Antimatter
In modern physics, antimatter is defined as matter composed of the antiparticles of the corresponding particles in "ordinary" matter, and can be thought of as matter with reversed charge and parity, or going backward in time. Antimatter occurs in natural processes like cosmic ray collisions and some types of radioactive decay, but only a tiny fraction of these have successfully been bound together in experiments to form antiatoms. Minuscule numbers of antiparticles can be generated at particle accelerators, but total artificial production has been only a few nanograms. No macroscopic amount of antimatter has ever been assembled due to the extreme cost and difficulty of production and handling. Nonetheless, antimatter is an essential component of widely available applications related to beta decay, such as positron emission tomography, radiation therapy, and industrial imaging.
In theory, a particle and its antiparticle have the same mass, but opposite electric charge, and other differences in quantum numbers.
A collision between any particle and its anti-particle partner leads to their mutual annihilation, giving rise to various proportions of intense photons, neutrinos, and sometimes less-massive particleantiparticle pairs. The majority of the total energy of annihilation emerges in the form of ionizing radiation. If surrounding matter is present, the energy content of this radiation will be absorbed and converted into other forms of energy, such as heat or light. The amount of energy released is usually proportional to the total mass of the collided matter and antimatter, in accordance with the mass–energy equivalence equation,.
Antiparticles bind with each other to form antimatter, just as ordinary particles bind to form normal matter. For example, a positron and an antiproton can form an antihydrogen atom. The nuclei of antihelium have been artificially produced, albeit with difficulty, and are the most complex anti-nuclei so far observed. Physical principles indicate that complex antimatter atomic nuclei are possible, as well as anti-atoms corresponding to the known chemical elements.
There is strong evidence that the observable universe is composed almost entirely of ordinary matter, as opposed to an equal mixture of matter and antimatter. This asymmetry of matter and antimatter in the visible universe is one of the great unsolved problems in physics. The process by which this inequality between matter and antimatter particles is hypothesised to have occurred is called baryogenesis.
Definitions
Antimatter particles carry the same charge as matter particles, but of opposite sign. That is, an antiproton is negatively charged and an antielectron is positively charged. Neutrons do not carry a net charge, but their constituent quarks do. Protons and neutrons have a baryon number of +1, while antiprotons and antineutrons have a baryon number of –1. Similarly, electrons have a lepton number of +1, while that of positrons is –1. When a particle and its corresponding antiparticle collide, they are both converted into energy.The French term for "made of or pertaining to antimatter",, led to the initialism "C.T." and the science fiction term, as used in such novels as Seetee Ship.
History
Conceptual
The idea of negative matter appears in past theories of matter that 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" and sinks of the flow of aether. The squirts represented normal matter and the sinks represented negative matter. 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, in which he coined the term. He hypothesized antiatoms, as well as 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 began 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. Although Dirac had laid the groundwork for the existence of these "antielectrons" he initially failed to pick up on the implications contained within his own equation. He freely gave the credit for that insight to J. Robert Oppenheimer, whose seminal paper "On the Theory of Electrons and Protons" drew on Dirac's equation and argued for the existence of a positively charged electron, which as a counterpart to the electron should have the same mass as the electron itself. This meant that it could not be, as Dirac had in fact suggested, a proton. Dirac further postulated the existence of antimatter in a 1931 paper which referred to the positron as an "anti-electron". These were discovered by Carl D. Anderson in 1932 and named positrons from "positive electron". Although Dirac did not himself use the term antimatter, its use follows on naturally enough from antielectrons, antiprotons, etc.
The Feynman–Stueckelberg interpretation states that antiparticles obey the same equations of motion as regular particles but with charge, parity and time inverted. This allows analysis of antiparticles using Feynman diagrams with time reversed.
Particle discoveries
After Carl Anderson's discovery of the positron in 1932, it was 22 years before the next antimatter particle was found.In 1955 Chamberlain, Segrè, Wiegand, Ypsilantis announced in a publication the discovery of the antiproton, in 1956 Cork, Lambertson, Piccioni, Wenzel discovered the antineutron; both using the Bevatron in Berkeley, California in 1954 via Ernest Lawrence.
Notation
One way to denote an antiparticle is by adding a bar over the particle's symbol. For example, the proton and antiproton are denoted as and, respectively. The same rule applies if one were to address a particle by its constituent components. A proton is made up of quarks, so an antiproton must therefore be formed from antiquarks. Another convention is to distinguish particles by positive and negative electric charge. Thus, the electron and positron are denoted simply as and respectively. To prevent confusion, however, the two conventions are never mixed.Properties
There is no difference in the gravitational behavior of matter and antimatter. In other words, antimatter falls down when dropped, not up. This was confirmed with the thin, very cold gas of thousands of antihydrogen atoms that were confined in a vertical shaft surrounded by superconducting electromagnetic coils. These can create a magnetic bottle to keep the antimatter from coming into contact with matter and annihilating. The researchers then gradually weakened the magnetic fields and detected the antiatoms using two sensors as they escaped and annihilated. Most of the anti-atoms came out of the bottom opening, and only one-quarter out of the top.There are compelling theoretical reasons to believe that, aside from the fact that antiparticles have different signs on all charges, matter and antimatter have exactly the same properties. This means a particle and its corresponding antiparticle must have identical masses and decay lifetimes. It also implies that, for example, a star made up of antimatter will shine just like an ordinary star. This idea was tested experimentally in 2016 by the ALPHA experiment, which measured the transition between the two lowest energy states of antihydrogen. The results, which are identical to that of hydrogen, confirmed the validity of quantum mechanics for antimatter.
Origin and asymmetry
Most things observable from the Earth seem to be made of matter rather than antimatter. If antimatter-dominated regions of space existed, the gamma rays produced in annihilation reactions along the boundary between matter and antimatter regions would be detectable.Antiparticles are created everywhere in the universe where high-energy particle collisions take place. High-energy cosmic rays striking Earth's atmosphere produce minute quantities of antiparticles in the resulting particle jets, which are immediately annihilated by contact with nearby matter. They may similarly be produced in regions like the center of the Milky Way and other galaxies, where very energetic celestial events occur. The presence of the resulting antimatter is detectable by the two gamma rays produced every time positrons annihilate with nearby matter. The frequency and wavelength of the gamma rays indicate that each carries 511 keV of energy.
Observations by the European Space Agency's INTEGRAL satellite may explain the origin of a giant antimatter cloud surrounding the Galactic Center. The observations show that the cloud is asymmetrical and matches the pattern of X-ray binaries, 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 kinetic energy while falling into a stellar remnant.
Antimatter may exist in relatively large amounts in far-away galaxies due to cosmic inflation in the primordial time of the universe. Antimatter galaxies, if they exist, are expected to have the same chemistry and absorption and emission spectra as normal-matter galaxies, and their astronomical objects would be observationally identical, making them difficult to distinguish. NASA is trying to determine if such galaxies exist by looking for X-ray and gamma ray signatures of annihilation events in colliding superclusters.
In October 2017, scientists working on the BASE experiment at CERN reported a measurement of the antiproton magnetic moment to a precision of 1.5 parts per billion. It is consistent with the most precise measurement of the proton magnetic moment, which supports the hypothesis of CPT symmetry. This measurement represents the first time that a property of antimatter is known more precisely than the equivalent property in matter.
Antimatter quantum interferometry has been first demonstrated in 2018 in the Positron Laboratory of Rafael Ferragut in Como, by a group led by Marco Giammarchi.