In General > s.a. quantum gravity, canonical
formulation / quantum-gravity phenomenology; spacetime [philosophical].
* Idea: Spacetime is not a manifold, but that picture enables us to talk about it without knowing what is going on; In particular, below Planck scale, events should be fuzzed out, and topology as well as other structures, can change; Beyond the first general points, there are many different approaches, differing on what structures are fundamental, how to treat their dynamics, and whether matter is to be identified with elementary objects, or built out of more fundamental stuff, etc; Events can be intersections of world-lines.
* History: Riemann had some early ideas, but the field did not start developing until the 1930s; Around the 1960s l = 10–12–10–14 cm was still considered a reasonable fundamental length! 1990s, Quantum geometry in lqg; 2000s, Structure of spacetime near singularities.
* Pregeometry: Geometry is not fundamental, but emerges at large scales; In some views, it does not really exist.
* Nature of theories: Most proposals are realistic (they assume the existence of physical entities endowed with concrete properties), and objective (they can be formulated without any reference to knowing subjects or sensorial fields); Many are also relational (spacetime is not a thing, but a complex of relations among things).
@ Reviews: Antonsen pr(94); Gibbs ht/95 [bibliography]; Kempf ht/98-proc; Markopoulou gq/02-ch; Gross et al ed-07; Ambjørn et al SA(08)jul; Ashtekar a0810-ch; news nat(13)aug [approaches]; Bahns et al a1501 [and algebraic quantum field theory, rev]; Licata ed-16 [and quantum theory].
@ General references: Wigner RMP(57); Zimmerman AJP(62)feb; Castell et al 75, 77, 79; Townsend PRD(77); Finkelstein & Rodriguez in(86); Namsrai FdP(88); Rüger HSPS(88); Anandan in(97)gq; Rovelli in(01)gq/99; Callender & Huggett ed-01; Crane a0706 [motivation, and quantum topos]; Bojowald a1002-conf [and lqg, lqc]; Hedrich a1101; Noldus a1305 [universal construction]; Bennett & Nielsen a1306-proc [need for new fundamental physics]; Svozil a1401 [and entanglement]; Amelino-Camelia & Astuti IJMPD(15)-a1404 [empty spacetime is not physically meaningful, Snyder space example]; Di Casola et al FP(15)-a1405 [no "mesoscopic" spacetime]; Wüthrich a1405-in [in lqg].
@ Pregeometry and emergence: Wheeler in(80); Stuckey in(00); Botta Cantcheff gq/04-GRF; Singh a0905 [gravity as a thermodynamic limit]; Piazza FP(10); Gao a1001-wd [gravity as fundamental]; > s.a. emergent gravity; gravitational thermodynamics.
Theoretical Aspects > s.a. information and spacetime / gravity; quantum black holes.
* Evidence: Area of quantum black holes, with ΔAmin ~ 4 ln2 lP2; Indications of UV cutoff from existence of fixed point for G; Arguments re quantum uncertainty and black holes.
@ Theoretical evidence: Cacciatori et al PLB(98)ht/97 [gas of wormholes]; Sidharth CSF(00)qp/99 [??]; Doplicher AIP(01)ht.
@ New Planck scale physics: Brandenberger gq/95; Jacobson PTPS(99)ht/00; Botta Cantcheff ht/00; Niemeyer & Parentani PRD(01)ap [and inflation]; Kowalski-Glikman MPLA(02)ht/01 [κ-Poincaré symmetry]; Arcioni et al JHEP(01)ht [with cosmological constant]; Adler AJP(10)sep [elementary arguments]; Bose a1011-wd [and Lorentz-invariant cutoff]; 't Hooft a1605-in; > s.a. inflation.
@ Non-local correlations: Requardt ht/02 [and non-commutative geometry]; > s.a. locality.
@ And decoherence: Ellis et al MPLA(97) [stringy fluctuations]; Zurek Nat(01)aug; Brody & Hughston qp/06-proc [emergence of classical spacetime].
@ Axioms, paradigms: Pérez Bergliaffa et al IJTP(98)gq/97 [axiomatic]; Kornai IJTP(03) [finitism].
@ Related topics: Bacry 88 [localizability and space]; Mazur & Nair GRG(89) [topological features]; Schiffer GRG(92) [horizons]; Nodland MPLA(98)ht [matter]; Sidharth CSF(00)qp/98 [time asymmetry]; Bernal et al FP(02)gq/00 [clock/rods]; Markopoulou JPCS(07)gq [collective excitations]; Bradonjić a0905, a1103/FP [spacetime geometry above the electroweak scale]; Cohen-Tannoudji a1402 [quanta of action, information and spacetime area].
> Quantum-field-theory approach: see quantum field theory, in curved backgrounds and phenomenology [short-distance structure].
> Discreteness, minimal length: see discrete geometries; GUP; Minimal Length; quantum-gravity and geometry; spacetime [points].
> Related topics: see dimensionality; Event; quantum gravity and geometry and matter; oscillators [model for spacetime decondensation].
Some Approaches and Issues > s.a. models of spacetime;
quantum spacetime proposals/types;
semiclassical general relativity; spacetime foam.
* Idea: Quantize only part of the metric, and/or replace smooth manifolds by a slightly more general structure.
* Coordinate operators: If the spectra are Lorentz-invariant and the operators commute, we get a continuum; If they don't commute, we get the Snyder proposal; If we impose Poincaré invariance, again we get a continuum.
@ Fluctuations: Redington gq/97; Rosales & Sánchez-Gómez gq/97 [conformal]; Acebal et al PLB(98), Miller JMP(99) [stochastic]; Zizzi IJTP(99)ht/98 [and Planck-size euclidean-black-hole foam]; Hogan PRD(08)-a0806 [based on wave optics]; Shalyt-Margolin a1306, Ent(16)-a1603 [and minimal length].
@ Signature dynamics: Greensite PLB(93)gq/92; Carlini & Greensite PRD(94)gq/93; Elizalde et al CQG(94)ht/93; Dzhunushaliev GRG(01)gq/99; > s.a. modified electromagnetism.
@ Metric degeneracy: Percacci in(92) [mean-field approach, \(\langle\)|g|1/2\(\rangle\)vev ≠ 0].
@ Deformed spaces: Toller PRA(99)qp/98 [quantum coordinates]; Quesne & Tkachuk PRA(10)-a0906 [composite systems]; > s.a. deformation quantization; discrete spacetime; generalized uncertainty relations; minkowski spacetime; modified lorentz symmetry; non-commutative geometry.
"Everybody thinks spacetime should be an output rather than an input of a final theory" – Nathan Seiberg, NYT 26.06.2001.
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