# Antimatter

For the physics of antimatter, see the article on antiparticles.

Antimatter is matter that is composed of the antiparticles of those that constitute normal matter. If a particle and its antiparticle come in contact with each other, the two annihilate and produce a burst of energy, which results in the production of other particles and antiparticles or electromagnetic radiation. In these reactions, rest mass is not conserved, although (as in any other reaction) energy (E=mc²) is conserved.

## History

In 1928 Paul Dirac developed a relativistic equation for the electron, now known as the Dirac equation. Curiously, the equation was found to have negative energy solutions in addition to the normal positive ones. This presented a problem, as electrons tend toward the lowest possible energy level; energies of negative infinity are nonsensical. As a way of getting around this, Dirac proposed that the vacuum can be considered a "sea" of negative energy, the Dirac sea. Any electrons would therefore have to sit on top of the sea.

Thinking further, Dirac found that a "hole" in the sea would have a positive charge. At first he thought that this was the proton, but Hermann Weyl pointed out the hole should have the same mass as the electron. The existence of this particle, the positron, was confirmed experimentally in 1932 by Carl D. Anderson.

Today's standard model shows that every particle has an antiparticle, for which each additive quantum number has the negative of the value it has for the normal matter particle. The sign reversal applies only to quantum numbers (properties) which are additive, such as charge, but not to mass, for example. The positron has the opposite charge but the same mass as the electron. An atom of antihydrogen is composed of a negatively-charged antiproton being orbited by a positively-charged positron .

## Antimatter production

Scientists in 1995 succeeded in producing antiatoms of hydrogen, and also antideuteron nuclei, made out of an antiproton and an antineutron, but no antiatom more complex than antideuterium has been created yet. In principle, antiatoms of any element could be built from readily available sources of antiparticles. Such antiatoms would have exactly the same properties as their normal-matter counterparts. The production of antielements in bulk quantities seems unlikely to ever become achievable, however.

Positrons and antiprotons can individually be stored in a device called a Penning trap, which uses a combination of magnetic field and electric fields to hold charged particles in a vacuum. Two international collaborations, ATRAP and ATHENA, used these devices to store thousands of slowly moving antihydrogen atoms in 2002. It is the goal of these collaborations to probe the energy level structure of antihydrogen to compare it with that of hydrogen as a test of the CPT theorem. One way to do this is to confine the antiatoms in an inhomogenous magnetic field (one cannot use electric fields since the antiatoms are neutral) and interrogate them with lasers. If the anti-atoms have too much kinetic energy they will be able to escape the magnetic trap, and it is therefore essential that the anti-atoms are produced with as little energy as possible. This is the key difference between the antihydrogen that ATRAP and ATHENA produced, which was made at very low temperatures, and the antihydrogen produced in 1995 which was moving at a speed close to the speed of light.

Antimatter/matter reactions have practical applications in medical imaging, such as positron emission tomography (PET). In some kinds of beta decay, a nuclide loses surplus positive charge by emitting a positron (in the same event, a proton becomes a neutron, and neutrinos are also given off). Nuclides with surplus positive charge are easily made in a cyclotron and are widely generated for medical use.

Antiparticles are created everywhere in the universe where high-energy particle collisions take place, such as in the center of our galaxy, but none have been detected that are residual from the Big Bang, as most normal matter is [1]. The unequal distribution between matter and antimatter in the universe has long been a mystery. The solution likely lies in the violation of CP-symmetry by the laws of nature; see baryogenesis.

## Notation

Physicists need a notation to distinguish particles from antiparticles. One way is to denote an antiparticle by adding a bar (or macron) over the symbol for the particle. For example, the proton and antiproton are denoted as $\displaystyle \mathrm{p}\,$ and $\displaystyle \bar{\mathrm{p}}$ , respectively.

Another convention is to distinguish particles by their electric charge. Thus, the electron and positron are denoted simply as e and e+. Adding a bar over the e+ symbol would be redundant and is not done.

## Antimatter as fuel

In antimatter-matter collisions, the entire rest mass of the particles is converted to energy. The energy per unit mass is about 10 orders of magnitude greater than chemical energy, and about 2 orders of magnitude greater than nuclear energy that can be liberated today using chemical reactions or nuclear fission/fusion. The reaction of 1 kg of antimatter with 1 kg of matter would produce 1.8×1017 J (180 quadrillion Joules) of energy (by the equation E=mc²). In contrast, burning a kilogram of gasoline produces 4.2×107 J, and nuclear fusion of a kilogram of hydrogen would produce 2.6×1015 J. Not all of that energy can be utilized by any realistic technology, because as much as 50% of energy produced in reactions between nucleons and antinucleons is carried away by neutrinos, so, for all intents and purposes, it can be considered lost. [2]

The scarcity of antimatter means that it is not readily available to be used as fuel, although it could be used in antimatter catalyzed nuclear pulse propulsion. Generating a single antiproton is immensely difficult and requires particle accelerators and vast amounts of energy—millions of times more than is released after it is annihilated with ordinary matter, due to inefficiencies in the process. Known methods of producing antimatter from energy also produce an equal amount of normal matter, so the theoretical limit is that half of the input energy is converted to antimatter. Counterbalancing this, when antimatter annihilates with ordinary matter energy equal to twice the mass of the antimatter is liberated—so energy storage in the form of antimatter could (in theory) be 100% efficient. Antimatter production is currently very limited, but has been growing at a nearly geometric rate since the discovery of the first antiproton in 1955[3]. The current antimatter production rate is between 1 and 10 nanograms per year, and this is expected to increase dramatically with new facilities at CERN and Fermilab. With current technology, it is considered possible to attain antimatter for \$25 million per gram by optimizing the collision and collection parameters (given current electricity generation costs). Antimatter production costs, in mass production, are almost linearly tied in with electricity costs, so economical pure-antimatter thrust applications are unlikely to come online without the advent of such technologies as deuterium-deuterium fusion power. Several NASA Institute for Advanced Concepts-funded studies [4] are exploring whether the antimatter that occurs naturally in the Van Allen belts of Earth, and ultimately, the gas giants like Jupiter, might be able to be collected with magnetic scoops, at hopefully a lower cost per gram.

Since the energy density is vastly higher than these other forms, the thrust to weight equation used in antimatter rocketry and spacecraft would be very different. In fact, the energy in a few grams of antimatter is enough to transport an unmanned spacecraft to Mars in about a month—the Mars Global Surveyor took eleven months to reach Mars. It is hoped that antimatter could be used as fuel for interplanetary travel or possibly interstellar travel, but it is also feared that if humanity ever gets the capabilities to do so, there could be the construction of antimatter weapons.

## The Antiuniverse

Dirac himself was the first to consider the existence of antimatter in an astronomical scale. But it was only after the confirmation of his theory, with the discovery of the positron, antiproton and antineutron that real speculation began on the possible existence of an antiuniverse. In the following years, motivated by basic symmetry principles, it was believed that the universe must consist of both matter and antimatter in equal amounts. If, however there were an isolated system of antimatter in the universe, free from interaction with ordinary matter, no earthbound observation could distinguish its true content, as photons (being their own antiparticle) are the same whether they are in a “universe” or an “antiuniverse”.

But assuming large zones of antimatter exist, there must be some boundary where antimatter atoms from the antimatter galaxies or stars will come into contact with normal atoms. In those regions a powerful flux of gamma rays would be produced. This has never been observed despite deployment of very sensitive instruments in space to detect them.

It is now thought that symmetry was broken in the early universe when charge and parity symmetry was violated (CP-violation). Standard Big Bang cosmology tells us that the universe initially contained equal amounts of matter and antimatter: however particles and antiparticles evolved slightly differently. It was found that a particular heavy unstable particle, which is its own antiparticle, decays slightly more often to positrons (e+) than to electrons (e-). How this accounts for the preponderance of matter over antimatter has not been completely explained. The Standard Model of particle physics does have a way of accommodating a difference between the evolution of matter and antimatter, but it falls short of explaining the net excess of matter in the universe by about 10 orders of magnitude.

After Dirac, the sci-fi writers had a field day with visions of antiworlds, antistars and antiuniverses, all made of antimatter, and it is still a common plot device, however suppositions of the existence a coeval, antimatter duplicate of this universe are not taken seriously in modern cosmology.

## Antimatter in popular culture

A famous fictional example of antimatter in action is in the science fiction franchise Star Trek, where it is a common energy source for starships. Antimatter engines also appear in various books of the Dragonriders of Pern series by Anne McCaffrey. In Niven's Ringworld series, antimatter appears as a weapon useful against even the super-dense matter scrith. Dan Brown explores the use of antimatter as a weapon in his novel Angels and Demons, where terrorists threaten to destroy the Vatican with potentially unstable antimatter stolen from CERN. In The Night's Dawn Trilogy by Peter F. Hamilton, antimatter is characterized as the most dangerous substance imaginable and outlawed across the Galaxy. Antimatter is also briefly referenced in the 1966 movie "Batman," (several evil henchmen are turned into antimatter when they are revived using "heavy water" from the batcave), but the concept remains completely unexplained in this example. Also, in the episode of Doctor Who, "The Planet of Evil", the scientist Dr Sorenson is transformed into an 'antiman' due to exposure to antimatter.

In comic books produced by DC Comics, the notion of an antiuniverse, or in DC's parlance Anti-Matter Universe, was first utilized in the Green Lantern series in the 1960s. The Anti-Matter Universe contains a world known as Qward, home to the Green Lantern Corps' sworn enemies, the Weaponers of Qward.

In 1985, a powerful, twisted denizen of the Anti-Matter Universe known as the Anti-Monitor succeeded in destroying most of the DC Multiverse during the events of the twelve-issue limited series Crisis on Infinite Earths