Physicists have long been embarrassed by the fact that most of the universe is made up of so-called dark matter - matter that we cannot see but has a gravitationally-evident presence. Many candidates for the nature of this matter have been proposed, and one serious contender could involve the universe having a mirror image of itself.

This refers to the particle physics theory of mirror matter, also known as shadow matter, or 'Alice' matter, and it actually attempts to solve a whole host of major cosmological issues, as shall be discussed in this entry.

The concept of 'symmetry' is a cherished feature of the cosmos among physicists. This refers to the mirror and rotational symmetry of particles and particle interactions, both in time and space. However, in 1956 it was discovered that there was a distinct violation of mirror symmetry in the universe: sub-atomic fundamental particles called neutrinos only ever 'spin' in a left-handed direction. Tsung Dao Lee and Chen Ning Yang, two Chinese-American researchers, proposed that each particle that was known to physicists so far might have a mirror 'twin' that is entirely invisible and beyond detection, but restores the mirror symmetry. However, the idea was largely forgotten.

The Nature of Mirror Matter

Over thirty years later, Robert Foot of the University of Melbourne and Sergei Gninenko of CERN¹ spurred a renewed interest in the mirror matter conjecture. Each type of fundamental particle has a 'twin' in their theory, where ordinary photons, for example, can interact with ordinary electrons, but never with mirror electrons or mirror photons. Likewise, the mirror particles can only interact with each other. Right-handed neutrinos exist in the mirror universe, with the same mass as a left-handed neutrino.

The reason mirror matter is invisible is because objects are only seen when photons, the particles associated with light, reflect off them and into our eyes; however, mirror matter uses mirror photons, and our eyes are made of ordinary matter. The two are incompatible. The same is true with all of the other forces. The strong nuclear force is carried by 'gluons', but the mirror force is carried by 'mirror gluons' meaning that they are also incompatible. Thus, mirror matter is also undetectable, to a certain extent.

All particles, however, both mirror and ordinary, feel the force of gravity. This is because gravity is an inherent part of the fabric of space-time, as Albert Einstein proved. Gravity is not exchanged through particles like the other forces, but is caused by 'warps' and 'ripples' in space that curve it into a fourth unseen dimension, in a similar way to how snooker balls on a rubber sheet curve it downwards.

Because of the way mirror particles still exert gravitational pulls, they are a perfect candidate for dark matter. Mirror matter is not the same as anti-matter, not least because mirror matter is invisible and undetectable by its nature; you could walk straight through a field of mirror photons and never know it.

Mirror matter solves the symmetry-breaking problem because it provides a natural mirror image of everything. Foot uses the analogy of a person with one arm. To restore the symmetry, you can remove the other arm, add a new arm, or imagine an identical twin that has one arm but on the other side of his/her body. The same could be true of sub-atomic particles like the neutrino that break the symmetry, whereby their twin is made of mirror matter.

Evidence for Mirror Matter

One of the reasons why Foot and Gninenko sparked research into mirror matter again was a curious experiment performed in 1990 by physicists at the University of Michigan, Ann Arbor. The experiment was an attempt at measuring the life-time of ortho-positronium. Ortho-positronium is a pseudo-atom made from a positron and electron attracted by their electromagnetic pull. It is expected to decay in 1.42*10^-10 seconds, but the experiment proved that it was 0.1% faster than this.

In March 2000, the experiments were repeated by Gregory Adkins of Franklin and Marshall College in Pennsylvania, Richard Fell of Brandeis University in Massachusetts and Jonathan Sapirstein of Notre Dame University in Indiana. The discrepancy in the life-time was still observed.

In 1985, Bob Holdom of the University of Toronto claimed that there may be an unknown force that links ordinary matter with mirror matter. The reason is that physicists believe the universe is actually very simple, and that the fundamental forces should be facets of one force, or 'superforce' that ties everything we know about particles together. Holdom believed the same unification should be true of matter and mirror matter.

The carrier of the Holdom force, or 'H particle' would be very weak because otherwise its effects would have been measured. Mirror particles and ordinary particles would be able to change into each other via use of an H particle. But what is the evidence for this ever occurring?

Such evidence has been found at the Sudbury Neutrino Observatory (SNO) in Canada. Neutrinos are elementary particles that exist in three types called 'flavours': muon neutrinos, tau neutrinos and electron neutrinos. Studies at the SNO seem to suggest the validity of 'neutrino oscillations' where neutrinos can change their flavour. To test this, beams of neutrinos produced in particle accelerators are monitored to see if other flavours are present in the beam, showing that the neutrinos have oscillated. In 1996, a beam purely of muon neutrinos was monitored, and twenty-two electron neutrinos were found in it.

Furthermore, evidence was found that the number of muon neutrinos in cosmic-ray interactions in the atmosphere is not enough, suggesting that they are oscillating into other flavours. Foot believes that they could be oscillating into their mirror counterparts. To do this, an ordinary particle may 'spit' out an H particle to become a mirror particle, while mirror particles gain H particles to become ordinary matter.

There exists a theory called the 'conservation of electric charge' stating that energy can never be created or destroyed. So, amounts put into a reaction are equal on their output. Therefore, if an electron spat out an H particle to become a mirror electron, it would lose its charge because mirror electrons have a mirror charge instead. This is forbidden. Thus, interactions between mirror and ordinary matter can only ever exist between particles with no electric charge. One such particle with zero electric charge is the photon. So mirror photons are permitted to converse with ordinary photons.

Now, the Heisenberg Uncertainty Principle is another cherished feature of quantum physics, and it is this, along with the Holdom Force, that may prove the existence of mirror matter through the life-time discrepancy in ortho-positronium. The uncertainty principle is the theory allowing particles in a quantum state to change their identity as long as they are not observed (this causes what is known as 'decoherence' where the particle in its state of 'superposition²' chooses what state it is in via a quantum wave function, determined by Erwin Schrödinger's equations). If a particle changes into another particle that has more energy, then this energy must be repaid quickly before decoherence occurs. The particle in question is called a 'virtual particle' because it only exists for a short time.

An Analogy

Here is how it all fits in: imagine that you are desperate to drive to your friend's house in the middle of the night to warn them of something. The only way that you can do this is by taking your dad's car, which has more momentum than you. Therefore, you must return the car home before your dad notices. You borrow the car from a convenient quantum vacuum and travel to your friend's house. However, before you return again to replace the car, there is a small chance that you will take your friend's dad's car back, because it looks exactly the same. Strangely, this is precisely what you do. The trouble is, on your journey back, the car breaks down, and falls into three parts.

You are acting like the ortho-positronium, which desperately wants to turn into a photon. As pointed out by Sheldon Glashow of Harvard University in 1986, this means that ortho-positronium is very sensitive to the mirror world because photons can feel the Holdom force. When you switch cars, this is analogous to an H particle being spat out from the photon to turn it into a mirror photon. In reality, the mirror photon changes into a mirror electron and a mirror positron, which spin around each other to produce mirror ortho-positronium. Also, the particles keep changing from ordinary ortho-positronium to mirror ortho-positronium every 3*10^-10 seconds. Midway between this oscillation (or while on the road), the substance is half ortho-positronium and half mirror ortho-positronium. It is at this point that the chance arises for it to decay into the three mirror photons (or the car breaking down and falling apart). These mirror photons are undetectable, and effectively disappear into the mirror world. It is this that could be deduced as a shorter life-time of the ortho-positronium.

In short:

1. Starts as ortho-positronium
2. Borrows the momentum of a virtual photon
3. Spits out an H particle
4. Becomes a mirror photon
5. Decays into a mirror electron and a mirror positron
6. Becomes mirror ortho-positronium
7. Borrows momentum of a virtual mirror photon
8. Gains an H particle
9. Becomes a photon
10. Decays into an electron and positron
11. Becomes ortho-positronium
12. Repeats from step two for a random time
13. When in a superposition of ortho-positronium and mirror ortho-positronium, it decays into three mirror photons, which are from then on undetectable by non-mirror physicists.

The ultimate test for this is to observe the decay in an empty vacuum. A single particle of matter interfering with the test matter could cause decoherence, after all. Indeed, this happened in 1995 when physicists at the University of Tokyo did not observe the life-time discrepancy. Gninenko has proposed a new experiment that monitors the energy of a vacuum containing ortho-positronium. Missing energy is a signature of its disappearance into mirror matter.

Cosmic Mysteries that Mirror Matter Could Solve

Additionally, Foot proposes that mirror particles may play a part in the problem of exoplanets. Over a hundred planets beyond our solar system have been found by testing 'wobbles' in stars that show they are being influenced by planets. All of these planets are believed to be Jupiter-sized gas planets, but they cannot be observed directly because they are too far away and therefore too faint. However, they all appear to be very close to their parent stars. One, for example, is eight times closer to its sun than Mercury is to our sun.

Foot says that perhaps solar systems form that are mostly ordinary matter, but a small percentage is mirror matter. Because this mirror matter does not interact with the other matter, it is drawn by the gravity of the central star, and clumps together to form a 'mirror gas planet' close to the star.

This speculation incurs a further conclusion of the mirror matter theory. Since mirror particles have the same capabilities as normal particles, this means that they may form larger objects similar to the ones we observe in ordinary experience. Therefore, mirror matter could merge to form mirror planets and stars and an entire mirror universe! Mirror asteroids could be floating around space, and interestingly, they could collide with Earth without our noticing.

Foot refers to the Tunguska event of 1908, where a giant explosion occurred above a region of Siberia, devastating 2000 miles worth of forestry. However, what makes the Tunguska event so strange is that nobody has ever found the remains of a body that could have collided with the Earth, and there does not appear to be any chemical traces in the area either. The culprit of the incident could have been a mirror asteroid or a mirror comet. In this case, the air molecules would go straight through the mirror body, heating it up and causing it to melt at low altitudes and create a blast wave. The fragments of the mirror body would still be present in the area, but undetectable because of their mirror nature. Ray Volkas, a colleague of Foot's, says that testing the mass, volume and density of samples from the region may prove the mirror matter's existence.

Mirror stars may also be proved by looking for mirror supernovae. Supernovae emit neutrinos, so mirror supernovae emit mirror neutrinos, which could, in theory, oscillate into ordinary neutrinos. Places such as the SNO could detect this burst of neutrinos, as a neutrino detector did in 1987, which proved to be from an ordinary supernova ³.

In the same way that mirror planets may orbit ordinary stars, the reverse may be true. We would observe 'isolated' planets in space, seemingly orbiting nothing that was in fact a mirror star. Such a planet was found in the Sigma Orionis cluster in 2000. Periodic changes in their light, or 'Doppler shifts', would reveal that they are moving planets, thus proving that they are affected by an invisible object.

It has even been speculated that extra-terrestrials have never been observed because they exist as mirror matter. There could even be mirror people walking on the Earth with us. Robert Foot openly admits that he has a 'vivid imagination'.

Creation of Mirror Matter

It is thought that this type of matter formed in the very early universe, when gravity 'split off' from the other forces of the 'superforce', just after the Planck time. There is no reason why there should be equal quantities of mirror matter and ordinary matter, which is why the dark matter of the universe could be accounted for by mirror matter even though there is more dark than light matter. The trouble with these theories is that they are extremely vague, which is why Robert Foot has been able to explain so many cosmic mysteries with it.

The highly-valued superstring theory is a theory that hopes to unify the forces of the universe. Superstring theory incorporates a theory of supersymmetry, and it seems that there may only be one solution to these equations; one that produces a looking glass universe.

Further Information

• Full details of the Mirror Matter theory.

• An Entry on Quantum Mechanics.

• The Standard Model of Particle Physics Entry could also be useful.

• Further Reading: Shadowlands: The Quest for Mirror Matter in the Universe by Robert Foot himself.