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 is conserved.
In particle physics, 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, a positron (the antiparticle of the electron or e+) and an antiproton (p) can form an antihydrogen atom in the same way that an electron and a proton form a normal matter hydrogen atom. Furthermore, mixing matter and antimatter can lead to the annihilation of both in the same way that mixing antiparticles and particles does, thus giving rise to high-energy photons (gamma rays) or other particleantiparticle pairs.
There is considerable speculation as to why the observable universe is apparently almost entirely matter, whether there exist other places that are almost entirely antimatter instead, and what might be possible if antimatter could be harnessed. 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 by which this asymmetry between particles and antiparticles developed is called baryogenesis.
IThe idea of negative matter has appeared in past theories of matter, theories which have now been abandoned. Using the once popular vortex theory of gravity the possibility of matter with negative gravity was discussed by William Hicks in the 1880s. Between the 1880s and the 1890s, Karl Pearson proposed the existence of "squirts" (sources) and sinks of the flow of aether. The squirts represented normal matter and the sinks represented negative matter, a term which Pearson is credited with coining. Pearson's theory required a fourth dimension for the aether to flow from and into.
The term antimatter was first used by Arthur Schuster in two rather whimsical letters to Nature in 1898,[2] in which he coined the term. He hypothesized antiatoms, whole antimatter solar systems and discussed the possibility of matter and antimatter annihilating each other. Schuster's ideas were not a serious theoretical proposal, merely speculation, and like the previous ideas, differed from the modern concept of antimatter in that it possessed negative gravity.
The modern theory of antimatter begins in 1928, with a paper by Paul Dirac. Dirac realised that his relativistic version of the Schrödinger wave equation for electrons predicted the possibility of antielectrons. These were discovered by Carl D. Anderson in 1932 and named positrons (a contraction of "positive electrons"). Although Dirac did not himself use the term antimatter, its use follows on naturally enough from antielectrons, antiprotons, etc. A complete periodic table of antimatter was envisaged by Charles Janet in 1929.
Scientists in 1995 succeeded in producing antiatoms of hydrogen, and also antideuterium 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 be 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.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 - baryogenesis.
Baryogenesis is the generic designation for the hypothetical physical processes that generated an asymmetry between baryons and anti-baryons in the very early universe.
Baryogenesis theories deal with different sub-fields of physics to describe the possible mechanisms for generating baryons. Most important are:
The fundamental difference between baryogenesis theories is the description of the interactions between fundamental particles.
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 belt around Earth discovered by Pamela craft BBC - August 7, 2011
A thin band of antimatter particles called antiprotons enveloping the Earth has been spotted for the first time. The find, described in Astrophysical Journal Letters, confirms theoretical work that predicted the Earth's magnetic field could trap antimatter. The team says a small number of antiprotons lie between the Van Allen belts of trapped "normal" matter. The researchers say there may be enough to implement a scheme using antimatter to fuel future spacecraft.
Fundamental matter-antimatter symmetry confirmed PhysOrg - July 28, 2011
International collaboration including MPQ scientists sets a new value for the antiproton mass relative to the electron with unprecedented precision.
Physicists Weigh Antimatter with Amazing Accuracy Live Science - July 28, 2011
A new measurement provides the most accurate weight yet of antimatter, revealing the mass of the antiproton (the proton's antiparticle) down to one part in a billion, researchers announced today (July 28). To give a sense of just how accurate their measurement was, researcher Masaki Hori said: "Imagine measuring the weight of the Eiffel Tower. The accuracy we've achieved here is roughly equivalent to making that measurement to within less than the weight of a sparrow perched on top. Next time it will be a feather."
Could the Big Bang have been a quick conversion of antimatter into matter? PhysOrg - July 19, 2011
Suppose at some point the universe ceases to expand, and instead begins collapsing in on itself (as in the Big Crunch scenario), and eventually becomes a supermassive black hole. The black hole¹s extreme mass produces an extremely strong gravitational field. Through a gravitational version of the so-called Schwinger mechanism, this gravitational field converts virtual particle-antiparticle pairs from the surrounding quantum vacuum into real particle-antiparticle pairs. If the black hole is made from matter (antimatter), it could violently repel billions and billions of antiparticles (particles) out into space in a fraction of a second, creating an ejection event that would look quite similar to a Big Bang.
Thunderstorms hurling antimatter into space caught by Fermi PhysOrg - January 11, 2011
Scientists using NASA's Fermi Gamma-ray Space Telescope have detected beams of antimatter produced above thunderstorms on Earth, a phenomenon never seen before.
Invention Traps Mysterious Antimatter Live Science - December 7, 2010
The trouble with studying antimatter is keeping it around without letting the odd substance come in contact with regular matter because if that happens, the two will destroy each other in an explosive annihilation. Now researchers at the European Organization for Nuclear Research (CERN) in Geneva have unveiled a new trap that they say can store a significant amount of antihydrogen atoms.
Colliding Particles Shed Light on Antimatter Mystery Live Science - August 20, 2010
Why We Exist: Matter Wins Battle Over Antimatter Space.com - May 20, 2010
New clue to anti-matter mystery BBC - May 19, 2010
A US-based physics experiment has found a clue as to why the world around us is composed of normal matter and not its shadowy opposite: anti-matter.
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