Inmodern physics,antimatter is defined asmatter composed of theantiparticles (or "partners") of the correspondingparticles in "ordinary" matter, and can be thought of as matter with reversed charge and parity, or going backward in time (seeCPT symmetry). Antimatter occurs in natural processes likecosmic ray collisions and some types ofradioactive 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 atparticle accelerators, but total artificial production has been only a fewnanograms.[1] Nomacroscopic 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 tobeta decay, such aspositron emission tomography,radiation therapy,[2] and industrial imaging.
A collision between any particle and its anti-particle partner leads to their mutualannihilation, giving rise to various proportions of intensephotons (gamma rays),neutrinos, and sometimes less-massive particle–antiparticle pairs. The majority of the total energy of annihilation emerges in the form ofionizing 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 themass–energy equivalence equation,E=mc2.[3]
Antiparticles bind with each other to form antimatter, just as ordinary particles bind to form normal matter. For example, apositron (the antiparticle of theelectron) and an antiproton (the antiparticle of the proton) can form anantihydrogen atom. Thenuclei ofantihelium have been artificially produced, albeit with difficulty, and are the most complex anti-nuclei so far observed.[4] 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 theobservable universe is composed almost entirely of ordinary matter, as opposed to an equal mixture of matter and antimatter.[5] Thisasymmetry of matter and antimatter in the visible universe is one of the greatunsolved problems in physics.[6] The process by which this inequality between matter and antimatter particles is hypothesised to have occurred is calledbaryogenesis.
Antimatter particles carry the same charge as matter particles, but of opposite sign. That is, an antiproton is negatively charged and an antielectron (positron) is positively charged. Neutrons do not carry a net charge, but their constituentquarks do. Protons and neutrons have abaryon number of +1, while antiprotons and antineutrons have a baryon number of –1. Similarly, electrons have alepton number of +1, while that of positrons is –1. When a particle and its corresponding antiparticle collide, they are both converted into energy.[7][8][9]
TheFrench term for "made of or pertaining to antimatter",contraterrene, led to the initialism "C.T." and the science fiction termseetee,[10] as used in such novels asSeetee Ship.[11]
The idea ofnegative matter appears in past theories of matter that have now been abandoned. Using the once popularvortex theory of gravity, the possibility of matter with negative gravity was discussed byWilliam Hicks in the 1880s. Between the 1880s and the 1890s,Karl Pearson proposed the existence of "squirts"[12] and sinks of the flow ofaether. 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.[13]
The term antimatter was first used byArthur Schuster in two rather whimsical letters toNature in 1898,[14] 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 possessednegative gravity.[15]
The modern theory of antimatter began in 1928, with a paper[16] byPaul Dirac. Dirac realised that hisrelativistic version of theSchrödinger wave equation for electrons predicted the possibility ofantielectrons. 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 toJ. Robert Oppenheimer, whose seminal paper "On the Theory of Electrons and Protons" (Feb 14th 1930) drew on Dirac's equation and argued for the existence of a positively charged electron (a positron), 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".[17][18] These were discovered byCarl D. Anderson in 1932 and namedpositrons from "positive electron". Although Dirac did not himself use the term antimatter, its use follows on naturally enough from antielectrons, antiprotons, etc.[19]
The discovery of antimatter lead to both scientific and science fiction speculations on astronomical antimatter bodies:antiplanets, antistars, andantigalaxies.Leon Lederman atColumbia University even speculated anti-planets might harbor intellegent life. The antigalaxy idea was also used to explain thebaryon asymmetry problem: the particle physics of theBig Bang model cannot explain why our universe has more matter than antimatter.Maurice Goldhaber suggested that our universe consists of matter rather than antimatter because the one universe of each type split at the beginning of time. By the 1970s the asymmetry problem was accepted as unexplained and the possibility of antigalaxies largely abandoned.[23]
One way to denote anantiparticle is by adding a bar over the particle's symbol. For example, the proton and antiproton are denoted asp andp, respectively. The same rule applies if one were to address a particle by its constituent components. A proton is made up ofuudquarks, so an antiproton must therefore be formed fromuudantiquarks. Another convention is to distinguish particles by positive and negativeelectric charge. Thus, the electron and positron are denoted simply ase− ande+ respectively. To prevent confusion, however, the two conventions are never mixed.
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 ofantihydrogen atoms that were confined in a vertical shaft surrounded by superconducting electromagnetic coils. These can create amagnetic 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.[24]
There are compelling theoretical reasons to believe that, aside from the fact that antiparticles have different signs on all charges (such as electric and baryon charges), matter and antimatter have exactly the same properties.[25][26] This means a particle and its corresponding antiparticle must have identical masses and decay lifetimes (if unstable). It also implies that, for example, a star made up of antimatter (an "antistar") will shine just like an ordinary star.[27] This idea was tested experimentally in 2016 by theALPHA experiment, which measured the transition between the two lowest energy states ofantihydrogen. The results, which are identical to that of hydrogen, confirmed the validity of quantum mechanics for antimatter.[28][29]
A video showing how scientists used the Fermi Gamma ray Space Telescope's gamma ray detector to uncover bursts of antimatter from thunderstorms
There are some 500 terrestrialgamma ray flashes daily. The red dots show those spotted by theFermi Gamma-ray Space Telescope in 2010. The blue areas indicate where potential lightning can occur for terrestrialgamma ray flashes.
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.[30]
Antiparticles are created everywhere in theuniverse where high-energy particle collisions take place. High-energycosmic rays strikingEarth's atmosphere (or any other matter in theSolar System) produce minute quantities of antiparticles in the resultingparticle jets, which are immediately annihilated by contact with nearby matter. They may similarly be produced in regions like thecenter of theMilky Way and other galaxies, where very energetic celestial events occur (principally the interaction ofrelativistic jets with theinterstellar medium). The presence of the resulting antimatter is detectable by the twogamma rays produced every timepositrons annihilate with nearby matter. Thefrequency andwavelength of the gamma rays indicate that each carries 511 keV of energy (that is, therest mass of anelectron multiplied byc2).
Observations by theEuropean Space Agency'sINTEGRAL 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 ofX-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 kinetic energy while falling into astellar remnant.[31][32]
Antimatter may exist in relatively large amounts in far-away galaxies due tocosmic inflation in the primordial time of the universe. Antimatter galaxies, if they exist, are expected to have the same chemistry andabsorption and emission spectra as normal-matter galaxies, and theirastronomical objects would be observationally identical, making them difficult to distinguish.[33]NASA is trying to determine if such galaxies exist by looking for X-ray and gamma ray signatures of annihilation events incollidingsuperclusters.[34]
In October 2017, scientists working on theBASE experiment atCERN reported a measurement of the antiprotonmagnetic moment to a precision of 1.5 parts per billion.[35][36] It is consistent with the most precise measurement of the proton magnetic moment (also made by BASE in 2014), which supports the hypothesis ofCPT 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 (L-NESS) of Rafael Ferragut inComo (Italy), by a group led by Marco Giammarchi.[37]
Positrons are produced naturally in β+ decays of naturally occurring radioactive isotopes (for example,potassium-40) and in interactions of gamma quanta (emitted by radioactive nuclei) with matter.Antineutrinos are another kind of antiparticle created by natural radioactivity (β− decay). Many different kinds of antiparticles are also produced by (and contained in)cosmic rays. In January 2011, research by theAmerican Astronomical Society discovered antimatter (positrons) originating abovethunderstorm clouds; positrons are produced in terrestrial gamma ray flashes created by electrons accelerated by strong electric fields in the clouds.[38][39]Antiprotons have also been found to exist in theVan Allen Belts around the Earth by thePAMELA module,[40][41] and similar antiproton belts may exist around Jupiter, Saturn, Neptune, and Uranus.[42][43]
Antiparticles are also produced in any environment with a sufficiently high temperature (mean particle energy greater than thepair production threshold). It is hypothesized that 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 detectable remaining antimatter,[44] is calledbaryon asymmetry. The exact mechanism that produced this asymmetry during baryogenesis remains an unsolved problem. One of thenecessary conditions for this asymmetry is theviolation of CP symmetry, which has been experimentally observed in theweak interaction.
Recent observations indicate black holes and neutron stars produce vast amounts of positron-electron plasma via the jets.[45][46]
Satellite experiments have found evidence ofpositrons and a few antiprotons in primary cosmic rays, amounting to less than 1% of the particles in primary cosmic rays. This antimatter cannot all have been created in the Big Bang, but is instead attributed to have been produced by cyclic processes at high energies. For instance, electron-positron pairs may be formed inpulsars, as a magnetized neutron star rotation cycle shears electron-positron pairs from the star surface. Therein the antimatter forms a wind that crashes upon the ejecta of the progenitor supernovae. This weathering takes place as "the cold, magnetized relativistic wind launched by the star hits the non-relativistically expanding ejecta, a shock wave system forms in the impact: the outer one propagates in the ejecta, while a reverse shock propagates back towards the star."[47] The former ejection of matter in the outer shock wave and the latter production of antimatter in the reverse shock wave are steps in a space weather cycle.
Preliminary results from the presently operatingAlpha Magnetic Spectrometer (AMS-02) on board theInternational Space Station show that positrons in the cosmic rays arrive with no directionality, and with energies that range from 10GeV to 250 GeV. In September, 2014, new results with almost twice as much data were presented in a talk at CERN and published in Physical Review Letters.[48][49] A new measurement of positron fraction up to 500 GeV was reported, showing that positron fraction peaks at a maximum of about 16% of total electron+positron events, around an energy of 275 ± 32 GeV. At higher energies, up to 500 GeV, the ratio of positrons to electrons begins to fall again. The absolute flux of positrons also begins to fall before 500 GeV, but peaks at energies far higher than electron energies, which peak about 10 GeV.[50] These results on interpretation have been suggested to be due to positron production in annihilation events of massivedark matter particles.[51]
Cosmic ray antiprotons also have a much higher energy than their normal-matter counterparts (protons). They arrive at Earth with a characteristic energy maximum of 2 GeV, indicating their production in a fundamentally different process from cosmic ray protons, which on average have only one-sixth of the energy.[52]
There is an ongoing search for larger antimatter nuclei, such asantihelium nuclei (that is, anti-alpha particles), in cosmic rays. The detection of natural antihelium could imply the existence of large antimatter structures such as an antistar. A prototype of theAMS-02 designatedAMS-01, was flown into space aboard theSpace ShuttleDiscovery onSTS-91 in June 1998. By not detecting anyantihelium at all, theAMS-01 established an upper limit of 1.1×10−6 for the antihelium to heliumflux ratio.[53] AMS-02 revealed in December 2016 that it had discovered a few signals consistent with antihelium nuclei amidst several billion helium nuclei. The result remains to be verified, and as of 2017[update], the team is trying to rule out contamination.[54]
Positrons were reported[55] in November 2008 to have been generated byLawrence Livermore National Laboratory in large numbers. Alaser droveelectrons through agold target'snuclei, which caused the incoming electrons to emitenergyquanta that decayed into both matter and antimatter. Positrons were detected at a higher rate and in greater density than ever previously detected in a laboratory. Previous experiments made smaller quantities of positrons using lasers and paper-thin targets; newer simulations showed that short bursts of ultra-intense lasers and millimeter-thick gold are a far more effective source.[56]
In 2023, the production of the first electron-positron beam-plasma was reported by a collaboration led by researchers atUniversity of Oxford working with theHigh-Radiation to Materials (HRMT)[57] facility atCERN.[58] The beam demonstrated the highest positron yield achieved so far in a laboratory setting. The experiment employed the 440 GeV proton beam, with protons, from theSuper Proton Synchrotron, and irradiated a particle converter composed ofcarbon andtantalum. This yielded a total electron-positron pairs via aparticle shower process. The produced pair beams have a volume that fills multipleDebye spheres and are thus able to sustain collective plasma oscillations.[58]
The existence of the antiproton was experimentally confirmed in 1955 byUniversity of California, BerkeleyphysicistsEmilio Segrè andOwen Chamberlain, for which they were awarded the 1959Nobel Prize in Physics.[59] An antiproton consists of two up antiquarks and one down antiquark (uud). The properties of the antiproton that have been measured all match the corresponding properties of the proton, with the exception of the antiproton having opposite electric charge and magnetic moment from the proton. Shortly afterwards, in 1956, the antineutron was discovered in proton–proton collisions at theBevatron (Lawrence Berkeley National Laboratory) byBruce Cork and colleagues.[60]
In addition to antibaryons, anti-nuclei consisting of multiple bound antiprotons and antineutrons have been created. These are typically produced at energies far too high to form antimatter atoms (with bound positrons in place of electrons). In 1965, a group of researchers led byAntonino Zichichi reported production of nuclei ofantideuterium at the Proton Synchrotron atCERN.[61] At roughly the same time, observations of antideuterium nuclei were reported by a group of American physicists at the Alternating Gradient Synchrotron atBrookhaven National Laboratory.[62]
In 1995,CERN announced that it had successfully brought into existence nine hot antihydrogen atoms by implementing theSLAC/Fermilab concept during thePS210 experiment. The experiment was performed using theLow Energy Antiproton Ring (LEAR), and was led by Walter Oelert and Mario Macri.[63] 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 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, namely,ATHENA andATRAP.
In 1999, CERN activated theAntiproton Decelerator, a device capable of decelerating antiprotons from3.5 GeV to5.3 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.[64] The ATRAP project released similar results very shortly thereafter.[65] The antiprotons used in these experiments were cooled by decelerating them with the Antiproton Decelerator, passing them through a thin sheet of foil, and finally capturing them in aPenning–Malmberg trap.[66] The overall cooling process is workable, but highly inefficient; approximately 25 million antiprotons leave the Antiproton Decelerator and roughly 25,000 make it to the Penning–Malmberg trap, which is about1/1000 or 0.1% of the original amount.
The antiprotons are still hot when initially trapped. To cool them further, they are mixed into an electron plasma. The electrons in this plasma cool via cyclotron radiation, and then sympathetically cool the antiprotons viaCoulomb collisions. Eventually, the electrons are removed by the application of short-duration electric fields, leaving the antiprotons with energies less than100 meV.[67] While the antiprotons are being cooled in the first trap, a small cloud of positrons is captured fromradioactivesodium in a Surko-style positron accumulator.[68] This cloud is then recaptured in a second trap near the antiprotons. Manipulations of the trap electrodes then tip the antiprotons into the positron plasma, where some combine with antiprotons to form antihydrogen. This neutral antihydrogen is unaffected by the electric and magnetic fields used to trap the charged positrons and antiprotons, and within a few microseconds the antihydrogen hits the trap walls, where it annihilates. Some hundreds of millions of antihydrogen atoms have been made in this fashion.
Most of the sought-after high-precision tests of the properties of antihydrogen could only be performed if the antihydrogen were trapped, that is, held in place for a relatively long time. While antihydrogen atoms are electrically neutral, thespins of their component particles produce amagnetic moment. These magnetic moments can interact with an inhomogeneous magnetic field; some of the antihydrogen atoms can be attracted to a magnetic minimum. Such a minimum can be created by a combination of mirror and multipole fields.[70] Antihydrogen can be trapped in such a magnetic minimum (minimum-B) trap; in November 2010, the ALPHA collaboration announced that they had so trapped 38 antihydrogen atoms for about a sixth of a second.[71][72] This was the first time that neutral antimatter had been trapped.
On 26 April 2011, ALPHA announced that they had trapped 309 antihydrogen atoms, some for as long as 1,000 seconds (about 17 minutes). This was longer than neutral antimatter had ever been trapped before.[73] ALPHA has used these trapped atoms to initiate research into the spectral properties of antihydrogen.[74]
In 2016, a new antiproton decelerator and cooler called ELENA (extra low energy antiproton decelerator) was built. It takes the antiprotons from the antiproton decelerator and cools them to 90 keV, which is "cold" enough to study. This machine works by using high energy and accelerating the particles within the chamber. More than one hundred antiprotons can be captured per second, a huge improvement, but it would still take several thousand years to make ananogram of antimatter.
The biggest limiting factor in the large-scale production of antimatter is the availability of antiprotons. Recent data released by CERN states that, when fully operational, their facilities are capable of producing ten million antiprotons per minute.[75] Assuming a 100% conversion of antiprotons to antihydrogen, it would take 100 billion years to produce 1 gram or 1mole of antihydrogen (approximately6.02×1023 atoms of antihydrogen). However, CERN only produces 1% of the antimatter Fermilab does, and neither are designed to produce antimatter. According to Gerald Jackson, using technology already in use today we are capable of producing and capturing 20 grams of antimatter particles per year at a yearly cost of 670 million dollars per facility.[76][77]
Antihelium-3 nuclei (3 He, i.e. two antiprotons and one antineutron) were first observed in the 1970s in proton–nucleus collision experiments at the Institute for High Energy Physics by Y. Prockoshkin's group (Protvino near Moscow, USSR)[78] and later created in nucleus–nucleus collision experiments.[79] Nucleus–nucleus collisions produce antinuclei through the coalescence of antiprotons and antineutrons created in these reactions. In 2011, theSTAR detector reported the observation of artificially created antihelium-4 nuclei (anti-alpha particles) (4 He) from such collisions.[80]
Antimatter cannot be stored in a container made of ordinary matter because antimatter reacts with any matter it touches, annihilating itself and an equal amount of the container. Antimatter in the form ofcharged particles can be contained by a combination ofelectric andmagnetic fields, in a device called aPenning trap. This device cannot, however, contain antimatter that consists of uncharged particles, for whichatomic traps are used. In particular, such a trap may use thedipole moment (electric ormagnetic) of the trapped particles. At highvacuum, the matter or antimatter particles can be trapped and cooled with slightly off-resonant laser radiation using amagneto-optical trap ormagnetic trap. Small particles can also be suspended withoptical tweezers, using a highly focused laser beam.[83]
In 2011,CERN scientists were able to preserve antihydrogen for approximately 17 minutes.[84] The record for storing antiparticles is currently held by the TRAP experiment at CERN: antiprotons were kept in a Penning trap for 405 days.[85] A proposal was made in 2018 to develop containment technology advanced enough to contain a billion anti-protons in a portable device to be driven to another lab for further experimentation.[86]
Scientists claim that antimatter is the costliest material to make.[87] In 2006, Gerald Smith estimated $250 million could produce 10 milligrams of positrons[88] (equivalent to $25 billion per gram); in 1999, NASA gave a figure of $62.5 trillion per gram of antihydrogen.[87] This is because production is difficult (only very few antiprotons are produced in reactions in particle accelerators) and because there is higher demand for other uses ofparticle accelerators. According to CERN, it has cost a few hundred millionSwiss francs to produce about 1 billionth of a gram (the amount used so far for particle/antiparticle collisions).[89] In comparison, to produce the first atomic weapon, the cost of theManhattan Project was estimated at $23 billion with inflation during 2007.[90]
Several studies funded byNASA Innovative Advanced Concepts are exploring whether it might be possible to use magnetic scoops to collect the antimatter that occurs naturally in theVan Allen belt of the Earth, and ultimately the belts of gas giants likeJupiter, ideally at a lower cost per gram.[91]
Matter–antimatter reactions have practical applications in medical imaging, such aspositron emission tomography (PET). In positivebeta decay, anuclide loses surplus positive charge by emitting a positron (in the same event, a proton becomes a neutron, and aneutrino is also emitted). Nuclides with surplus positive charge are easily made in acyclotron and are widely generated for medical use. Antiprotons have also been shown within laboratory experiments to have the potential to treat certain cancers, in a similar method currently used for ion (proton) therapy.[92]
Not all of that energy can be utilized by any realistic propulsion technology because of the nature of the annihilation products. While electron–positron reactions result in gamma ray photons, these are difficult to direct and use for thrust. In reactions between protons and antiprotons, their energy is converted largely into relativistic neutral and chargedpions. Theneutral pions decay almost immediately (with a lifetime of 85attoseconds) into high-energy photons, but thecharged pions decay more slowly (with a lifetime of 26 nanoseconds) and can bedeflected magnetically to produce thrust.
Charged pions ultimately decay into a combination ofneutrinos (carrying about 22% of the energy of the charged pions) and unstable chargedmuons (carrying about 78% of the charged pion energy), with the muons then decaying into a combination of electrons, positrons and neutrinos (cf.muon decay; the neutrinos from this decay carry about 2/3 of the energy of the muons, meaning that from the original charged pions, the total fraction of their energy converted to neutrinos by one route or another would be about0.22 + (2/3)⋅0.78 = 0.74).[96]
Antimatter has been considered as a trigger mechanism for nuclear weapons.[97] A major obstacle is the difficulty of producing antimatter in large enough quantities, and there is no evidence that it will ever be feasible.[98] Nonetheless, theU.S. Air Force funded studies of the physics of antimatter in theCold War, and began considering its possible use in weapons, not just as a trigger, but as the explosive itself.[99]
Alfvén–Klein cosmology – Non-standard model of the universe; emphasizes the role of ionized gasesPages displaying short descriptions of redirect targets
^Tsan, Ung Chan (2013). "Mass, Matter, Materialization, Mattergenesis and Conservation of Charge".International Journal of Modern Physics E.22 (5): 1350027.Bibcode:2013IJMPE..2250027T.doi:10.1142/S0218301313500274.Matter conservation means conservation of baryonic numberA and leptonic numberL,A andL being algebraic numbers. PositiveA andL are associated to matter particles, negativeA andL are associated to antimatter particles. All known interactions do conserve matter.
^Tsan, U. C. (2012). "Negative Numbers And Antimatter Particles".International Journal of Modern Physics E.21 (1):1250005-1 –1250005-23.Bibcode:2012IJMPE..2150005T.doi:10.1142/S021830131250005X.Antimatter particles are characterized by negative baryonic numberA or/and negative leptonic numberL. Materialization and annihilation obey conservation ofA andL (associated to all known interactions).
^Dirac, Paul A. M. (1965).Physics Nobel Lectures(PDF). Vol. 12. Amsterdam-London-New York: Elsevier. pp. 320–325.Archived(PDF) from the original on 10 October 2019. Retrieved10 October 2019.
^"Antimatter".Science Fiction Encyclopedia.Archived from the original on 28 July 2019. Retrieved10 October 2019.
^As Dirac said in 1933It is quite possible that for some of the stars it is the other way about, these stars being built up mainly of positrons and negative protons. In fact, there may be half the stars of each kind. The two kinds of stars would both show exactly the same spectra, and there would be no way of distinguishing them by present astronomical methods.Dirac 1965, p. 325
^Efthymiopoulos, I; Hessler, C; Gaillard, H; Grenier, D; Meddahi, M; Trilhe, P; Pardons, A; Theis, C; Charitonidis, N; Evrard, S; Vincke, H; Lazzaroni, M (2011)."HiRadMat: A New Irradiation Facility for Material Testing at CERN".2nd International Particle Accelerator Conference.
^Steigerwald, B. (14 March 2006)."New and Improved Antimatter Spaceship for Mars Missions".NASA.Archived from the original on 6 August 2011. Retrieved11 June 2010."A rough estimate to produce the 10 milligrams of positrons needed for a human Mars mission is about 250 million dollars using technology that is currently under development," said Smith.
^Schmidt, G. R. (1999). "Antimatter Production for Near-Term Propulsion Applications".35th Joint Propulsion Conference and Exhibit. American Institute of Aeronautics and Astronautics.doi:10.2514/6.1999-2691.
^(compared to theformation of water at1.56×107 J/kg, for example)
^Gsponer, Andre; Hurni, Jean-Pierre (1987). "The physics of antimatter induced fusion and thermonuclear explosions". In Velarde, G.; Minguez, E. (eds.).Proceedings of the International Conference on Emerging Nuclear Energy Systems, Madrid, June/July, 1986. Vol. 4.World Scientific. pp. 66–169.arXiv:physics/0507114.Bibcode:2005physics...7114G.
