Most charged particles have corresponding anti-particles: a positron is the anti-matter counterpart to the electron; they possess an equal but opposite charge. For the proton, the anti-matter counterpart is called an anti-proton. Most particles are indistinguishable, apart from their opposite charges, from their anti-particles.

The first element of the anti-periodic table containing anti-matter was made for the first time in 1995. Nine atoms of anti-hydrogen, consisting of an anti-electron bound to an anti-proton, were made by colliding a beam of xenon atoms into a beam of anti-protons within an accelerator. The anti-hydrogen atoms produced were travelling so fast that they were annihilated after coming into contact with ordinary matter within 30 nano seconds. Matter and anti-matter both have positive mass, therefore both will be gravitationally attractive, not repulsive.

Apart from it being made of anti-particles, anti-hydrogen is expected to have properties indistinguishable from ordinary hydrogen. It will be a great test of quantum theory to try to find any difference at all between hydrogen and its anti-matter counterpart. If anti-matter atoms can be slowed sufficiently to be caught in a magneto-optic trap, then laser Doppler cooling could further reduce the thermal velocities to enable precise measurements to be taken.

There is a huge in-balance between the plentiful matter and the scarce anti-matter in the Universe so there must be some difference between the two. And indeed there is; in 2004 experimenters have found a difference in the decay rate of B-mesons and the anti-B-meson. The difference in decay rates of these two particles is about 70% and is a 100,000 times greater difference than that seen for any other particle/anti-particle pair, but is still nowhere near great enough to explain why the anti-matter created in the big bang disappeared before it could interact with the matter created at the same time, thereby preventing the mutual annihilation of both which would have resulted in a huge flash leaving the Universe inhabited by just photons [if that is indeed what happened; others would have us believe that matter was created in the Big Bang in slightly greater proportions than was anti-matter, and that after all the anti-matter was annihilated by most of the matter, a residual amount of matter remained: the present Universe]. The B-mesons contain bottom quarks, which belong to the third generation of quarks.

The annihilation of matter with anti-matter is the most energy efficient process known, converting 100% of the mass into energy, by way of the formula E=mc2. This is much more efficient at converting mass to energy than the fusion of deuterium to produce helium-4, which converts just 0.6% of the mass into energy. Scotties' matter/antimatter reactor is a considerable way off yet, as a way has to be found firstly to produce enough anti-matter, secondly to make it stationary, and thirdly, to contain it without it coming into contact with ordinary matter. Present production techniques require a great deal more energy to create anti-matter than will be released when it annihilates with ordinary matter, so a way will need to be found for creating it efficiently. Preliminary investigations with the reaction of anti-matter with matter suggest that, after the initial annihilation of most atoms, a small proportion seems able to co-exist without annihilation, which is puzzling.

Positrons annihilate with electrons only when the two are on a head-on collision, the likelihood of annihilation being increased if the two are travelling at the same speed, with the same kinetic energy, as then they do not need to find some means of dissipating the excess kinetic energy. At other angles of incidence either they don't annihilate, or, rarely, they bond together forming positronium. The interaction of positrons with ordinary matter (consisting of neutral atoms) involves some surprises. For many molecules, and aromatic compounds in particular, the annihilation cross section (the probability of annihilation) is up to a million times larger than that between isolated electrons and positrons, and for anthracene molecules, it is 10 million times higher. Anthracene has just a few hundred electrons, insufficient alone to account for the 10 million fold annihilation probability enhancement. The positrons were, it turns out, forming novel bonds with the molecule, orbiting as if they were bound like electrons are, the large number of orbits thus increasing the probability of interaction with an electron. Also, the multiplicity of vibrational energy states in a large molecule gives increased opportunity for the kinetic energy difference between the electron and positron to be dissipated, which also increases the likelihood of annihilation. For instance, positrons with a kinetic energy of 0.3keV are much more likely to annihilate with a molecule of butane because butane has a vibrational energy state close to 0.3keV, where the kinetic energy of the in-coming positron can be dumped.

Calculations of the interaction of a positron with a lithium atom reveal that the positron quantumly binds with the atom before the inevitable annihilation perhaps one nanosecond later, after many trillions of orbits. Positrons also avoid the Pauli Exculsion Principle; a positron with exactly the same energy levels and quantum numbers as one of the orbiting electrons can co-exist in orbit around an atom, increasing the stability and binding energy of such a combination. Such composite atoms could get involved in extremely un-usual and revolutionary chemistry, and may provide pathways for the synthesis of novel compounds. A stream of positrons is being emitted by some object near the centre of our galaxy, the Milky Way, and it may be involved in such exotic chemistry as it strikes a nearby cold molecular cloud.

Shown is a hydrogen atom and its anti-matter counterpart, anti-hydrogen.