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Antimatter is matter that is composed of the antiparticles of those that constitute normal matter. An atom of anti-hydrogen, for instance, is composed of a negatively-charged antiproton being orbited by a positively-charged positron. If a particle/antiparticle pair comes in contact with each other, the two annihilate and produce a burst of electromagnetic radiation.

Scientists in 1995 succeeded in producing anti-atoms of hydrogen, and also anti-deuteron nuclei, made out of an anti-proton plus an anti-neutron, but not yet more complex antimatter. 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 an asymmetry in the properties of B-mesons and anti-B-mesons [1].

Positrons and antiprotons can individually be stored in a device called a Penning trap, which uses a combination of magnetic and electric fields to hold the particles near the center of a vacuum. Traps for holding complete antihydrogen atoms require intense magnetic fields and very low temperatures for the antiparticles; the first such traps were developed by the ATRAP and ATHENA projects.

The symbol used to denote an antiparticle is the same symbol used to denote its normal matter counterpart, but with an overstrike. For example, a proton is denoted with a "p", and an antiproton is denoted by a "p" with a line over its top .

See also CPT theorem

Antimatter as fuel

In antimatter-matter collisions, the entire possible mass of the particles is converted to energy. This is much greater than the chemical energy or even nuclear energy that can be converted 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 of energy (by the equation E=mc2). In contrast, burning a kilogram of petrol produces 4.2×107 J, and nuclear fusion of a kilogram of hydrogen would produce 2.6×1015 J.

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. 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 dollars per gram (roughly 1,000 times the equivalent cost of the equivalent amount of energy in 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.

The most famous fictional example of this kind of power source in action is in the media franchise, Star Trek, where antimatter is a common energy source for starships.

See also: ambiplasma

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