Unified
Theories |

**In General** > s.a. paradigms in physics; physical theories [ToEs, Theories of Everything]; quantum gravity and approaches [gravity and quantum theory].

* __Idea__: Unification may
refer to the merging of conceptual and technical frameworks from different
theories, a tradition
that can be traced back to the celestial/earthly world unification of the 1500s
and 1600s, or to the unification of two interactions that results when they
are included in a larger theory.

* __Examples__: Keplerian orbits
+ Galilean physics → Newtonian
mechanics; Maxwell theory + Galilean relativity → special
relativity; Special relativity + non-relativistic quantum mechanics → antiparticles;
Special
relativity + Newtonian gravity → general relativity; Quantum theory + general
relativity → ???

@ __References__: Rueger BJPS(05)
[and scientific realism]; Schellekens RPP(08)-a0807 [rev];
Hawking & Mlodinow SA(10)oct [ToE]; news nat(11)mar [status]; Emam 11; Kim a1301-conf [historical approach]; Wilczek a1512-talk [Maxwell and the unity of field and substance].

**Unification of Fundamental Interactions** > s.a. GUTs; interactions; Relativity Principle; symmetries in physics.

* __1800s__: Unification
in the modern sense started with Maxwell's unification of electricity and
magnetism.

* __1900s__: Kaluza, Klein,
and Einstein worked on unifying electromagnetism with gravitation when those
were the only two known forces; Because of the lack of a clear success (despite
interesting
ideas) and the interest in nuclear interactions, after 1955 not many people
worked on unification until the 1970s; 1970s, Salam-Weinberg electroweak
theory, and higher-dimensional supersymmetry and supergravity; 1984, Strings
replace *d* = 11 supergravity as favorite TOE; 1996, M-theory replaces
the 5 consistent string theories; 1999, Randall-Sundrum and other models introduce
the brane-world idea.

* __2008__: Garrett Lisi
proposed a theory based on an E8 connection; After a few months, consensus
seems to be that it won't work.

* __Scales__: Electroweak
unification at about 10^{3} GeV, GUT unification
at perhaps 10^{16} GeV, with gravity at possibly 10^{19} GeV,
the Planck energy; The disparity between these energy scales gives rise to
the hierarchy problem.

* __Rem__: In the most common unified scenarios, interactions are unified at very high energies when the various coupling constants approach the same value and the theory has a symmetry group that includes the ones of the individual interactions; But this picture may lose its motivation if the continuum disappears at high energies.

@ __History__: Gell-Mann et al in(79)-a1307 [several possibilities]; Goenner LRR(04);
Verbin & Nielsen GRG(05)phy/04 [Kaluza's
idea]; Ducheyne SHPSA(05)
[and Newton]; Battimelli EJP(05)
[and pre-Einstein electromagnetism]; Ellis pw(05)jan
[Einstein]; Regge JPA(07)
[thoughts on future]; Collins SA(08)apr [Lisi's proposal]; van Dongen 10 [Einstein's attempts, r Isis(11)#4 Schweber]; Goenner LRR(14) [part II, 1930-1965]; Nath 16; > s.a. GUTs; history of physics.

@ __Geometrical aspects__: Cirilo-Lombardo IJTP-a1411 [possible fermionic representations, algebraic and geometrical viewpoints]; Roldan & Barros a1603.

@ __Related topics__: Weinberg SA(99)dec [future];
Myrvold PhSc(03)apr
[advantages, Bayesian view]; Ha a1007-conf [status, issues]; Ferreira et al PLB(13) [using the radio source PKS1413+135]; 't Hooft FP(14) [and the ontological interpretation of quantum mechanics]; Finster & Kleiner JPCS(15)-a1502, Finster a1505 [causal fermion systems].

**Specific Unified Theories** > s.a. gravity + electromagnetism; gravity
theories; particle physics; string theory.

* __Gravity + standard model__: One common approach is to treat gravity as a gauge theory with tetrads and spin-connections as variables.

@ __Gravity + electroweak__: Verwimp JMP(90);
Gitner & Dehnen NCB(00)gq/97 [parity
violation]; Batakis PLB(97)ht/96;
Cianfrani & Montani IJTP(07)gq/06,
gq/06-MGXI
[geometrical, Kaluza-Klein approach]; Glinka & Pervushin ONCP(08)-a0705;
Nesti & Percacci JPA(08)-a0706 [graviweak];
Alexander a0706 [isogravity];
Pandres GRG(09)-a1006 [replacing diffeomorphisms with larger group]; Scholz AdP(11)-a1102 [extended Weyl theory]; Hsu MPLA(11)-a1106; Onofrio MPLA(14)-a1412 [gravitoweak unification].

@ __Gravity + standard model__: Pawłowski hp/96;
Gillan ht/01 [with 3 timelike dimensions]; Patwardhan ht/04 [gauge
theory
of all interactions]; Lisi gq/05 [Clifford-bundle formulation], a0711 +
pw(08)jul
[E8 connection formulation]; Nesti a0706 [using
spinors]; Jones SHPMP(09)
[evidence for one does not disconfirm the other]; Distler & Garibaldi CMP(10)-a0905 [E8
is not enough]; Lisi et al JPA(10)-a1004;
Lisi a1006-proc [embedding
in E8]; Lisi & Weatherall SA(10)dec;
Andrianov et al PRL(13)-a1302 [Universal Landau Pole];
Hsu ChJP-a1309 [based on gauge theory of the translation group];
Delbourgo IJMPA(13) [with fermionic Lorentz scalar variables];
Finster a1409 [from continuum limit of system with massive Dirac particles and neutrinos]; Stack & Delbourgo IJMPA(15)-a1511 [gravity + chromodynamics with 3 anticommuting color coordinates]; Delbourgo a1602 [bosonic spacetime plus fermionic property space]; Mannheim a1603-GRF [geometrization of all interactions, using conformal gravity]; Chkareuli a1703 [Poincaré gauge gravity]; > s.a. spin-foam models.

@ __Gravity + Yang-Mills theory__: Peldán NPB(93)gq/92; Ackermann & Tolksdorf ht/95 [+
Higgs field]; Gogoladze et al PRL(03)
[higher-dimensional]; Smolin PRD(09)-a0712 [extended Plebański action]; Torres-Gomez & Krasnov PRD(10)-a0911 [+
Higgs field]; Elyasi & Boroojerdian a1109 [Lie-algebroid structures];
Alexander et al PLB(12)-a1105 [SU(2) and 3D general relativity, spin-foam approach];
Gerhardt ATMP(14)-a1207, a1301 [and spinor fields]; Stack & Delbourgo IJMPA(15)-a1411 [with anticommuting coordinates in the spacetime metric]; Chamseddine & Mukhanov JHEP(16)-a1602 [higher-dimensional Lorentz group as the symmetry of the tangent space].

@ __Gravity + other theories__: Bergmann in(81);
Dell & Smolin in(86)
[metric-connection];
Alexandre et al NJP(10)-a0906 [*N* =
8, *D* = 4 supergravity
as ToE]; Pavšič JPCS(10)-a0912; > s.a. spin-foam models.

@ __Related topics__: Chruściel AIHP(85); Özer
gq/99, gq/00 [and
equivalence principle, cosmological constant]; Aldaya et al JPA(02)ht/01
[group cohomology]; Aranda & Wudka PRD(10)-a1008 [gauge-Higgs]; Vieira et al PRD(12)-a1206 [solar-type main-sequence stars as probes]; Ootsuka a1206 [in terms of Kawaguchi areal geometry]; Furey PRD(12) [unified theory of ideals]; Cirilo-Lombardo et al IJTP-a1404 [based on affine geometry].

> __Related topics__:
see electroweak theory; GUTs [electroweak and strong interactions]; non-commutative gravity and gauge theories.

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