antimatter n : matter consisting of elementary particles that are the antiparticles of those making up normal matter

English

Pronunciation

• /ˈæntimæˌtɚ/, /ˈæn.taɪ.mæˌtɚ/

Noun

antimatter

1. Matter that is composed of the antiparticles of those that constitute normal matter.

Translations

• Catalan: antimatèria
• Croatian: antimaterija
• French: antimatière
• German: Antimaterie
• Greek: αντιύλη
• Icelandic: andefni
• Spanish: antimateria

Adjective

antimatter (no comparative or superlative)

1. Consisting of antimatter.

In particle physics and quantum chemistry, antimatter is the extension of the concept of the antiparticle to matter, where antimatter is composed of antiparticles in the same way that normal matter is composed of particles. For example an antielectron (a positron, an electron with a positive charge) and an antiproton (a proton with a negative charge) could form an antihydrogen atom in the same way that an electron and a proton form a normal matter hydrogen atom. Furthermore, mixing of matter and antimatter would lead to the annihilation of both in the same way that mixing of antiparticles and particles does, thus giving rise to high-energy photons (gamma rays) or other particle–antiparticle pairs. The particles resulting from matter-antimatter annihilation are endowed with energy equal to the difference between the rest mass of the products of the annihilation and the rest mass of the original matter-antimatter pair, which is often quite large.

There is considerable speculation both in science and science fiction as to why the observable universe is apparently almost entirely matter, whether other places are almost entirely antimatter instead, and what might be possible if antimatter could be harnessed, but at this time the apparent asymmetry of matter and antimatter in the visible universe is one of the greatest unsolved problems in physics. The process developing particles and antiparticles is called baryogenesis.

History

In December 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 is filled with a "sea" of negative-energy electrons, the Dirac sea. Any real electrons would therefore have to sit on top of the sea, having positive energy.

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 that 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. During this period, antimatter was sometimes also known as "contraterrene matter".

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. For particles whose additive quantum numbers are all zero, the particle may be its own antiparticle; such particles include the photon and the neutral pion.

Notation

One way to denote an antiparticle is by adding a bar (or macron) over the particle's symbol. For example, the proton and antiproton are denoted as p and p, respectively. The same rule applies if you were to address a particle by its constituent components. A proton is made up of u u d quarks, so an antiproton must therefore be formed from u u d antiquarks. Another convention is to distinguish particles by their electric charge. Thus, the electron and positron are denoted simply as e− and e+ respectively.

Origin (Naturally occurring production)

Antimatter could be produced together with matter at the creation of our Universe.

Asymmetry

Most objects, observable from the Earth, seem to be built of matter rather than antimatter. There is no current reasoning over why matter prevailed over antimatter, but many believe it was the result of asymmetry, and some scientists believe that the ratio of this asymmetry was 1,000,000 antimatter particles to 1,000,001 matter particles. Antiparticles are created everywhere in the universe where high-energy particle collisions take place. High-energy cosmic rays impacting Earth's atmosphere (or any other matter in the solar system) produce minute quantities of antimatter in the resulting particle jets, which are immediately annihilated by contact with nearby matter. It may similarly be produced in regions like the center of the Milky Way Galaxy and other galaxies, where very energetic celestial events occur (principally the interaction of relativistic jets with the interstellar medium). The presence of the resulting antimatter is detectable by the gamma rays produced when positrons annihilate with nearby matter. The gamma rays' frequency and wavelength indicate that each carries 511 keV of energy (i.e. the rest mass of an electron or positron multiplied by c2). Recent observations by the European Space Agency’s INTEGRAL (International Gamma-Ray Astrophysics Laboratory) satellite may explain the origin of a giant cloud of antimatter surrounding the galactic center. The observations show that the cloud is asymmetrical and matches the pattern of X-ray binaries, binary star systems containing black holes or neutron stars, mostly on one side of the galactic center. While the mechanism is not fully understood it is likely to involve the production of electron-positron pairs as ordinary matter gains tremendous energy while falling into a stellar remnant.

Antihelium

The Balloon-borne Experiment with Superconducting Spectrometer (BESS) is searching for larger antinuclei, in particular antihelium, that are very unlikely to be produced by collisions. (One of the current experiments, under assumptions of current theory, would take 15 billion years on average to encounter a single antihelium atom made that way.)

Antihelium Isotope, ³He was created.

Artificial production

Antiparticles are also produced in any environment with a sufficiently high temperature (mean particle energy greater than the pair production threshold). The period of baryogenesis, when the universe was extremely hot and dense, matter and antimatter were continually produced and annihilated. The presence of remaining matter, and absence of detectable remaining antimatter, also called baryon asymmetry, is attributed to violation of the CP-symmetry relating matter and antimatter. The exact mechanism of this violation during baryogenesis remains a mystery.

Positrons are also produced via the radioactive beta+ decay, but this mechanism can be considered as "natural" as well as "artificial".

Antihydrogen

The artificial production of atoms of antimatter (specifically antihydrogen) first became a reality in the early 1990s. An atom of antihydrogen comprises a negatively-charged antiproton being orbited by a positively-charged positron. Stanley Brodsky, Ivan Schmidt and Charles Munger at SLAC realized that an antiproton, traveling at relativistic speeds and passing close to the nucleus of an atom, would have the potential to force the creation of an electron-positron pair. It was postulated that under this scenario the antiproton would have a small chance of pairing with the positron (ejecting the electron) to form an antihydrogen atom.

In 1995 CERN announced that it had successfully created nine antihydrogen atoms by implementing the SLAC/Fermilab concept during the PS210 experiment. The experiment was performed using the Low Energy Antiproton Ring (LEAR), and was led by Walter Oelert and Mario Macri. Fermilab soon confirmed the CERN findings by producing approximately 100 antihydrogen atoms at their facilities.

The antihydrogen atoms created during PS210, and subsequent experiments (at both CERN and Fermilab) were extremely energetic ("hot") and were not well suited to study. To resolve this hurdle, and to gain a better understanding of antihydrogen, two collaborations were formed in the late 1990s — ATHENA and ATRAP. In 2005, ATHENA disbanded and some of the former members (along with others) formed the ALPHA Collaboration, which is also situated at CERN. The primary goal of these collaborations is the creation of less energetic ("cold") antihydrogen, better suited to study.

In 1999 CERN activated the Antiproton Decelerator, a device capable of decelerating antiprotons from 3.5 GeV to 5.3 MeV — still too "hot" to produce study-effective antihydrogen, but a huge leap forward. In late 2002 the ATHENA project announced that they had created the world's first "cold" antihydrogen. The antiprotons used in the experiment were cooled sufficiently by decelerating them (using the Antiproton Decelerator), passing them through a thin sheet of foil, and finally capturing them in a Penning trap. The antiprotons also underwent stochastic cooling at several stages during the process.

The ATHENA team's antiproton cooling process is effective, but highly inefficient. Approximately 25 million antiprotons leave the Antiproton Decelerator; roughly 10 thousand make it to the Penning trap. In early 2004 ATHENA researchers released data on a new method of creating low-energy antihydrogen. The technique involves slowing antiprotons using the Antiproton Decelerator, and injecting them into a Penning trap (specifically a Penning-Malmberg trap). Once trapped the antiprotons are mixed with electrons that have been cooled to an energy potential significantly less than the antiprotons; the resulting Coulomb collisions cool the antiprotons while warming the electrons until the particles reach an equilibrium of approximately 4 K.

While the antiprotons are being cooled in the first trap, a small cloud of positron plasma is injected into a second trap (the mixing trap). Exciting the resonance of the mixing trap’s confinement fields can control the temperature of the positron plasma; but the procedure is more effective when the plasma is in thermal equilibrium with the trap’s environment. The positron plasma cloud is generated in a positron accumulator prior to injection; the source of the positrons is usually radioactive sodium.

Once the antiprotons are sufficiently cooled, the antiproton-electron mixture is transferred into the mixing trap (containing the positrons). The electrons are subsequently removed by a series of fast pulses in the mixing trap's electrical field. When the antiprotons reach the positron plasma further Coulomb collisions occur, resulting in further cooling of the antiprotons. When the positrons and antiprotons approach thermal equilibrium antihydrogen atoms begin to form. Being electrically neutral the antihydrogen atoms are not affected by the trap and can leave the confinement fields.

Using this method ATHENA researchers predict they will be able to create up to 100 antihydrogen atoms per operational second. ATHENA and ATRAP are now seeking to further cool the antihydrogen atoms by subjecting them to an inhomogeneous field. While antihydrogen atoms are electrically neutral, their spin produces magnetic moments. These magnetic moments vary depending on the spin direction of the atom, and can be deflected by inhomogeneous fields regardless of electrical charge.

The biggest limiting factor in the production of antimatter is the availability of antiprotons. Recent data released by CERN states that when fully operational their facilities are capable of producing 107 antiprotons per second. Assuming an optimal conversion of antiprotons to antihydrogen, it would take two billion years to produce 1 gram of antihydrogen (approximately 6.02×1023 atoms of antihydrogen.) Another limiting factor to antimatter production is storage. As stated above there is no known way to effectively store antihydrogen. The ATHENA project has managed to keep antihydrogen atoms from annihilation for tens of seconds — just enough time to briefly study their behaviour.

Hydrogen atoms are simplest objects, that can be considered as "matter" rather than as just particles. Simultaneous trapping of antiprotons and antielectrons was reported and the cooling is achieved; there are patents on the way of production of antihydrogen. In spite of this progress, the confinment time is not yet long, and the antimatter is not yet available at the market.

Preservation

Antimatter cannot be stored in a container made of ordinary matter because antimatter reacts with any matter it touches, annihilating itself and the container. Antimatter that is composed of charged particles can be contained by a combination of an electric field and a magnetic field in a device known as a Penning trap. This device cannot, however, contain antimatter that consists of uncharged particles, and atomic traps are used. In particular, such a trap may use the dipole moment (electrical or magnetic) of the trapped particles; at high vacuum, the matter or anti-matter particles can be trapped (suspended) and cooled with slightly off-resonant laser radiation (see, for, example, magneto-optical trap and Magnetic trap). Small particles can be also suspended by just intensive optical beam in the optical tweezers.

Cost

Antimatter is said to be the most expensive substance in existence, with an estimated cost of $300 billion per milligram. This is because production is difficult (only a few atoms are produced in reactions in particle accelerators), and because there is higher demand for the other uses of particle accelerators. According to CERN, it has cost a few hundred million Swiss Francs to produce about 1 billionth of a gram.

Several NASA Institute for Advanced Concepts-funded studies are exploring whether it might be possible to use magnetic scoops to collect the antimatter that occurs naturally in the Van Allen belts of Earth, and ultimately, the belts of gas giants like Jupiter, hopefully at a lower cost per gram.

Uses

Medical

Antimatter-matter 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 neutrinos are also given off). Nuclides with surplus positive charge are easily made in a cyclotron and are widely generated for medical use.

Fuel

In antimatter-matter collisions resulting in photon emission, the entire rest mass of the particles is converted to kinetic energy. The energy per unit mass (9×1016 J/kg) is about 10 orders of magnitude greater than chemical energy (compared to TNT at 4.2×106 J/kg, and formation of water at 1.56×107 J/kg), about 4 orders of magnitude greater than nuclear energy that can be liberated today using nuclear fission (about 40 MeV per 238U nucleus transmuted to Lead, or 1.5×1013 J/kg), and about 2 orders of magnitude greater than the best possible from fusion (about 6.3×1014 J/kg for the proton-proton chain). The reaction of 1 kg of antimatter with 1 kg of matter would produce 1.8×1017 J (180 petajoules) of energy (by the mass-energy equivalence formula E = mc²), or the rough equivalent of 47 megatons of TNT. For comparison, Tsar Bomba, the largest nuclear weapon ever detonated produced an estimated 57 mt and was capable of over 100mt, but utilized hundreds of kg's of fissile material.

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.

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 to between 3 and 30 nanograms per year by 2015 or 2020 with new superconducting linear accelerator facilities at CERN and Fermilab. Some researchers claim that with current technology, it is possible to obtain antimatter for US$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-tritium fusion power (assuming that such a power source actually would prove to be cheap). Many experts, however, dispute these claims as being far too optimistic by many orders of magnitude. They point out that in 2004; the annual production of antiprotons at CERN was several picograms at a cost of $20 million. This means to produce 1 gram of antimatter, CERN would need to spend 100 quadrillion dollars and run the antimatter factory for 100 billion years. Storage is another problem, as antiprotons are negatively charged and repel against each other, so that they cannot be concentrated in a small volume. Plasma oscillations in the charged cloud of antiprotons can cause instabilities that drive antiprotons out of the storage trap. For these reasons, to date only a few million antiprotons have been stored simultaneously in a magnetic trap, which corresponds to much less than a femtogram. Antihydrogen atoms or molecules are neutral so in principle they do not suffer the plasma problems of antiprotons described above. But cold antihydrogen is far more difficult to produce than antiprotons, and so far not a single antihydrogen atom has been trapped in a magnetic field.

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 a few minutes, 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, as a side-effect of antimatter propulsion, the design of antimatter weapons might become an equal reality.

One researcher of the CERN laboratories, which produces antimatter on a regular basis, said: Until now, the use of anti-matter as a source of energy is mentioned more often in fiction, than in technological projects.

Antimatter in fiction

Existence of anti-particles is more "sci-" than "fiction" . However, until now, the anti-world, or even macroscopic amounts of antimatter exist rather in jokes and sci-fi novels, than in laboratories. One of these novels being Dan Brown's "Angels and Demons" in which the whole story centers around production of antimatter at the CERN facility and its possible use in terrorism because of the awesome power of an annihilation . While the book may be a stretch, the description of antimatter and its production included in the book are interesting.

Pop culture

The 1960s hit television show The Man from U.N.C.L.E. dealt with the potential of antimatter in an episode called The Suburbia Affair. Pianist and comedian Victor Borge played a pianist and scientist who had come up with a formula for antimatter and, fearing its destructive potential, hid in a bizarre suburban development populated by single adults and couples who hate children. Borge's character, Dr. Rutter, disguised his formula in a dissonant piece of music until forced to reveal it to the evil organization, THRUSH (Technological Hierarchy for the Removal of Undesirables and Subjugation of Humanity). The good guys, Illya Kuryakin and Napoleon Solo, arrive to save the day and, after a plea from the wounded Rutter, destroy the computer where the antimatter formula has been stored. In another 20 years, Rutter warns, someone else will come up with the formula.

Military

Because of its potential to release immense amounts of energy in contact with normal matter, there has been interest in various weapon uses, potentially enabling miniature warheads of pinhead-size to be more destructive than modern-day nuclear weapons. An antimatter particle colliding with a matter particle releases 100% of the energy contained within the particles, while a hydrogen bomb only releases about 0.7% of this energy. This gives a clue to how effective and powerful this force is. However, this development is still in early planning stages, though antimatter weapons are popular in science fiction such as in Peter F. Hamilton's Night's Dawn Trilogy , Dan Brown's Angels and Demons and Star Trek where the production of antimatter leads to the possibility of use as both a fuel and highly effective weapon. At the moment, the traps are not very efficient, and

it is more constructive to just create all the antimatter at the moment it would be used.

Stanisław Lem

In the novel The Cyberiad, Stanisław Lem describes the building up the antimatter in the following way:

• The machine, however, had already begun. First it manufactured antiprotons, then antielectrons, antineutrons, antineutrinos, and labored on, until from out of all this antimatter an antiworld took shape, ..

In another novel, The Invincible , the researchers fail to fight the self-organizing microrobots, even though they use antimatter as a weapon. In the novel Eden , humans use the antimatter as weapon, but it does not help them to understand anything about the civilization they met.

Isaac Asimov

In stories by Isaac Asimov (1940s), mankind creates a new generation of robots with "positronic brains" as complex as those of humans. .

See also

• Ambiplasma
• Particle accelerator
• Antiparticle
• antihydrogen

References

External links

• Links to top sites on AntiMatter
• Freeview Video 'Antimatter' by the Vega Science Trust and the BBC/OU
• CERN Webcasts (Realplayer required)
• What is Antimatter? (from the Frequently Asked Questions at the Center for Antimatter-Matter Studies)
• FAQ from CERN with lots of information about antimatter aimed at the general reader, posted in response to antimatter's fictional portrayal in Angels and Demons
• What is direct CP-violation?''
• Animated illustration of antihydrogen production at CERN from the Exploratorium.
• Mystery of antimatter source solved - Maybe