Editing Antiparticle (section)

== History ==

=== Experiment ===

In 1932, soon after the prediction of [[positron]]s by [[Paul Dirac]], [[Carl D. Anderson]] found that cosmic-ray collisions produced these particles in a [[cloud chamber]] – a [[particle detector]] in which moving [[electron]]s (or positrons) leave behind trails as they move through the gas. The electric charge-to-mass ratio of a particle can be measured by observing the radius of curling of its cloud-chamber track in a [[magnetic field]]. Positrons, because of the direction that their paths curled, were at first mistaken for electrons travelling in the opposite direction.  Positron paths in a cloud-chamber trace the same helical path as an electron but rotate in the opposite direction with respect to the magnetic field direction due to their having the same magnitude of charge-to-mass ratio but with opposite charge and, therefore, opposite signed charge-to-mass ratios.

The [[antiproton]] and [[antineutron]] were found by [[Emilio Segrè]] and [[Owen Chamberlain]] in 1955 at the [[University of California, Berkeley]].<ref>{{cite web|url=https://www.nobelprize.org/prizes/physics/1959/summary/|title=The Nobel Prize in Physics 1959}}</ref> Since then, the antiparticles of many other subatomic particles have been created in particle accelerator experiments. In recent years, complete atoms of [[antimatter]] have been assembled out of antiprotons and positrons, collected in electromagnetic traps.<ref>{{cite web|url=http://news.nationalgeographic.com/news/2010/11/101118-antimatter-trapped-engines-bombs-nature-science-cern/|archive-url=https://web.archive.org/web/20101120181454/http://news.nationalgeographic.com/news/2010/11/101118-antimatter-trapped-engines-bombs-nature-science-cern/|url-status=dead|archive-date=November 20, 2010|title=Antimatter Atoms Trapped for First Time – 'A Big Deal'|date=19 November 2010}}</ref>

=== Dirac hole theory ===
{{quote box|quote=... the development of [[quantum field theory]] made the interpretation of antiparticles as holes unnecessary, even though it lingers on in many textbooks.|source=[[Steven Weinberg]]<ref>{{cite book|last=Weinberg|first=Steve|title=The quantum theory of fields, Volume 1 : Foundations|isbn=0-521-55001-7|page=[https://archive.org/details/quantumtheoryoff00stev/page/14 14]|year=1995|publisher=Cambridge University Press |url-access=registration|url=https://archive.org/details/quantumtheoryoff00stev/page/14}}</ref>|width=300px}}
Solutions of the [[Dirac equation]] contain negative energy quantum states. As a result, an electron could always radiate energy and fall into a negative energy state. Even worse, it could keep radiating infinite amounts of energy because there were infinitely many negative energy states available. To prevent this unphysical situation from happening, Dirac proposed that a "sea" of negative-energy electrons fills the universe, already occupying all of the lower-energy states so that, due to the [[Pauli exclusion principle]], no other electron could fall into them. Sometimes, however, one of these negative-energy particles could be lifted out of this [[Dirac sea]] to become a positive-energy particle. But, when lifted out, it would leave behind a ''[[electron hole|hole]]'' in the sea that would act exactly like a positive-energy electron with a reversed charge. These holes were interpreted as "negative-energy electrons" by Paul Dirac and mistakenly  identified  with [[proton]]s in his 1930 paper ''A Theory of Electrons and Protons''<ref>
{{cite journal
 |last1=Dirac |first1=Paul
 |date=1930
 |title=A Theory of Electrons and Protons
 |journal=[[Proceedings of the Royal Society A]]
 |volume=126 |issue= 801|pages=360–365
 |doi=10.1098/rspa.1930.0013
|bibcode = 1930RSPSA.126..360D |doi-access=free}}</ref> However, these "negative-energy electrons" turned out to be [[positron]]s, and not [[proton]]s.

This picture implied an infinite negative charge for the universe{{snd}}a problem of which Dirac was aware. Dirac tried to argue that we would perceive this as the normal state of zero charge. Another difficulty was the difference in masses of the electron and the proton. Dirac tried  to argue that this was due to the electromagnetic interactions with the sea, until [[Hermann Weyl]] proved that hole theory was completely symmetric between negative and positive charges. Dirac also predicted a reaction {{Subatomic particle|Electron}}&nbsp;+&nbsp;{{Subatomic particle|Proton+}}&nbsp;→&nbsp;{{Subatomic particle|Photon}}&nbsp;+&nbsp;{{Subatomic particle|Photon}}, where an electron and a proton annihilate to give two photons. [[Robert Oppenheimer]] and [[Igor Tamm]], however,  proved that this would cause ordinary matter to disappear too fast. A year later, in 1931, Dirac modified his theory and postulated the [[positron]], a new particle of the same mass as the electron. The discovery of this particle the next year removed the last two objections to his theory.

Within Dirac's theory, the problem of infinite charge of the universe remains. Some [[boson]]s also have antiparticles, but since bosons do not obey the [[Pauli exclusion principle]] (only [[fermion]]s do), hole theory does not work for them. A unified interpretation of antiparticles is now available in [[quantum field theory]], which solves both these problems by describing antimatter as negative energy states of the same underlying matter field, i.e. particles moving backwards in time.<ref>{{Cite book|url=https://books.google.com/books?id=Y-0kAwAAQBAJ&pg=PA61|title=Quantum Field Theory for the Gifted Amateur|last1=Lancaster|first1=Tom|last2=Blundell|first2=Stephen J.|last3=Blundell|first3=Stephen|date= 2014|publisher=OUP Oxford|isbn=978-0199699339|page=61|language=en}}</ref>