Antimatter

**Antimatter** is defined as matter composed of the antiparticles (or "partners") of the corresponding particles in "ordinary" matter. A particle and its antiparticle (for example, a proton and an antiproton) have the same mass, but opposite electric charge, and other differences in quantum numbers. Minuscule numbers of antiparticles are generated daily in natural processes like cosmic ray collisions and some types of radioactive decay.

A collision between any particle and its anti-particle partner leads to their mutual annihilation, giving rise to various proportions of intense photons (gamma rays), neutrinos, and sometimes less-massive particle–antiparticle pairs. The majority of the total energy of annihilation emerges in the form of ionizing radiation. If surrounding matter is present, the energy content of this radiation will be absorbed and converted into other forms of energy, such as heat or light.

Notoriously difficult to mine and even more difficult to produce, primarily due to the associated cost, antimatter production is nonetheless common throughout the Settled Systems; the antimatter industry is one of the most lucrative scientific and engineering fields in history despite the immense cost. It is utilized in a variety of fields - from fuel, to power generation, to medicine - and is required for the reaction that produces HZQ, the fuel that enables FTL travel. Research has also been carried out into antimatter weapons, although development of such devices is officially outlawed in both the United Colonies and the Herschel Nations.

## Antiparticles

### Antiproton

The antiproton is the antiparticle of the proton. Antiprotons are stable, but they are typically short-lived, since any collision with a proton will cause both particles to be annihilated in a burst of energy.

### Positron

The positron is the antiparticle of the electron. It has an electric charge of +1 *e*, a spin of 1/2 (the same as the electron), and the same mass as an electron.

### Antielements

Like matter, antimatter particles can be bound together to create "antiatoms" and thus "antielements", although such endeavours are a difficult process. The antielements of the first twenty-six elements have been synthesized - from antihydrogen all the way up to antiiron - and many have similar properties to their ordinary matter counterparts. However, only antihydrogen is widely produced, and is used as fuel in pion rocket engines.

## Annihilation

Annihilation is the process that occurs when a subatomic particle collides with its respective antiparticle to produce other particles, such as an electron colliding with a positron to produce two photons. The total energy and momentum of the initial pair are conserved in the process and distributed among a set of other particles in the final state. Annihilation is essential in the modern world for powering space stations, roemer drives and other forms of technology with high energy demands.

### Proton–antiproton annihilation

When a proton encounters an antiproton, the reaction is not as simple as electron–positron annihilation. Unlike an electron, a proton is a composite particle consisting of three "valence quarks" and an indeterminate number of "sea quarks" bound by gluons. Thus, when a proton encounters an antiproton, one of its quarks, usually a constituent valence quark, may annihilate with an antiquark to produce a gluon, after which the gluon, together with the remaining quarks, antiquarks, and gluons, will undergo a complex process of rearrangement into a number of mesons (mostly pions and kaons) that will share the total energy and momentum.

The newly created mesons are unstable and, unless they encounter and interact with some other material, will decay in a series of reactions that ultimately produce only photons, electrons, positrons, and neutrinos. In a pion rocket engine, the charged pions are directed by a magnetic nozzle to the rear of the starship, generating immense thrust; as a result, a heavy ship can take off and achieve orbit under its own power without having to shed weight. This type of reaction will occur between any baryon (particle consisting of three quarks) and any antibaryon consisting of three antiquarks, one of which corresponds to a quark in the baryon.

## Uses

### Fuel

Isolated and stored antimatter is widely used as a fuel for interplanetary or interstellar travel, either as part of an antimatter-catalyzed nuclear pulse propulsion or an antimatter rocket engine. Since the energy density of antimatter is higher than that of conventional fuels, antimatter-fueled spacecraft have a higher thrust-to-weight ratio than a conventional spacecraft. The reaction of 1 kg of antimatter with 1 kg of matter produces 1.8×1017 J (180 petajoules) of energy, or the rough equivalent of 43 megatons of TNT.

However, not all of that energy is utilized because of the nature of the annihilation products. While electron–positron reactions result in gamma ray photons, these are difficult to direct and use for thrust. In reactions between protons and antiprotons, their energy is converted largely into relativistic neutral and charged pions. The neutral pions decay almost immediately (with a lifetime of 85 attoseconds) into high-energy photons, but the charged pions decay more slowly (with a lifetime of 26 nanoseconds) and can be deflected magnetically to produce thrust; this is the process that occurs inside a pion rocket engine.

### Medicine

Matter–antimatter reactions have practical applications in medical imaging, such as positron emission tomography (PET). In positive beta decay, a nuclide loses surplus positive charge by emitting a positron (in the same event, a proton becomes a neutron, and a neutrino is also emitted). Nuclides with surplus positive charge are easily made in a cyclotron and are widely generated for medical use. Antiprotons have also been shown within laboratory experiments to have the potential to treat certain cancers.

### Weapons

Since its production became commonplace in the 22nd century, antimatter has been researched as the propellant, power source or explosive for a weapon of mass destruction. The paramount advantage of such a weapon is that antimatter and matter collisions result in the entire sum of their mass energy equivalent being released as energy, which is at least two orders of magnitude greater than the energy release of the most efficient fusion weapons (100% vs 0.4-1%).

Active development of antimatter weapons began during the Herschel Wars by the Free Republics, which sought to build a weapon that could swiftly end the war and secure their independence from the United Colonies. Since their weapons technology was years behind the Herschel Nations, the U.C. had no choice but to stand down and recognise the H.N.'s independence rather than risk a devastating escalation. Further development of antimatter weapons was outlawed as part of the U.C.-H.N. peace agreement and both sides officially ended their research; this, however, did not stop clandestine efforts by both sides to build such a device.

The first deployment of an antimatter weapon was by the Hanseatic League, which used such a device to destroy Luna in an attack called "The Lesson". The attack triggered the Great Galactic War and the resumption of public development of antimatter weapons by the U.C. The final time they were deployed as near the end of the war during the Bombardment of Laconia, after which antimatter weapons were outlawed once again.