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# Antimatter

Antimatter is matter that is composed of the antiparticles of those that constitute normal matter. In 1929-31, Paul Dirac put forward a theory that for each type of particle, there is 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. So, the antiparticle of the normal electron is called the positron, as it has a positive charge, but the same mass as the electron. An atom of antihydrogen, for instance, is composed of a negatively-charged antiproton being orbited by a positively-charged positron. Paul Dirac's theory has been experimentally verified and today a wide range of antiparticles have been detected. This is one of the few examples of a fundamental particle being predicted in theory and later discovered by experiment.

If a particle/antiparticle pair comes in contact with each other, the two annihilate and produce a burst of energy, which may manifest itself in the form of other particles and antiparticles or electromagnetic radiation. In these reactions, rest mass is not conserved, although (as in any other reaction), mass-energy is conserved.

Scientists in 1995 succeeded in producing anti-atoms of hydrogen, and also anti-deuteron nuclei, made out of an antiproton and an antineutron, but not yet more complex antimatter. In principle, sufficiently large quantities of antimatter could produce anti-nuclei of other elements, which would have exactly the same properties as their positive-matter counterparts. However, such a "periodic table of anti-elements" is thought to be, at best, highly unlikely, as the quantities of antimatter required would be, quite literally, astronomical.

Antiparticles are created elsewhere in the universe where there are high-energy particle collisions, 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 [2].

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 produce 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 anti-atoms in an inhomogenous magnetic field (one cannot use electric fields since the anti-atom is 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, see 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.

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## 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 p and $\bar{\mathrm{p}}$, 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 respectively. The reaction of 1 kg of antimatter with 1 kg of matter would produce 1.8×1017 J 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. [3]

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[4]. 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 billion per gram (roughly 1,000 times more costly than current space shuttle propellants) 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.

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 a small ship to the moon. 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.

## Antimatter in Popular Culture

The most famous fictional example of this kind of power source in action is in the science fiction franchise, Star Trek, where antimatter is a common energy source for starships. Antimatter as a weapon is also explored in the fictional work by author Dan Brown in the novel 'Angels and Demons'.