### Disciplines

Contemporary physics research is divided into various disciplines that study different aspects of the physical world.

Field(s) | Disciplines | Main theories | Some concepts |
---|---|---|---|

Astrophysics and Mechanics | Cosmology, Planetology, Plasma Physics, Big Bang Astroparticles | Big Bang, Cosmic Inflation, General Relativity, Dark Matter, Cosmic Rays | Black Hole, Galaxy, Gravity, Gravitational Wave, Planet, Solar System, Star, Universe |

Quantum physics and Wave physics | Atomic physics, Molecular physics, Optics, Photonics | Quantum optics | Diffraction, Electromagnetic wave, Laser, Polarization, Interference |

Particle physics | Particle Accelerator, Nuclear Physics | Standard Model, Grand Unification Theory, String Theory, M Theory | Elementary Interaction (Gravity, Electromagnetism, Weak Interaction, Strong Interaction), Elementary Particle, Antiparticle, Spin, Spontaneous symmetry breaking |

Statistical Physics and Condensed Matter Physics | Thermodynamics, Solid State Physics, Materials Science, Polymer Physics, Soft Matter, Mesoscopic Physics, Disordered System, Biophysics | Superconductivity, Bloch Wave, Fermionic Condensate, Fermi Liquid | State of Matter (Solid , Liquid, Gas), Plasma, Bose-Einstein Condensate, Supercritical, Superfluid), Conductive, Magnetism, Self-organization |

### Theories

Although physics is concerned with a wide variety of systems, some theories can only be related to physics as a whole and not to any of its fields. Each is assumed to be correct, within some area of validity or applicability. For example, the theory of classical mechanics faithfully describes the motion of an object, provided that

- its dimensions are much larger than those of an atom,
- its speed is much lower than the speed of light,
- it is not too close to a large mass, and
- it is devoid of electric charge.

Old theories, such as Newtonian mechanics, have evolved giving rise to original research subjects, particularly in the study of complex phenomena (example: chaos theory). Their fundamental principles form the basis of all research in physics and any student of physics, whatever his specialty, is expected to acquire the bases of each of them.

Theory | Large domains | Concepts |
---|---|---|

Newtonian mechanics | Kinematics, Newton’s laws of motion, Analytical mechanics, Fluid mechanics, Point mechanics, Solid mechanics, Galileo transformations, Continuous mechanics | Dimension, Space, Time, Frame of reference, Length, Velocity, Relative velocity, Mass, Angular momentum, Force, Energy, Angular momentum, Torque, Law of conservation, Harmonic oscillator, Wave, Work, Power, Balance |

Electromagnetism | Electrostatics, Electricity, Magnetism, Maxwell’s equations | Electric charge, Electric current, Electric field, Magnetic field, Electromagnetic field, Electromagnetic wave |

Statistical Physics and Thermodynamics | Thermal machine, Kinetic theory of gases | Boltzmann constant, Entropy, Free energy, Heat, Partition function, Temperature, Thermodynamic equilibrium, Reversibility |

Quantum mechanics | Path integral, Schrödinger’s equation, Quantum field theory | Hamiltonian, Boson, Fermion, Identical particles, Planck constant, Quantum harmonic oscillator, Wave function, Zero point energy |

Theory of relativity | Galilean Relativity, Restricted Relativity, General Relativity | Equivalence principle, Quadrivector, Space-time, Speed of light, Relative speed, Lorentz invariance |

### Method

#### Theory and experience

(Cross sections of the first orbitals of the hydrogen atom, the color code representing the probability amplitude of the electron (black: zero amplitude, white: maximum amplitude).)

Physicists observe, measure and model the behavior and interactions of matter through space and time in order to bring out general quantitative laws. Time – defined by duration, interval and the correlative construction of scales – and space – the set of places where movement takes place and where the material being or cluster, that is, say the particle, the molecule or the grain, the body of matter… or even the operator, are positioned at a given moment – are real facts observed, transformed into measurable abstract mathematical and physical entities to be logically integrated into the scientific scheme. It is only from these constructions that it is possible to develop secondary concepts with explanatory values. Thus energy, an abstract description of states, a force field or a fractal dimension, can characterize various “physical phenomena”. Metrology is thus a major intermediate branch of physics.

A theory or a model – called a diagram once patiently supported by solid experiments and verified until its ultimate logical consequences, is a mathematically formalized conceptual set, in which physical parameters that are assumed to be independent (load, energy and time, for example) are expressed as variables (*q*, *E* and *t*) and measured with appropriate units (coulomb, joule and second). The theory relates these variables by one or more equations (for example, E = mc^{2}). These relationships make it possible to quantitatively predict the outcome of experiments.

An experiment is a material protocol making it possible to measure certain phenomena of which the theory gives a conceptual representation. It is illusory to isolate an experiment from the associated theory. The physicist obviously does not measure things at random; he must have in mind the conceptual universe of a theory. Aristotle never thought of calculating the time it takes for a dropped stone to reach the ground, simply because his conception of the sublunar world did not envisage such a quantification. This experiment had to wait for Galileo to be done. Another example of an experiment clearly dictated by a theoretical conceptual framework is the discovery of quarks within the framework of particle physics. The particle physicist Gell-Mann noticed that the particles subjected to the strong force were distributed according to an elegant mathematical structure, but that three fundamental positions (in the mathematical sense of the theory of representations) of this structure were not realized. He therefore postulated the existence of particles more fundamental (in the physical sense) than protons and neutrons. Experiments subsequently made it possible, by following this theory, to demonstrate their existence.

Conversely, better or new experiences do not coincide or clash with theory. They can :

- either question the theory – as was the case with the problem of the black body and the representations of light which provoked the advent of quantum mechanics and special and general relativities, in a way analogous to the shaking of the foundations of vitalism in chemistry or the collapse of spontaneous generation in biology.
- or else not fit into accepted theories. The example of the discovery of Neptune is enlightening in this regard. Astronomers could measure the trajectory of Uranus but Newton’s theory gave a different trajectory from that observed. To maintain the theory, Urbain Le Verrier and, independently, John Adams postulated the existence of a new planet, and from this hypothesis predicted its position. The German astronomer Johann Gottfried Galle verified in September 1846 that the calculations of Le Verrier and Adams were correct by observing Neptune at the predicted place. It is clear that the interpretation of the first experiment depends on the theory, and the second could never have taken place without this same theory and its calculation. Another example is the existence of the neutrino, supposed by Pauli to explain the continuous spectrum of beta decay, as well as the apparent non-conservation of angular momentum.
- finally it can give birth to the theory in a purely fortuitous way (serendipity): thus the physicist Henri Becquerel discovered radioactivity in 1895 by accidentally storing uranium salts near a blank photographic plate.

#### The research

The culture of research in physics differs significantly from that of other sciences with regard to the separation between theory and experiment. Since the twentieth century, the majority of physicists have specialized in either theoretical physics or experimental physics. In contrast, almost all renowned theorists in chemistry or biology are also experimenters.

Numerical simulation has occupied a very important place in physics research since the beginnings of computer science. It indeed allows the approximate resolution of mathematical problems which cannot be treated analytically. Many theorists are also ”numericians”.

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