Editing Richard Feynman (section)

=== Physics ===
At Caltech, Feynman investigated the physics of the [[superfluid]]ity of supercooled [[liquid helium]], where helium seems to display a complete lack of [[viscosity]] when flowing. Feynman provided a quantum-mechanical explanation for the Soviet physicist [[Lev Landau]]'s theory of superfluidity.{{sfn|Gleick|1992|pp=299–303}} Applying the Schrödinger equation to the question showed that the superfluid was displaying quantum mechanical behavior observable on a macroscopic scale. This helped with the problem of [[superconductivity]], but the solution eluded Feynman.<ref>{{cite journal|last1=Pines|first1=David|title=Richard Feynman and Condensed Matter Physics|journal=Physics Today|volume=42|page=61|year=1989|doi=10.1063/1.881194|bibcode = 1989PhT....42b..61P|issue=2}}</ref> It was solved with the [[BCS theory]] of superconductivity, proposed by [[John Bardeen]], [[Leon Neil Cooper]], and [[John Robert Schrieffer]] in 1957.{{sfn|Gleick|1992|pp=299–303}}

[[File:RichardFeynman-PaineMansionWoods1984 copyrightTamikoThiel bw.jpg|alt=Feynman standing among trees|thumb|left|Feynman at the [[Robert Treat Paine Estate]] in [[Waltham, Massachusetts]], in 1984]]
Feynman, inspired by a desire to quantize the Wheeler–Feynman absorber theory of electrodynamics, laid the groundwork for the path integral formulation and Feynman diagrams.{{sfn|Mehra|1994|pp=92–101}}

With [[Murray Gell-Mann]], Feynman developed a model of [[weak decay]], which showed that the current coupling in the process is a combination of vector and axial currents (an example of weak decay is the decay of a neutron into an electron, a proton, and an [[antineutrino]]). Although [[E. C. George Sudarshan]] and Robert Marshak developed the theory nearly simultaneously, Feynman's collaboration with Gell-Mann was seen as seminal because the [[weak interaction]] was neatly described by the vector and axial currents. It thus combined the 1933 [[beta decay]] theory of [[Enrico Fermi]] with an explanation of [[parity violation]].{{sfn|Gleick|1992|pp=330–339}}

Feynman attempted an explanation, called the [[parton model]], of the [[strong interaction]]s governing nucleon scattering. The parton model emerged as a complement to the [[quark model]] developed by Gell-Mann. The relationship between the two models was murky; Gell-Mann referred to Feynman's partons derisively as "put-ons". In the mid-1960s, physicists believed that quarks were just a bookkeeping device for symmetry numbers, not real particles; the statistics of the [[omega-minus particle]], if it were interpreted as three identical strange quarks bound together, seemed impossible if quarks were real.{{sfn|Gleick|1992|pp=387–396}}{{sfn|Mehra|1994|pp=507–514}}

The [[SLAC National Accelerator Laboratory]] [[deep inelastic scattering]] experiments of the late 1960s showed that [[nucleon]]s (protons and neutrons) contained point-like particles that scattered electrons. It was natural to identify these with quarks, but Feynman's parton model attempted to interpret the experimental data in a way that did not introduce additional hypotheses. For example, the data showed that some 45% of the energy momentum was carried by electrically neutral particles in the nucleon. These electrically neutral particles are now seen to be the [[gluon]]s that carry the forces between the quarks, and their three-valued color quantum number solves the omega-minus problem. Feynman did not dispute the quark model; for example, when the fifth quark was discovered in 1977, Feynman immediately pointed out to his students that the discovery implied the existence of a sixth quark, which was discovered in the decade after his death.{{sfn|Gleick|1992|pp=387–396}}{{sfn|Mehra|1994|pp=516–519}}

After the success of quantum electrodynamics, Feynman turned to [[quantum gravity]]. By analogy with the photon, which has spin 1, he investigated the consequences of a free massless spin 2 field and derived the [[Einstein field equation]] of general relativity, but little more. The computational device that Feynman discovered then for gravity, "ghosts", which are "particles" in the interior of his diagrams that have the "wrong" connection between spin and statistics, have proved invaluable in explaining the quantum particle behavior of the [[Yang–Mills theory|Yang–Mills theories]], for example, [[quantum chromodynamics]] and the [[electro-weak]] theory.{{sfn|Mehra|1994|pp=505–507}} He did work on all four of the [[fundamental interactions]] of nature: [[electromagnetic force|electromagnetic]], the [[weak force]], the [[strong force]] and gravity. John and Mary Gribbin state in their book on Feynman that "Nobody else has made such influential contributions to the investigation of all four of the interactions".{{sfn|Gribbin|Gribbin|p=189|1997}}

Partly as a way to bring publicity to progress in physics, Feynman offered $1,000 prizes for two of his challenges in nanotechnology; one was claimed by [[William McLellan (nanotechnology)|William McLellan]] and the other by [[Tom Newman (scientist)|Tom Newman]].{{sfn|Gribbin|Gribbin|1997|p=170}}

Feynman was also interested in the relationship between physics and computation. He was also one of the first scientists to conceive the possibility of [[quantum computer]]s.<ref name="mike_ike">{{Cite book|last1=Nielsen|first1=Michael A.|author-link1=Michael Nielsen |last2=Chuang|first2=Isaac L. |author-link2=Isaac Chuang |title=Quantum Computation and Quantum Information|title-link=Quantum Computation and Quantum Information|publisher=[[Cambridge University Press]]|location=Cambridge|year=2010|edition=10th anniversary|oclc=844974180 |isbn=978-1-107-00217-3 |page=7}}</ref><ref>{{Cite book|title-link= Quantum Computing: A Gentle Introduction |title=Quantum Computing: A Gentle Introduction|last1=Rieffel|first1=Eleanor G.|last2=Polak|first2=Wolfgang H.|date=March 4, 2011|publisher=MIT Press|isbn=978-0-262-01506-6|language=en|author-link=Eleanor Rieffel |page=44}}</ref>{{sfn|Deutsch|1992|pp=57–61}} In the 1980s he began to spend his summers working at [[Thinking Machines Corporation]], helping to build some of the first parallel supercomputers and considering the construction of quantum computers.{{sfn|Hillis|1989|pp=78–83}}<ref>{{cite journal |last=Feynman |first=Richard |title=Simulating Physics with Computers |journal=International Journal of Theoretical Physics |volume=21 |pages=467–488 |year=1982 |doi=10.1007/BF02650179 |bibcode=1982IJTP...21..467F |issue=6–7|citeseerx = 10.1.1.45.9310 |s2cid=124545445}}</ref>

Between 1984 and 1986, he developed a variational method for the approximate calculation of path integrals, which has led to a powerful method of converting divergent perturbation expansions into convergent strong-coupling expansions ([[variational perturbation theory]]) and, as a consequence, to the most accurate determination<ref>{{cite journal |title=Specific heat of liquid helium in zero gravity very near the lambda point |last=Kleinert |first=Hagen |journal=Physical Review D |volume=60 |page=085001 |year=1999 |doi=10.1103/PhysRevD.60.085001 |arxiv=hep-th/9812197 |bibcode=1999PhRvD..60h5001K |author-link=Hagen Kleinert |issue=8|s2cid=117436273}}</ref> of [[critical exponent]]s measured in satellite experiments.<ref>{{cite journal |title=Specific heat of liquid helium in zero gravity very near the lambda point |last1=Lipa |first1=J. A. |journal=Physical Review B |volume=68 |page=174518 |year=2003 |doi=10.1103/PhysRevB.68.174518 |last2=Nissen |first2=J. |last3=Stricker |first3=D. |last4=Swanson |first4=D. |last5=Chui |first5=T. |arxiv=cond-mat/0310163 |bibcode=2003PhRvB..68q4518L |issue=17|s2cid=55646571}}</ref> At Caltech, he once chalked "What I cannot create I do not understand" on his blackboard.<ref name="Way2017">{{cite journal|last1=Way|first1=Michael|title=What I cannot create, I do not understand|journal=Journal of Cell Science|volume=130|issue=18|year=2017|pages=2941–2942|issn=1477-9137|doi=10.1242/jcs.209791|pmid=28916552|s2cid=36379246|doi-access=free}}</ref>