Physical Theories

Structure of Theories > s.a. computation; history of physics; Interpretation; logic.
* Method: Knowledge in physics comes from interplay of theory and experiment; In the theory, one simplifies the system and considers simple, closed ones, supposes that observers are not important, identifies the simplest measurements, e.g., m, l, t, and relates to mathematical description; Later, one tries to get rid of ideal elements by making them dynamical, or giving a natural choice (Leibniz's principle of sufficient reason).
* Ideal elements: Formal element which are contingent (a different choice is possible) and play a role in the evolution of the physical degrees of freedom but are non-dynamical, absolute; Examples: Correspondence observables-operators, time, inner product in insert; Number of spacetime dimensions, topology in general relativity; Preferred class of inertial observers in pre-general-relativity physics; > s.a. general relativity, inertia.
* Structure: A theory has a lattice of propositions (including assumptions), structural equations and equations of other origin, about some structure which constitutes a model (or metaphor) for the systems under consideration; P Duhem considered the metaphor itself as an educational tool, not a part of science, while J Bernstein and J Ziman view them as an integral part of science [@ pw(00)nov]; As with any metaphor, a key issue is to establish how far each theory can go.
* Dynamics: It is described in terms of physical laws and initial conditions; This dichotomy appeared with Newton, and modern physics has extended the notion of initial conditions to internal degrees of freedom and fields; some quantization methods try to overcome the distinction.
@ Books: Holton & Roller 58; Ripley 64 [simple]; Cooper 68; Tonti 76; Shive & Weber 82 [II]; Sklar 85; Pavsic 01 [overview].
@ General references: Caianello RNC(92); Foy qp/00 [logical basis]; Tarantola & Mosegaard mp/00 [use of inference].
@ Physical laws: Peres FP(80) [physicist's role]; Yamakawa & Kreinovich IJTP(99) [why second-order equations]; Caticha gq/01-in [as laws of inference]; Butterfield phy/04 [laws and models]; Frisch PhSc(04) [and initial conditions, electrodynamics]; Winsberg PhSc(04) [and statistical mechanics]; Davies in(07)qp [laws as software for universe and limitations]; Crease pw(07)jul [laws expressing impossibilities]; > s.a. physics [nature of laws].

Construction of Theories > s.a. Operationalism.
* Approaches: The main distinction is between operational and deductive ones; The danger with an operational approach is that one may get stuck with technical difficulties and make little progress; The danger with a deductive approach is that progress in the right direction is more likely to be impeded by idealizations.
* Remark: It is important to understand which are the right variables; Which questions we can ask and which make no sense (see Newton's laws, Einstein's relativity, Bohr-Heisenberg principle).
@ References: Corry 04 [Hilbert and axiomatization of physics]; Emch SHPMP(07).
> Specific theories: see cosmology, particle physics, etc.

Criteria for Physical Theories > s.a. Consistency; paradigms.
* Traditional: Adequacy, i.e., verifiability, and good agreement with experiment.
* Stability under variations: Often assumed as a dogma and not discussed explicitly.
* Also: Accuracy, elegance and simplicity (XX cy aesthetic judgment), scope, symmetries.
* Naturalness: Structural and numerical naturalness (no fine tuning); The former is more subjective, and has generated many wrong theories; This criterion might be rejected at some deep scales (see, e.g. Linde's smörgåsbord picture of many universes created after inflation), and needs to be treated with care.
* Examples: Interpretation of positron in Dirac theory, according to "truth" of knowledge at the time it was the proton (Dirac), according to "beauty" it was not (Weyl).
* On non-observable quantities: Around 1926, W Heisenberg advocated using only directly observable quantities in the theory; The point of view was picked up by G Chew in his S-matrix approach to quantum field theory.
* Occam's razor: The principle according to which scientific explanations should be neat, with as few arbitrary assumptions as possible; Avoid doing with more what can be done with less; A.k.a. principle of economy; > s.a. Explanations [truth and simplicity].
@ General considerations on types of theories: Nelson AS(85); Von Weizsäcker 85; Cushing 90; Tavakol BJPS(91) [fragility]; Elby et al FP(93); Barrett PhSc(03) [our best physical theories are false]; Streater 07 ["lost causes"].
@ Occam's razor: Sorkin IJTP(83)ap/05; Garrett PW(91)may; Jefferys & Berger AS(92); Soklakov mp/00; Standish FPL(04) [justification].
@ Stability: Bouligand ARB(35); Destouches ARB(35); Duhem 54; Thom 67; Vilela Mendes JPA(94).
@ Criteria: Einstein JFI(36); McAllister AS(98) [beauty]; Mermin PT(00)mar [elegance]; Norton SHPMP(00) [Einstein and simplicity]; Falmagne FP(04) [meaningfulness + order-invariance]; Tsallis PhyA(04) [beauty, truth and new ideas].

Foundations > s.a. computation; matter; Metaphysics; philosophy of physics; quantum mechanics.
@ Proposals re foundations: Barbour FP(89); Stein 96; Svozil qp/97-in; Collins FP(05); > s.a. physics.
@ Theoretical foundations: Frank 46; Krieger 96 [matter]; Wilczek PT(04)dec [mass and force].
@ Physics and mathematics: Enz & Mehra 74; Oldershaw AJP(88); > s.a. mathematical physics.
@ Development of ideas: Dirac SA(63)may; Ryde 94; Rohrlich FP(96) [unreasonable effectiveness of physical intuition].

Metatheories, Theories of Physical Theories > s.a. Deformations.
* Change of level: When going from one physical theory to a deeper one, the singularities at the former level tend to be dissolved (see atomic stability from classical to quantum mechanics).
* Types of theories: Einstein [@ London Times 1919] distinguished between theories of principles, and theories of constructs.
* Examples: The PPN formalism (> see modified newtonian gravity).
@ References: Giere PhSc(94) [cognitive structure]; Smith BJPS(98) [approximate truth]; Halvorson SHPMP(04)qp/03 [insert, state space and information theory]; D'Agostini phy/04-in [probabilistic reasoning]; Baumann PhSc(05) [re "better theories"]; 't Hooft a0707-in [grand view]; Matravers CP(07); Page a0712-in [Bayesian meta-theories, "sensible quantum mechanics", and quantum cosmology].

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