Aphysical theory is a model of physical events. It is judged by the extent to which its predictions agree with empirical observations. The quality of a physical theory is also judged on its ability to make new predictions which can be verified by new observations. A physical theory differs from amathematical theorem in that while both are based on some form ofaxioms, judgment of mathematical applicability is not based on agreement with any experimental results.[3][4] A physical theory similarly differs from amathematical theory, in the sense that the word "theory" has a different meaning in mathematical terms.[b]
A physical theory involves one or more relationships between various measurable quantities.Archimedes realized that a ship floats by displacing its mass of water,Pythagoras understood the relation between the length of avibrating string and the musical tone it produces.[5][6] Other examples includeentropy as a measure of the uncertainty regarding the positions andmotions of unseenparticles and thequantum mechanical idea that (action and)energy are not continuously variable.[citation needed]
Theoretical physics consists of several different approaches. In this regard,theoretical particle physics forms a good example. For instance: "phenomenologists" might employ (semi-)empirical formulas andheuristics to agree with experimental results, oftenwithout deep physical understanding.[c] "Modelers" (also called "model-builders") often appear much like phenomenologists, but try to model speculative theories that have certain desirable features (rather than on experimental data), or apply the techniques ofmathematical modeling to physics problems.[d] Some attempt to create approximate theories, calledeffective theories, because fully developed theories may be regarded as unsolvable ortoo complicated. Other theorists may try tounify, formalise, reinterpret or generalise extant theories, or create completely new ones altogether.[e] Sometimes the vision provided by pure mathematical systems can provide clues to how a physical system might be modeled;[f] e.g., the notion, due toRiemann and others, thatspace itself might be curved. Theoretical problems that need computational investigation are often the concern ofcomputational physics.
Physical theories become accepted if they are able to make correct predictions and no (or few) incorrect ones. The theory should have, at least as a secondary objective, a certain economy and elegance (compare tomathematical beauty), a notion sometimes called "Occam's razor" after the 13th-century English philosopherWilliam of Occam (or Ockham), in which the simpler of two theories that describe the same matter just as adequately is preferred (but conceptual simplicity may mean mathematical complexity).[10] They are also more likely to be accepted if they connect a wide range of phenomena. Testing the consequences of a theory is part of thescientific method.[11]
All of these achievements depended on the theoretical physics as a moving force both to suggest experiments and to consolidate results — often by ingenious application of existing mathematics, or, as in the case of Descartes and Newton (withLeibniz), by inventing new mathematics.Fourier's studies of heat conduction led to a new branch of mathematics:infinite, orthogonal series.[14]
Modern theoretical physics attempts to unify theories and explain phenomena in further attempts to understand theUniverse, from thecosmological to theelementary particle scale. Where experimentation cannot be done, theoretical physics still tries to advance through the use of mathematical models.[citation needed]
Mainstream theories (sometimes referred to ascentral theories) are the body of knowledge of both factual and scientific views and possess a usual scientific quality of the tests of repeatability, consistency with existing well-established science and experimentation. There do exist mainstream theories that are generally accepted theories based solely upon their effects explaining a wide variety of data, although the detection, explanation, and possible composition are subjects of debate.[citation needed]
Theproposed theories of physics are usually relatively new theories which deal with the study of physics which include scientific approaches, means for determining the validity of models and new types of reasoning used to arrive at the theory. However, some proposed theories include theories that have been around for decades and have eluded methods of discovery and testing. Proposed theories can include fringe theories in the process of becoming established (and, sometimes, gaining wider acceptance). Proposed theories usually have not been tested. In addition to the theories like those listed below, there are also differentinterpretations of quantum mechanics, which may or may not be considered different theories since it is debatable whether they yield different predictions for physical experiments, even in principle. For example,AdS/CFT correspondence,Chern–Simons theory,graviton,magnetic monopole,string theory,theory of everything.[citation needed]
Fringe theories include any new area of scientific endeavor in the process of becoming established and some proposed theories. It can include speculative sciences. This includes physics fields and physical theories presented in accordance with known evidence, and a body of associated predictions have been made according to that theory.[citation needed]
Some fringe theories go on to become a widely accepted part of physics. Other fringe theories end up being disproven. Some fringe theories are a form ofprotoscience and others are a form ofpseudoscience. The falsification of the original theory sometimes leads to reformulation of the theory.[citation needed]
"Thought" experiments are situations created in one's mind, asking a question akin to "suppose you are in this situation, assuming such is true, what would follow?". They are usually created to investigate phenomena that are not readily experienced in every-day situations. Famous examples of such thought experiments areSchrödinger's cat, theEPR thought experiment,simple illustrations of time dilation, and so on. These usually lead to real experiments designed to verify that the conclusion (and therefore the assumptions) of the thought experiments are correct. The EPR thought experiment led to theBell inequalities, which were thentested to various degrees of rigor, leading to the acceptance of the current formulation ofquantum mechanics andprobabilism as aworking hypothesis.[citation needed]
^There is some debate as to whether or not theoretical physics uses mathematics to build intuition and illustrativeness to extract physical insight (especially when normalexperience fails), rather than as a tool in formalizing theories. This links to the question of it using mathematics in a less formally rigorous, and more intuitive orheuristic way than, say,mathematical physics.
^Sometimes the word "theory" can be used ambiguously in this sense, not to describe scientific theories, but research (sub)fields and programmes. Examples: relativity theory, quantum field theory, string theory.
^Arguably these are the most celebrated theories in physics: Newton's theory of gravitation, Einstein's theory of relativity and Maxwell's theory of electromagnetism share some of these attributes.
^This approach is often favoured by (pure) mathematicians and mathematical physicists.
^See 'Correspondence of Isaac Newton, vol.2, 1676–1687' ed. H W Turnbull, Cambridge University Press 1960; at page 297, document #235, letter from Hooke to Newton dated 24 November 1679.
Duhem, Pierre.La théorie physique - Son objet, sa structure, (in French). 2nd edition - 1914. English translation:The physical theory - its purpose, its structure. Republished byJoseph Vrin philosophical bookstore (1981),ISBN2711602214.
Famous series of books dealing with theoretical concepts in physics covering 10 volumes, translated into many languages and reprinted over many editions. Often known simply as "Landau and Lifschits" or "Landau-Lifschits" in the literature.
