Editing Antimatter (section)

{{Short description|Material composed of antiparticles of the corresponding particles of ordinary matter}}
{{Distinguish|dark matter}}
{{Redirect|Anti-nucleus|the antibodies|Antinuclear antibody}}
{{Other uses}}
{{Use dmy dates|date=July 2020}}
[[File:PositronDiscovery.png|thumb|A [[cloud chamber]] photograph of the first observed [[positron]], 2 August 1932.]]
{{Antimatter}}
{{Modern physics}}

In [[modern physics]], '''antimatter''' is defined as [[matter]] composed of the [[antiparticle]]s (or "partners") of the corresponding [[subatomic particle|particles]] in "ordinary" matter, and can be thought of as matter with reversed charge and parity, or going backward in time (see [[CPT symmetry]]). 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 accelerator]]s, but total artificial production has been only a few [[nanogram]]s.<ref>{{cite news|access-date=2018-11-08|title=Ten things you might not know about antimatter|url=https://www.symmetrymagazine.org/article/april-2015/ten-things-you-might-not-know-about-antimatter|newspaper=symmetry magazine|archive-date=8 November 2018|archive-url=https://web.archive.org/web/20181108224807/https://www.symmetrymagazine.org/article/april-2015/ten-things-you-might-not-know-about-antimatter|url-status=live}}</ref> No [[Macroscopic scale|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]],<ref>{{Cite journal |last1=Bittner |first1=Martin-Immanuel |last2=Grosu |first2=Anca-Ligia |last3=Wiedenmann |first3=Nicole |last4=Wilkens |first4=Jan J. |date=2014-01-16 |title=A systematic review of antiproton radiotherapy |journal=Frontiers in Physics |language=English |volume=1 |doi=10.3389/fphy.2013.00037 |doi-access=free |issn=2296-424X}}</ref> and industrial imaging.

In theory, a particle and its antiparticle (for example, a [[proton]] and an [[antiproton]]) have the same [[mass]], but opposite [[electric charge]], and other differences in [[quantum number]]s.

A collision between any particle and its anti-particle partner leads to their mutual [[annihilation]], giving rise to various proportions of intense [[photon]]s ([[gamma ray]]s), [[neutrino]]s, and sometimes less-massive particle{{ndash}}antiparticle 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 notable [[mass–energy equivalence]] equation, {{math|{{nowrap|''E''{{=}}''mc''<sup>2</sup>}}}}.<ref>{{cite web |date=11 August 2011 |title=Smidgen of Antimatter Surrounds Earth |work=Discovery News |url=http://news.discovery.com/space/pamela-spots-a-smidgen-of-antimatter-110811.html |archive-url=https://web.archive.org/web/20110926174944/http://news.discovery.com/space/pamela-spots-a-smidgen-of-antimatter-110811.html |archive-date=26 September 2011|url-status=live}}</ref>

Antiparticles bind with each other to form antimatter, just as ordinary particles bind to form normal matter. For example, a [[positron]] (the antiparticle of the [[electron]]) and an antiproton (the antiparticle of the proton) can form an [[antihydrogen]] atom. The [[Atomic nucleus|nuclei]] of [[#Antihelium|antihelium]] have been artificially produced, albeit with difficulty, and are the most complex anti-nuclei so far observed.<ref>{{cite journal |last=Agakishiev |first=H. |display-authors=etal |collaboration=STAR Collaboration |date=2011 |title=Observation of the antimatter helium-4 nucleus |journal=[[Nature (journal)|Nature]] |volume=473 |issue=7347 |pages=353–356 |arxiv=1103.3312 |bibcode=2011Natur.473..353S |doi=10.1038/nature10079 |pmid=21516103|s2cid=118484566 }}</ref> 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.<ref>{{cite journal |last=Canetti |first=L. |display-authors=etal |date=2012 |title=Matter and Antimatter in the Universe |journal=New J. Phys. |volume=14 |issue=9 |pages=095012 |arxiv=1204.4186 |doi=10.1088/1367-2630/14/9/095012 |bibcode=2012NJPh...14i5012C|s2cid=119233888 }}</ref> This [[baryon asymmetry|asymmetry of matter and antimatter]] in the visible universe is one of the great [[unsolved problems in physics]].<ref>{{Cite web |last=Tenenbaum |first=David |date=28 December 2012 |title=One step closer: UW-Madison scientists help explain scarcity of antimatter |url=http://www.news.wisc.edu/21376 |work=University of Wisconsin–Madison News |archive-url=https://web.archive.org/web/20121228043843/http://www.news.wisc.edu/21376|archive-date=28 December 2012}}</ref> The process by which this inequality between matter and antimatter particles is hypothesised to have occurred is called [[baryogenesis]].