Antimatter

For the physics of antimatter, see the article on antiparticles; for the Cubanate album, see Antimatter (album); for the band, see Antimatter (band)

Antimatter or contra-terrene matter 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

Artificial production

The artificial production of antimatter (specifically antihydrogen) first became a reality in the early 1990s. Charles Munger of the SLAC, and associates at Fermilab, realised that an antiproton, travelling 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 9 antihydrogen atoms by implementing the SLAC/Fermilab concept during the PS210 experiment. The experiment was preformed using the Low-Energy Antiproton Ring (LEAR), and was lead 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. 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/c 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 worlds 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 traps 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 effected by the trap and can leave the confinement fields.

Using this method ATHENA researchers predict they will be able to create 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 (up or down) 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 $10^{7}$ antiprotons per second. Assuming an optimal conversion of antiprotons to antihydrogen (which is far from true) it would take two billion years (give or take a few thousand) to produce 1 gram 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 10s of seconds - just enough time to briefly study their behaviour.

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.

Naturally occurring production

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 is immediately destroyed by contact with nearby matter. It may similarly be produced in regions like the center of the Milky Way Galaxy, where very energetic celestial events occur. The presence of the resulting antimatter is detected by the gamma rays produced when it annihilates with nearby matter.

Antiparticles are also produced in any environment with a sufficiently high temperature (mean particle energy greater than the pair production threshold). The region of space near a black hole's event horizon can be thought of as being such an environment, with the resulting matter and antimatter being a component of Hawking radiation. During 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 detection of remaining antimatter^[1], is attributed to violation of the CP-symmetry relating matter and antimatter. The exact mechanism of this violation during baryogenesis remains a mystery.

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 $\mathrm {p} \,$ 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 nuclear fission/fusion. The reaction of 1 kg of antimatter with 1 kg of matter would produce 1.8×10¹⁷ J (180 petajoules) 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 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. However, it should be noted 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 million trillion dollars and run the antimatter factory for 100 billion years.

Several NASA Institute for Advanced Concepts-funded studies are exploring whether the antimatter that occurs naturally in the Van Allen belts of Earth, and ultimately, the belts of gas giants like Jupiter, might be able to be collected with magnetic scoops, at hopefully a lower cost per gram.^[4]

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.

See also: What is direct CP-violation?

Antimatter in popular culture

The extremely large amount of energy released by matter/antimatter annihilation has inspired many appearances in fiction:

• 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; large reactors generate power by mixing supercooled deuterium and antideuterium, with the annihilation reaction regulated by dilithium crystals. It is also used as a weapon, as in photon torpedoes.
• 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 briefly referenced in the 1966 movie Batman: The Movie, (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.
• In the episode of Doctor Who, "The Planet of Evil," the scientist Dr. Sorenson is transformed into an 'antiman' due to exposure to antimatter.
• Late in The Rocky Horror Picture Show, Riff confirms to Dr. Furter that the pitchfork-like weapon he has pointed at him is "a laser, capable of emitting a beam of pure antimatter." This misuse of the term led to the audience-response line, "Then it's not a laser."
• 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 the City of Heroes comic book, the superhero Positron is capable of generating anti-matter, and utilizing it as a weapon.
• 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.
• The Protoss race of Starcraft uses antimatter for both propulsion and weaponry.

See also

• Gravitational interaction of antimatter
• Elementary particle
• Positron

References

• Tipler, Paul (2002). Modern Physics (4th ed. ed.). W. H. Freeman. ISBN 0716743450. {{cite book}}: |edition= has extra text (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)

Footnotes

1. ^ ""What's the Matter with Antimatter?". NASA Science News. 2000. Retrieved January 3. {{cite web}}: Check date values in: |accessdate= (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
2. ^ Stanley K. Borowski (1987). "Comparison of Fusion/Antiproton Propulsion Systems for Interplanetary Travel" (PDF). National Aeronautics and Space Administration. Retrieved December 7. {{cite web}}: Check date values in: |accessdate= (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
3. ^ Tyler Freeman (2003). "The History of Antimatter". Antimatter: The Science Fact. Retrieved December 7. {{cite web}}: Check date values in: |accessdate= (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
4. ^ Jim Bickford. "Extraction of Antiparticles in Planetary Magnetic Fields" (PDF). NASA Institute for Advanced Concepts. Retrieved December 7. {{cite web}}: Check date values in: |accessdate= (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)

External links and references

• CERN Webcasts (Realplayer required)
• What is Antimatter? (from the Frequently Asked Questions at the Center for Antimatter-Matter Studies)
• Some interesting FAQs from CERN that contain lots of information about antimatter aimed at the general reader