From Citizendium
Revision as of 16:32, 25 January 2007 by Kristof Moors (Talk | contribs)

Jump to: navigation, search

Physics is science of matter and energy, and is the most basic of the natural sciences. Physics deals with the fundamental constituents of matter and their interactions, as well as how matter is organized on all possible length and time scales. Physics aims to provide unified descriptions of the behavior of matter and energy, from fundamental principles as much as possible, while describing a wide-variety of phenoma. Physics has considerable overlap with other sciences and engineering fields, most notably biology, chemistry, mathematics, electrical engineering and materials science

Physicists study a wide range of physical phenomena, from quarks to galaxies, from individual atoms to macroscopic biological systems.

Physics and Other disciplines

Physics finds applications throughout the other natural sciences as they regard the basic principles of nature. Physics is often said to be the "fundamental science", because the other sciences deal with material systems that obey the laws of physics. For example, chemistry is the science of matter (such as atoms and molecules) and the chemical substances that they form in the bulk. The structure, reactivity, and properties of a chemical compound are determined by the properties of the underlying molecules, which can be described by areas of physics such as quantum mechanics (in the applied subfiled of quantum chemistry), thermodynamics, and electromagnetism.

Physics is closely related to mathematics, which provides the logical framework in which physical laws can be precisely formulated and their predictions quantified. Physical definitions, models and theories are invariably expressed using mathematical relations. A key difference between physics and mathematics is that because physics is ultimately concerned with descriptions of the material world, it tests its theories by observations (called experiments), whereas mathematics does not have such requirements. The distinction, however, is not always clear-cut. This large area of research intermediate between physics and mathematics is known as mathematical physics.

Physics is also closely related to engineering and technology. For instance, electrical engineering is the study of the practical application of electromagnetism. Statics, a subfield of mechanics, is responsible for the building of bridges. Further, physicists, or practitioners of physics, invent and design processes and devices, such as the transistor, whether in basic or applied research. Experimental physicists design and perform experiments with particle accelerators, nuclear reactors, telescopes, barometers, synchrotrons, cyclotrons, spectrometers, lasers, and other equipment.

Central theories

While physics deals with a wide variety of systems, there are certain theories that are basis for physics. Each of these theories has been experimentally tested numerous times and found correct as an approximation of nature (within a certain domain of validity). These theories continue to be areas of active research; for instance, a remarkable aspect of classical mechanics known as chaos was discovered in the 20th century, three centuries after the original formulation of classical mechanics by Isaac Newton (16421727). These "central theories" are important tools for research into more specialized topics, and any physicist, regardless of his or her specialization, is expected to be literate in them.

  • Quantum mechanics is the branch of physics treating atomic and subatomic systems and their interaction with radiation in terms of observable quantities. It is based on the observation that all forms of energy are released in discrete units or bundles called quanta. Quantum theory typically permits only probable or statistical calculation of the observed features of subatomic particles, understood in terms of wave functions. Quantum mechanics itself has several levels of approximation.
  • Classical mechanics is a model of the physics of forces acting upon bodies. As opposed to quantum mechanics, classical mechanics is deterministic. Classical mechanics is usually regarded as a limit of quantum mechanics, although this has not been proven in general. Classical mechanics can be divided into two parts: Newtonian, after Newton and his laws of motion, and relativistic, due to Einstein and his theory of relativity.

Research and fields within physics

Physics can be subdivided in a variety of different manners; for teaching, for historical purposes, or for research purposes.

Contemporary research in physics is divided into several subfields. The following is based on the units the members of the American Physical Society organize themselves into in order of size. Although many physicists are members of multiple units

  • Particle physics, also known as "high-energy physics". This branch is concerned with the properties of subatomic particles much smaller than atoms, including the elementary particles from which all other units of matter are constructed.
  • Atomic, molecular, and optical physics (AMO physics) which deals with the behavior of individual atoms and molecules, and including the ways in which they absorb and emit light. Molecular physics is sometimes considered a branch of chemical physics. Laser science may be considered a subfield of AMO or as a separate field.
  • Fluid dynamics concerns itself with the study of the moving fluids; liquids or gases.
  • Biophysics or biological physics. Many members of this field are actually biologists and chemists who are members of other societies. As evidenced by the size of the Biophysical Society, ~7000 members when compared to the biological physics unit of the APS , ~1800 members.

Classical and quantum physics

Further information: Classical physics, Quantum physics, Modern physics, Semiclassical

Another overall classification of physicial theories is classical vs. quantum. All theories should ultimately become quantum, but classical theories are often both sufficiently valid and accurate so as to avoid the need for quantum theories. Additionally, for some classical theories, e.g., relativity, quantum analogs have yet to be fully determine.

Quantum physics refers to those theories which follow the postulates of quantum mechanics. To this effect, all results that are not quantized are called classical: this includes the special and general theories of relativity. Simply because a result is classical does not mean that it was discovered before the advent of quantum mechanics.

Both classical and quantum physics are active areas of research. However, there exist problems in physics in which classical and quantum aspects must be combined to attain some approximation or limit that may acquire several forms as the passage from classical to quantum mechanics is often difficult — such problems are termed semiclassical.

Theoretical and experimental physics

Most individual physicists specialize in either theoretical physics or experimental physics. There have been few exceptions, such as great Italian physicist Enrico Fermi (19011954), who made fundamental contributions to both theory and experimentation.

Roughly speaking, theorists seek to develop theories, through mathematical and computational models, that can both describe and interpret existing experimental results and successfully predict future results, while experimentalists devise and perform experiments to explore new phenomena and test theoretical predictions. Although theory and experiment can be developed separately, they are strongly dependent on each other. Progress in physics frequently comes about when experimentalists make a discovery that existing theories cannot account for, necessitating the formulation of new theories, or when theorists make predictions that experimentalists test.

Current research directions

For more information, see: Unsolved problems in physics.

Research in physics is progressing constantly on a large number of fronts, and is likely to do so for the foreseeable future. A sampling of a few current directions include.

In condensed matter physics, the biggest unsolved theoretical problem is the explanation for high-temperature superconductivity. Strong efforts, largely experimental, are being put into making workable spintronics and quantum computers.

In particle physics, the first pieces of experimental evidence for physics beyond the Standard Model have begun to appear. Foremost amongst these are indications that neutrinos have non-zero mass. These experimental results appear to have solved the long-standing solar neutrino problem in solar physics. The physics of massive neutrinos is currently an area of active theoretical and experimental research. In the next several years, particle accelerators will begin probing energy scales in the TeV range, in which experimentalists are hoping to find evidence for the Higgs boson and supersymmetric particles.

Theoretical attempts to unify quantum mechanics and general relativity into a single theory of quantum gravity, a program ongoing for over half a century, have not yet borne fruit. The current leading candidates are M-theory, superstring theory and loop quantum gravity.

Many astronomical and cosmological phenomena have yet to be satisfactorily explained, including the existence of ultra-high energy cosmic rays, the baryon asymmetry, the acceleration of the universe and the anomalous rotation rates of galaxies.

Although much progress has been made in high-energy, quantum, and astronomical physics, many everyday phenomena, involving complexity, chaos, or turbulence are still poorly understood. Complex problems that seem like they could be solved by a clever application of dynamics and mechanics, such as the formation of sandpiles, nodes in trickling water, the shape of water droplets, mechanisms of surface tension catastrophes, or self-sorting in shaken heterogeneous collections are unsolved. These complex phenomena have received growing attention since the 1970s for several reasons, not least of which has been the availability of modern mathematical methods and computers which enabled complex systems to be modeled in new ways. The interdisciplinary relevance of complex physics has also increased, as exemplified by the study of turbulence in aerodynamics or the observation of pattern formation in biological systems.

Two rapidly-growing applied fields to which physics makes contributions are biophysics, or biological physics, and nanotechnology.

See also

Further reading

Popular reading

University-level textbooks


  • Hewitt, Paul (2001). Conceptual Physics with Practicing Physics Workbook (9th ed.). Addison Wesley. ISBN 0-321-05202-1. 
  • Serway, Raymond A.; Jewett, John W. (2004). Physics for Scientists and Engineers (6th ed.). Brooks/Cole. ISBN 0-534-40842-7. 


  • Thornton, Stephen T.; Marion, Jerry B. (2003). Classical Dynamics of Particles and Systems (5th ed.). Brooks Cole. ISBN 0-534-40896-6. 
  • Kroemer, Herbert; Kittel, Charles (1980). Thermal Physics (2nd ed.). W. H. Freeman Company. ISBN 0-7167-1088-9. 
  • Tipler, Paul; Llewellyn, Ralph (2002). Modern Physics (4th ed.). W. H. Freeman. ISBN 0-7167-4345-0. 
  • Arfken, George B.; Weber, Hans J. (2000). Mathematical Methods for Physicists (5th ed.). Academic Press. ISBN 0-12-059825-6. 


  • Jackson, John D. (1998). Classical Electrodynamics (3rd ed.). Wiley. ISBN 0-471-30932-X. 
  • Huang, Kerson (1990). Statistical Mechanics. Wiley, John & Sons, Inc. ISBN 0-471-81518-7. 
  • Merzbacher, Eugen (1998). Quantum Mechanics. Wiley, John & Sons, Inc. ISBN 0-471-88702-1. 
  • Peskin, Michael E.; Schroeder, Daniel V. (1994). Introduction to Quantum Field Theory. Perseus Publishing. ISBN 0-201-50397-2. 
  • Wald, Robert M. (1984). General Relativity. University of Chicago Press. ISBN 0-226-87033-2. 


  • Cropper, William H. (2004). Great Physicists: The Life and Times of Leading Physicists from Galileo to Hawking. Oxford University Press. ISBN 0-19-517324-4. 
  • Heilbron, John L. (2005). The Oxford Guide to the History of Physics and Astronomy. Oxford University Press. ISBN 0-19-517198-5. 
  • Weaver, Jefferson H. (editor) (1987). The World of Physics. Simon and Schuster. ISBN 0-671-49931-9.  A selection of 56 articles, written by physicists. Commentaries and notes by Lloyd Motz and Dale McAdoo.

External links


Template:Natural sciences-footer