Note that on larger bodies, such as the Moon, the dominant source of small craters are secondary impacts. On such bodies the size distribution of craters should show the usual behavior. Still the crater rates on the Moon does suggest the presence of mirror matter space bodies in our solar system.
A recent sky survey detected far fewer potential earth
crossing asteroids than had been expected according to earlier
estimates by the late Shoemaker. He arrived at a much higher
estimate by studying the cratering record on the Moon. Finally there are a number of observations of anomalous meteoritic events that could be caused by impacting mirror matter objects.
in the Lagrangian, where F (F') is the (mirror)
electromagnetic field tensor, gives every charged mirror particle
an effective ordinary charge that is epsilon times as small.
Epsilon would have to be smaller than about 10^(-6). This bound arises from experiments
measuring the lifetime of orthopositronium.
Positronium is a bound state consisting of an electron
and a positron. Positronium with total spin 1 is called
othopositronium. A nonzero value for epsilon would
cause the eigenstates of the Hamiltonian to be linear
combinations of orthopositronium and mirror orthopositronium. So,
if you start at t = 0 with orthopositronium, part of it will have
oscillated into mirror orthopositronium at a later time t > 0.
Once orthopositronium has oscillated into
mirror orthopositronium it has effectively disappeared, because it
will subsequently decay into three invisible mirror photons. Experiments measuring
the decay rate of orthopositronium should thus measure a faster decay rate than predicted.
However, it makes a difference if the experiment is performed in vacuum or
in some other kind of medium. In a medium containing, say, gas,
the frequent collisions between orthopositronium and the gas
molecules will inhibit the oscillation of orthopositronium into
mirror orthopositronium. This effect is known as the quantum Zeno
effect. A nonzero value for epsilon thus manifest itself in a shorter
lifetime of orthopositronium in vacuum, while leaving the lifetime in a medium unaffected.