Wave-Function Collapse as a Dynamical Process |

**In General**
> s.a. wave-function collapse [including some models]; zeno effect.

* __Idea__: The point of view
according to which wave-function collapse (quantum-state reduction) is a physical
process, as opposed to just being related to our knowledge and description of
the system; In a relativistic scenario the collapse will then happen at a finite
speed, as opposed to the instantaneous collapse of the conventional view.

* __Tests__: One way to test it is
to look for quantum behaviour in larger and larger objects; If collapse models
are correct, then quantum effects will not be apparent above a certain mass;
2015, Physicists have already carried out double-slit interference experiments
with large molecules; Various groups are planning to search for such a cut-off
using metal clusters and nanoparticles, and hope to see results within a decade;
2018, Work on bulk heating experiments in spontaneous collapse.

* __Motivation__: Using this
point of view it becomes easier to explain how quantum fluctuations in
the early universe might have become classical.

@ __General references__: Lewin FP(09) [as an effect of field quantization];
Omnès a1006,
a1601
[derivation of collapse from quantum dynamics];
Carlesso & Donadi a1907-proc [rev].

@ __Speed / time for collapse__:
Squires PLA(90);
Pegg PLA(91);
Zurek qp/03 ["decoherence timescale"];
Ohanian a1311 [atom-interferometer test].

@ __State recovery / uncollapse__: Katz et al PRL(08)-a0806;
Jordan & Korotkov CP(10)-a0906 [undoing quantum measurements];
news PhysOrg(13)nov.

@ __Objective collapse, and retina / mind__:
Georgiev qp/02;
Thaheld qp/06/PRA,
BioSys(08)qp/06;
Ghirardi & Romano JPCS-a1401 [collapse models and perceptual processes];
Ghirardi IJTP(15)-a1411 [reply to Elio Conte].

@ __Constraints__: Jones et al FP(04)qp [SNO experiment];
Curceanu et al JAP(15)-a1502 [from X-ray experiments];
Helou et al PRD(17)-a1606,
Carlesso et al PRD(16)-a1606 [from gravitational-wave detectors].

@ __Other phenomenology__:
Squires PLA(91),
Pearle et al FP(99)qp/00 [and radiation];
Pearle FP(12)-a1003 [and cosmogenesis];
Donadi et al FP(13)-a1207 [and neutrino oscillations],
FP(13)-a1207 [and flavor oscillations];
Okon & Sudarsky FP(14)-a1309 [advantages for cosmology and quantum gravity];
Donadi & Bassi JPA(15)-a1408 [and electromagnetic radiation];
Toroš & Bassi JPA(18)-a1601 [and matter-wave interferometry];
Simonov & Hiesmayr PRA(16)-a1606 [and neutral meson flavor oscillations];
Adler a1712 [minimum temperature for solid objects],
PRA(18)-a1801 [heating by phonon excitation];
Carlesso & Paternostro a1906-ch [opto-mechanical tests];
Vinante et al PRA(19) [tests with levitated nanoparticles].

@ __Collapse over a finite time__:
Marchewka & Schuss a1103;
Ignatiev JPCS(13)-a1204;
Moreno et al a1809
[incompatible with non-local correlations and non-signaling].

@ __Related topics__: Bassi et al JPA(05)qp [and *E* non-conservation],
JPA(07) [and Hilbert space operator formalism];
Bondoni a1006-wd [measurement as mathematical vs phenomenological.process];
Tumulka a1102 [paradoxes and primitive ontology];
Brouzakis et al PLB(12)-a1109;
Weinberg PRA(12)-a1109;
Rizos & Tetradis JHEP(12)-a1112;
Simpson FP(11);
Melkikh a1311 [and quantum field theory];
McQueen SHPMP(15)-a1501 [four tails problems];
Yun a1606 [entangling-speed threshold];
Bedingham & Maroney PRA(17)-a1607 [and time-reversal symmetry].

**And Gravity** > s.a. models of
decoherence [gravity-related]; semiclassical gravity.

* __History__: In the first lecture
of his 1962 course on gravitation Feynman speculated that gravity would enforce
classical behavior for masses larger than the Planck mass; The idea is related
to work on the quantum-classical divide and attempts at building and experimenting
with hybrid quantum-classical devices.

* __Gravity produces wave function collapse__:
A wave function collapses under the influence of gravity in a region where the
matter density reaches a certain value (masses and lengths of the order of bacterial
scales); Gravity provides a decoherence mechanism; Penrose's proposal is motivated
by a fundamental conflict between the superposition principle of quantum mechanics
and the principle of general covariance.

* __Wave function collapse produces gravity__:
2017, An idea proposed by Antoine Tilloy is that when a GRW-type flash collapses a wave
function and causes a particle to be localized in one place, it creates a gravitational
field at that event; Gravity remains classical.

@ __General references__:
Károlyházy NC(66);
Diósi PLA(84)-a1412;
Károlyházy in(85),
et al in(86);
Diósi PRA(89);
Ellis et al PLB(89);
Fivel qp/97;
Anandan FP(99)gq/98;
Christian gq/98-ch;
De Filippo qp/00,
qp/01;
De Filippo et al PhyA(03)qp [simulation];
Mureika PRD(06) [and large extra dimensions];
Adler JPA(07)qp/06;
Salart et al PRL(08)-a0803 [and Bell inequalities];
Diósi JPCS(09)-a0902 [collapse causes gravity];
Diósi JPCS(13)-a1302;
Sharma & Singh GRG(14)-a1403 [Ricci identities and Dirac equations];
Diósi FP(14),
NJP(14)-a1404 [in bulk matter];
Singh JPCS-a1503 [survey];
Banerjee et al IJMPD(15)-a1505-GRF [and the cosmological constant and time].

@ __Relativistic models__: Quandt-Wiese a0912;
Stoica Quanta(16)-a1601 [continuous evolution];
Quandt-Wiese a1701,
a1701 [from semiclassical gravity,
wave function evolution in a dynamically expanding spacetime];
Gasbarri et al PRD(17)-a1701 [mechanism];
Okon & Sudarsky a1701 [in cosmology and quantum gravity].

@ __Other models__: Melko & Mann gq/00 [*D*-dimensional Schrödinger-Newton equations];
Gao IJTP(10)-a1001 [and spacetime discreteness];
Adler a1401-ch
[state vector reduction as driven by spacetime foam];
Bera et al FP(17)-a1608 [stochastic modification of the Schrödinger-Newton equation];
Brody & Hughston a1611-in [energy-driven stochastic master equation for the density matrix];
Korbicz & Tuziemski a1612;
Tilloy PRD(18)-a1709 [GRW with massive flashes];
Laloë a1905
[small imaginary component in the gravitational coupling].

@ __Penrose's proposal__: Penrose in(81),
in(86);
Penrose GRG(96);
Penrose in(00);
Gao SHPMP(13)-a1305 [critique];
Oosterkamp & Zaanen a1401-conf [thought experiment];
Penrose FP(14);
Bahrami et al PRA(14)-a1408 [cutoff, and dissipative generalization].

@ __Gravitational self-interaction__: Colin et al CQG(14)-a1403 [approximate dynamics and experimental setting].

@ __Tests__: Christian PRL(05)qp [with cosmic neutrinos];
van Wezel et al PhilM(08)-a0706 [towards an experimental test];
Quandt-Wiese a1701 [proposal].

> __Phenomenology__:
see matter phenomenology in quantum gravity;
models of dark energy;
quantum cosmological perturbations.

**Continuous Spontaneous Localization**

* __Idea__: CSL models of
spontaneous wave function collapse modify the linear Schrödinger
equation by adding stochastic non-linear terms to it, which leads to
non-conservation of energy for the system under consideration; The
larger a system is, the faster a state superposition will collpase;
> s.a. schrödinger equation.

* __GRW mechanism__: Gives a quantum
theory without observers; In fact, two different ones by using either the matter
density ontology (GRWm) or the flash ontology (GRWf); Testable deviations from
quantum mechanics are known for both theories, but the difference is so small
that no decisive experiment has been performed (2007).

@ __General references__: Pearle IJTP(79),
PRA(89) [randomly fluctuating interaction];
Pearle & Soucek FPL(89) [path integral];
Ghirardi & Pearle in(90);
Nicrosini & Rimini FP(90) [stochastic processes in \(\cal H\)];
Squires PLA(91) [without stochastic field];
Pearle in(97)qp/98,
in(97)qp/98,
LNP(99)qp,
PRA(99)qp,
FP(00)qp [and conservation laws],
PRA(04)qp/03 [energy-driven];
Santos & Escobar qp/98 [beable interpretation];
Hansson qp/00 [from non-abelian non-linearity];
Bassi & Ghirardi PRP(03)qp;
Dowker & Herbauts FPL(05)qp/04 [without wave function];
Lewis SHPMP(05) [interpretation];
Morikawa & Nakamichi PTP(06)qp/05 [as spontaneous symmetry breaking];
Pearle JPA(07)qp/06,
qp/06 [overview];
Bassi JPCS(07)qp [rev];
Bassi et al RMP(13)-a1204 [rev, and tests],
a1212/RMP [stochastic methods];
Bassi & Ulbricht JPCS(14)-a1401 [rev];
Bhatt et al a1808 [quantum to classical transition].

@ __GRW mechanism__:
Ghirardi et al PRA(90),
PRA(90);
Tessieri et al PRA(95);
Clifton & Monton BJPS(99),
Bassi & Ghirardi BJPS(99),
BJPS(99) [and enumeration principle];
Monton SHPMP(04) [interpretation];
Allori et al BJPS(08)qp/06 [and pilot-wave theory];
Vacchini JPA(07) [GRW master equation and decoherence];
Frigg & Hoefer SHPMP(07) [and probabilities];
Goldstein et al JSP(12)-a0710 [testable predictions];
Tumulka RVMP(09)-a0711 [flash ontology];
Bedingham JPA(11) [hidden-variable interpretation];
Esfeld & Gisin a1310 [John Bell's GRW flash theory];
Wallace a1407 [tails of the GRW wave function].

@ __GRW mechanism, variations__: Smirne et al PRA(14)-a1408 [dissipative extension];
Egg & Esfeld a1410 [GRW matter density theory (GRWm)];
Jones et al a1907
[not extendible to relativistic particles].

@ __For quantum field theory__: Tumulka PRS(06)qp/05 [GRW];
Diósi AIP(06)qp [QED];
Derakhshani PLA(13)-a1304 [Newtonian theory of semiclassical gravity];
Pearle a1404-wd,
PRD(15)-a1412 [scalar field, relativistic model];
Pearle a1610-in [for photons].

@ __Astrophysics and cosmology__:
Das et al a1302-MG13 [cosmology];
Tilloy & Stace a1901,
comment Adler et al PRD(19)-a1901 [test with neutron stars];
Martin & Vennin a1906 [cosmology, and the cmb];
> s.a. black-hole information.

@ __Other types of systems__: Bassi JPA(05)qp/04 [single free particle];
Pearle PRA(05)qp [quasirelativistic quasilocal model];
Laloë et al PRA(14)-a1409 [trapped ultra-cold atoms];
Diósi PRL(15)-a1411 [classical mechanical oscillators];
Tilloy a1910-ch [on finite-dimensional Hilbert spaces].

@ __Other phenomenology__: Nimmrichter et al PRA(11) [tests with matter-wave interferometry];
Lochan et al PRD(12) [constraints from cmb spectral distortions];
Carlesso et al NJP(18)-a1708 [non-interferometric test];
Mishra et al a1807 [estimates for bulk heating experiments];
Singh ZfN-a1806 [and emergence of spacetime].

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